Patent Publication Number: US-2023150098-A1

Title: Rotary impact tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Pat. Application No. 16/738,113, filed Jan. 9, 2020, now U.S. Pat. No. 11,554,468, which claims priority to U.S. Provisional Pat. Application No. 62/816,263 filed on Mar. 11, 2019, and U.S. Provisional Pat. Application No. 62/790,350 filed on Jan. 9, 2019, the entire contents of all of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to power tools, and more specifically to rotary impact tools. 
     BACKGROUND OF THE INVENTION 
     Rotary impact tools utilize a motor and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. Some rotary impact tools include an electric motor and an onboard battery for powering the electric motor. 
     SUMMARY OF THE INVENTION 
     The present invention provides, in one aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece of at least 900 ft-lbs of fastening torque. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a battery pack supported by the housing for providing power to the motor. The battery pack has a nominal voltage of at least 18 Volts and a nominal capacity of at least 5 Ah. The rotary impact tool has an overall weight including the battery pack that is less than or equal to 7.5 pounds. A ratio of the fastening torque to the overall weight is greater than or equal to 120 ft-lbs per pound. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a battery pack supported by the housing for providing power to the motor. The battery pack has a nominal voltage of at least 18 Volts and a nominal capacity of at least 5 Ah. The rotary impact tool has an overall weight including the battery pack that is less than or equal to 7.5 lbs. A peak output speed of the drive assembly to the overall weight is greater than or equal to 280 revolutions per minute per pound. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a battery pack supported by the housing for providing power to the motor. The battery pack has a nominal voltage of at least 18 Volts and a nominal capacity of at least 5 Ah. The rotary impact tool has an overall weight including the battery pack that is less than or equal to 7.5 pounds. A ratio of peak impact frequency provided by the drive assembly to the overall weight is greater than or equal to 350 impacts per minute per pound. 
     The present invention provides, in another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece of at least 975 ft-lbs of fastening torque. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a battery pack supported by the housing for providing power to the motor. The battery pack has a nominal voltage of at least 18 Volts and a nominal capacity of at least 9 Ah. The rotary impact tool has an overall weight including the battery pack that is less than or equal to 8.5 pounds. A ratio of the fastening torque to the overall weight is greater than or equal to 114 ft-lbs per pound. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a battery pack supported by the housing for providing power to the motor. The battery pack has a nominal voltage of at least 18 Volts and a nominal capacity of at least 5 Ah. The rotary impact tool has an overall weight including the battery pack that is less than or equal to 7.5 lbs. A mechanism efficiency of the rotary impact tool is defined as: 
     
       
         
           
             
               η 
               a 
             
             = 
             
               
                 B 
                 P 
                 M 
                 × 
                 K 
                 
                   E 
                   
                     H 
                     a 
                     m 
                     m 
                     e 
                     r 
                     , 
                     D 
                     r 
                     i 
                     l 
                     l 
                     i 
                     n 
                     g 
                   
                 
               
               
                 V 
                 o 
                 l 
                 t 
                 a 
                 g 
                 
                   e 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
                 × 
                 C 
                 u 
                 r 
                 r 
                 e 
                 n 
                 
                   t 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
               
             
           
         
       
     
      BPM is the number of impacts per minute, KE Hammer,   Drilling  is a kinetic energy of the hammer during a loaded condition and prior to impact with the anvil, Voltage motor  is a voltage across the motor, and Current motor  is a current drawn by the motor. A first performance ratio (PR 1 ) of the rotary impact tool is defined as: 
     
       
         
           
             
               
                 PR 
               
               1 
             
             = 
             
               
                 
                   
                     
                       η 
                       a 
                     
                   
                   
                     I 
                     n 
                     e 
                     r 
                     t 
                     i 
                     
                       a 
                       
                         h 
                         a 
                         m 
                         m 
                         e 
                         r 
                       
                     
                   
                 
               
             
             × 
             
               
                 
                   1 
                   
                     216 
                     , 
                     000 
                   
                 
               
             
           
         
       
     
      Inertia hammer  is a moment of inertia of the hammer. The first performance ratio of the rotary impact tool is greater than 1. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a battery pack supported by the housing for providing power to the motor. The battery pack has a nominal voltage of at least 18 Volts and a nominal capacity of at least 5 Ah. The rotary impact tool has an overall weight including the battery pack that is less than or equal to 7.5 lbs. A mechanism efficiency of the rotary impact tool is defined as: 
     
       
         
           
             
               η 
               a 
             
             = 
             
               
                 B 
                 P 
                 M 
                 × 
                 K 
                 
                   E 
                   
                     H 
                     a 
                     m 
                     m 
                     e 
                     r 
                     , 
                     D 
                     r 
                     i 
                     l 
                     l 
                     i 
                     n 
                     g 
                   
                 
               
               
                 V 
                 o 
                 l 
                 t 
                 a 
                 g 
                 
                   e 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
                 × 
                 C 
                 u 
                 r 
                 r 
                 e 
                 n 
                 
                   t 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
               
             
           
         
       
     
      BPM is the number of impacts per minute, KE Hammer,   Drilling  is a kinetic energy of the hammer during a loaded condition and prior to impact with the anvil, Voltage motor  is a voltage across the motor, and Current motor  is a current drawn by the motor. A second performance ratio (PR 2 ) of the rotary impact tool is defined as: 
     
       
         
           
             
               
                 PR 
               
               2 
             
             = 
             
               
                 
                   
                     
                       η 
                       a 
                     
                     × 
                     R 
                     P 
                     
                       M 
                       
                         n 
                         o 
                         − 
                         l 
                         o 
                         a 
                         d 
                       
                     
                   
                   
                     I 
                     n 
                     e 
                     r 
                     t 
                     i 
                     
                       a 
                       
                         h 
                         a 
                         m 
                         m 
                         e 
                         r 
                       
                     
                   
                 
               
             
             × 
             
               
                 
                   1 
                   
                     216 
                     , 
                     000 
                     , 
                     000 
                   
                 
               
             
           
         
       
     
      RPM no-load  is a rotational frequency of the impact mechanism under a no-load condition and Inertia hammer  is a moment of inertia of the hammer. The second performance ratio of the rotary impact tool is greater than 2. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a battery pack supported by the housing for providing power to the motor. The battery pack has a nominal voltage of at least 18 Volts and a nominal capacity of at least 5 Ah. The rotary impact tool has an overall weight including the battery pack that is less than or equal to 7.5 lbs. A mechanism efficiency of the rotary impact tool is defined as: 
     
       
         
           
             
               η 
               a 
             
             = 
             
               
                 B 
                 P 
                 M 
                 × 
                 K 
                 
                   E 
                   
                     H 
                     a 
                     m 
                     m 
                     e 
                     r 
                     , 
                     D 
                     r 
                     i 
                     l 
                     l 
                     i 
                     n 
                     g 
                   
                 
               
               
                 V 
                 o 
                 l 
                 t 
                 a 
                 g 
                 
                   e 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
                 × 
                 C 
                 u 
                 r 
                 r 
                 e 
                 n 
                 
                   t 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
               
             
           
         
       
     
      BPM is the number of impacts per minute, KE Hammer,   Drilling  is a kinetic energy of the hammer during a loaded condition and prior to impact with the anvil, Voltage motor  is a voltage across the motor, and Current motor  is a current drawn by the motor. A third performance ratio (PR 3 ) of the rotary impact tool is defined as: 
     
       
         
           
             
               
                 PR 
               
               3 
             
             = 
             
               
                 
                   
                     
                       η 
                       a 
                     
                   
                   
                     M 
                     a 
                     s 
                     
                       s 
                       
                         h 
                         a 
                         m 
                         m 
                         e 
                         r 
                       
                     
                   
                 
               
             
             × 
             
               
                 
                   1 
                   
                     60 
                   
                 
               
             
           
         
       
     
      Mass hammer  is a mass of the hammer. The third performance ratio of the rotary impact tool is greater than 2. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a battery pack supported by the housing for providing power to the motor. The battery pack has a nominal voltage of at least 18 Volts and a nominal capacity of at least 5 Ah. The rotary impact tool has an overall weight including the battery pack that is less than or equal to 7.5 lbs. A mechanism efficiency of the rotary impact tool is defined as: 
     
