Patent Publication Number: US-11641794-B2

Title: Transmission disconnection by wheel rotation for walk-behind machine

Description:
RELATED APPLICATION INFORMATION 
     This application is a continuation of U.S. patent application Ser. No. 16/371,267 filed Apr. 1, 2019, which claims the benefit of Chinese Patent Application No. 201611205394.X, filed on Dec. 23, 2016, and Chinese Patent Application No. 201711093658.1, filed on Nov. 8, 2017, each of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF DISCLOSURE 
     The present disclosure relates to a walk-behind machine. 
     BACKGROUND 
     Lawn mowers, snow throwers, etc. are typical walk-behind machines. A walk-behind machine includes a motor, a plurality of wheels, and a transmission mechanism. The motor drives the wheels to rotate through the transmission mechanism so that the walk-behind machine moves relative to the ground. 
     The walk-behind machine requires the right wheel and the left wheel to rotate at different speeds during steering. The walk-behind machine is provided with a clutch to enable the right wheel and the left wheel to rotate asynchronously. Generally, the rotation speed of a wheel is greater than the other wheel&#39;s. The clutch is also called a differential device. For the typical walk-behind machine with that clutch, when the motor shaft stops rotating, the machine can be pushed forward to make the wheels continue to rotate forward, and the clutch can cut off the transmission between the wheels and the motor shaft. When the motor stops rotating and the machine is pulled backward to rotate the wheel backward, the clutch cannot disconnect the transmission between the wheels and the motor shaft, and the wheels will drive the motor shaft to rotate. At this point where the differential device is in a driving state, a large force is required to pull the walk-behind machine to move backward, which is known as the “lock-up” phenomenon. After the motor stops rotating, in order to disconnect the transmission between the wheels and the motor shaft and in order to make the differential device enter into an unlocked state, the machine needs to be pushed forward to unlock the differential device, then the wheels can rotate freely relative to the motor shaft. Pulling the walk-behind machine backward cannot make the machine “unlocked,” which brings great inconvenience to users. 
     After the motor stops and when the machine is pushed or is pulled, the wheels will rotate and drive the motor shaft to rotate through the transmission mechanism. In that condition, a great force is required to push or pull the machine to move. 
     SUMMARY 
     In one aspect of the disclosure, a walk-behind machine, includes a chassis, a handle, wheels, a first motor, a working element, a second motor, and a transmission mechanism. The handle is connected to the chassis. The wheels are for supporting the chassis and can rotate relative to the chassis. The first motor is mounted to the chassis, for providing a driving force to rotate the wheels. The working element moves relative to the chassis to implement the machine&#39;s function. The second motor is for driving the working element to move. The transmission mechanism connects the first motor with the wheels and includes a clutch for realizing one-way transmission between the first motor and the wheels, wherein the clutch has a driving state where the first motor drives the wheels to rotate and an unlocked state where the wheels freely rotate relative to the first motor. The clutch includes a movable member and a driving member. The movable member is capable of moving between a locked position and an unlocked position. The driving member transfers power with the wheels by means of friction. When the movable member is in the locked position, the clutch is in the driving state where the first motor is capable of driving the wheels to rotate in a first direction. When the movable member is in the unlocked position the clutch is in the unlocked state. When the first motor stops rotating, the wheels rotate in a second direction opposite to the first direction, and can drive the driving member to move so that the driving member pushes the movable member to move from the locked position to the unlocked position. 
     In one or more examples, the first motor includes a motor shaft for outputting a driving force. The clutch includes a transmission shaft, a fixing member and an outer ring member. The transmission shaft is connected to the motor shaft and is driven by the motor shaft to rotate. The fixing member is coupled to the transmission shaft or is a part of the transmission shaft and forms a driving surface capable of driving the movable member. The outer ring member is sleeved on the transmission shaft, is connected to the wheels to drive the wheels to rotate and forms a mounting groove. The movable member is located between the wall of the mounting groove and the driving surface. 
     In one or more examples, the wall of the mounting groove is an annular surface, and the movable member is a pin. 
     In one or more examples, the driving member includes a plurality of driving portions, and the driving portions get into the mounting groove and are capable of contacting the movable member to push the movable member to move. 
     In one or more examples, the clutch includes a plurality of movable members and the number of the movable members is the same as the number of driving portions. The driving portions and the movable members are alternatively arranged. 
     In one or more examples, the fixing member forms a plurality of driving surfaces and the number of the driving surfaces is the same as the number of movable members, the driving surfaces are in one-to-one correspondence with the movable members. 
     In one or more examples, the frictional force between the wheels and the driving member is greater than the frictional force between the fixing member and the movable member when the movable member is in the locked position. 
     In one or more examples, the wheels form a first wheel gear or a first wheel gear is mounted on the wheels. The walk-behind machine further includes a first transmission gear meshing with the first wheel gear and driven by the transmission shaft to drive the wheel to rotate. When the motor shaft actively rotates, the transmission shaft drives the first transmission gear to rotate so as to rotate the wheels and the wheels exert a force, whose direction is opposite to the rotation direction of the first transmission gear, to the driving member. 
     In one or more examples, the wheels form a second wheel gear or a second wheel gear is mounted on the wheels. The walk-behind machine further includes a second transmission gear meshing with the second wheel gear. When the wheels rotate, the first transmission gear and the second transmission gear rotate in opposite directions and the second transmission gear exerts a force, whose direction is opposite to the rotation direction of the first transmission gear, to the driving member. 
     In one or more examples, the walk-behind machine further includes a friction plate disposed between the second transmission gear and the driving member and realizing the friction transmission between the second transmission gear and the driving member. 
     In one or more examples, the clutch includes a magnet exerting a magnetic attractive force to the movable member. 
     In one or more examples, the clutch includes a plurality of the movable members and the magnets, whose number is equivalent to the number of the movable members. The transmission shaft rotates about a central axis, in the direction of which magnets exert a magnetic attractive force to the movable members of the central axis. 
     In one or more examples, the magnets are fixed to the outer ring member and are evenly distributed in the circumferential direction of the central axis. 
     In one or more examples, the magnet is annular and surrounds the transmission shaft. 
