GB2084917A - Improvements in or relating to percussive tools - Google Patents

Abstract

A percussive tool such as an electric or pneumatic hammer including a reciprocating striker 10 striking an anvil 13 which is slidably mounted in a cylinder and which in turn strikes an implement, incorporates a device for preventing rebound of the anvil towards the striker when the implement is moved away from the workpiece allowing the anvil to move forward beyond its normal operating range of movement. The device comprises a venting valve formed jointly by a land 28 on a reduced-diameter portion 21 of the anvil and an annular constriction 29 in the bore in which portion 21 reciprocates. The venting valve operates to vent the cylinder space 30 at the rearward side of the anvil in its most forward and rearward positions only. Thus, rebounding of the anvil from its idle, i.e. most forward, position seals the cylinder space 30 and causes the pressure therein to rise and resist further rearward movement of the anvil. <IMAGE>

Description

SPECIFICATION Improvements in or relating to percussive tools This invention relates to percussive tools such as electric and pneumatic hammers.

According to this invention there is provided a percussive tool comprising a striker which is mounted for guided reciprocating movement, an anvil mounted for limited axial sliding movement coaxially with the striker and arranged to be struck by the striker during the reciprocating movement of the latter and to transmit the impacts to a working implement, said anvil comprising a piston portion which is slidably engaged in a cylinder bore in the tool and a reduced-diameter portion which is slidably engaged in a reduceddiameter portion of the bore and projects through said portion towards the striker, and means provided for venting the cylinder space between the piston portion and the reduceddiameter portion of the bore when the anvil is at each extremity of said limited axial movement and to seal said cylinder space when the anvil is displaced from its end position further from the striker in a direction towards the striker, which end position is an idle position of the anvil beyond the range of its normal working movement.

In a preferred construction according to the invention, the means for venting said cylinder space comprises a peripheral constriction in the reduced-diameter part of the bore and a peripheral land on said reduced-diameter part of the anvil, a seal being formed intermediate the end positions of the anvil by engagement of the land in the constricted part of the bore, and a venting space being formed between the reduced-diameter part of the anvil and the reduced-diameter portion of the bore when the land is axially spaced in either direction from the constriction. The constriction may be formed by a sealing ring disposed in a peripheral recess in the reduced diameter portion of the bore.

Preferably means is provided, in a known manner, for retaining the striker resiliently in a position forward of its normal working position to immobilize it if the implement moves forward appreciably beyond its normal working position.

One embodiment of the invention will now be described in more detail with reference by way of example to the accompanying drawings in which: Figures 1 to 3 show in axial section the forward portions of a percussive tool according to the invention in three working positions respectively.

The percussive tool shown is an electric hammer of the general kind described in more detail in our Patent Specification No.

1030136 in which a striker 10 is mounted for reciprocating movement in a cylinder 11 in the body 1 2 of the hammer. The striker is hollow over the major part of its length to form within it a cylinder bore which is closed at its forward end and in which a driver piston (not shown) is reciprocatable. The piston is driven by an electric motor through a connecting rod. A cushion of air is trapped, within the striker, between the driver and striker 10 so that reciprocation of the driving causes the striker also to reciprocate. Axial grooves (not shown) are formed in the wall of cylinder 11 to allow air to move to and fro past the striker so that there is no build-up of air pressure to impede the striker movement. At the forward end of its travel the striker hits an anvil 1 3 which is thus driven forward and hits the working implement 14.So long as the implement is pressed against the workpiece, the implement and anvil are pressed back towards the striker to receive working impacts from the striker. However, if the implement is withdrawn from the workpiece or the load pressure is otherwise removed from the implement the striker and anvil, the anvil rebounds and hits the striker, and the striker moves beyond its normal travel and hit a fixed part of the tool, producing unnecessary noise and wear.

To reduce this noise and wear due to the striker many available hammers incorporate means which operates, if the striker moves an appreciable distance forward beyond its most forward normal working position, to hold the striker resiliently in its idle forward position until the working implement is again pressed against the workpiece so that the implement pushes the striker, indirectly through the anvil, back to its normal working position. The hammer then re-commences to work normally.

One such means operating to hold the striker in its advanced idle position by air pressure is described in our Patent Specification No.

1030136 but mechanical means are known which achieve the same end. We have however found that although the striker when idling is held immobile by such means the anvil may bounce back from the forward end of its travel and upset the operation of the means for immobilising the striker. The drawings show a construction we have devised to prevent such bouncing by holding the anvil immobile when the implement is idle.

In the illustrated construction the anvil 1 3 is slidably mounted in a cylinder member 17 which is recessed into the forward end of the main body part 1 2 of the tool. The anvil has a piston portion 1 9 which has a peripheral sealing ring 20 and which is slidable in the cylinder bore, and which is slidable in the cylinder bore, and a reduced-diameter tail rod 21 which projects through a reduced-diameter end portion 22 of the cylinder bore, the end of the tail rod being arranged to be struck by the striker 10. There is a radial clearance between the surface of the tail rod and the reduced-diameter portion 22 of the bore. This clearance is increased over the free end part 26 of the length of the tail rod and over a short length 27 forming a groove axially spaced from the part 26 so that a land 28 is formed.A sealing ring 29 is received in a recess in the portion 22 of the bore and forms a seal with the land to prevent air from moving through the radial clearance into or out of the cylinder space indicated at 30 in Fig. 2 when the anvil is in a position intermediate its end positions. In each of the two end positions of the anvil illustrated in Figures 1 and 3 respectively, air can move into and out of the cylinder space 30 along the radial clearance.

