GB2099748A - A hammer drill - Google Patents

Abstract

An air cushion percussion mechanism (18) for a hammer drill has a cylindrical hollow piston (39) reciprocated in a guide and a piston- like striker (42) arranged in a bore (41) in the hollow piston (39) and which is entrained by the hollow piston (39) through an enclosed air cushion. To compensate for air losses existing during compression, a transfer duct formed as a bore (49) in the wall of the hollow piston (39) is provided which, at times, can be connected through sealing means (46) to the space (48) containing the air cushion, through a longitudinal bore (47) in the striker (42). <IMAGE>

Description

SPECIFICATION A hammer drill State of the Art The invention originates from a percussion mechanism for a hammer drill according to the preamble to the main claim. With one known hammer drill, the overflow duct to the air surrounding the percussion mechanism and controlled by the striker is formed as an elongate groove closed at both ends and arranged on the inner surface of the wall of the hollow piston. The length of this elongate groove is so calculated that the groove extends by a small amount beyond the sealing contact surface of the peripheral surface of the striker at both ends. Thus, an air loss possibly occurring during the compression of the rear air cushion during percussive operation at the instant of the axial impact of the striker on the tool, can be compensated through the overflow duct.This percussion mechanism, in which the hollow piston as well as the striker consist of steel, operates satisfactorily. However, it is necessary, particularly for small hammer drills, to design percussion mechanisms of low weight and particularly small dimensions. However, due to the small dimensions, the sealing contact surfaces between the peripheral surface of the striker and the inner surface of the wall of the hollow piston are necessarily much smaller so that, due to the insufficient sealing, the occurring air losses are increased. The air losses can be especially high when the hollow piston consists of aluminium or plastics - a material with a relatively high coefficient of expansion -- and the striker consists of steel material with a relatively low coefficient of expansion.

Advantages of the Invention As opposed to this, the percussion mechanism in accordance with the invention comprising the characterising features of the main claim has the advantage that, due to the use of resilient sealing means, the sealing is very good even with relatively large gaps between the striker and the hollow piston. The inner outlet from the bore forming the overflow duct may be very well deburred which is necessary when using resilient sealing means for preventing excessive wear on the latter.

Advantageous further developments and improvements of the percussion mechanism set forth in the main claim are made possible by the measures set forth in the sub-claims. It is particularly advantageous when the axial length of the (sealing) contact surface of the peripheral surface of the striker with the inner surface of the wall of the hollow piston, is 0.7 to 1.3 times as long as the stroke of the hollow piston.

Drawing Three embodiments of the invention are illustrated in the drawing and are described in detail in the following specification. Figure 1 shows a hammer drill in partial longitudinal section, Figure 2 is a longitudinal section through an intermediate shaft of the hammer drill extending parallel to the longitudinal section of Figure 1, Figure 3 is a longitudinal section through a second embodiment and Figure 4 is a longitudinal section through a third embodiment of a percussion mechanism for a hammer drill.

Description of the Embodiments The hammer drill illustrated in Figures 1 and 2 has a gear housing 1 consisting of metal which is arranged in an outer plastics shell 2. At its front end, the plastics shell changes into a cylindrical housing extension 3 which is formed for the rigid clamping of additional devices or a handle. At the forward end of the housing extension 3, a tool holder is arranged on the hammer drill which serves for the reception of a tool - in this case a drill 5. The shank of the drill 5 has at least one longitudinal groove 6 closed at both ends in which engages an associated radially displaceable locking element -- ball 7 - in the tool holder 4.

The ball 7 is guided in a radial bore in the tool holder tube 8 and is prevented from moving radially by means of a spring loaded sliding sleeve 9. As can be seen from Figure 1, the drill 5 can be moved axially in the tool holder tube 8. The length of this axial movement is established by the axial length of the longitudinal groove 6.

A pistol grip 10 is extended from the housing shell 2 at the rear end remote from the tool holder 4. A switch provided with a push button 11 through which the hammer drill can be set in operation, is provided in the piston grip. A current supply cable 12 leads in at the lower end of the pistol grip 10 through a resilient nozzle.

