GB2387215A

GB2387215A - Drive mechanism having an elastomeric material located between meshing tooth forms

Info

Publication number: GB2387215A
Application number: GB0120941A
Authority: GB
Inventors: Adrian Coussins; Nick Tulip
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2001-08-29
Filing date: 2001-08-29
Publication date: 2003-10-08
Also published as: GB0120941D0

Abstract

A drive mechanism for a power tool wherein rotary drive from an electric motor is transmitted from a pinion 22 on a driving shaft to a spur gear 11 driving the power tool through a concentric inner drive unit. The spur gear 11 has inwardly directed tooth forms 19 meshing with outwardly directed tooth forms 21 of the inner drive unit, and elastomeric material 20 is disposed between the meshing tooth forms. The drive unit is preferably used in a hedge cutter to drive the cutting blades.

Description

238721 5

This invention relates to a driving mechanism for a power tool, such as a hedgecutter, wherein drive is transmitted from a pinion on the output shaft of a motor to a spur gear, and the drive from the spur gear is then transmitted to the working tool. The working tool may comprise, for example, the cutting blades of a hedgecutter.

A problem that arises when such a drive system is used in a hedgecutter is that the cutting blades can become momentarily jammed when attempting to cut a thick branch or a metal wire and this impact on the drive mechanism can damage the gear wheels and/or the motor. One way of overcoming the problem is to insert a slip 10 clutch in the drive mechanism so that the clutch slips when the load exceeds a predetermined level. However we have found that this can lead to a decrease in the cutting performance of the hedgecutter.

In a cylinder lawnmower, it is also known to smooth the transmission of drive to a IS drive belt by transmitting the drive through an elastomeric material.

According to one aspect of the present invention there is provided a drive mechanism for a power tool wherein rotary drive from an electric motor is transmitted from a pinion on the output shaft of the motor to a spur gear, and wherein rotary drive from 20 the spur gear is transmitted to a working tool through a solid body of elastomeric material whereby, if the load on the working tool momentarily exceeds a predetermined level sufficient to stall the motor, the motor continues to rotate the spur gear for a short period before the motor stalls.

25 By extending the time the motor is permitted to drive the spur gear before stalling, the impact on the gear wheel is significantly reduced and the risk of damaging the drive mechanism is correspondingly less.

In one embodiment of the invention the elastomeric material is interposed between an 30 internal driving surface of the spur gear and a driven surface of a concentric member coupled to the working tool and driven by the spur gear through the elastomer. The

elastomeric material is then compressed between the two surfaces if the working tool becomes jammed and the inner concentric member can no longer rotate. The driven concentric member may have an output drive shaft for driving the working tool, or it may rotate about a fixed spindle to drive the working tool.

s Preferably the elastomeric material is so arranged that it also provides electric insulation between the electric motor and the working tool.

In a hedgecutter embodying the invention, the output drive shaft or the inner 10 concentric member drive two eccentric discs that are attached to the respective cutting blades so that the blades are reciprocated relative to one another in a conventional mariner. A hedgecutter embodying the invention will now be described with reference to the 15 accompanying drawings wherein, by way of example only: Fig. 1 is a diagrammatic vertical cross-section of a drive mechanism for driving the cutting blades; 20 Fig. 2 is a diagrammatic vertical cross-section of an alternative drive mechanism, and Fig. 3 is a diagrammatic horizontal cross-section illustrating the main components of the drive mechanism in Fig. 1 and Fig.2.

25 Referring first to Figs. 1 and 3, an electric motor (not shown) has a pinion 10 rigidly coupled to an output shaft of the motor and meshing with external teeth 12 of a spur gear 1 1. The gear wheel 11 drives a concentric shaft 13 which rotates in upper and lower bearings 14, 15 located inside the hedgecutter housing (not shown) to drive a pair of reciprocating cutter blades (not shown) attached to respective upper and lower 30 eccentric metal discs 16, 17 received on the shaft 13 and separated by a metal shim 31.