     A walk-behind machine, includes a chassis, a working element, a handle, wheels, a motor and a clutch. The working element moves relative to the chassis to implement the machine&#39;s function. The handle is connected to the chassis. The wheels are for supporting the chassis and can rotate relative to the chassis. The motor is capable of providing a driving force for rotating the wheels and includes a motor shaft for outputting the driving force. The clutch is capable of one-way transmitting between the motor shaft and the wheels. The clutch has a driving state where the motor shaft drives the wheels to rotate and an unlocked state where the wheels freely rotate relative to the motor shaft. The clutch includes a movable member and a driving member. The movable member is capable of moving between a locked position and an unlocked position. The driving member could be driven by the wheels. When the movable member is in the locked position the clutch is in the driving state where the motor shaft drives the wheels to rotate in a first direction. When the movable member is in the unlocked position the clutch is in the unlocked state. When the motor shaft stops rotating, the wheels rotate in a second direction opposite to the first direction and can drive the driving member to move so that the driving member pushes the movable member to move from the locked position to the unlocked position. 
     In one or more examples, the wheels drive the driving member by friction. 
     In one or more examples, the clutch includes a transmission shaft, a fixing member and an outer ring member. The transmission shaft is coupled to the motor shaft and is driven to rotate by the motor shaft. The fixing member is coupled to the transmission shaft or is a part of the transmission shaft and forms a driving surface capable of driving the movable member. The outer ring member is sleeved on the transmission shaft and is connected to the wheels to drive the wheels to rotate. The outer ring member forms a mounting groove between the wall of which and the driving surface the movable member is located. 
     In one or more examples, the wall of the mounting groove is an annular surface, and the movable member is a pin. 
     In one or more examples, the driving member includes a plurality of driving portions, and the driving portions get into the mounting groove and are capable of contacting the movable member to push the movable member to move. 
     In one or more examples, the clutch includes a plurality of movable members and the number of the movable members is equivalent to the number of driving portions. The driving portions and the movable members are alternatively arranged. 
     In one or more examples, the fixing member forms a plurality of driving surfaces and the number of the driving surfaces is the same as the number of movable members, the driving surfaces are in one-to-one correspondence with the movable members. 
     In one or more examples, the wheels form a first wheel gear or a first wheel gear is mounted on the wheels, and the walk-behind machine further includes a first transmission gear meshing with the first wheel gear and driven by the transmission shaft to drive the wheel to rotate. When the motor shaft actively rotates, the transmission shaft drives the first transmission gear to rotate, thereby driving the wheels to rotate and the wheels exert a force, whose direction is opposite to the rotation direction of the first transmission gear, to the driving member. 
     In one or more examples, the wheels form a second wheel gear or a second wheel gear is mounted on the wheels, and the walk-behind machine further includes a second transmission gear meshing with the second wheel gear. When the wheels rotate, the first transmission gear and the second transmission gear rotate in opposite directions and the second transmission gear exerts a force whose direction is opposite to the rotation direction of the first transmission gear to the driving member. 
     In one or more examples, the walk-behind machine further includes a friction plate disposed between the second transmission gear and the driving member and realizing the friction transmission between the second transmission gear and the driving member. 
     In one or more examples, the walk-behind machine has a self-propelled forward-moving mode in which the motor drives the wheels to rotate in the first direction and a self-propelled backward-moving mode. When the motor is turned off, the walk-behind machine is out of the self-propelled forward-moving mode and the walk-behind machine is pushed forward to make the wheels rotate in the first direction, the wheels drive the movable member to move from the locked position to the unlocked position and the clutch turns into the unlocked state. When the walk-behind machine is in the self-propelled backward-moving mode, the motor drives the wheels to rotate in the second direction opposite to the first direction. When the motor is turned off, the walk-behind machine is out of the self-propelled backward-moving mode and the walk-behind machine is pushed to make the wheels rotate in the second direction, the wheels drive the movable member to move from the locked position to the unlocked position and the clutch into the unlocked state. 
     In one or more examples, when the motor is turned off, the walk-behind machine is out of the self-propelled forward-moving mode and the walk-behind machine is pushed backward to make the wheels rotate in the second direction, the wheels drive the movable member to move from the locked position to the unlocked position and the clutch turns into the unlocked state. When the motor is turned off, the walk-behind machine is out of the self-propelled backward-moving mode and the walk-behind machine is pushed forward to make the wheels rotate in the first direction, the wheels drive the movable member to move from the locked position to the unlocked position and the clutch turns into the unlocked state. 
     In one or more examples, the walk-behind machine has a self-propelled forward-moving mode in which the motor drives the wheels to rotate in the first direction and a self-propelled backward-moving mode in which the motor drives the wheels to rotate in the second direction opposite to the first direction. When the motor is turned off and the walk-behind machine is pushed to drive the wheels to turn at an angle in any direction, the wheels drive the movable member to move from the locked position to the unlocked position and the clutch turns into the unlocked state. 
     In one or more examples, the walk-behind machine further includes a transmission mechanism connecting the motor shaft and the wheels, wherein the transmission mechanism includes the clutch and a gearbox connecting the motor shaft and the clutch. 
     In one or more examples, the clutch includes a magnet exerting a magnetic attractive force to the movable member. 
     In one or more examples, the clutch includes a plurality of the movable members and the magnets, whose number is the same as the number of magnets and the transmission shaft rotates about a central axis, in the direction of which magnets exert a magnetic attractive force to the movable members of the central axis. 
     In one or more examples, the magnets are fixed to the outer ring member and are evenly distributed in the circumferential direction of the central axis. 
     In one or more examples, the magnet is annular and surrounds the transmission shaft. 
     In one or more examples, the working element is driven to move by the motor. 
     In one or more examples, the walk-behind machine further includes a second motor for driving the working element to move. 
