Vibration reduction apparatus for power tool and power tool incorporating such apparatus

The present invention relates to a vibration reduction apparatus for a power tool and to a power tools incorporating such apparatus. The invention relates particularly, but not exclusively, to vibration reduction apparatus for powered hammers, and to hammers incorporating such apparatus.

Electrically driven hammers are known in which a driving member in the form of a flying mass is reciprocally driven in a piston, and impact of the flying mass against the end of the piston imparts a hammer action to a bit of the hammer. Such an arrangement is disclosed in European patent application EP1252976 and is shown in 10 Figure 1.

Referring in detail to Figure 1, the prior art demolition hammer comprises an electric motor 2, a gear arrangement and a piston drive arrangement which are housed within a metal gear housing 5 surrounded by a plastic housing 4. A rear 15 handle housing incorporating a rear handle 6 and a trigger switch arrangement 8 is fitted to the rear of the housings 4, 5. A cable (not shown) extends through a cable guide 10 and connects the motor to an external electricity supply. When the cable is connected to the electricity supply when the trigger switch arrangement 8 is depressed, the motor 2 is actuated to rotationally drive the armature of the motor. A 20 radial fan 14 is fitted at one end of the armature and a pinion is formed at the opposite end of the armature so that when the motor is actuated the armature rotatingly drives the fan 14 and the pinion. The metal gear housing 5 is made from magnesium with steel inserts and rigidly supports the components housed within it.

The motor pinion rotatingly drives a first gear wheel of an intermediate gear arrangement which is rotatably mounted on a spindle, which spindle is mounted in an insert to the gear housing 5. The intermediate gear has a second gear wheel which

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-2rotatingly drives a drive gear. The drive gear is non-rotatably mounted on a drive spindle mounted within the gear housing 5. A crank plate 30 is non-rotatably mounted at the end of the drive spindle remote from the drive gear, the crank plate being formed with an eccentric bore for housing an eccentric crank pin 32. The 5 crank pin 32 extends from the crank plate into a bore at the rearward end of a crank arm 34 so that the crank arm can pivot about the crank pin 32. The opposite forward end of the crank arm 34 is formed with a bore through which extends a trunnion pin 36 so that the crank arm 34 can pivot about the trunnion pin 36. The trunnion pin 36 is fitted to the rear of a piston 38 by fitting the ends of the trunnion pin 36 into 10 receiving bores formed in a pair of opposing arms which extend to the rear of the piston 38. The piston is reciprocally mounted in cylindrical hollow spindle 40 so that it can reciprocate within the hollow spindle. An O-ring seal 42 is fitted in an annular recess formed in the periphery of the piston 38 so as to form an airtight seal between the piston 38 and the internal surface of the hollow spindle 40.

When the motor 2 is actuated, the armature pinion rotatingly drives the intermediate gear arrangement via the first gear wheel and the second gear wheel of the intermediate gear arrangement rotatingly drives the drive spindle via the drive gear. The drive spindle rotatingly drives the crank plate 30 and the crank arm 20 arrangement comprising the crank pin 32, the crank arm 34 and the trunnion pin 36 converts the rotational drive from the crank plate 30 to a reciprocating drive to the piston 38. In this way the piston 38 is reciprocatingly driven back and forth along the hollow spindle 40 when the motor is actuated by a user depressing the trigger switch

8.

The spindle 40 is mounted in magnesium housing 42 from the forward end until an annular rearward facing shoulder (not shown) on the exterior of the spindle butts up against a forward facing annular shoulder (not shown) formed from a set of ribs in the interior of the magnesium casing 42. The ribs enable air in the chamber 30 surrounding the spindle 40 to circulate freely in the region between ram 58 and beat

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-3piece 64. An increased diameter portion on the exterior of the spindle fits closely within a reduced diameter portion on the interior of the magnesium casing 42. Rearwardly of the increased diameter portion and the reduced diameter portion an annular chamber is formed between the external surface of the spindle 40 and the 5 internal surface of the magnesium casing 42. This chamber is open at its forward and rearward ends. At its forward end the chamber communicates via the spaces between the ribs in the magnesium casing with a volume of air between the ram 58 and the beat piece 64. At its rearward end the chamber communicates via the spaces between the ribs 7 and the recess of the gear casing 5 with a volume of air in 10 the gear casing 5.

The volume of air in the gear casing 5 communicates with the air outside of the hammer via a narrow channel 9 and a filter 11. The air pressure within the hammer, which changes due to changes in the temperature of the hammer, is thus equalised 15 with the air pressure outside of the hammer. The filter 11 also keeps the air within the hammer gear casing 5 relatively clean and dust free.