As best seen in Fig. 3, the gear wheel 11 consists of an outer metal sleeve 18 having four inwardly directed dog teeth 19, the space between each pair of dog teeth 1 9 being occupied by elastomeric material 20 together with corresponding meshing teeth 21 of 5 an inner concentric driven dog 22 push-fitted on a splined portion of the drive shaft 13; Accordingly drive from the gear wheel 11 is transmitted through the elastomeric material 20 to the inner driven dog 22 and thereby to the drive shaft 13 for reciprocating the cutter blades. The elastomeric material may consist of rubber and preferably has a hardness factor of 3050 measured on the Shore scale. It can be 10 formed as an outset moulding in the recessed portions of the outer metal sleeve 18 between the teeth 19.

As shown in Fig.1, the outer metal sleeve 18 has a base flange 23 extending inwardly toward the central drive shaft 13, and the inner driven dog 22 is clamped between this 15 flange 23 and a steel ring 24 by threaded bolts 25 inserted in holes formed in the outer sleeve. A plastic washer 26 is inserted between the steel ring 24 and the driven dog 22, and another plastic washer 27 is inserted between the flange 23 of the outer sleeve and the upper eccentric disc 16. In addition, the elastomeric material 20 extends beneath the driven dog 22 and the flange 23 of the outer metal sleeve 1 8 so that the 20 drive shaft 13 is fully insulated from the electric motor, a clearance 32 being provided between the flange 23 and the shaft 13. Altematively the plastic washer 27 could form a bushing around the shaft 13 to replace the clearance 32.

In the alternative embodiment illustrated in Fig.2, where like components are denoted 25 by like references, the shaft 13 is fixed in the hedgecutter housing and the gear wheel 11 drives the inner dog 22 about the fixed shaft on bearings 29. In this case the upper eccentric disc 16 Comes an integral part of the inner dog 22 and the lower eccentric disc 17 is a keyed pushfit with the inner dog 22 so that it is driven by the inner dog 22 about bearings 30 surrounding the shaft 13.

In use, should the cutter blades become jammed for any reason, there will be a short period of a few milliseconds when the motor will continue to rotate the gear wheel 11 as the elastomeric material 20 is compressed. This period is crucial as it substantially reduces the impact on the gear wheels and therefore the risk of any damage to the 5 drive mechanism. Once the motor stalls, the energy supply is cut off and the cause of the jamming can be removed.

An additional advantage of the described drive mechanism is that the elastomeric material in the mechanism results in reduced vibration and a quieter operation.

Claims

CLAIMS:

1. A drive mechanism for a power tool wherein rotary drive from an electric motor is transmitted from a pinion on a driving shaft to a spur gear driving the power 5 tool through a concentric inner drive unit, the spur gear having inwardly directed tooth forms meshing with outwardly directed tooth forms of the inner drive unit, and wherein elastomeric material is interposed between the meshing tooth forms.

2. A drive mechanism according to claim 1 wherein the elastomeric material 10 provides electrical insulation.

3. A drive mechanism according to claim 1 or claim 2, wherein the inner drive unit is driven by the spur gear about a fixed central shaft.

15

4. A drive mechanism according to claim 1 or claim 2, wherein a central circular portion of the spur gear is recessed to accommodate the inner drive unit.

5. A drive mechanism according to claim 4, wherein the recessed portion of the spur gear includes a base and elastomeric material is further interposed between the 20 base and the inner drive unit.

6. A drive mechanism according to claim 4 or claim 5, wherein the inwardly directed tooth forms of the spur gear project into the recessed portion of the spur gear to mesh with the tooth forms of the inner drive unit.

7. A drive mechanism according to claim 5 or claim 6 wherein the spur gear includes a wall extending inwardly from the circumference of the spur gear to a central shaft of the drive mechanism.

30

8. A drive mechanism according to claim 1 or claim 2, the mechanism being carried by a central drive shaft for rotation therewith.

9. A drive mechanism according to any one of the preceding claims wherein the inner drive unit includes a pair of eccentric drive transmission components for driving a corresponding pair of reciprocating cutter blades.

s

10. A hedgecutter including a pair of reciprocating cutter blades and a drive mechanism according to claim 9 for driving the cutter blades.