     A walk-behind machine, including a chassis, a handle, wheels, a motor, and a transmission mechanism. The handle is connected to the chassis. The wheels are for supporting the chassis and capable of rotating relative to the chassis. The motor is capable of providing a driving force for rotating the wheels. The transmission mechanism connects the motor with the wheels and includes a clutch for realizing one-way transmission between the motor and the wheels. The clutch has a driving state where the motor drives the wheels to rotate and an unlocked state where the wheels freely rotate relative to the motor. The clutch includes a movable member capable of moving between a locked position and an unlocked position. When the movable member is in the locked position the clutch is in the driving state where the motor actively rotates to drive the wheels to rotate. When the movable member is in the unlocked position the clutch is in the unlocked state. When the motor stops rotating, the wheels rotate at an angle in any direction and drive the movable member to move from the locked position to the unlocked position. 
     In one or more examples, the clutch further includes a driving member. When the clutch is in the driving state, the motor can drive the wheels to rotate in a first direction. When the motor stops rotating, the wheels rotate in a second direction opposite to the first direction, and can drive the driving member to move so that the driving member pushes the movable member to move from the locked position to the unlocked position. 
     In one or more examples, the wheels can drive the driving member to move by friction force. 
     In one or more examples, the motor includes a motor shaft for outputting a driving force. The clutch includes a transmission shaft, a fixing member and an outer ring member. The transmission shaft is connected to the motor shaft and is driven by the motor shaft to rotate. The fixing member is coupled to the transmission shaft or is a part of the transmission shaft and forms a driving surface capable of driving the movable member. The outer ring member is sleeved on the transmission shaft, is connected to the wheels to drive the wheel to rotate and forms a mounting groove. The movable member is located between the wall of the mounting groove and the driving surface. 
     In one or more examples, the wall of the mounting groove is an annular surface, and the movable member is a pin. 
     In one or more examples, the wheels form a first wheel gear or a first wheel gear is mounted on the wheels. The walk-behind machine further includes a first transmission gear meshing with the first wheel gear and driven by the transmission shaft to drive the wheels to rotate. When the motor shaft actively rotates, the transmission shaft drives the first transmission gear to rotate so as to rotate the wheels and the wheels exert a force, whose direction is opposite to the rotation direction of the first transmission gear, to the driving member. 
     In one or more examples, the wheels form a second wheel gear or a second wheel gear is mounted on the wheels. The walk-behind machine further includes a second transmission gear meshing with the second wheel gear. When the wheels rotate, the first transmission gear and the second transmission gear rotate in opposite directions and the second transmission gear exerts a force, whose direction is opposite to the rotation direction of the first transmission gear, to the driving member. 
     In one or more examples, the walk-behind machine further includes a friction plate disposed between the second transmission gear and the driving member to realize the friction transmission between the second transmission gear and the driving member. 
     In one or more examples, the walk-behind machine has a self-propelled forward mode and a self-propelled backward mode. When the walk-behind machine is in the self-propelled forward mode, the motor drives the wheels to rotate in the first direction. When the motor is turned off, the walk-behind machine is out of the self-propelled forward mode and the walk-behind machine is pushed forward to make the wheels rotate in the first direction, the wheels drive the movable member to move from the locked position to the unlocked position and the clutch gets into the unlocked state. When the walk-behind machine is in the self-propelled backward mode, the motor drives the wheels to rotate in the second direction opposite to the first direction. When the motor is turned off, the walk-behind machine is out of the self-propelled backward mode and the walk-behind machine is pushed backward to make the wheels rotate in the second direction, the wheels drive the movable member to move from the locked position to the unlocked position and the clutch gets into the unlocked state. 
     In one or more examples, when the motor is turned off, the walk-behind machine is out of the self-propelled forward-moving mode and the walk-behind machine is pushed backward to make the wheels rotate in the second direction, the wheels drive the movable member to move from the locked position to the unlocked position and the clutch turns into the unlocked state. When the motor is turned off, the walk-behind machine is out of the self-propelled backward-moving mode and the walk-behind machine is pushed forward to make the wheels rotate in the first direction, the wheels drive the movable member to move from the locked position to the unlocked position and the clutch turns into the unlocked state. 
     In one or more examples, the walk-behind machine has a self-propelled forward-moving mode in which the motor drives the wheels to rotate in the first direction and a self-propelled backward-moving mode in which the motor drives the wheels to rotate in the second direction opposite to the first direction. When the motor is turned off, the walk-behind machine could be pushed to drive the wheels to turn at an angle in any direction and the wheels drive the movable member to move from the locked position to the unlocked position, the clutch turning into the unlocked state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of an example walk-behind machine of the present disclosure. 
         FIG.  2    is a schematic view of a chassis, a motor, a transmission mechanism and a plurality of wheels of the walk-behind machine of  FIG.  1   . 
         FIG.  3    is a schematic view of the transmission mechanism separated from the chassis of  FIG.  2   . 
         FIG.  4    is a schematic view of an example motor protection shield located in a motor receiving slot of  FIG.  2   . 
         FIG.  5    is a schematic view of an example anti-wrap and an example gearbox of  FIG.  4   . 
         FIG.  6    is a schematic view of the motor protection shield of the walk-behind machine of  FIG.  1   . 
         FIG.  7    is a schematic view of another perspective of the structure of  FIG.  6   . 
         FIG.  8    is a cross-sectional view of the structure of  FIG.  6   . 
         FIG.  9    is an exploded view of the structure of  FIG.  6   . 
         FIG.  10    is a schematic view showing the internal structure of the gearbox of the walk-behind machine of  FIG.  1   . 
         FIG.  11    is a schematic view of the motor, the transmission mechanism and the plurality of wheels of the walk-behind machine of  FIG.  1   . 
         FIG.  12    is a schematic view of the structure of  FIG.  11    after removing a wheel cover. 
         FIG.  13    is a schematic view showing an example first transmission gear and an example second transmission gear of the transmission mechanism of  FIG.  12    respectively engaged with a first wheel gear and a second wheel gear. 
         FIG.  14    is a schematic view of the transmission mechanism of  FIG.  13   . 
         FIG.  15    is a cross-sectional view of the transmission mechanism of  FIG.  14   . 
         FIG.  16    is an exploded view of the transmission mechanism of  FIG.  14   . 
         FIG.  17    is an exploded view of another perspective of the transmission mechanism of  FIG.  14   . 
         FIG.  18    is a schematic view of the first transmission gear, an outer ring member and a driving member of the transmission mechanism of  FIG.  14   . 
