GB2084916A

GB2084916A - Improvements relating to percussive tools

Info

Publication number: GB2084916A
Application number: GB8130253A
Authority: GB
Original assignee: Kango Electric Hammers Ltd
Current assignee: Kango Electric Hammers Ltd
Priority date: 1980-10-08
Filing date: 1981-10-07
Publication date: 1982-04-21

Abstract

A percussive tool incorporating a reciprocating driver piston and a striker 11 employs a pneumatic device for retaining the striker substantially immobile when it moves forward beyond its range of normal working movement. A vent passage 13 operates under working conditions to vent the forward end space of the cylinder but, when the striker moves forward under idling conditions, initially seals the vent passage to decelerate the striker, releasing the pressure in the cylinder end space through a recess 18 as the striker moves to allow communication between the recess and the vent passage. A small rearward movement of the striker causes this communication to be interrupted and a suction effect occurs, impeding further rearward movement of the striker. The suction effect is overcome when the tool is pressed against a work-piece. <IMAGE>

Description

SPECIFICATION Improvement relating to percussive tools This invention relates to percussive tools and has a particularly useful but not exclusive application in automatic hammers such as electric hammers.

According to this invention there is provided a percussive tool in which a striker disposed in a fixed cylinder is reciprocated by virtue of the presence of a column of air trapped between the striker and a motorreciprocated driver piston, said striker having a piston portion which slides in the cylinder and a reduced-diameter piston rod portion which is slidably mounted in a bore opening to the forward end of the cylinder and which seals the bore with respect to the cylinder, said piston portion of the striker in its working movement reciprocating in a range of movement spaced away from the forward end of the cylinder but when idling moving forward beyond said range of movement, and in which venting means is formed by co-operation of the piston portion of the striker and the cylinder and operates to vent the space between the piston portion of the striker and the forward end of the cylinder permanently when the striker is substantially in its range of working movement and when the piston portion of the striker is in positions adjacent the forward end of the cylinder but to interrupt venting of said space when said piston portion moves rearward of said positions.

The invention will now be described in more detail with reference by way of example to the accompanying diagrammatic drawings in which: Figure 1 is a compound sectional view of the striker piston of an automatic hammer incorporating the invention and illustrates the range of movement of the piston in normal operation, Figure 2 is a view corresponding to Fig. 1 but showing respectively the striker in an intermediate position, Figure 3 is a compound sectional view similar to Fig. 1 but shows the range of idling movement of the striker piston.

Referring to the drawings, the automatic hammer includes a driver piston (not shown) slidably mounted in known manner in one end portion of a cylinder 10 which is fixed against axial movement. The driver piston is reciprocated by a motor (not shown) and in known manner causes a striker piston 11 which is mounted in the forward end portion of the cylinder to reciprocate by reason of the presence of a column of air A trapped between the two pistons. As the striker piston reaches the forward end of its reciprocating movement it strikes the end of either a tool or an anvil 1 2 and rebounds. If, however, there is no external force urging the tool or anvil rearward the striker piston moves forward beyond its normal range of movement and reciprocates idly under the influence of the column of air A.If this idling movement is not damped or suppressed it results in the striker piston clattering noisily and in unnecessary wear and possible damage to the striker piston or the components which it strikes.

In order to prevent or suppress this idling movement of the striker various solutions have been proposed including the use of spring latches to hold the striker piston releasably in its idling position, and porting arrangement which provide a pneumatic force holding the striker piston resiliently in its idling position.

In the present arrangement the cylinder is provided with a vent port 1 3 at a location spaced from the forward end of the cylinder.

This end of the cylinder is sealed by the piston rod 14 of the striker piston which is slidably mounted in a bore 1 5 at the forward end of the cylinder and which imparts the blow to the tool or anvil 12. In Fig. 1, the parts of the piston 11 shown above and below the centre line indicate the forward and rearward limits of movement of the piston during its normal working movement, of length L1, and it will be seen that the vent 1 3 is so positioned that it vents the part of the cylinder space forward of the piston 11 to atmosphere in all positions of the piston 11 in its working movement but is, under these conditions, out of communication with the part of the cylinder between the driver and striker pistons.The space 1 6 between the forward end of the piston rod 14 and the tool or anvil 1 2 is vented to atmosphere by a vent port 1 7 which opens to the reduced diameter bore 1 5 at a position level with the forward end of the piston rod when the striker piston is at the forward end of its normal working movement, i.e. when it strikes the tool or anvil.