A bearing seat for a front bearing, formed as a ball bearing 14, of an armature shaft 1 5 of an electric motor, is arranged substantially centrally in a transverse wall 13 of the gear housing 1. At its end remote from the electric motor, the angle of transverse wall 13 carries a tubular extension 16 in which is moved a percussion mechanism 18.

At its forward end facing the tool holder 4, the tubular extension 1 6 carries a flange 19 which engagingly supports the gear housing in an associated fitting 20 in the interior of the plastics shell 2.

The tubular extension 1 6 and the armature shaft 1 5 are arranged in the longitudinal central plane of the hammer (section plane of Figure 1).

At its forward free end, the armature shaft 1 5 carries a motor pinion 21 which meshes with a gear wheel 22 which is mounted on an intermediate shaft 23 for rotation therewith. The intermediate shaft 23 is arranged in a plane (Figure 2) laterally displaced with respect to the longitudinal central plane (Figure 1). Over practically its entire length, it carries an external spline 24 and is supported on the one hand in a grooved ball bearing 25 and on the other hand in a needle bearing 26.

A hub member 27 of a swash plate drive for the percussion mechanism 1 8 is arranged near to the gearwheel 22 and likewise connected to the intermediate shaft for rotation therewith. At its outer end, the hub member 27 has an annular continuous track 28 for balls 29 lying in a plane inclined with respect to the axis of the hub member 27.

Each of the hub member 27 and the gear wheei 22 has an inner spline 30, 31 on its internal bore which engage with the outer spline 24 on the intermediate shaft 23. Axially, the hub member 27 and the gearwheel 22 are supported on the one hand by a locking ring 32 inserted in an associated groove in the external spline 24 and on the other hand by the inner ring of the grooved ball bearing 25.

The forward portion of the spline 24 facing the needle bearing 26 forms the driving pinion 33 for the intermediate shaft 22 which meshes with a gearwheel 34. The gearwheel 34 is arranged at the rear end of the tool holder tube 8.

An outer track 36 machined on the inside of ring 35 is associated with the track 28 on the hub member 27, between which the balls 29 are guided. In order to maintain the balls at a definite spacing, they are guided in a cage 37 well known in ball bearings. A swash pin 38 is formed integrally with the ring 35 and drives the air cushion percussion mechanism 1 8 of the hammer drill to and fro.

The percussion mechanism 18 of the hammer drill is arranged in the interior of the rotary tool holder tube 8 which certainly forms a guide tube therefor. The percussion mechanism 1 8 has a hollow piston 39 sealingly and slidingly guided in the cylindrical inner chamber 40 of the tool holder tube 8. The hollow piston consists of an aluminium alloy, a material having a relatively high coefficient of expansion. A striker 42 consisting of steel and formed as a free moving piston is guided in the cylindrical bore 41 of the hollow piston 39.

At its end remote from the base 43 of the hollow piston 39, The striker has an extension 44 facing the tool 5 which has the same size of cylindrical cross section over its entire length. At its front free end, it is provided with a short transmission cone.

A resilient sealing means formed as an O-ring 46 is inserted in an associated recess 45 formed as an annular groove in the peripheral surface of the striker 42 at its forward end facing the extension 44. This O-ring 46 outwardly seals the space 48, in which an air cushion is formed, enclosed between the base 43 of the hollow piston 39 and the striker 42. This space 48 is connected to the recess 45 through an axially parallel longitudinal bore 47 extending in the striker 42.

An overflow duct, formed as a bore 49, is arranged in the cylindrical wall of the hollow piston 39 and through which the air cushion can be relieved to the air surrounding the percussion mechanism (atmosphere). Viewed in an axial direction, the bore 49 is also located so far in front of the base 43 of the hollow piston 39 that, during the percussive operation illustrated in Figure 1 the tool 5 is pushed into the tool holder tube 8 up to its inner abutment -- it lies opposite the forward dead point position of the recess 45 whereby the space 48 is connected to the bore 49. The outlet from the bore 49 on the outside of the hollow piston 39 cooperates with a control groove 50 arranged in the wall in the interior 40 of the tool holder tube 8 and which is connected through a passage 51 to the air surrounding the percussion mechanism.The axial length S of the control groove 50 formed as an annular groove is shorter than the axial length L of the (sealing) contact surface of the peripheral surface of the striker 42. Preferably the axial length L of the (sealing) contact surface of the peripheral surface of the striker 42 with the wall of the cylindrical bore 41 of the hollow piston 39 amounts to 70 to 130% of the stroke path H of the hollow piston 39.