         FIG.  19    is a plan view of another perspective of the structure of  FIG.  18   . 
         FIG.  20    is a cross-sectional view of the structure of  FIG.  18    along line A-A. 
         FIG.  21    is a cross-sectional view of the structure of  FIG.  18    along line B-B. 
         FIG.  22    is a schematic view of the movable member of  FIG.  20    in an unlocked position. 
         FIG.  23    is a schematic view of the outer ring member, a plurality of magnets and a spacer of  FIG.  16   . 
         FIG.  24    is an exploded view of the structure of  FIG.  23   . 
         FIG.  25    is a schematic view of another power tool. 
         FIG.  26    is a schematic illustration of a transmission shaft, the outer ring member, a plurality of pins and the plurality of magnets of the power tool of  FIG.  25   . 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIG.  1    through  FIG.  3   , a walk-behind machine  100  includes a chassis  10 , a handle  20 , a plurality of wheels  30 , a first motor  40 , and a transmission mechanism  50 . The walk-behind machine  100  may also be called a power tool. 
     The chassis  10  is used to mount the first motor  40 . The plurality of wheels  30  are used to support the chassis  10 . As the plurality of wheels  30  contact the ground and rotate around a first axis  102  relative to the chassis  10 , the walk-behind machine  100  rotate relative to the ground. The handle  20  is coupled to the chassis  10 . Users can push the handle  20  to move the chassis  10  relative to the ground, thereby moving the walk-behind machine  100  relative to the ground. Users can control the walk-behind machine  100  by manipulating the handle  20 . 
     The first motor  40  includes a motor shaft  41 . The first motor  40  is capable of driving the plurality of wheels  30  to rotate. The motor shaft  41  is used for outputting a driving force to the plurality of wheels  30 . The motor shaft  41  rotates around a rotation axis  103  which is parallel to the first axis  102 . The motor shaft  41  drives the plurality of wheels  30  to rotate. In the illustrated example, the first motor  40  is an electronic motor and the motor shaft  41  is an electronic motor shaft. The first motor  40  may also be called a self-propelling motor. The walk-behind machine  100  also includes a battery pack that powers the first motor  40 . As an alternative example, the first motor  40  may also be an internal combustion engine powered by fuel combustion. 
     The transmission mechanism  50  transmits power between the first motor  40  and the plurality of wheels  30 . The transmission mechanism  50  connects the motor shaft  41  and the plurality of wheels  30  and transmits motion between the motor shaft  41  and the plurality of wheels  30 . 
     The transmission mechanism  50  transfers motion in one-way between the motor shaft  41  and the plurality of wheels  30 . The transmission mechanism  50  drives the plurality of wheels  30  to rotate as the motor shaft  41  actively rotates. The transmission mechanism  50  includes a clutch  50   a . The clutch  50   a  has a drive state and an unlocked state. In the driving state, the motor shaft  41  drives the plurality of wheels  30  to rotate; in the unlocked state, the plurality of wheels  30  are free to rotate relative to the motor shaft  41 . 
     When the first motor  40  is in an off state, if the walk-behind machine  100  is pushed to make the plurality of wheels  30  rotate by a certain angle in any direction, the clutch  50   a  will turn to the unlocked state. That means the clutch  50   a  can be unlocked when the walk-behind machine  100  is pushed forward or backward to rotate the plurality of wheels  30  clockwise or counterclockwise. 
     The transmission mechanism  50  can realize the “unlocking” by rotating the plurality of wheels  30  in any direction, thus it is convenient and quick for operation. 
     The walk-behind machine  100  has a self-driving forward mode and a self-driving backward mode. In the self-driving forward mode, the rotation speed of the plurality of wheels  30  is greater than that of the plurality of wheels  30  in the self-driving backward mode. 
     As an alternative example, the walk-behind machine  100  includes two start switches for respectively activating the self-driving forward mode and the self-driving backward mode. 
     As another alternative example, the walk-behind machine  100  includes a switch that switches between the self-driving forward mode and the self-driving backward mode. 
     The walk-behind machine  100  has a manually-pushing state and a self-driving state. The walk-behind machine  100  can be conveniently manually pushed forward or backward in the manually-pushing state and be propelled to move by a motor instead of manual pushing force in the self-driving state. 
     As an alternative example, the walk-behind machine  100  turns into the self-driving forward mode when pushed forward and turns into the self-driving backward mode when pushed backward. As an alternative example, the walk-behind machine  100  is provided with a switch that switches between the manually-pushing state and the self-driving state. 
     In the self-driving forward mode, the first motor  40  drives the plurality of wheels  30  to rotate in the first direction and in the self-drive back mode, drives the plurality of wheels  30  to rotate in the second direction opposite to the first direction. 
     When the first motor  40  is turned off to make the walk-behind machine  100  exit the self-driving forward mode and the walk-behind machine  100  is pushed forward to rotate the plurality of wheels  30  in the first direction, the clutch  50   a  turns into the unlocked state. The walk-behind machine  100  can enter the manually-pushing state automatically, not by users&#39; additional operations such as operating an unlock trigger or unlocking switch to unlock the clutch  50   a.    
     When the self-propelling motor is turned off to make the walk-behind machine  100  exit the self-driving backward mode and the walk-behind machine  100  is pushed backward to rotate the plurality of wheels  30  in the second direction, the clutch  50   a  turns into the unlocked state. The walk-behind machine  100  can enter the manually-pushing state automatically, not by users&#39; additional operations such as operating an unlock trigger or unlocking switch to unlock the clutch  50   a . The self-propelling motor is turned off to make the walk-behind machine  100  exit the self-driving forward mode which means the self-propelling motor does not continue to drive the plurality of wheels  30  to rotate in the first direction. The self-propelling motor is turned off to make the walk-behind machine  100  exit the self-driving backward mode which means the self-propelling motor does not continue to drive the plurality of wheels  30  to rotate in the second direction. 
     After the first motor  40  is turned on, the walk-behind machine  100  can enter the self-driving state and can also easily switch between the manually-pushing state and the self-driving state. 