       
         
           
             
               η 
               a 
             
             = 
             
               
                 B 
                 P 
                 M 
                 × 
                 K 
                 
                   E 
                   
                     H 
                     a 
                     m 
                     m 
                     e 
                     r 
                     , 
                     D 
                     r 
                     i 
                     l 
                     l 
                     i 
                     n 
                     g 
                   
                 
               
               
                 V 
                 o 
                 l 
                 t 
                 a 
                 g 
                 
                   e 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
                 × 
                 C 
                 u 
                 r 
                 r 
                 e 
                 n 
                 
                   t 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
               
             
           
         
       
     
      BPM is the number of impacts per minute, KE Hammer,   Drilling  is a kinetic energy of the hammer during a loaded condition and prior to impact with the anvil, Voltage motor  is a voltage across the motor, and Current motor  is a current drawn by the motor. A fourth performance ratio (PR 4 ) of the rotary impact tool is defined as: 
     
       
         
           
             
               
                 PR 
               
               4 
             
             = 
             
               
                 
                   
                     
                       η 
                       a 
                     
                     × 
                     R 
                     P 
                     
                       M 
                       
                         n 
                         o 
                         − 
                         l 
                         o 
                         a 
                         d 
                       
                     
                   
                   
                     M 
                     a 
                     s 
                     
                       s 
                       
                         h 
                         a 
                         m 
                         m 
                         e 
                         r 
                       
                     
                   
                 
               
             
             × 
             
               
                 
                   1 
                   
                     3 
                     , 
                     600 
                   
                 
               
             
           
         
       
     
      RPM no-load  is a rotational frequency of the impact mechanism under a no-load condition and Mass hammer  is a mass of the hammer. The fourth performance ratio of the rotary impact tool is greater than 65. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a battery pack supported by the housing for providing power to the motor. The battery pack has a nominal voltage of at least 18 Volts and a nominal capacity of at least 9 Ah. The rotary impact tool has an overall weight including the battery pack that is less than or equal to 8.5 lbs. A mechanism efficiency of the rotary impact tool is defined as: 
     
       
         
           
             
               η 
               a 
             
             = 
             
               
                 B 
                 P 
                 M 
                 × 
                 K 
                 
                   E 
                   
                     H 
                     a 
                     m 
                     m 
                     e 
                     r 
                     , 
                     D 
                     r 
                     i 
                     l 
                     l 
                     i 
                     n 
                     g 
                   
                 
               
               
                 V 
                 o 
                 l 
                 t 
                 a 
                 g 
                 
                   e 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
                 × 
                 C 
                 u 
                 r 
                 r 
                 e 
                 n 
                 
                   t 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
               
             
           
         
       
     
      BPM is the number of impacts per minute, KE Hammer,   Drilling  is a kinetic energy of the hammer during a loaded condition and prior to impact with the anvil, Voltage motor  is a voltage across the motor, and Current motor  is a current drawn by the motor and a voltage across the motor. A first performance ratio (PR 1 ) of the rotary impact tool is defined as: 
     
       
         
           
             
               
                 PR 
               
               1 
             
             = 
             
               
                 
                   
                     
                       η 
                       a 
                     
                   
                   
                     I 
                     n 
                     e 
                     r 
                     t 
                     i 
                     
                       a 
                       
                         h 
                         a 
                         m 
                         m 
                         e 
                         r 
                       
                     
                   
                 
               
             
             × 
             
               
                 
                   1 
                   
                     216 
                     , 
                     000 
                   
                 
               
             
           
         
       
     
      Inertia hammer  is a moment of inertia of the hammer. The first performance ratio of the rotary impact tool is greater than 1. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a battery pack supported by the housing for providing power to the motor. The battery pack has a nominal voltage of at least 18 Volts and a nominal capacity of at least 9 Ah. The rotary impact tool has an overall weight including the battery pack that is less than or equal to 8.5 lbs. A mechanism efficiency of the rotary impact tool is defined as: 
     
       
         
           
             
               η 
               a 
             
             = 
             
               
                 B 
                 P 
                 M 
                 × 
                 K 
                 
                   E 
                   
                     H 
                     a 
                     m 
                     m 
                     e 
                     r 
                     , 
                     D 
                     r 
                     i 
                     l 
                     l 
                     i 
                     n 
                     g 
                   
                 
               
               
                 V 
                 o 
                 l 
                 t 
                 a 
                 g 
                 
                   e 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
                 × 
                 C 
                 u 
                 r 
                 r 
                 e 
                 n 
                 
                   t 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
               
             
           
         
       
     
      BPM is the number of impacts per minute, KE Hammer,   Drilling  is a kinetic energy of the hammer during a loaded condition and prior to impact with the anvil, Voltage motor  is a voltage across the motor, and Current motor  is a current drawn by the motor and a voltage across the motor. A second performance ratio (PR 2 ) of the rotary impact tool is defined as: 
     
       
         
           
             
               
                 PR 
               
               2 
             
             = 
             
               
                 
                   
                     
                       η 
                       a 
                     
                     × 
                     R 
                     P 
                     
                       M 
                       
                         n 
                         o 
                         − 
                         l 
                         o 
                         a 
                         d 
                       
                     
                   
                   
                     I 
                     n 
                     e 
                     r 
                     t 
                     i 
                     
                       a 
                       
                         h 
                         a 
                         m 
                         m 
                         e 
                         r 
                       
                     
                   
                 
               
             
             × 
             
               
                 
                   1 
                   
                     216 
                     , 
                     000 
                     , 
                     000 
                   
                 
               
             
           
         
       
     
      RPM no-load  is a rotational frequency of the impact mechanism under a no-load condition and Inertia hammer  is a moment of inertia of the hammer. The second performance ratio of the rotary impact tool is greater than 2. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a battery pack supported by the housing for providing power to the motor. The battery pack has a nominal voltage of at least 18 Volts and a nominal capacity of at least 9 Ah. The rotary impact tool has an overall weight including the battery pack that is less than or equal to 8.5 lbs. A mechanism efficiency of the rotary impact tool is defined as: 
     
       
         
           
             
               η 
               a 
             
             = 
             
               
                 B 
                 P 
                 M 
                 × 
                 K 
                 
                   E 
                   
                     H 
                     a 
                     m 
                     m 
                     e 
                     r 
                     , 
                     D 
                     r 
                     i 
                     l 
                     l 
                     i 
                     n 
                     g 
                   
                 
               
               
                 V 
                 o 
                 l 
                 t 
                 a 
                 g 
                 
                   e 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
                 × 
                 C 
                 u 
                 r 
                 r 
                 e 
                 n 
                 
                   t 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
               
             
           
         
       
     
      BPM is the number of impacts per minute, KE Hammer,   Drilling  is a kinetic energy of the hammer during a loaded condition and prior to impact with the anvil, Voltage motor  is a voltage across the motor, and Current motor  is a current drawn by the motor and a voltage across the motor. A third performance ratio (PR 3 ) of the rotary impact tool is defined as: 
     
       
         
           
             
               
                 PR 
               
               3 
             
             = 
             
               
                 
                   
                     
                       η 
                       a 
                     
                   
                   
                     M 
                     a 
                     s 
                     
                       s 
                       
                         h 
                         a 
                         m 
                         m 
                         e 
                         r 
                       
                     
                   
                 
               
             
             × 
             
               
                 
                   1 
                   
                     60 
                   
                 
               
             
           
         
       
     
      Mass hammer  is a mass of the hammer. The third performance ratio of the rotary impact tool is greater than 2. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a battery pack supported by the housing for providing power to the motor. The battery pack has a nominal voltage of at least 18 Volts and a nominal capacity of at least 9 Ah. The rotary impact tool has an overall weight including the battery pack that is less than or equal to 8.5 lbs. A mechanism efficiency of the rotary impact tool is defined as: 
     
       
         
           
             
               η 
               a 
             
             = 
             
               
                 B 
                 P 
                 M 
                 × 
                 K 
                 
                   E 
                   
                     H 
                     a 
                     m 
                     m 
                     e 
                     r 
                     , 
                     D 
                     r 
                     i 
                     l 
                     l 
                     i 
                     n 
                     g 
                   