Moreover, the stroke path is defined as the path which the hollow piston 39 takes from its rear to its forward dead point With the embodiment illustrated in Figure 1 in which the overflow duct formed by the bore 49 is controlled on the one hand by the striker 42 and on the other hand by the axial limitation of the control groove 50, the axial length L of the sealing contact surface when the percussive energy relationships permits it - can move nearer to the shorter length of the given longitudinal region.

The rear end of the hollow piston 39 remote from the tool holder 4 is formed like a fork and carries a pivot pin 55. A transverse bore in which the swash pin 38 engages with a slight movement clearance, is arranged centrally in the pivot pin 55.

Thus, the swash pin 38 can be moved slightly axially within the transverse bore. The position of the swash pin 38 at the rear dead point of the hollow piston 39, is illustrated in dotted lines in Figure 1. The distance H indicates the stroke path of the piston.

The inner end of a fixed support sleeve 56 extends into the forward end region of the cylindrical bore 41 in the hollow piston 39 remote from the swash pin 38. An intermediate dolly 57 is sealingly and slidingly guided in the axial bore in the support sleeve 56. The intermediate dolly 57 is formed like a piston and has a cylindrical piston extension extending towards the striker 42. The diameter of the piston extension is substantially smaller than the extension 44 of the striker 42.

Due to an annular rib 58' projecting onto the axial bore in the support sleeve 56, there is a guarantee that the intermediate dolly 57 cannot leave the axial bore in a direction towards the striker 42. A brake ring 58 is inserted in an annular groove cut into the wall of the axial bore in the supporting sleeve 56 at the free end facing the striker 42. The brake ring 58 forms a trapping device for the striker 42 in its idling position. In the idling position, the tool 5 is located in its most forward position limited by the longitudinal groove 6, at least pushed well into the tool holder tube 8.

In this position, the extension 44 on the striker 42 is forced into the brake ring 58 which has expanded radially and thus retains the striker 42.

In this position, the space 48 located between the piston base 43 and the striker 42 is relieved permanently through the overflow duct 49 so that an air cushion cannot be built up.

A rotary motion of the hub member 27 generates a reciprocating movement of the hollow piston 39. The striker is likewise displaced in a reciprocating movement by the air cushion, which acts as an energy store, built up between the base 43 of the hollow piston 39 and the striker 42 in the percussive position. In the forward dead point of the pot piston and also of the striker 42 (Figure 1) the air losses unavoidably occurring during the forward movement of the hollow piston 39 are displaced through the open overflow duct (passage 51, bore 49, recess 45 and longitudinal bore 47).During the following return movement of the hollow piston 39, the striker 42 then closes the bore 49 at its inner opening and then in addition the hollow piston 39 closes the bore 49 at its outer opening: The striker 42 is entrained rearwardly by the negative pressure cushion now built up in the isolated space 48. Finally, the hollow piston 39 reaches its rear dead point after performing the stroke path H whereupon the air enclosed in the space 48 is compressed until the striker 42 also reaches its rear dead point and is accelerated forwards once again.

The second embodiment of a percussion mechanism illustrated in Figure 3 also has a hollow piston 69 in which is guided a striker 72.