     When the first motor  40  is turned off to make the walk-behind machine  100  exit the self-driving forward mode, the walk-behind machine  100  is pushed backward to rotate the plurality of wheels  30  in the second direction and the clutch  50   a  turns into the unlocked state. When the first motor  40  is turned off to make the walk-behind machine  100  exit the self-driving backward mode, the walk-behind machine  100  is pushed forward to rotate the plurality of wheels  30  in the first direction and the clutch  50   a  turns into the unlocked state 
     When the walk-behind machine  100  exits the self-driving forward mode, the first motor  40  is turned off. The clutch  50   a  can turn into the unlocked state when the walk-behind machine  100  is pushed to drive the plurality of wheels  30  to rotate in any direction. That is, the clutch  50   a  can turn into the unlocked state when the walk-behind machine  100  is pushed forward or backward to drive the plurality of wheels  30  to rotate in the first or second direction. 
     When the walk-behind machine  100  exits the self-driving backward mode, the first motor  40  is turned off. The clutch  50   a  can turn into the unlocked state when the walk-behind machine  100  is pushed to drive the plurality of wheels  30  to rotate in any direction. That is, the clutch  50   a  can turn into the unlocked state when the walk-behind machine  100  is pushed forward or backward to drive the plurality of wheels  30  to rotate in the first or second direction. 
     The walk-behind machine  100  includes a working element  70 . The working element  70  is used to perform the functions of the walk-behind machine  100 . In the illustrated example, the working element  70  is a mowing blade and the walk-behind machine  100  is a lawn mower. The chassis  10  forms a cutting cavity  11 . The mowing blade rotates within the cutting cavity  11 . As another example, the working element  70  is an auger and the walk-behind machine  100  is a snow thrower. 
     In the illustrated example, the working element  70  and the plurality of wheels  30  are respectively driven by different motors. The walk-behind machine  100  includes a second motor  60 , and the second motor  60  is mounted to the chassis  10 . The second motor  60  drives the working element  70 . The second motor  60  may be an internal combustion engine powered by fuel combustion or a motor powered by electricity. In this example, the second motor  60  is an electronic motor. The battery pack supplies power to the second motor  60  and the first motor  40 . As shown in  FIG.  1   , the walk-behind machine  100  includes a power trigger  61  and a drive trigger  42 . The power trigger  61  is used to activate the second motor  60  and the drive the trigger  42  is used to activate the first motor  40 . The walk-behind machine  100  may be a lawn mower and the second motor  60  can be called a mowing motor which drives the mowing blade to rotate. When the mowing motor is powered by electricity, the mowing motor can also be called a mowing electric motor. 
     As an alternative example, the working element and the wheels can be driven by the same motor. That is, a motor, such as the first motor  40 , drives the working element and also drives the wheels. 
     As shown in  FIG.  11    to  FIG.  13   , a first wheel gear  31  is mounted and fixed to the plurality of wheels  30 . The first wheel gear  31  is fixedly coupled to the plurality of wheels  30  to drive the plurality of wheels  30  to rotate. A second wheel gear  32  is mounted and fixed to the plurality of wheels  30 . The first wheel gear  31  and second wheel gear  32  rotate in synchronization with the plurality of wheels  30 . The walk-behind machine  100  also includes a wheel cover  33 . The plurality of wheels  30  form a cavity in which the first wheel gear  31  and the second wheel gear  32  are located. The cavity is provided with an opening which is covered by the wheel cover  33  to prevent dust from entering the cavity to contaminate the first wheel gear  31  and the second wheel gear  32 . 
     As another example, the first wheel gear may also be a part of the wheels, that is to say, be formed with the wheels. The second wheel gear may also be a part of the wheels, that is to say, be formed with the wheels. 
     The transmission mechanism  50  includes the clutch  50   a  and a gearbox  80 . As shown in  FIG.  12    to  FIG.  17   , the clutch  50   a  includes a transmission shaft  51 , a movable member  52 , a driving member  53 , and an outer ring member  56 . 
     The transmission shaft  51  is driven to rotate by the motor shaft  41  and then drives the plurality of wheels  30  to rotate. The gearbox  80  connects the transmission shaft  51  and the motor shaft  41  to make the motor shaft  41  drive the transmission shaft  51  to rotate and the rotation speed of the transmission shaft  51  is lower than the rotation speed of the motor shaft  41 . As shown in  FIG.  9    and  FIG.  10   , the gearbox  80  includes a first driving gear  81 , a first driven gear  82 , a second driving gear  83 , a second driven gear  84 , and an outer housing  85 . The first driving gear  81  is fixed to the motor shaft  41 . The first driven gear  82  meshes with the first driving gear  81 . The first driven gear  82  and the second driving gear  83  rotate coaxially. The second driving gear  83  and the second driven gear  84  mesh with each other. The second driven gear  84  is fixed to the transmission shaft  51 . 
     As shown in  FIGS.  12 - 19   , the transmission mechanism  50  further includes a first transmission gear  54 , a second transmission gear  55 , a fixing member  57 , a friction member  58 , and an elastic member  59 . 
     The first transmission gear  54  meshes with and rotates in synchronization with the first wheel gear  31 . When the first motor  40  is working or the motor shaft  41  rotates actively, the transmission shaft  51  is driven to rotate and then drives the first transmission gear  54  to rotate, which further drives the plurality of wheels  30  to rotate. The second transmission gear  55  meshes with and rotates in synchronization with the second wheel gear  32 . When the plurality of wheels  30  rotate, the first transmission gear  54  and the second transmission gear  55  rotate in opposite directions, and the second transmission gear  55  exerts a force opposite to the rotation direction of the first transmission gear  54  to the driving member  53 . 
     In the illustrated example, the first wheel gear  31  is an external gear, the second wheel gear  32  is an internal gear, the first transmission gear  54  is an external gear, and the second transmission gear  55  is an external gear. The transmission shaft  51  rotates about a central axis  101 . The rotation axis  103  of the motor shaft is parallel to the central axis  101 . The central axis  101  is parallel or coincident with the first axis  102 . The first transmission gear  54  and the second transfer gear  55  rotate about the central axis  101 . The rotational axis of the first transmission gear  54  coincides with the rotational axis of the second transmission gear  55  so that the transmission mechanism  50  is compact. 
