GB2350728A

GB2350728A - Preventing dust jamming commutator brushes in a hand-held machine tool

Info

Publication number: GB2350728A
Application number: GB0008906A
Authority: GB
Inventors: Gerhard Meixner; Csaba Kreiter; Harald Schindler
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1999-04-20
Filing date: 2000-04-11
Publication date: 2000-12-06
Also published as: DE19917903A1; JP2000350418A; GB0008906D0

Abstract

A hand-held machine tool motor, having a commutator (14) in electrical contact with at least one pair of brushes (17), each of the brushes (17) being guided in a longitudinally displaceable manner in relation to the commutator (14) by means of a brush-holder (15) forming a longitudinal guide (20), the guide having a plurality of discontinuities (23), viewed in the direction of displacement (22) of the brushes (17) which trap the dust. The discontinuities may be transverse slots 24 or holes or other punched structures and may include guide parts 26,29. The discontinuities may be formed on the brush itself.

Description

2350728 Electric motorparticularly for a hand--held machine too Pnor art

The starting point of the invention is an electric motor according to the pre-characterising clause of claim 1. An electric motor which has a commutator which is in contact with a pair of brushes IS known from DE 195 929 B1. Each of the brushes is guided in a longitudinally displaceable manner by means of a brush-holder, the latter and the appertaining brush forming a longitudinal guide. The two brush-holders are of quiver-like shape, with four lateral faces an7anged at right angles to one another. Under these circumstances, the longitudinal guidance takes place via the lateral faces of the brushholder and via the brush itself Apart from a window in the lateral faces which serves for assembly purposes and as a wear-indicator, no discontinuities are provided in the longitudinal guide, viewed in the longitudinal direction.

The brushes are constructed, in each case, as sliding brushes which make contact electrically with the collector. To this end, the brushes are pretensioned in the direction of the collector by a pressure spring in each case. Because of the rotating collector, the brushes are subjected, when the electric motor is running, to abrasive wear which necessitates adjustment of the brushes in the direction of the collector. The longitudinal guide must therefore be so constituted that there is sliding clearance between the brush and the brush-holder. Depending upon the working erivironment but particularly in an atrnosphere containing drilling or grinding dust, particles of dust are able to pass into the region of the longitudinal guide and form a deposit between the brush and the brushholder. This can result in sticking andjamming ofthe brush in the brush-holder, with the consequence that the adjusting movement of the carbon brush is hindered and the contact between the carbon brush and the collector is interrupted after even a short time, as a result of which the electric motor breaks down.

2 Attempts have already been made to counteract the problem oflamming of the brushes with the aid of various variant configurations of the brush arrangement. Thus, for example, quiver-like brush-holders are known which have, transversely to the direction of displacement, an undulating profile which results in a longitudinal guide of more linear construction instead of a predominantly laminar longitudinal guide. Other forms of embodiment have strip-like engagement between the brush-holder and the brush (rail-type guidance), which likewise reduces the extent ofthe laminar contact between the brush and thebrush-holder. However, the jamming problems cannot be remedied to a satisfactory degree by any of the aforesaid brusb/brush-holder combinations.

Advantages of the invention The electric motor according to the invention having the features in claim I has the advantage that the problems of jamming between the brush and the brush-holder are counteracted in an effective manner. Tbrough the fact that the longitudinal guide has a plurality of discontinuities between the brush and the brush-holder, viewed in the direction of displacement of the brush, non-crifical possibilities for the deposition of the grains of dust that cause jamming can be provided along the longitudinal guide, a fact which considerably reduces the risk of the electric motor breaking down.

Advantageous finther developments o and improvements to the electric motor according to the invention are possible as a result of the measures set out in the dependent claims. It is advantageous if the discontinuities are produced at intervals which are regular and as short as possible. In this connection, it is particularly advantageous to provide the discontinuities in the brush-holding arrangement, although in principle they can also be constructed in the brush. Construction ofthe discontinuities in the brush-bolder guarantees satisfactory stability and can, in any case, be achieved with little outlay on production, for example by using a punchingibending part made of metal. Particularly good results have 3 already been achieved in practical tests with a guide length, between adjacent discontinuities, of 2 nun or less in each case, without the stability of the brush-holding arrangement being significantly impaired.

Drawings Five exemplified embodiments of the invention are represented in the drawings and explained in greater detail in the following description. In the drawings: figure I shows, diagrarnmatically, a perspective representation of a handheld machine tool with an electric motor, with the housing shell lying open, figure 2 shows a cross-section through a brush-holder belonging to the electric motor, with the brush inserted therein, according to a first exemplified embodiment, figure 3 shows a partial section through the brush-holder with the brush, along the line 11-11 in figure 2, figure 4 shows a plan view of an inner lateral face of a brush-holder according to a second exemplified embodiment, figure 5 shows a section through the lateral face of the brush-holder according to figure 4, figure 6 shows a plan view of an inner lateral face of a brushholder according to a third exemplified embodiment figure 7 shows a plan view of an inner lateral face of a brush-holder according to a fourth exemplified embodiment and figure 8 shows a plan view of an inner lateral face of a brush-holder according to a fifth exemplified embodiment.