                 
               
               
                 V 
                 o 
                 l 
                 t 
                 a 
                 g 
                 
                   e 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
                 × 
                 C 
                 u 
                 r 
                 r 
                 e 
                 n 
                 
                   t 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
               
             
           
         
       
     
      BPM is the number of impacts per minute, KE Hammer,   Drilling  is a kinetic energy of the hammer during a loaded condition and prior to impact with the anvil, Voltage motor  is a voltage across the motor, and Current motor  is a current drawn by the motor and a voltage across the motor. A fourth performance ratio (PR 4 ) of the rotary impact tool is defined as: 
     
       
         
           
             
               
                 PR 
               
               4 
             
             = 
             
               
                 
                   
                     
                       η 
                       a 
                     
                     × 
                     R 
                     P 
                     
                       M 
                       
                         n 
                         o 
                         − 
                         l 
                         o 
                         a 
                         d 
                       
                     
                   
                   
                     M 
                     a 
                     s 
                     
                       s 
                       
                         h 
                         a 
                         m 
                         m 
                         e 
                         r 
                       
                     
                   
                 
               
             
             × 
             
               
                 
                   1 
                   
                     3 
                     , 
                     600 
                   
                 
               
             
           
         
       
     
      RPM no-load  is a rotational frequency of the impact mechanism under a no-load condition and Mass hammer  is a mass of the hammer. The fourth performance ratio of the rotary impact tool is greater than 65. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing defining a rear of the rotary impact tool and a top of the rotary impact tool, an electric motor supported within the housing, a handle having a first end coupled to the housing and an opposite second end, a battery receptacle coupled to the second end of the handle, and a battery pack attachable to the battery receptacle. The battery pack defines a bottom of the rotary impact tool and provides power to the motor when attached to the battery receptacle. The rotary impact tool further includes a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The distal end of the anvil defines a front of the rotary impact tool. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. A tool length is defined between the rear of the rotary impact tool and the front of the rotary impact tool. A tool height is defined between the bottom of the rotary impact tool and the top of the rotary impact tool. A ratio of the tool length to the tool height is less than or equal to 1. 
     The present invention provides in yet another aspect, a rotary impact tool comprising a housing defining a top of the rotary impact tool, an electric motor supported within the housing, and a handle having a first end coupled to the housing and an opposite second end. The handle has a foot at the second end. The rotary impact tool further comprises a battery receptacle coupled to the foot of the handle and a battery pack attachable to the battery receptacle. The battery pack defines a bottom of the rotary impact tool and provides power to the motor when attached to the battery receptacle. The rotary impact tool further comprises a trigger on the handle to activate the motor. The trigger has a bottom lip in facing relationship with the foot of the handle. The rotary impact tool further comprises a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The distal end of the anvil defines a front of the rotary impact tool. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. A handle height is defined between a top surface of the foot and the bottom lip of the trigger and a tool height is defined between the bottom and the top of the rotary impact tool. A ratio of the handle height to the tool height is greater than or equal to 0.3. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having a bore in a distal end thereof for receipt of the workpiece or a tool bit for performing work on the workpiece. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and has a nominal width of 7/16 inches. The drive assembly further includes a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil. The drive assembly also includes a spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further includes a collar having a body surrounding the anvil. The collar is moveable along the anvil between a first position, in which the tool bit is locked within the anvil, and a second position, in which the tool bit is removable from the anvil. The collar is biased towards the first position. The collar includes knurling on an outer surface of the body and a lip extending away from the rotational axis that is graspable by a user for moving the collar from the first positon to the second position. 
     The present invention provides, in yet another aspect, a rotary impact tool comprising a housing, an electric motor supported in the housing, and a drive assembly for converting a continuous torque input from the motor to consecutive rotational impacts upon a workpiece. The drive assembly includes an anvil having an outer surface and a longitudinal bore in a distal end of the anvil configured to receive a tool bit for performing work on the workpiece. The tool bit has a bit recess. The bore defines a hexagonal cross-sectional shape in a plane oriented transverse to a rotational axis of the anvil and the bore has a nominal width of 7/16 inches. The drive assembly further includes a plunger detent aperture extending radially inward from the outer surface to the bore, a bit detent aperture extending radially inward from the outer surface to the bore, a hammer that is both rotationally and axially movable relative to the anvil for imparting the consecutive rotational impacts upon the anvil, and a hammer spring for biasing the hammer in an axial direction toward the anvil. The rotary impact tool further comprises a bit detent arranged in the bit detent aperture. The bit detent is moveable between a first bit detent position, in which the bit detent is at least partially in the bore, and a second bit detent position, in which the bit detent is out of the bore. The rotary impact tool further comprises a plunger in the bore. The plunger has a plunger detent recess. The rotary impact tool further comprises a plunger detent arranged in the plunger detent aperture. The plunger detent is moveable between a first plunger detent position, in which the plunger detent is at least partially in the plunger detent recess, and a second plunger detent position, in which the plunger detent is out of the plunger detent recess. The rotary impact tool further comprises a plunger spring biasing the plunger toward the distal end of the anvil, an O-ring at least partially arranged in the bit detent aperture, and a collar surrounding the anvil. The collar is moveable along the anvil between a first collar position, in which the plunger detent is inhibited by the collar from moving from the first plunger detent position to the second plunger detent position, and the bit detent is inhibited by the collar from moving from the first bit detent position to second bit detent positon, and a second collar position, in which the plunger detent is moveable by the plunger from the first plunger detent position to the second plunger detent position, and the bit detent is moveable from the first bit detent position to the second bit detent position. The collar is biased towards the first collar position. When the collar is in the second collar position and the tool bit is inserted into the bore, the O-ring is deformable by the bit detent, such that the bit detent is moveable by the bit from the first bit detent position to the second bit detent position. When the collar is in the first collar position and the tool bit is in the bore, the bit detent is in the bit recess, such that the tool bit is locked within the bore. When the collar is moved from the first collar position to the second collar position when the tool bit is in the bore, the tool bit is ejectable from the bore by the plunger. 
     Other features and aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a rotary impact driver in accordance with an embodiment of the invention. 
         FIG.  2    is a plan view of the impact driver of  FIG.  1   . 
         FIG.  3    is a partial cross-sectional view of the impact driver of  FIG.  1   . 
         FIG.  4    is a perspective view of a tool bit for use with the impact driver of  FIG.  1   . 
         FIG.  5    is a cross-sectional view of a battery pack for use with the impact driver of  FIG.  1   . 
         FIG.  6    is a perspective view of a bit retention assembly of the impact driver of  FIG.  1   . 
         FIG.  7    is an enlarged perspective view of the impact driver of  FIG.  1   , with portions removed. 
         FIG.  8    is a cross-sectional view of the bit retention assembly of  FIG.  6   , with a collar in a first collar position. 
         FIG.  9    is a cross-sectional view of the bit retention assembly of  FIG.  6   , with the collar in a second collar position. 
         FIG.  10    is an enlarged perspective view of the impact driver of  FIG.  1   , with a bracket and ring removed. 
         FIG.  11    is an enlarged plan view of an anvil of the impact driver of  FIG.  1   . 
         FIG.  12    is a perspective view of another embodiment of a bit retention assembly for use with the impact driver of  FIG.  1   . 
         FIG.  13    is a perspective view of another embodiment of an anvil for use with the impact driver of  FIG.  1   , incorporating features of the bit retention assembly of  FIG.  12   . 
         FIG.  14    is a cross-sectional view of the bit retention assembly of  FIG.  12    shown in a bit-locking state. 
         FIG.  15    is a cross-sectional view of the bit retention assembly of  FIG.  12    shown in a bit-release state. 
     