     The movable member  52  is movable between a locked position and an unlocked position relative to the transmission shaft  51 . The movable member  52  may be pins. The movable member  52  in  FIG.  20    is in the locked position. The movable member  52  in  FIG.  22    is in the unlocked position. The transmission shaft  51  drives the plurality of wheels  30  to rotate when the movable member  52  is in the locked position. The plurality of wheels  30  are free to rotate relative to the transmission shaft  51  when the movable member  52  is in the unlocked position, that is, the transmission shaft  51  is not driven to rotate whether the plurality of wheels  30  rotate clockwise or counterclockwise. 
     The driving member  53  is driven by the plurality of wheels  30  to make the movable member  52  move between the locked position and the unlocked position. The driving member  53  is driven by the plurality of wheels  30  to drive the movable member  52  to move from the locked position to the unlocked position. 
     As an example, when the plurality of wheels  30  rotate, the second transmission gear  55  is driven to rotate by the second wheel gear  32 . The transmission between the second transmission gear  55  and the driving member  53  is a friction transmission such that the second transmission gear  55  drives the driving member  53  to move to make the movable member  52  move between the locked position and the unlocked position. 
     The transmission between the plurality of wheels  30  and the driving member  53  is a friction transmission. The plurality of wheels  30  exert a force opposite to the rotation direction of the first transmission gear  54  to the driving member  53 . The friction member  58 , realizing the friction transmission between the plurality of wheels  30  and the driving member  53 , is disposed between the second transmission gear  55  and the driving member  53 . The elastic member  59  exerts a force to the second transmission gear  55  to cause the driving member  53  and the second transmission gear  55  to clamp the friction member  58 . The driving member  53  includes a friction portion  532  which is in contact with the friction member  58  to transmit a frictional force. 
     As an alternative example, the second wheel gear, the second transmission gear and the friction member may not be removed and the wheels contact the driving member directly and exert a force opposite to the rotation direction of the first transmission gear to the driving member directly to realize the friction transmission between them. 
     In the illustrated example, the fixing member  57  is coupled to and rotates in synchronization with the transmission shaft  51 . The fixing member  57  forms a driving surface  571 . The fixing member  57  is separately provided to help mount and remove the transmission mechanism  50 . As another alternative example, the transmission mechanism may also be provided without a fixing member, and the driving surface, as part of the transmission shaft, is formed with the transmission shaft. 
     The outer ring member  56  is sleeved on the transmission shaft  51 . The outer ring member  56  forms a mounting groove  561  which accommodates the fixing member  57  and the movable member  52 . The movable member  52  are pins. A plurality of pins is disposed in the mounting groove  561 . The number of the driving surfaces is the same as in number as that of the pins. The pins are located between the groove wall  562 , an annular surface, of the mounting groove  561  and the transmission shaft  51 . 
     In the illustrated example, as shown in  FIG.  16   ,  FIG.  17   ,  FIG.  23    and  FIG.  24   , the power tool includes magnets  52   a . The magnets  52   a  exert a magnetic attractive force to the movable member  52 . The magnets  52   a  exert an attractive force in the direction of the central axis  101  to the pins. 
     The magnets  52   a  reduce the noise generated during the moving of the movable member  52 . The magnets  52   a  make the movable member  52  less likely to topple or deflect during the movement to prevent the clutch  50   a  from being accidentally locked, keeping transmission reliable. 
     The number of magnets  52   a  is equivalent to the number of pins. A plurality of magnets  52   a  are evenly distributed in the circumferential direction of the central axis  101 . 
     During the movement of the pins, each pin is subjected to the same force of the magnets, which keeps the transmission reliable. As another alternative example, the magnets may be annular. 
     The power tool also includes spacer  52   b . The spacer  52   b  is located between the pins and the magnets  52   a  and disposed in the mounting groove  561 . The spacer  52   b  enables the pin to move smoothly within the mounting groove  561 . One end of the pin is in contact with the spacer  52   b . The outer ring member  56  forms recesses  52   c  in which the magnets  52   a  are disposed. 
     As an alternative example, the outer ring member is located between the pins and the magnets in the direction of the central axis. That is, the magnets are disposed outside the mounting groove. As an example, as shown in  FIG.  25    and  FIG.  26   , the power tool  200  includes a motor  201 , a housing  202 , and a work head  203 . The motor  201  drives the working head  203  to rotate. The working head  203  is used to mount a working element. The housing  202  forms a handle  204  for users to hold. The power tool  200  includes a clutch. The clutch includes a transmission shaft  205 , a driving member  206 , an outer ring member  207 , pins  208 , and magnets  209 . The driving member  206  includes a plurality of driving portions  2061 . The driving member  206 , the outer ring member  207  and the pins  208  respectively have the same structure and the same mounting way with the driving member, the outer ring member and the pins of the power tool shown in  FIG.  1    to  FIG.  24   . The power tools in  FIG.  25    and  FIG.  26    are hammer and drill tools. The clutch is also called a shaft-lock structure. The clutch of  FIG.  25    and  FIG.  26    differs from the clutch of  FIG.  1    to  FIG.  24    in structure and in the mounting way of the magnets. The magnets  209  may be annular and surround the transmission shaft  51 . The magnets  209  are located outside the mounting groove and are fixed to the outer ring member  207 . The outer ring member  207  is located between the pins and the magnets  209 . 
     The first transmission gear  54  is mounted to the outer ring member  56 , and the outer ring member  56  drives the first transmission gear  54  to rotate. The movable member  52  is disposed between the driving surface  571  and the groove wall  562  of the mounting groove  561 . The driving member  53  includes a plurality of driving portions  531 . The driving portions  531  get into the mounting groove  561  and contact with the movable member  52  to push the movable member  52  to move. A plurality of driving portions  531  and a plurality of pins are alternatively arranged. 
     As shown in  FIG.  20   , the movable member  52  is in the locked position. The movable member  52  is simultaneously in contact with the groove wall  562  and the driving surface  571  of the mounting groove  561 . 