A ram 58 is located within the hollow spindle 40 forwardly of the piston 38 so that it can also reciprocate within the hollow spindle 40. An O-ring seal 60 is located 20 in a recess formed around the periphery of the ram 58 so as to form an airtight seal between the ram 58 and the spindle 40. In the operating position of the ram 58 (shown in the upper half of Figure 1), with the ram located behind bores 62 in the spindle, a closed air cushion is formed between the forward face of the piston 38 and the rearward face of the ram 58. Reciprocation of the piston 38 thus reciprocatingly 25 drives the ram 58 via the closed air cushion. When the hammer enters idle mode (i.e. when the hammer bit is removed from a work piece), the ram 58 moves forwardly, past the bores 62 to the position shown in the bottom half of Figure 1. This vents the air cushion and so the ram 58 is no longer reciprocatingly driven by the piston 38 in idle mode, as is known to persons skilled in the art.

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-4However, known hammer drills of this type suffer from the drawback that the hammer action generates significant vibrations, which can be harmful to users of the apparatus, and can cause damage to the apparatus itself.

It is known to reduce the effect of vibrations on users of power tools by providing absorbent material around handles of the tool, the absorbent material acting as passive vibration damping material. However, the effectiveness of such materials in reducing the transmission of vibrations to the user of the apparatus is limited.

Active vibration reduction apparatus are known in which rotatable masses are driven about respective axes of rotation, the centres of mass of the rotatable masses being spaced from the axes of rotation such that rotation of the masses about the axes of rotation generates vibrations. By controlling the frequency of rotation of the 15 masses, and the relative phases between the centres of mass of the masses, vibrations can be generated which can be used to counteract unwanted vibrations, for example in diesel motors. Such arrangements are disclosed in FR 2606110, WO 88/06687, FR 2550471, EP 0840191, EP 0505976 and EP 0337040. However, it has not to date been considered feasible to apply such techniques to the reduction of 20 unwanted vibrations generated in power tools.

Accordingly, there is provided a power tool comprising a housing; a motor in the housing adapted to cause reciprocating motion of a piston located within a cylinder; and vibration reduction means in the housing, wherein said vibration 25 reduction means comprises at least one driveable mass adapted to be driven in a reciprocating manner by the motor relative to the housing, such that vibration of said vibration reduction means at least partially cancels vibration of said housing caused by said movement of the piston, wherein the at least one said driveable mass is driven by means of air displaced by the piston.

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-5By providing vibration reduction means caused to vibrate by the motor, such that vibration of said vibration reduction means at least partially cancels vibration of said housing caused by said movement of the piston, this provides the advantage of enabling vibration of the housing cause by operation of the tool to be significantly 5 more effectively reduced than in prior art power tools.

An embodiment of the present invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:10

Figure 1 is a partially cut away side view of a prior art demolition hammer;

Figure 2 is a cross sectional side view of a first design of vibration reduction apparatus;

Figure 3 is a cross sectional view, corresponding to Figure 2, of a second 15 design of vibration reduction apparatus;

Figure 4 is an exploded perspective view of a third design of vibration reduction apparatus; and

Figure 5 is a side cross sectional view, corresponding to Figure 2, of a vibration reduction apparatus of an embodiment of the present invention.

Three alternative designs of vibration reduction mechanisms will now be described with reference to Figures 2 to 4. These are not embodiments of the present invention. However, they are included to aid the reader in understanding the embodiment of the invention described later on with reference to Figure 5.

Referring now to Figure 2, a design of compact vibration reduction apparatus 501 for use with a tool having a hammer piston 520 is shown. The hammer piston 520 is caused to reciprocate in cylinder 530 by rotation of a gear wheel 523 about axis 524, the hammer piston 520 being mounted to the gear wheel 523 via piston 30 arm 531 and pivot pin 522. Reciprocating motion of the hammer piston causes a

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-6flying mass 569 to be driven along cylinder 530 to impart a hammer action to a bit (not shown) of the tool.

The gear wheel 523 is caused to rotate about axis 524 by meshing of gear 514 5 on drive shaft 511 with gear teeth 532 on the periphery of gear wheel 523, the drive shaft 511 being caused to rotate by a motor (not shown) of the tool. .

The vibration reduction mechanism 501 has a cam 533 rigidly mounted around gear wheel 523, a counterweight 521 surrounding piston cylinder 530, and a cam 10 follower 534 on counterweight 521. The cam follower 534 on counterweight 521 is urged into contact with cam 533 by means of a compression spring 535. The external profile of the cam 533 is such that rotation of gear wheel 523 to cause reciprocating motion of hammer piston 520 causes oscillation of counterweight 521 relative to piston cylinder 530 in antiphase to motion of the hammer piston 520.