    
    
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION 
       FIGS.  1 - 3    illustrate a power tool in the form of a rotary impact tool or impact driver  10 . The impact driver  10  includes a motor housing  14  in which an electric motor  18  is supported ( FIG.  3   ), an end cap  20  coupled to a rear end of the motor housing  14 , a gear case  22  at least partially housing a gear train  26 , and an impact housing  30  housing an impact mechanism  32 . The gear train  26  and impact mechanism  32  are part of a drive assembly  33  for converting a continuous torque input from the motor  18  to consecutive rotational impacts upon a workpiece, as described in further detail below. 
     The impact mechanism  32  includes an anvil  34  upon which a quick-release collar  35  of a bit retention assembly  36  is supported, which facilitates retention and removal of a tool bit  37  ( FIG.  4   ) from the anvil  34 , as described in further detail below. As also described in further detail below and shown in  FIG.  3   , the gear train  26  transfers torque from the motor  18  to the impact mechanism  32 , which transfers torque to the tool bit  37  retained within the anvil  34 . As shown in  FIGS.  1  and  2   , the impact driver  10  further includes a bracket  38  that is removably mounted to the gear case  22  to secure a support member, such as a ring  40 , to the impact driver  10 , as described in further detail below. 
     With reference to  FIGS.  1  and  2   , the impact driver  10  also includes a handle  42  having a first end  39  coupled to the motor housing  14  and a second end  41  extending away from the motor housing  14 . The second end  41  includes a foot  43  having a battery receptacle  44  that receives a battery pack  46 . As shown in  FIG.  2   , the motor housing  14  defines the top  48  of the impact driver  10 , and when the battery pack  46  is coupled to the battery receptacle  44 , the battery  46  defines the bottom  50  of the impact power driver  10 , such that an overall height H1 of the impact driver  10  (excluding the bracket  38  and ring  40 ) is defined between the top  48  and bottom  50  of the impact driver  10 . A distal end of the anvil  34  defines the front  54  of the impact driver  10  and the end cap  38  defines the rear  56  of the impact driver  10 , such that an overall length L is defined between the front  54  and rear  56  of the impact driver  10 . 
     In some embodiments, the overall height H1 is 250 mm and the overall length L is 203 mm, such that a ratio of the overall length L to the overall height H is 0.81. Because the ratio of overall length L to overall height H is less than 1, the impact driver  10  is easier to hold and manipulate by an operator because when the operator is grasping the handle  42 , the operator’s hand is proximate a center of gravity CG ( FIGS.  2  and  3   ) of the impact driver  10 . Thus, the moment created by the center of gravity CG while the impact driver  10  is being held is reduced, improving the operator’s control and comfort while using the impact driver  10 . 
     With continued reference to  FIG.  2   , the handle  42  includes a rear side  60  and a trigger  62  that selectively electrically connects the motor  18  and the battery pack  46  to provide DC power to the motor  18  when the battery pack  46  is attached to the battery receptacle  44 . The trigger  62  has a front side  64  and a bottom lip  66  that is in facing relationship with the foot  43 . A minimum “trigger to back handle” distance D1 is defined between the rear side  60  of the handle  42  and the front side  64  of the trigger  62 . A handle height H2 is defined between the bottom lip  66  of the trigger  62  and a top surface  72  of the foot  43 . In some embodiments, the handle height H2 is 87 mm, such that a ratio of the handle height H2 to the overall height H1 is 0.34. With the ratio of the handle height H2 to the overall height H1 being greater than 0.3, the impact driver  10  is easier to manipulate because the handle  42  accounts for nearly a third or greater than a third of the overall height H1. In some embodiments, the trigger to back handle distance D1 is 63 mm or less, making the impact driver  10  more user friendly for operators with smaller hands. 
     As shown in  FIG.  5   , the battery pack  46  includes a housing  73  enclosing a plurality of battery cells  74  that are electrically connected to provide the desired output (e.g., nominal voltage, current capacity, etc.) of the battery pack  46 . Each battery cell  74  may have a nominal voltage between about 3 Volts (V) and about 5 V. The battery pack  46  is rechargeable, and the cells may have a Lithium-based chemistry (e.g., Lithium, Lithium-ion, etc.) or any other suitable chemistry. The battery pack  46  has a nominal output voltage of at least 18 V and a nominal capacity of at least 5 Amp-hours (Ah) (e.g., with two strings of five series-connected battery cells (a “5S2P” pack)). In other embodiments, the impact driver  10  may utilize a battery pack that has a nominal capacity of at least 9 Ah (e.g., with three strings of five series-connected battery cells (a “5S3P pack”). 
     The motor  18 , supported within the motor housing  14 , receives power from the battery pack  46  when the battery pack  46  is coupled to the battery receptacle  44  ( FIG.  2   ). The motor  18  is preferably a brushless direct current (“BLDC”) motor with a stator  76  that has a plurality of stator windings  78  ( FIG.  3   ). The motor  18  also includes a rotor  80  having a plurality of permanent magnets (not shown). The stator  76  has a nominal diameter of at least 60 mm and the stator  76  has a stack length of at least 18 mm. For example, in one embodiment, the motor  18  is a BL60-18 motor having a nominal diameter of 60 mm and a stack length of 18 mm. The motor  18  has an approximate peak power of 950 Watts when powered by the 5 Ah battery pack  46  (the 5S2P pack). 
     The rotor  80  is rotatable about an axis  84  and includes a motor output shaft  85  for driving the gear train  26 , and the impact mechanism  32  is coupled to an output of the gear train  26 . The gear train  26  may be configured in any of a number of different ways to provide a speed reduction between the output shaft  85  and an input of the impact mechanism  32 . With reference to  FIG.  3   , the illustrated gear train  26  includes a helical pinion  86  formed on the motor output shaft  85 , a plurality of helical planet gears  88  meshed with the helical pinion  86 , and a helical ring gear  90  meshed with the planet gears  88  and rotationally fixed within the gear case  22 . The planet gears  88  are mounted on a camshaft  92  of the impact mechanism  32  such that the camshaft  92  functions as a planet carrier. Accordingly, rotation of the output shaft  85  rotates the planet gears  88 , which then rotate along the inner circumference of the ring gear  90  and thereby rotate the camshaft  92 . The output shaft  85  is rotatably supported by a first or forward bearing  96  and a second or rear bearing  100  that is supported by the end cap  20 . 
     The impact mechanism  32  of the impact driver  10  will now be described with reference to  FIG.  3   . The impact mechanism  32  includes the anvil  34 , which extends from the impact housing  30 . As noted above, the tool bit  37  can be coupled to the anvil  34  for performing work on a workpiece (e.g., a fastener). The impact mechanism  32  is configured to convert the continuous rotational force or torque provided by the motor  18  and gear train  26  to a striking rotational force or intermittent applications of torque to the anvil  34  when the reaction torque on the anvil  34  (e.g., due to engagement between the tool element and a fastener being worked upon) exceeds a certain threshold. In the illustrated embodiment of the impact driver  10 , the impact mechanism  32  includes the camshaft  92 , a hammer  104  supported on and axially slidable relative to the camshaft  92 , and the anvil  34 . 
     The impact mechanism  32  further includes a hammer spring  108  biasing the hammer  104  toward the front of the impact driver  10  (i.e., toward the right in  FIG.  3   ). In other words, the hammer spring  108  biases the hammer  104  in an axial direction toward the anvil  34 , along the axis  84 . A thrust bearing  112  and a thrust washer  116  are positioned between the hammer spring  108  and the hammer  104 . The thrust bearing  112  and the thrust washer  116  allow for the hammer spring  108  and the camshaft  92  to continue to rotate relative to the hammer  104  after each impact strike when lugs  118  ( FIG.  7   ) on the hammer  104  engage with corresponding anvil lugs  120  and rotation of the hammer  104  momentarily stops. 
     The camshaft  92  further includes cam grooves  124  in which corresponding cam balls  128  are received ( FIG.  3   ). The cam balls  128  are in driving engagement with the hammer  104  such that movement of the cam balls  128  within the cam grooves  124  allows for relative axial movement of the hammer  104  along the camshaft  92  when the hammer lugs  118  and the anvil lugs  120  are engaged, rotation of the anvil  34  is seized, and the camshaft  92  continues to rotate. 
     In other embodiments (not shown), the impact mechanism includes a cylinder coupled to the electric motor  18  to receive torque therefrom, causing the cylinder to rotate. The cylinder at least partially defines a chamber that contains an incompressible fluid (e.g., hydraulic fluid, oil, etc.). The hydraulic fluid in the chamber reduces the wear and the noise of the impact assembly that is created by impacting the hammer and the anvil. The hammer and anvil are both positioned at least partially within the chamber. The hammer includes an aperture to permit the hydraulic fluid in the chamber to pass through the hammer. A hammer spring biases the hammer toward the anvil. Such an impact mechanism is described in U.S. Provisional Pat. Application No. 62/699,911, filed on Jul. 18, 2018, the entire contents of which is incorporated herein by reference. 