A recoil buffer 32 is disposed in cylinder space 30 against the shoulder formed at the reduction of the cylinder bore diameter. A buffer ring 33 registers in the opposite end of the cylinder bore and is abutted axially at its other side by an idle blow buffer 34 disposed in a counterbore in a fixed annular holder 35 for the implement 1 4. Buffer ring 33 can slide a short distance axially to compress the buffer.

In working operation of the tool the anvil reciprocates between positions approximating to those in which it is shown in Figs. 1 and 2.

In the position shown in Fig. 1, the piston portion 1 9 of the anvil is disposed against the recoil buffer 32 and the part 26 is struck by the striker 10. The groove 27 is disposed radially opposite the sealing ring 29 and is axially longer than the sealing ring so that the cylinder space 30 is vented through the radial clearance and the space between the striker 10 and the cylinder member 1 7 to the cylinder 11 which is at a pressure approximating to atmospheric pressure by reason of the axial grooves previously mentioned. When struck by the striker, the anvil moves away from the buffer 32 and the land 28 comes into sliding sealing engagement with the sealing ring 29, so that further forward movement of the anvil creates in the cylinder space 30, now very small, a depression tending to pull the body of the hammer towards the workpiece.The rebound of the implement from the workpiece knocks the anvil back from approximately the position shown in Fig. 2 to the position shown in Fig. 1 and so on. If there is no rebound, the anvil continues to move forward until it strikes the buffer ring 33 and idle blow buffer 34, see Fig. 3. In this position, the land 28 has moved completely past the sealing ring 29 so that the cylinder space 30 is vented to cylinder 11. If now the anvil moves away from buffer ring 33, the land 28 forms a seal again with the ring 29 so that further rightward movement of the anvil is resisted by the resulting rise in pressure of the air trapped in space 30. In effect, the anvil is held adjacent the buffer ring 33. When the implement is once more pressed firmly against the workpiece, the air resistance to the movement of the anvil is overcome and working operation of the tool re-commences.

In alternative arrangements the sealing ring may be provided on the reduced-diameter part of the anvil and the land may be formed in the reduced-diameter part 22 of the bore.

In other but less preferred alternative arrangements the inner cylindrical wall of the cylinder member 1 7 has two axially-spaced rings of axial grooves disposed respectively at positions corresponding to the positions of the sealing ring 20 when the piston portion 1 9 is at opposite extremities of its movement. In these arrangements the tail rod 21 of the anvil forms a sliding seal with the cylinder member in all positions of the anvil. The grooves in each set have a greater axial length than the piston and serve to vent the cylinder space 30 to atmosphere via the slideway for the implement 40 when the piston portion is at either extremity of its movement. Intermediate these extreme positions the cylinder space 30 is substantially sealed. The operation is thus very similar to that of the arrangement illustrated, but the venting of the cylinder space 30 is to atmosphere via the slideway for the implement 14 instead of to the space between the striker and the cylinder member 17, which space is itself vented as previously described.

Claims (5)

1. A percussive tool comprising a striker which is mounted for guided reciprocating movement, an anvil mounted for limited axial sliding movement coaxially with the striker and arranged to be struck by the striker during the reciprocating movement of the latter and to transmit the impacts to a working implement, said anvil comprising a piston portion which is slidably engaged in a cylinder bore in the tool and a reduced-diameter portion which is slidably engaged in a reduceddiameter portion of the bore and projects through said portion towards the striker, and means being provided for venting the cylinder space between the piston portion and the reduced-diameter portion of the bore when the anvil is at each extremity of said limited axial movement and to seal said cylinder space when the anvil is displaced from its end position further from the striker in a direction towards the striker, which end position is an idle position of the anvil beyond the range of its normal working movement.

2. A percussive tool as claimed in claim 1, wherein the means for venting said cylinder space comprises a peripheral constriction in the reduced-diameter part of the bore and a peripheral land on the reduced-diameter portion of the anvil on said tail rod, a seal being formed intermediate the end positions of the anvil by engagement of the land in the constricted part of the bore, and venting space being formed between the reduced-diameter portion of the anvil and the reduced-diameter portion of the bore when the land is axially spaced in either direction from the constriction:

3. A percussive tool as claimed in claim 2, wherein the constriction is formed by a sealing ring disposed in a peripheral recess in the reduced-diameter portion of the bore.

4. A percussive tool as claimed in any one of claims 1 to 3, wherein a buffer ring is disposed in each end of the cylinder.

5. A percussive tool substantially as herein before described with reference to and as illustrated in the accompanying drawings.