Since the hammer piston 520 and the counterweight 521 are driven by the same drive shaft 511, this ensures that the counterweight 521 is driven at the same frequency as the hammer piston 520. Because the counterweight 521 surrounds the cylinder 530, the mechanism 501 can be made more compact, and twisting torque produced by operation of the mechanism is minimised.

In order to deactivate the vibration reduction mechanism 501 when the gear wheel 523 is still rotating while the hammer action of the tool is deactivated (for example when the bit of the tool is removed from the work piece ), the counterweight 521 is locked in position relative to the cylinder 530 in its furthest position to the left as shown in Figure 2. This is achieved by a ball bearing 536 in a housing 537 surrounding the piston cylinder 530 becoming aligned with recess 539 in the external surface of the counterweight 521. The ball bearing 536 normally allowing sliding movement of the counterweight 521 relative to the piston cylinder 530. In order to lock the counterweight 521 in position relative to the piston cylinder 530 when the ball bearing 536 and recess 539 are aligned, a pin 538 is displaced to the right as shown 30 in Figure 2, as a result of which the ball bearing 536 is displaced upwards into

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-7engagement with recess 539 in the counterweight 521 to prevent axial movement of the counterweight 521 .In this position, the cam 533 rotates freely on gear wheel 523, but the cam follower 534 is prevented from moving into engagement with the cam 533.

Referring to Figure 3, in which parts common to the design of Figure 2 are denoted by like reference numerals but increased by 100, a vibration reduction mechanism 601 of a second design has a cam 633 which is normally rigidly locked to gear wheel 623 by means of a pin 640 which is urged downwards as shown in Figure 10 3 by means of torsional spring 641 which in turn urges a ball bearing 642 outwards of the pin 640 to lock the cam plate 633 to the gear wheel 623.

In order to deactivate the vibration reduction mechanism 601, pin 638 is urged to the left as shown in Figure 3 against the action of compression spring 643 to move 15 pin 640 upwards so that ball bearing 642 can be accommodated in narrowed portion 644 of pin 640. As a result, cam 633 can rotate freely on gear wheel 623, as a result of which the cam 633 does not displace cam follower 634. The counterweight 621 therefore does not move relative to piston cylinder 630.

Referring to Figure 4, in which parts common to the design of Figure 3 are denoted by like reference numerals but increased by 200, a cam 833 is mounted to a gear plate 823, and a cam follower 834 is mounted to pivot arm 860. Pivot arm 860 pivots about pivot pin 861 and is urged by spring 835 into engagement with cam 833. A counterweight 821 is slidably mounted to piston cylinder 830 by means of pin 862, 25 and hammer piston 820 is mounted to gear wheel 823 by means of crank arm 831 and eccentric pin 822. The hammer piston 820 is driven in a reciprocating manner by means of a gear (not shown) on a drive shaft engaging with gear teeth 832 on gear wheel 823, and rotation of gear wheel 823 with cam 833 in engagement with cam follower 834 causes reciprocating motion of counterweight 821 generally in 30 antiphase with hammer piston 820.

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-8Counterweight 821 is mounted to pin 862 by means of a ball bearing 842 which is urged into engagement with a recess 863 on a lower part of pin 862 received in bore 864 of counterweight 821 by means of a pin 840. In order to deactivate the 5 vibration reduction mechanism 801, the pin 840 is urged downwards against the action of compression spring 841 to disengage ball bearings 842 from recess 863 in pin 862. This allows the pin 862 to slide freely in the bore 864 in counterweight 821.

An embodiment of the present invention will now be described with reference to 10 Figure 5 in which parts common to the design of Figure 3 are denoted by like reference numerals but increased by 100. The vibration reduction mechanism 701 has a counterweight 721 provided in a chamber 750 adjacent to piston cylinder 730. Hammer piston 720 is driven reciprocally by means of wobble plate 751 mounted to drive shaft 752 (the drive shaft 752 being connected to the motor), and displacement 15 of hammer piston 720 within piston cylinder 730 causes displacement of air in piston cylinder 730 and channel 753 connecting piston cylinder 730 and chamber 750 such that counterweight 721 is caused to move generally in antiphase with hammer piston 720.

Throughout this specification and claims which follow, unless the context 20 requires otherwise, the word comprise, and variations such as comprises or comprising, will be understood to imply the inclusion of a stated integer or group of integers or steps but not the exclusion of any other integer or group of integers.

The reference to any prior art in this specification is not, and should not be 25 taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge in Australia.

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