     The bit retention assembly  36  of the impact driver  10  will now be described with reference to  FIGS.  6 - 9   . Specifically, the distal end of the anvil  34  includes a longitudinal bore  132  in which the tool bit  37  is receivable. As shown in  FIG.  11   , the bore  132  has a hexagonal cross-sectional shape in a plane oriented transverse to the axis  84 , and has a nominal width  134  of 7/16 inches to receive the tool bit  37 , which has a corresponding nominal width of 7/16 inches. The anvil  34  also includes a single radial slot  136  that extends from the longitudinal bore  132  through the anvil  34 . The bit retention assembly  36  includes a ball detent  140  received in the radial slot  136 , the collar  35  slidably disposed on the anvil  34 , a collar spring  144  that biases the collar  35  in a rearward direction to a first collar position ( FIGS.  1 - 3 ,  6 , and  8   ), and a washer  148  and retaining ring  150  that maintain the collar spring  144  on the anvil  34 . The collar  35  includes a body portion  152  including knurling  156  on an outer surface thereof. The collar  35  also includes an annular lip  158  arranged on a distal end of the collar  35  that is farthest from the impact housing  30 . The lip  158  extends away from body portion  152  and the axis  84  so as to form a flared portion of the collar  35 . 
     The collar  35  also includes an interior ring  160  having an inner diameter sized to maintain at least a portion of the ball detent  140  within the longitudinal bore  132  which, in turn, is received within a circumferential groove  164  of the tool bit  37  ( FIG.  4   ) to secure the tool bit  37  within the anvil  34 . The bit retention assembly  14  also includes a detent spring  168  positioned around the anvil  34 . A U-shaped finger  172  of the detent spring  168  is received within the slot  136  for biasing the ball detent  140  toward the front of the slot  136  and toward the open end of the longitudinal bore  18 . The collar  35  is moveable along the anvil  34  between the first collar position ( FIGS.  1 - 3 ,  6 , and  8   ) and a second collar position ( FIG.  9   ), in which the collar  35  is pulled forwardly along the anvil  34  against the bias of the collar spring  144  until the interior ring  160  moves forward of the ball detent  140 , such that a recess  176  rearward of the interior ring  160  is axially aligned with the ball detent  140 . 
     In operation, to secure the tool bit  37  within the anvil  34 , while the collar  35  is in the first collar position, an operator needs only to insert the end of the tool bit  37  having the circumferential groove  164  within the longitudinal bore  132  and push the tool bit  37  toward the ball detent  140 . Continued insertion of the tool bit  37  causes the tool bit  37  to engage the ball detent  140  and push the ball detent  140  rearward against the bias of the detent spring  168 . After the ball detent  140  is pushed far enough to clear the interior ring  160  on the collar  35 , the ball detent  140  is pushed radially outwardly in the slot  136  and into the recess  176  by the tool bit  37 . The tool bit  37  may then slide under the ball detent  140  until the ball detent  140  is received within the circumferential groove  164  in the tool bit  37 , at which time the detent spring  168  at least partially rebounds to push the ball detent  140  underneath the interior ring  160 . Since the collar  35  is not required to be moved to the second collar position to secure the tool bit  37  within the anvil  34 , the operator of the impact driver  10  needs only to use a single hand to insert and secure the tool bit  37  within the anvil  34 . 
     To release the tool bit  37 , the operator may grasp the knurling  156  on the body portion  152  and/or the lip  158  of the collar  35  to move the collar  35  from the first collar position to the second collar position, such that the recess  176  is axially aligned with the ball detent  140 . The tool bit  37  may then be pulled from the anvil  34 , during which time the tool bit  37  forces the ball detent  140  to displace radially outwardly into the recess  176 . Once the tool bit  37  has moved passed the ball detent  140 , the detent spring  168  at least partially rebounds to push the ball detent  140  underneath the interior ring  160 . The operator may then release the collar  35 , allowing the collar spring  144  to return the collar  35  to the first collar position. The knurling  156  enhances the operator’s grip on the collar  35  by permitting more friction to be developed between the collar  35  and the operator’s fingers when grasping the collar  35 . Similarly, the lip  158  facilitates the operator’s grasp the collar  35  for moving it from the first collar position to the second collar position because the lip  158  provides a flared portion against which the operator can apply force in a direction parallel to the axis  84 . 
     As noted above, the bracket  38  is removably mounted to the gear case  22  to secure the ring  40  to the impact driver  10 . With reference to  FIGS.  3  and  10   , the gear case  22  includes an upwardly-extending mounting portion  184  that is arranged between the motor housing  14  and the impact housing  30 . The mounting portion  184  includes a pair of mounting bores  188  extending through a mounting surface  192 . The mounting portion  184  protrudes radially through the motor housing  14  such that the bores  188  are exposed to the exterior of the impact driver  10 . As shown in  FIGS.  1  and  2   , the bracket  38  can be removably coupled to the mounting portion  184  via a pair of bracket fasteners  196 . Before fastening the bracket  38  to the mounting portion  184 , the ring  40  can be arranged between the bracket  38  and the mounting surface  192 . The ring  40  is configured to receive a lanyard  200  ( FIG.  1   ) that is attached to a user’s belt, for example, to tether the impact driver  10  to the user. As such, the lanyard  200 , ring  40 , and bracket  38  will cooperate to prevent the impact driver  10  from hitting the ground if dropped by the operator. The ring  40  is configured to pivot within the bracket  38 , providing flexibility in how the lanyard  200  tethers the impact driver  10  to the operator. 
     As shown in  FIG.  1   , four housing fasteners  204  extend respectively, in the following order, through each of the impact housing  30 , the gear case  22 , and the motor housing  14 , starting through the impact housing  30  and terminating in the motor housing  14 . In this manner, the motor housing  14  is coupled to the impact housing  30  and the gear case  22  is secured (i.e., clamped) between the motor housing  14  and the impact housing  30 . Because the bracket  38  is secured to the mounting portion  184  with only the bracket fasteners  196 , removal of the housing fasteners  204  that join the motor housing  14  and gear case  22  to the impact housing  30  is not required to remove the bracket  38  from the mounting portion  184 . This arrangement thus affords the operator greater convenience when removing the bracket  38  to service or remove the ring  40 . Also, because the bracket  38  is not secured to the impact driver  10  via the housing fasteners  204 , the bracket  38  is more easily shared across different tools having an arrangement of mounting bores that are similar to the arrangement of the mounting bores  188  of the mounting portion  184 . 
     In operation of the impact driver  10 , the operator first inserts the tool bit  37  into the anvil  36 , as described above. The operator then depresses the trigger switch  62  to activate the motor  18 , which continuously drives the gear train  26  and the camshaft  92  via the output shaft  85 . As the camshaft  92  rotates, the cam balls  128  drive the hammer  104  to co-rotate with the camshaft  92 , and the hammer lugs  118  engage, respectively, driven surfaces of the anvil lugs  120  to provide an impact and to rotatably drive the anvil  34  and the tool bit  37 . After each impact, the hammer  104  moves or slides rearward along the camshaft  92 , away from the anvil  34 , so that the hammer lugs  118  disengage the anvil lugs  120 . The hammer spring  108  stores some of the rearward energy of the hammer  104  to provide a return mechanism for the hammer  104 . After the hammer lugs  118  disengage the respective anvil lugs  120 , the hammer  104  continues to rotate and moves or slides forwardly, toward the anvil  34 , as the hammer spring  108  releases its stored energy, until the drive surfaces of the hammer lugs  118  re-engage the driven surfaces of the anvil lugs  120  to cause another impact. As defined herein, “impact frequency” means the number of impacts imparted by the hammer  104  upon the anvil  34  per unit time, measured in “impacts per minute.” Once finished with the impact driving operation, the operator may remove the tool bit  37  from the anvil  34 , as described above. 
     During operation of the impact driver  10  under a no-load condition, when the anvil  34  is not being used to apply torque to a fastener, the co-rotation of the camshaft  92 , the hammer  104 , and the anvil  34  define an “output speed” of the impact driver  10  measured in revolutions per minute. 
     The impact driver  10  has a weight of 5.9 pounds, the 5 Ah battery pack  46  (the 5S2P pack) has a weight of 1.55 pounds, and the 9 Ah battery pack (5S3P) has a weight of 2.4 pounds. Thus, when the 5 Ah battery pack  46  is coupled to the impact driver  10 , the impact driver  10  has an overall weight of 7.45 pounds, and when the 9 Ah battery pack is coupled to the impact driver  10 , the impact driver  10  has an overall weight of 8.3 pounds. As defined herein, the term “fastening torque” means torque applied to a fastener in a direction increasing tension (i.e. in a tightening direction). 
     The first and second rows of TABLE 1 below list the overall weight, the peak output speed, the peak fastening torque, and the peak impact frequency (measured in impacts per minute) achieved by known prior art 7/16 inch impact wrenches that use a 5 Ah battery pack. The third and fourth rows of TABLE 1 below list the peak output speed, the peak fastening torque, and the peak impact frequency achieved by the impact driver  10  when respectively using the battery pack  46  (the 5S2P pack - 5 Ah) or the 5S3P (9 Ah) battery pack. The peak fastening torque is measured by fastening a 1-¼″ zinc plated, Grade 8 bolt. TABLE 1 below also lists the ratios of peak output speed to overall weight, calculated by dividing peak output speed by the overall weight. TABLE 1 below also lists the ratio of peak fastening torque to overall weight, calculated by dividing the peak fastening torque by the overall weight. TABLE 1 below also lists the ratio of peak impact frequency to the overall weight, calculated by dividing the peak impact frequency by the overall weight.  
     