If the tool moves forward so as no longer to be struck by the striker piston, or when resistance to the tool ceases, the striker piston moves foward beyond its normal forward limiting position and through an intermediate range of positions, illustrated by Fig. 2, in which the piston 11 closes off the vent 13, causing the air trapped in front of the piston to commence to become compressed and to decelerate the piston.

However the internal diameter of the cylinder is increased from a point just forward of the vent 1 3 to the forward end of the cylinder, and is of an axial length such that the annular space 1 8 which is thus formed about the striker piston comes into communication with the space A between the two pistons and with the vent 1 3. Although the space between the two pistons is vented, the reciprocating movement of the driver piston causes pressure oscillations which tend to cause reciprocation of the striker piston 11 also, but if the striker piston moves rearward beyond the point at which it forms a seal with the main part of the cylinder bore, the pressure of the air thus trapped forward of the striker piston falls, and the resulting pressure difference across the striker piston exerts an increasing force urging it forward. The free reciprocating movement of the striker piston in its idling position is thus rstricted to the small distance L2 shown in Fig. 3.

In alternative arrangements the invention can be employed in a percussive tool of the kind in which the striker is slidably mounted in a fixed cylinder and has within it an axial bore opening to the rearward end of the striker and in which the driver piston is slidably mounted in the bore of the striker, the air column thus being within the striker. In these alternative arrangements the striker still has a piston rod portion 16. In one such arrangement the vent passage 1 3 and recess 18 are employed but communication between the recess 1 8 and the vent passage when the striker is in its forward end position is provided by an annular groove in the external surface of the piston portion of the striker, the axial length of the groove being selected so that the required interruption of venting takes place when the striker moves rearwardly.In another such arrangement, one or more venting grooves extend axially along the external surface of the striker from a position spaced rearwardly from the forward end of the piston portion to its rearward edge and instead of the vent passage 13, one or more venting grooves extend axially rearwardly along the cylinder wall from a little way forward of the front edge of the striker when at the forward end of its normal working movement, and one or more further venting grooves in the cylinder wall extend axially rearward from the forward end of the cylinder and are slightly longer axially than the ungrooved front end portion of the piston portion of the striker.

The venting grooves in the cylinder wall are axially aligned with the venting groove or grooves in the striker.

In these alternative arrangements, any of the usual devices may be provided for venting the air-space between the driver and the striker piston when the latter moves forward beyond its normal working range of movement.

The arrangements described above can advantageously be used in conjunction with the arrangement for restricting the extent of idling movement of the anvil of a percussive tool, which arrangement is described and illustrated in our Patent Application No. 8130254 of even date.

Claims

1. A percussive tool in which a striker disposed in a fixed cylinder is reciprocated by virtue of the presence of a column of air trapped between the striker and a motorreciprocated driver piston, said striker having a piston portion which slides in the cylinder and a reduced-diameter piston rod portion which is slidably mounted in a bore opening to the forward end of the cylinder and which seals the bore with respect to the cylinder, said piston portion of the striker in its working movement reciprocating in a range of movement spaced away from the forward end of the cylinder but when idling moving forward beyond said range of movement, and in which venting means is formed by co-operation of the piston portion of the striker and the cylinder and operates to vent the space between the piston portion of the striker and the forward end of the cylinder permanently when the striker is substantially in its range of working movement and when the piston portion of the striker is in positions adjacent the forward end of the cylinder but to interrupt venting of said space when said piston portion moves rearward of said positions.

2. A tool as claimed in claim 1, wherein said venting means comprises a vent passage in the cylinder wall at a location axially spaced rearwardly of the forward end of the cylinder and forwardly of the range of working movement of the striker, a recess in the cylinder wall extending along the cylinder from the forward end of the cylinder to a location spaced forwardly of said vent passage, and a surface of said piston portion which blanks off communication between the recess and the vent passage when said piston portion moves rearward of said positions.

3. A percussive tool substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.