     At this time, the first motor  40  is working and the motor shaft  41  actively rotates. When the transmission shaft  51  rotates in a clockwise direction (the direction indicated by the arrow), the driving surface  571  drives the movable member  52  to rotate in the clockwise direction for driving the outer ring member  56  and the first transmission gear  54  to rotate in the clockwise direction, thereby driving the plurality of wheels  30  to rotate. 
     As shown in  FIG.  18    to  FIG.  21   , the movable member  52  is located at a locked position capable of simultaneously contacting the driving surface  571  and the groove wall  562  of the mounting groove  561 . 
     When the first motor  40  is working, the motor shaft  41  actively rotates, and at this time, the transmission shaft  51  rotates in a clockwise direction (the direction indicated by the arrow). The movable member  52  simultaneously contacts the groove wall  562  of the mounting groove  561  and the driving surface  571 . Thereby, the driving shaft  51  drives the outer ring member  56  to rotate clockwise. 
     The first transmission gear  54  is sleeved on the outer ring member  56 . The transmission between the first transmission gear  54  and the outer ring member  56  is realized through a flat portion. The clockwise rotation of the outer ring member  56  causes the first transmission gear  54  to rotate clockwise. The first transmission gear  54  meshes with the first wheel gear  31  to drive the plurality of wheels  30  to rotate. 
     When the plurality of wheels  30  rotate, the driving member  53  receives a force, opposite to the rotation direction of the first transmission gear  54 , from the plurality of wheels  30 . That is, the driving member  53  receives a force in a counterclockwise direction and is driven to the position shown in  FIG.  20    by the plurality of wheels  30 . At this time, the driving member  53  blocks the movement of the movable member  52 , so that the movable member  52  cannot move from the locked position to the unlocked position, or the movable member  52  keeps in the locked position. The rotation of the plurality of wheels  30  drives the second transmission gear  55  to rotate counterclockwise. That is, the plurality of wheels  30  drives the second transmission gear  55  to rotate in a direction opposite to the rotation direction of the first transmission gear  54 . The transmission between the second transmission gear  55  and the driving member  53  are friction transmission. The second transmission gear  55  exerts a force to the driving member  53  in a direction opposite to the rotation direction of the first transmission gear  54 , and the driving member  53  is in the way of the movable member  52  from the locked position to the unlocked position. The driving member  53  is driven to rotate to the position shown in  FIG.  20    by the plurality of wheels  30 . 
     When the movable member  52  is in the locked position and the clutch  50   a  is in the locked state, and the motor shaft  41  can drive the plurality of wheels  30  to rotate in the first direction to advance the machine. At this time, the walk-behind machine  100  is in the self-driving forward mode. When the first motor  40  is stopped, or in other words, the motor shaft  41  stops rotating and the walk-behind machine  100  was dragged backward, the plurality of wheels  30  actively rotate in a second direction opposite to the first direction and drive the first transmission gear  54  to rotate counterclockwise. The second transmission gear  55  rotates clockwise. The force exerted by the plurality of wheels  30  on the movable member  52  causes the movable member  52  to move from the locked position to the unlocked position. Further, the plurality of wheels  30  can drive the driving member  53  to move, then the driving member  53  drives the movable member  52  move from the locked position to the unlocked position. The plurality of wheels  30  exert a clockwise force to the driving member  53 . The transmission between the driving member  53  and the transmission shaft  51  is realized with a flat shaft portion engaging in a flat hole. When the transmission shaft  51  stops rotating, the driving member  53  can rotate relative to the transmission shaft  51 . At this time, the driving member  53  rotates by a certain angle under the force of the plurality of wheels  30 , and pushes the movable member  52  to move from the locked position to the unlocked position. 
     When the first motor  40  stops working, the walk-behind machine  100  is pushed forward and the plurality of wheels  30  actively rotate, the plurality of wheels  30  drive the first transmission gear  54  to rotate clockwise. The first transmission gear  54  drives the outer ring member  56  to rotate clockwise. The outer ring member  56  rotates clockwise relative to the transmission shaft  51  to disengage the movable member  52  from the locked position. The driving member  53  can block the movable member  52  from entering the locked position on the other side. At this time, the movable member  52  cannot simultaneously contact the groove wall  562  of the mounting groove  561  and the driving surface  571 . The outer ring member  56  is rotatable relative to the transmission shaft  51 . That is, the plurality of wheels  30  are rotatable relative to the transmission shaft. 
     As an alternative example, the first motor  40  has a forward rotation mode and a reverse rotation mode. In the forward rotation mode, the motor shaft  41  rotates in the first direction; in the reverse mode, the motor shaft  41  rotates in the second direction opposite to the first direction. That is, the rotation directions of the motor shaft  41  are opposite in the forward rotation mode and the reverse rotation mode. Furthermore, the current direction of the first motor  40  in the forward rotation mode is opposite to that in the reverse rotation mode. When walk-behind machine  100  exits the self-driving state, the motor shaft  41  of the first motor  40  enters the reverse rotation mode. The rotation direction of the motor shaft  41  is opposite to that in the self-driving state. The motor shaft  41  actively rotates by a certain angle to rotate the transmission shaft  51  by a certain angle so that the movable member  52  is moved from the locked position to the unlocked position to unlock the clutch. When the walk-behind machine  100  is stopped from traveling suddenly by a large resistance in the self-driving state and the movable member  52  is in the locked position, there is a large static friction between the movable member  52  and the outer ring member  56 , that is, the locking force of the movable member  52 . When the locking force of the movable member  52  is greater than the friction between the plurality of wheels  30  and the driving member  53 , the plurality of wheels  30  cannot drive the driving member  53  to push the movable member  52  to move from the locked position to the unlocked position. The reverse rotation of the motor shaft  41  reduces the friction between the movable member  52  and the outer ring member  56  or directly moves the movable member  52  to the unlocked position. The movable member  52  can smoothly move to the unlocked position. The walk-behind machine  100  can be pulled or pushed smoothly. 
     As shown in  FIG.  22   , the movable member  52  is located at an unlocked position where the movable member  52  cannot simultaneously contact the groove wall  562  of the mounting groove  561  and the driving surface  571 . 