       
         
          TABLE 1
           
               
               
               
               
               
               
               
               
             
               
                   
                 Overall Weight (pounds) 
                 Peak Output Speed (revolutions per minute) 
                 Peak Fastening Torque (ft-lbs) 
                 Peak Impact Frequency (impacts per minute) 
                 Ratio of Peak Output Speed to Overall Weight (revolution per minute per pound) 
                 Ratio of Peak Fastening Torque to Overall Weight (ft-lbs per pound) 
                 Ratio of Peak Impact Frequency to Overall Weight (impacts per minute per pound) 
               
             
            
               
                 First prior art impact wrench 
                 7.6 
                 1,900 
                 973 
                 2,400 
                 250.0 
                 128.0 
                 315.8 
               
               
                 Second prior art impact wrench 
                 8.2 
                 1,800 
                 1,054 
                 2,200 
                 219.5 
                 128.5 
                 268.3 
               
               
                 Impact driver  10  with 5 Ah battery pack  46 
 
                 7.45 
                 2,420 
                 920 
                 2,858 
                 324.8 
                 123.5 
                 383.6 
               
               
                 Impact driver  10  with 9 Ah battery pack 
                 8.3 
                 NA 
                 986 
                 NA 
                 NA 
                 118.7 
                 NA 
               
            
           
         
       
     
     As shown in TABLE 1, when using the 5 Ah battery pack  46 , and with a motor  18  capable of generating approximately 950 Watts of power with a stator  76  having a nominal diameter of only 60 mm and a stack length of only 18 mm, the impact driver  10  is capable of achieving a higher ratio of peak output speed to overall weight than either of the prior art impact wrenches while having a lower overall weight than either of the prior art impact wrenches. 
     Also, as shown in TABLE 1, when using the 5 Ah battery pack  46 , and with a motor  18  capable of generating approximately 950 Watts of power with a stator  76  having a nominal diameter of only 60 mm and a stack length of only 18 mm, the impact driver  10  achieves nearly the same ratio of peak fastening torque to overall weight as the prior art impact wrenches, while having a lower overall weight than the prior art impact wrenches. Therefore, on a per-unit weight basis, the impact driver  10  approximately matches the fastening torque performance of the heavier prior art impact wrenches. 
     Further, as shown in TABLE 1, when using the 5 Ah battery pack  46 , and with a motor  18  capable of generating approximately 950 Watts of power with a stator  76  having a nominal diameter of only 60 mm and a stack length of only 18 mm, the impact driver  10  achieves a higher ratio of impact frequency to overall weight than the prior art impact wrenches, while having a lower overall weight than the prior art impact wrenches. Thus, the impact driver  10  provides an operator with a lighter weight rotary impact tool for jobs while still achieving the nearly the same or better fastening performance characteristics than other known prior art 7/16-inch impact wrenches. 
     As used herein, the term “mechanism efficiency” (“η a ”) represents how well an impact driver produces work per unit of time per input unit of power. The mechanism efficiency is determined by multiplying the impact frequency, measured in impacts per minute (“BPM”) by the kinetic energy of the hammer  104  during a loaded condition and prior to impact with the anvil  34  (“KE Hammer,   Drilling ”, measured in Joules) divided by current drawn by the motor  18  (“Current motor ”, measured in Amperes) and the voltage across the motor  18  (“Voltage motor ”, measured in Volts), as shown in the below equation: 
     
       
         
           
             
               η 
               a 
             
             = 
             
               
                 B 
                 P 
                 M 
                 × 
                 K 
                 
                   E 
                   
                     H 
                     a 
                     m 
                     m 
                     e 
                     r 
                     , 
                     D 
                     r 
                     i 
                     l 
                     l 
                     i 
                     n 
                     g 
                   
                 
               
               
                 V 
                 o 
                 l 
                 t 
                 a 
                 g 
                 
                   e 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
                 × 
                 C 
                 u 
                 r 
                 r 
                 e 
                 n 
                 
                   t 
                   
                     m 
                     o 
                     t 
                     o 
                     r 
                   
                 
               
             
           
         
       
     
     When using the 5 Ah battery pack  46 , and with a motor  18  capable of generating approximately 950 Watts of power with a stator  76  having a nominal diameter of only 60 mm and a stack length of only 18 mm, the impact driver  10  is capable of achieving a variety of advantageous performance ratios, as described below. 
     For example, a first performance ratio (“PR 1 ”) measures the efficiency of the impact mechanism  32  per unit of inertia of the hammer  104 . The first performance ratio is determined by dividing the mechanism efficiency by the moment of inertia of the hammer  104  (“Inertia hammer ”, measured in kg-m 2 ) and a scaler of 216,000, as shown in the below equation: 
     
       
         
           
             
               
                 PR 
               
               1 
             
             = 
             
               
                 
                   
                     
                       η 
                       a 
                     
                   
                   
                     I 
                     n 
                     e 
                     r 
                     t 
                     i 
                     
                       a 
                       
                         h 
                         a 
                         m 
                         m 
                         e 
                         r 
                       
                     
                   
                 
               
             
             × 
             
               
                 
                   1 
                   
                     216 
                     , 
                     000 
                   
                 
               
             
           
         
       
     
     The scaler of 1/216,000 is used to reduce the first performance ratio to a manageable number of significant digits (e.g., three, as shown in Table 2 below). However, other scalers could be used. 
     A second performance ratio (“PR 2 ”) measures the ability of the impact mechanism  32  to maintain the level at which it’s performing work during a transition from a no-load state to a loaded state, per unit of inertia of the hammer  104 . Specifically, the second performance ratio is determined by multiplying the mechanism efficiency times the rotational frequency, measured in revolutions per minute, of the impact mechanism  32  under a no-load condition (“RPM no-load ”) divided by the moment of inertia of the hammer  104  and a scaler of 216,000,000, as shown in the below equation: 
     
       
         
           
             
               
                 PR 
               
               2 
             
             = 
             
               
                 
                   
                     
                       η 
                       a 
                     
                     × 
                     R 
                     P 
                     
                       M 
                       
                         n 
                         o 
                         − 
                         l 
                         o 
                         a 
                         d 
                       
                     
                   
                   
                     I 
                     n 
                     e 
                     r 
                     t 
                     i 
                     
                       a 
                       
                         h 
                         a 
                         m 
                         m 
                         e 
                         r 
                       