The striker 72 is mainly constructed just like the striker 42 of the first embodiment so that the same reference numerals are used for parts corresponding to the first embodiment. Once again, the hollow piston 69 is guided in the interior of a tool holder tube 68. An overflow duct formed as a bore 79 is located in the wall of the pot piston. The outlet from the bore 79 on the outside of the hollow piston 69 is, of course, permanently connected to the air surrounding the percussion mechanism through a longitudinal groove 80 provided on the outside of the hollow piston 69. Thus, with this embodiment, the overflow duct (bore 47, recess 45, bore 79, longitudinal groove 80) is only controlled by the striker 72.For this reason, the axial length L of the (sealing) contact surface of the peripheral surface of the striker 72 with the wall of the cylindrical bore in the hollow piston 69 should be with this embodiment greater than or at least the same as the stroke H of the pot piston 69. In this embodiment, the stroke H through which the pot piston is set in motion through a known crank drive, is twice as large as the eccentric radius of the crank disc 81. The percussion mechanism illustrated in Figure 3 can be used in a hammer drill which is known from German AS 22 42 944.

Once again, the third embodiment of a percussion mechanism illustrated in Figure 4 has a hollow piston 89 in which a striker 92 is sealingly and slidingly guided. Once again, the striker 92 corresponds mainly to the striker 42 of the first embodiment.

The pot piston 89 which, for example, can be used in a hammer drill as is known from German AS 2516.406, is guided in a fixed guide tube 96 in the bore of which is arranged a liner 97.

Once again, an overflow duct in the form of a bore 99 is arranged in the wall of the hollow piston 89. The outlet from the bore 99 on the outside of the hollow piston 89 cooperates with a control groove 1 00 arranged in the wall of the guide tube as in the first embodiment. In this embodiment, the control groove 100 is, of course, formed as an axially parallel extending groove since the guide tube 96 is arranged stationary.

Once again, the control groove 100 is connected to the atmosphere surrounding the percussion mechanism through a passage 101 in the wall of the guide tube 96.

The method of operation of this third embodiment is of course clearly the same as with the first embodiment.

Claims (9)

1. A percussion mechanism for a hammer drill in which a striker giving up its percussive energy to a tool on impact is guided in the cylindrical interior of a hollow piston axially reciprocated by a motor and is entrained during percussive operation by an air cushion enclosed between the base of the hollow piston and the striker and which is relieved to the air surrounding the percussion mechanism (atmosphere) during idling by at least one overflow duct controlled by the striker, characterised in that, the striker is sealed with respect to the inner surface of the wall of the hollow piston by preferably resilient sealing means, such as an O-ring which control the overflow duct formed as a bore in the wall of the hollow piston inserted in an associated recess arranged particularly at the forward end facing the tool and that the recess is connected to the space in the hollow piston forming the air cushion through a longitudinal bore in the striker.

2. A percussion mechanism according to claim 1, characterised in that, in the forward dead point position of the hollow piston during percussive operation, the bore in its wall lies at least partially opposite the recess.

3. A percussion mechanism according to claim 1 or 2, characterised in that, the axial length of the (sealing) contact surface of the peripheral surface of the striker with the wall of the cylindrical bore of the hollow piston is 0.7 to 1.3 times as long as the stroke of the hollow piston.

4. A percussion mechanism according to claim 2 or 3, characterised in that, the outlet from the bore on the outside of the hollow piston cooperates with a control groove which is connected to the air surrounding the percussion mechanism, arranged in the outside of the hollow piston or in the wall of the guide tube or of a liner.

5. A percussion mechanism according to claim 4, characterised in that, the axial length of the control groove is shorter than the axial length of the (sealing) contact surface of the peripheral surface of the striker.

6. A percussion mechanism according to claim 1 or 2, characterised in that, the outlet from the bore on the outside of the hollow piston is connected permanently to the air surrounding the percussion mechanism especially through a longitudinal groove provided in the outside of the hollow piston.

7. A percussion mechanism according to claim 6, characterised in that, the length of the (sealing) contact surface of the peripheral surface of the striker is greater than or the same size as the stroke of the hollow piston.

8. A percussion mechanism according to one of the preceding claims, characterised in that, the striker consists of steel and the hollow piston consists of aluminium.

9. A percussion mechanism for a hammer drill substantially as herein described with reference to Figures 1 and 2, Figure 3 or Figure 4 of the accompanying drawings.