Description of the exemplified embodiments

4 In figure 1, 10 designates an electrically operated hand-held machine tool which has a shell-sbaped housing I I in which an electric motor 12 is accommodated. The electric motor 12 has an armature shaft 13 on which a collector 14 is arranged in a torsion-proof manner. Two carbon brushes 17, which are mounted in a longitudinally displaceable manner in diagrammatically represented brush-holders 15 in the housing 11, are in electrically conductive contact with the collector 14.

In figure 2, one of the brush-holders 15 is represented in cross-section. The brush-holder 15 is of quiver-shaped construction With lateral faces 18a, b, c, d w1iich are located approximately orthogonally in relation to one another and forrn the walling system 19 of a receptacle 16 for a brush 17. In this case, the brush-holder 15 is manufactured from a suitably bent sheet-metal part, but it may also be produced, for example, as a moulded plastic part. In the case of the example, the walling system 19 has six inwardly directed undulatory proJections 21 which form a linear longitudinal guide 20 for the brush 17. Each of the brushes 17 is constructed approximately as a rectangular block which is longitudinally displaceable over the longitudinal guide 20 in relation to the brushholder 15.

As emerges from figure 3, the brush-holder 15 has, viewed in the longitudinal direction 22 of the brush 15, a plurality of discontinuities 23 which are located in the region of the longitudinal guide 20. In the case of the example, the discontinuities 23 are constructed as transverseslit-shaped clearances 24 at regular intervals in the walling system 19 of the brush-holder 15. The projections 21 which are left are thus arranged in a larnellar manner at a distance from one another.

The longitudinal guide 20 is thus interrupted a number of times by the clearances 24 in the direction of displacement of the brush 15, the projections 21 forming, in each case, guide regions 25 of the longitudinal guide 20. The discontinuities 23 serve as a receptacle for particles of dust deposited between the brush 17 and the brush-holder 15. In order to effectively diminish the risk of jamming, the guide regions 25 should be as short as possible. Particularly good results were achieved in tests in the case of an embodiment with a length of the individual guide regions 25 of about 2 min or less.

Figures 4 to 8 each show, by way of an example, a lateral face 18a which is constructed in the same way as the other lateral faces 18b, c, d. Parts of the lateral faces l8a-d of greater or lesser size contribute to the longitudinal guide 20, depending upon the embodiment of the brush- holder 15. Parts which are the same and have the same action with reference to figures I to 3 are identified by the same reference symbols.

In the exemplified embodiment according to figures 4 and 5, the guide regions 25 of the longitudinal guide 20 are formed by inwardly protruding tongues 26 which are cut out on three sides from a basic sheet-metal body forming the lateral face 18a, and are bent inwards in the direction of the brush 17. In this case, the tongues 26 form the guide regions 25 with an inner side 27 of the sheet-metal tabs which are located approximately parallel to the brushes 17.

In the two exemplified embodiments according to figures 6, 7 and 8, the clearances 24 are formed by portions punched out of a one-piece basic sheet-metal body 30. Webs 29, which form the guide regions 25, are left between adjacent punched-out portions. As regards the shape of the punched-out portions, a virtually unlin-uted number of variant configurations are conceivable, in which connection care should be taken to ensure a shape which recurs as regularly as possible and small web lengths, Viewed in the direction of displacement.

The invention is not restricted to the exemplified embodiments shown. Alternative embodiments, in which the longitudinal guide has discontinuities, are also conceivable.

6 Thus, the discontinuities for rmeiving the troublesome particle. s of dust could, in principle, also be constructed in the brushes. The construction of discontinuities is also possible when the brushes are guided in a rail-like manner.

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Claims

Claims

Electric motor, particularly for a hand-held machine tool, having a commutator (14) which is in electrical contact with at least one pair of brushes (17), each of the brushes (17) being guided in a longitudinally displaceable manner in relation to the commutator (14) by means of a brush-holder (15) tbrough the fact that the brush-holder (15) and the brush (17) form a longitudinal guide (20) with one another in each case, characterised in that the longitudinal guide (20) has a plurality of discontinuities (23), viewed in the direction of displacement (22) of the brushes (1 7
2. Electric motor according to claim 1, characterised in that the discontinuities (23) in the longitudinal guide (20) are formed by clearances (24) in the brush-holder (15).
3. Electric motor according to claim 2, characterised m that the discontinuities (23) are arranged at regular intervals.
4. Electric motor according to claim 3, characterised in that guide portions (25), which form the longitudinal guide (20) jointly with the brush (17), are provided between adjacent clearances (24) in the brush-holder (15) in each case.
5. Electric motor according to claim 4, characterised in that the brushholder (15) is formed in a quiver-like manner from four lateral walls (1 8a-d) which are arranged approximately at right angles to one another.

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6. Electric motor according to claim 5, characterised m that the guide portions (25) are formed by undulatory prOjections (2 1) which project inwards on the inner sides of the lateral walls (1 8a-d).
7. Electric motor according to claim 5, characterised m that the guide portions (25) are formed by tongues (26) which PrOject inwards from the lateral walls (1 8a-d).
8. Electric motor according to claim 5, chamcterised in that the guide portions (25) are formed by webs (29) which are an-anged between the clearances (24).
9. Electric motor according to claim 8, characterised in that the clearances (24) are formed by portions (28) punched out of a sheet-metal body, and the webs (29) represent guide portions (25) located between them.
10. An electric motor substantially as herein described with reference to the accompanying drawings.