                     
                   
                 
               
             
             × 
             
               
                 
                   1 
                   
                     216 
                     , 
                     000 
                     , 
                     000 
                   
                 
               
             
           
         
       
     
     The scaler of 1/216,000,000 is used to reduce the second performance ratio to a manageable number of significant digits (e.g., three, as shown in Table 2 below). However, other scalers could be used. 
     A third performance ratio (“PR 3 ”) measures the efficiency of the impact mechanism  32  per unit of mass of the hammer  104 . The third performance ratio is determined by dividing the mechanism efficiency by the mass of the hammer  104  (“Mass  hammer ”, measured in kg) and a scaler of  60 , as shown in the below equation: 
     
       
         
           
             
               
                 PR 
               
               3 
             
             = 
             
               
                 
                   
                     
                       η 
                       a 
                     
                   
                   
                     M 
                     a 
                     s 
                     
                       s 
                       
                         h 
                         a 
                         m 
                         m 
                         e 
                         r 
                       
                     
                   
                 
               
             
             × 
             
               
                 
                   1 
                   
                     60 
                   
                 
               
             
           
         
       
     
     The scaler of 1/60 is used to reduce the third performance ratio to a manageable number of significant digits (e.g., three, as shown in Table 2 below). However, other scalers could be used. 
     A fourth performance ratio (“PR 4 ”) measures the ability of the impact mechanism  32  to maintain the level at which it’s performing work during a transition from a no-load state to a loaded state, per unit of mass of the hammer  104 . Specifically, the fourth performance ratio is determined by multiplying the mechanism efficiency times the rotational frequency, measured in revolutions per minute, of the impact mechanism  32  under a no-load condition divided by the mass of the hammer  104  and a scaler of 3600, as shown in the below equation: 
     
       
         
           
             
               
                 PR 
               
               4 
             
             = 
             
               
                 
                   
                     
                       η 
                       a 
                     
                     × 
                     R 
                     P 
                     
                       M 
                       
                         n 
                         o 
                         − 
                         l 
                         o 
                         a 
                         d 
                       
                     
                   
                   
                     M 
                     a 
                     s 
                     
                       s 
                       
                         h 
                         a 
                         m 
                         m 
                         e 
                         r 
                       
                     
                   
                 
               
             
             × 
             
               
                 
                   1 
                   
                     3 
                     , 
                     600 
                   
                 
               
             
           
         
       
     
     The scaler of 1/3,600 is used to reduce the third performance ratio to a manageable number of significant digits (e.g., four, as shown in Table 2 below). However, other scalers could be used. 
     The first and second rows of TABLE 2 below list values for impact frequency (measured in impacts per minute), hammer kinetic energy (J), voltage (V), current (A), no-load speed (RPM), hammer inertia (kg-s2), hammer mass (kg), as well as the first, second, third, and fourth performance ratios respectively achieved by the first and second prior art 7/16-inch impact wrenches discussed in TABLE 1 above, using a 5 Ah battery pack in a drilling operation. The third row lists the same values for a third prior art 7/16-inch impact wrench using a 5 Ah battery pack in a drilling operation. The fourth and fifth rows of TABLE 2 below list the same values for the impact driver  10  when respectively using the battery pack  46  (the 5S2P pack - 5 Ah) or the 5S3P (9 Ah) battery pack.  
     
       
         
          TABLE 2
           
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 Impacts per minute 
                 Hammer Kinetic Energy (J) 
                 Voltage (V) 
                 Current (A) 
                 No-Load Speed (RPM) 
                 Hammer Inertia (kg-s 2 ) 
                 Hammer Mass (Kg) 
                 PR 1 
 
                 PR 2 
 
                 PR 3 
 
                 PR 4 
 
               
             
            
               
                 First prior art impact wrench 
                 2,671 
                 14.9 
                 19.3 
                 44.1 
                 1,883 
                 2.45×10 -4 
 
                 0.416 
                 0.89 
                 1.67 
                 1.88 
                 59.01 
               
               
                 Second prior art impact wrench 
                 2,161 
                 17.3 
                 19.1 
                 49.3 
                 1,662 
                 4.37×10 -4 
 
                 0.542 
                 0.42 
                 0.70 
                 1.22 
                 33.74 
               
               
                 Third prior art impact wrench 
                 2,599 
                 11.8 
                 19.4 
                 39.3 
                 1,632 
                 2.04×10 -4 
 
                 0.357 
                 0.91 
                 1.49 
                 1.88 
                 51.11 
               
               
                 Impact driver  10  with 5 Ah battery pack  46 
 
                 3,094 
                 14.8 
                 18.9 
                 58.3 
                 2,099 
                 1.82×10 -4 
 
                 0.306 
                 1.06 
                 2.23 
                 2.27 
                 79.49 
               
               
                 Impact driver  10  with 9 Ah battery pack 
                 3,212 
                 16.0 
                 18.4 
                 62.2 
                 2,099 
                 1.82×10 -4 
 
                 0.306 
                 1.14 
                 2.40 
                 2.44 
                 85.29 
               
            
           
         
       
