Patent Publication Number: US-2003221847-A1

Title: Pneumatic percussion operated mechanism

Description:
BACKGROUND OF THE INVENTION  
       [0001] The invention relates to a pneumatic percussion operated mechanism for an at least partially percussion driven hand tool device such as a chisel hammer or a hammer drill.  
       [0002] A pneumatic percussion operated mechanism has a free piston guided in a guide tube and driven by an air spring, the free piston conventionally impacting upon an anvil movable for a limited axial extent, itself abutting on the trailing end of a tool movable for a limited axial extent.  
       [0003] According to DE 2702128 a pneumatic percussion operated mechanism formed as short as possible for a hammer drill features a cup-shaped free piston in which an anvil penetrates axially inwardly. Other hollow, cup-shaped free pistons the arranged on the anvil side have been described in JP 8141937 and JP 7328955.  
       [0004] In the case of pneumatic percussion operated mechanisms and under certain working conditions such as inadequate counter pressure from the workpiece re-striking of the anvil by the tool may occur, which is prevented according to DE 3804026 by using air passages in the free piston and a rotationally displaceable guide tube.  
       SUMMARY OF THE INVENTION  
       [0005] The object of the invention is to provide a technically simple development of a pneumatic percussion operated mechanism that is short in its construction and free of idling.  
       [0006] Essentially, a pneumatic percussion operated mechanism for an at least partially percussion operated hand tool machine has a cup-shaped piston forming a hollow space, an anvil penetrates displaceably axially inwardly within an axial penetration length and, together with the latter forms a damping, pneumatically acting radially extending gap relative to the essentially gas-tight closed hollow space, which drives the anvil, driven by the piston within the penetration length, to the reversal movement.  
       [0007] By virtue of the damping, pneumatically acting radial gap due to the high flow resistance towards the hollow space, after the percussion strike of the piston on the anvil, there is a suction effect between the essentially gas-tight, closed hollow space of the piston and the anvil, whereby at the operating point there is substantially no braking of the anvil; however, in the case of a no-load strike it reverses its direction of movement before the contact on the trailing end of the tool, whereby with sufficient boring capacity an optimum idling behavior is obtained.  
       [0008] Advantageously the gap area of the radial annular gap between the anvil and the piston is 3% to 6% of the anvil cross-section area formed by the anvil inside the piston, whereby in the case of normal percussion frequencies of 50 Hz to 100 Hz a reversal of movement occurs within 10 mm.  
       [0009] Advantageously in the case of conventional pneumatic percussion operated mechanisms for hand tool machines having an anvil weighing approximately 50 g penetrating into the piston and a pneumatically acting diameter of approximately 10 mm, the gap thickness is in the range of 0.1 mm to 0.17 mm, whereby optimum results have been obtained in a series of trials. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010] The drawings show:  
     [0011]FIG. 1 is a sectional view of a percussion operated mechanism, and  
     [0012]FIG. 2 is an enlarged partial sectional view of a single unit II, in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0013]FIG. 1 is a pneumatic percussion operated mechanism of a percussion driven hand tool machine  1  having a cup-shaped piston  2  at the no-load strike movement reversal point, in which an anvil  3  is spaced radially inwardly of the cup-shaped piston and displaceably penetrates into an axial penetration length L, forming, over a portion of its outer sleeve length together with a portion of the inner sleeve length of the piston  2 , a damping, pneumatically acting radial gap X relative to an essentially gas-tight, closed hollow space  4  within the piston  2 .  
     [0014] According to FIG. 2, in the case of a pneumatically effective diameter D of the anvil  3  of approximately 10 mm, the thickness of the damping, radial gap X is the range of 0.1 mm to 0.17 mm.