     Since the movable member  52  cannot simultaneously contact the groove wall  562  of the mounting groove  561  and the driving surface  571 , the transmission shaft  51  and the outer ring member  56  can freely rotate. That is, the transmission shaft  51  can freely rotate relative to the plurality of wheels  30 . When the transmission shaft  51  stops rotating, and the plurality of wheels  30  rotate to drive the first transmission gear  54  and the outer ring member  56  to rotate in a counterclockwise direction (the direction indicated by the arrow), the second transmission gear  55  is driven to rotate clockwise by the plurality of wheels  30 . That is, the plurality of wheels  30  apply a force to the driving member  53  opposite to the rotation direction of the first transmission gear  54 , and the driving member  53  is driven to rotate by the plurality of wheels  30  to the position shown in  FIG.  22   . At this time, the driving member  53  blocks the movement of the movable member  52 , preventing the movable member  52  from moving from the unlocked position to the locked position, keeping the movable member  52  in the unlocked position. 
     The middle portion of the driving surface  571  corresponds to the unlocked position of the movable member  52 , and two locked positions are respectively on the right and left side of the unlocked position. The movable member  52  in  FIG.  20    is at the locked position on the left side. The motor shaft of the first motor  40  rotates in a direction to drive the transmission shaft  51  to rotate in the arrow direction, so that the walk-behind machine  100  moves forward, that is, gets into the self-driving forward mode. When the motor shaft of the first motor  40  rotates in opposite direction to drive the transmission shaft  51  to rotate in a direction opposite to the arrow direction, the movable member  52  moves to the locked position on the right side, thereby realizing the backward movement, the self-driving backward mode, of the walk-behind machine  100 . 
     The transmission principle and the unlocking principle of the clutch  50   a  are the same in the self-driving backward mode and the self-driving forward mode. 
     As shown in  FIG.  2    to  FIG.  9   , the lawn mower includes a motor shield  44 . The motor shield  44  accommodates the self-propelling motor. The motor shield  44  includes a motor guard portion  441  and a heat dissipating portion  442 . The heat dissipating portion  442  protrudes from the motor guard portion  441  in a direction away from the ground. The motor guard portion  441  forms a motor cavity  4411  in which the self-propelling motor is located. The heat dissipating portion  442  forms a heat dissipating cavity  4421  connected through with the motor cavity  4411 . The heat dissipating portion  442  is provided with heat dissipating hole  4422  which connects the heat dissipating cavity  4421  and the outside of the heat dissipating portion  442 . The heat dissipating portion  442  can avoid reducing the heat dissipating effect by preventing the grass cuttings from blocking the heat dissipating holes  4422  or from entering the motor cavity  4411 . 
     The heat dissipating portion  442  protrudes from the motor guard portion  441  in the radial direction of the rotational axis  103  of the motor shaft. The heat dissipating portion  442  is connected to one end of the motor guard portion  441  in the axial direction of the rotational axis  103  of the motor shaft. 
     The self-propelling motor further includes a heat dissipating fan  43  fixed to the motor shaft. The motor guard portion  441  forms an air hole  4412 ; and in the axial direction of the rotation axis  103  of the motor shaft, the position of the air hole  4412  corresponds to the position of the heat dissipating fan  43 . 
     The heat dissipating fan  43  rotates to drive air to flow into the motor shield  44  from the heat dissipating holes  4422  and be exhausted from the air hole  4412 . 
     The motor shield  44  forms two heat dissipating portions  442 ; the heat dissipating fan  43  is located between the two heat dissipating portions  442  in the axial direction of the rotational axis  103  of the motor shaft. In the axial direction of the rotational axis  103  of the motor shaft, the two heat dissipating portions  442  are respectively connected to the two ends of the motor guard portion  441 . 
     The motor guard portion  441  further forms an auxiliary heat dissipating hole  4413 , whose area is smaller than the area of the air hole  4412 . 
     The heat dissipating holes  4422  are formed in a strip shape; the long longitudinal direction of the strips coincides with the convex direction of the heat dissipating portion  442 . The heat dissipating hole  4422  is located on side wall of the heat dissipating portion  442 . 
     The heat dissipating portion  442  forms auxiliary air hole  4423  located at the top end of the heat dissipating portion  442 . 
     The lawn mower also includes an anti-wrapping sleeve  51   a  which is rotatably sleeved on the transmission shaft  51 . The self-propelling motor drives the transmission shaft  51  to rotate about the central axis  101 . 
     The anti-wrapping sleeve  51   a  can prevent the transmission shaft  51  from being entangled by the grass when it rotates. 
     The distance between the motor shield  44  and the anti-wrapping sleeve  51   a  is greater than 0 mm and less than or equal to 10 mm. The distance between the motor shield  44  and the anti-wrapping sleeve  51   a  is greater than 0 mm and less than or equal to 3 mm. 
     The anti-wrapping sleeve  51   a  is disposed at one end of the gearbox  80 . The anti-wrapping sleeve  51   a  and the self-propelling motor are located on the same side of the gearbox  80 . 
     An annular groove  851  is formed between the outer housing  85  and the transmission shaft  51 . One end of the anti-wrapping sleeve  51   a  is located in the annular groove  851 . 
     The anti-wrapping sleeve  51   a  defines a large end  511   a  and a small end  511   b . The diameter of the large end  511   a  is greater than that of the small end  511   b . The large end  511   a  of the anti-wrap  51   a  is closer to the gearbox  80  than the small end  511   b . The large end  511   a  of the anti-wrapping sleeve  51   a  is located in the annular groove  851 . The transmission shaft  51  extends through the gearbox  80 . 
     The chassis  10  forms a motor housing cavity  12  in which the self-propelling motor is located. The opening of the motor housing cavity  12  coincides with that of the cutting cavity  11 , and the opening of the motor housing cavity  12  is towards the ground. 
     In the direction of the central axis  101 , the anti-wrap  51   a  is located between the gearbox  80  and the wall of the motor housing cavity  12 . 
     The basic principles, main features and advantages of the present disclosure are described above. Those skilled in the art should understand that the above examples do not limit the disclosure in any way, and any technical solution of equivalent replacement or equivalent transformation is within the protection scope of the present disclosure.