     
     As can be seen in TABLE 2, as compared with the three prior art 7/16″ impact wrenches using a 5 Ah battery pack in a drilling operation, the impact driver  10  with the 5 Ah battery pack  46  is the only 7/16-inch impact driver able to achieve a first performance ratio that is greater than 1, a second performance ratio that is greater than 2, a third performance ratio that is greater than 2, and a fourth performance ratio that is greater than 65. Similarly, the impact driver  10  when using a 9 Ah battery pack in a drilling operation is able to achieve a first performance ratio that is greater than 1, a second performance ratio that is greater than 2, a third performance ratio that is greater than 2, and a fourth performance ratio that is greater than 65. 
     With respect to the first and third performance ratios, while the three prior art 7/16-inch impact drivers benefit from larger hammers than the impact driver  10  with respect to peak fastening torque (see TABLE 1), they are penalized in evaluation of the first and third performance ratios because the larger hammers also result in a higher moment of inertia. Because the impact driver  10  has a smaller and lighter hammer  104  yet still achieves a comparable mechanism efficiency as the three prior art 7/16-inch impact drivers, it achieves a first performance ratio that is greater than 1 and a third performance ratio that is greater than 2 because the moment of inertia of the hammer  104  is lower (relevant to the first performance ratio) due to the smaller and lighter hammer  104  (relevant to the third performance ratio). Thus, the efficiency of the impact mechanism  32  per unit of inertia of the hammer  104  of the impact driver  10  (first performance ratio) or per unit of mass of the hammer  104  (third performance ratio) is greater than the three prior art 7/16-inch impact drivers. 
     With respect to the second and fourth performance ratios, impact drivers that have a high no-load speed (at the beginning of an operation) and a high loaded speed (as evaluated by the kinetic energy of the hammer  104  in a loaded state, prior to impact) are favored, because during a drilling or fastening operation, it is advantageous for the impact mechanism  32  to possess both high initial (unloaded) speed and a high speed when in a loaded state (during the operation) that is continued through termination of the operation. Because the impact driver  10  has a smaller hammer  104  yet still achieves a higher no-load speed than the three prior art 7/16-inch impact drivers, it achieves a second performance ratio that is greater than 2 and a fourth performance ratio that is greater than 65. Thus, the impact mechanism  32  of the impact driver  10  is better able to maintain the level at which it’s performing work during a transition from a no-load state to a loaded state, per unit of inertia of the hammer  104  (second performance ratio) or per unit of mass of the hammer  104  (fourth performance ratio), compared to the three prior art 7/16-inch impact drivers identified in TABLE 2 above. 
     The impact driver  10  is particularly effective at drilling operations because it simultaneously achieves a first performance ratio that is greater than 1, a second performance ratio that is greater than 2, a third performance ratio that is greater than 2, and a fourth performance ratio that is greater than 65. 
     An alternative embodiment of a bit retention assembly  208  for the impact driver  10  will now be described with reference to  FIGS.  12 - 15   . A distal end  210  of an anvil  212  includes a longitudinal bore  216  in which the tool bit  37  is receivable. Like the bore  132  of the anvil  34 , the bore  216  of the anvil  212  has a hexagonal cross-sectional shape in a plane oriented transverse to the axis  84 , and has a nominal width of 7/16 inches to receive the tool bit  37 . The anvil  212  has an outer surface  220  and a circumferential groove  224  ( FIG.  13   ) for receipt of a clip  228  ( FIGS.  14  and  15   ). A bearing  232  is also arranged on the outer surface  220  for rotatably supporting the anvil  212  within the impact housing  30 . In some embodiments, the bearing  232  is press-fit to the anvil  212 . The anvil  212  also has a circumferential O-ring groove  236  ( FIG.  13   ) in which an O-ring  240  ( FIGS.  14  and  15   ) is retained. 
     The anvil  212  further includes a pair of radial plunger detent apertures  244  and a radial bit detent aperture  248 , all of which extend radially inward from the outer surface  220  to the bore  216  ( FIG.  13   ). The bit detent aperture  248  intersects the O-ring groove  236 , such that the O-ring  240  is at least partially arranged in the bit detent aperture  248 . As shown in  FIGS.  14  and  15   , a pair of plunger detents  252  are respectively arranged in the plunger detent apertures  244  and a bit detent  256  is arranged in the bit detent aperture  248 . As shown in  FIGS.  14  and  15   , a plunger  260  is arranged in the bore  216  and is biased toward the distal end  210  of the anvil  212  by a plunger spring  268  that is also arranged in the bore  216 . The plunger  260  includes a circumferential plunger detent recess  270 . 
     The bit retention assembly  208  includes the O-ring  240 , the bit detent  256  received in the bit detent aperture  248 , a collar  272  slidably disposed on the anvil  212 , a collar spring  276  that biases the collar  272  in a rearward direction to a first collar position ( FIGS.  12  and  14   ), and a washer  280  that maintains the collar spring  276  on the anvil  212 . As shown in  FIGS.  14  and  15   , the washer  280  is arranged between the O-ring  240  and the collar spring  276 , with the washer  280  being abutted with the O-ring  240 . As shown in  FIG.  12   , the collar  272  may include ribs  282  on an outer surface  283  thereof to enhance the operator’s grip on the collar  272 . The clip  228  limits the extent to which the collar spring  276  can push the collar  272  rearward, such that the first position is defined by the collar  272  being abutted against the clip  228 , as shown in  FIGS.  12  and  14   . 
     The collar  272  includes a first inner plunger detent surface  284  and a second inner plunger detent surface  288  that has a greater diameter than the first inner plunger detent surface  284 . The collar  272  also includes a first inner bit detent surface  292  and a second inner bit detent surface  296  that has a greater diameter than the first inner bit detent surface  292 . In the first collar position ( FIGS.  12  and  14   ), the first inner plunger detent surface  284  is axially aligned with the plunger detent apertures  244 , such that the plunger detents  252  are radially inhibited by the first inner plunger detent surface  284 , and the first inner bit detent surface  292  is axially aligned with the bit detent aperture  248 . As shown in  FIG.  14   , when the collar  272  is in the first collar position, the plunger spring  268  is maintained in a compressed state by virtue of the plunger detents  252  being inhibited from moving in a radially outward direction by the first inner plunger detent surface  284 . Thus, the plunger detents  252  are maintained in the plunger detent recess  270 , keeping the plunger  260  axially loaded against the plunger spring  268 . 
     The collar  272  is moveable along the anvil  212  between the first collar position ( FIGS.  12  and  14   ) and a second collar position ( FIG.  15   ), in which the collar  272  is pulled forwardly along the anvil  212  against the bias of the collar spring  276  until the first inner plunger detent surface  284  is axially forward of the plunger detent apertures  244 , the second inner plunger detent surface  288  is axially aligned with the plunger detent apertures  244 , the first inner bit detent surface  292  is axially forward of the bit detent aperture  248 , and the second inner bit detent surface  296  is axially aligned with the bit detent aperture  248 . 
     In operation, to secure the tool bit  37  within the anvil  212 , while the collar  272  is in the second collar position ( FIG.  15   ), an operator needs only to insert the end of the tool bit  37  having the circumferential groove  164  within the longitudinal bore  216  and push the tool bit  37  toward the plunger  260 . Continued insertion of the tool bit  37  causes the tool bit  37  to engage the bit detent  256  and push the bit detent  256  radially outward in the bit detent aperture  248  until it abuts the first inner bit detent surface  292 , causing the O-ring  240  to elastically deform until the bit detent  256  is pushed out of the longitudinal bore  216 . Once the bit detent  256  is pushed out of the longitudinal bore, the tool bit  37  may then slide past the bit detent  256  until the bit detent  256  is axially aligned with the circumferential groove  164  in the tool bit  37 , at which time the O-ring  240  elastically recovers to push the bit detent  256  into the circumferential groove  164 . The tool bit  37  is then locked within the bore  216 . 
     As the tool bit  37  moves rearwardly in the longitudinal bore  216 , the tool bit  37  also pushes the plunger  260  rearward, compressing the plunger spring  268 , such that the plunger detents  252  become axially aligned with the plunger detent recess  270 . The collar spring  276  is thus allowed to push the collar  272  rearward, causing the plunger detents  252  to be pushed into the plunger detent recess  270 . The collar spring  276  then continues pushing the collar  272  rearward until the first inner plunger detent surface  284  becomes axially aligned with the plunger detent apertures  244  and the collar  272  is in the first collar position. Since the operator does not need to manually move the collar  272  from the second collar position to the first collar position ( FIG.  14   ) to secure the tool bit  37  within the anvil  212 , the operator of the impact driver  10  needs only to use a single hand to insert and secure the tool bit  37  within the anvil  34 . 
     To release the tool bit  37 , the operator moves move the collar  272  from the first collar position to the second collar position. The ribs  282  facilitate the operator’s grasp on the collar  272  moving it from the first collar position to the second collar position because the ribs  282  provide flared portions against which the operator can apply force in a direction parallel with the axis  84 . Movement of the collar  272  to the second collar position causes the second inner plunger detent surface  288  to be axially aligned with the plunger detent apertures  244  and the second inner bit detent surface  296  to be axially aligned with the bit detent aperture  248 . 
     Because the plunger detents  252  are no longer radially constrained by the first inner plunger detent surface  288 , the plunger spring  268  is able to rebound, pushing the plunger  260  toward the distal end  210  of the anvil  212 , thus causing the plunger detents  252  to be moved radially outward in the plunger detent apertures  244  until they are out of the plunger detent recess  270  and abutting the second inner plunger detent surface  288  of the collar  272 . Because the bit detent  256  is no longer radially constrained by the first inner bit detent surface  292 , the tool bit  37  is no longer locked within the bore  216  and thus the plunger  260  ejects the tool bit  37  from the bore  216 . 
     As the tool bit  37  is ejected from the bore  216  by the plunger  260 , the bit detent  256  is pushed by the tool bit  37  radially outward in the bit detent aperture  248  until it abuts the second inner bit detent surface  296 . As the bit detent  256  is pushed radially outward by the tool bit  37 , the movement of the bit detent  256 , and thus the movement of the tool bit  37  as it is exiting the bore  216 , is resisted by the O-ring  240 , because the bit detent  256  must frictionally engage the o-ring  240  as it is moved toward the second inner bit detent surface  296 . Because the O-ring  240  resists the movement of the tool bit  37  from the bore  216 , the tool bit  37  is prevented from suddenly ejecting from the bore  216  when the collar  272  is moved to the second collar position. Thus, it is easier for an operator to grasp or retain the tool bit  37  as it is ejected from the bore  216 . 
     The operator may then release the collar  272 . When the collar  272  is released, the collar  272  is maintained in the second position by virtue of the plunger spring  268  keeping the plunger  260  pushed forward, such that the plunger detents  252  are maintained against an intermediate flat  300  of the plunger  260 , the diameter of which is greater than the plunger detent recess  270 . Thus, the plunger detents  252  are maintained against the second inner plunger detent surface  288  of the collar  272 , thereby preventing the collar spring  276  from returning the collar  272  to the first collar position. The collar  272  is thus maintained in the second collar position, ready for reinsertion of the tool bit  37 , as described above. 
     Various features of the invention are set forth in the following claims.