GB2405535A

GB2405535A - Contact interface between the brush and an associated commutator

Info

Publication number: GB2405535A
Application number: GB0419091A
Authority: GB
Inventors: Juergen Wiker
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2003-09-01
Filing date: 2004-08-26
Publication date: 2005-03-02
Anticipated expiration: 2024-08-26
Also published as: GB2405535B; GB0419091D0; CN1592003A; FR2862444A1; DE10340141A1; FR2862444B1; CN100524973C

Abstract

An electric motor, in particularfor hand tool machines, having at least one electrically conductive brush body 10 having its contact face 16 arranged in contact relative to the stationary commutator (12) such that its leading edge is spaced apart from the corresponding commutator contact face 14. The radius R2 of the contact surface has its axis offset from a plane 24 passing though the axis of rotation and the centre line of the brush such that when the commutator rotates causing brush tilt the leading edge 36 fig 5 comes into contact with the segments. Fig 4 has each half of the contact surface with an offset radius of curvature R2 to allow for rotation in either direction. The leading edge may have a chamfer of 5{ to 35{ with a width of 0.2 to 2 mm.. Planes 18,32,34 indicate the offset and R2 is at least 10% larger than the commutator radius.

Description

ROBERT BOSCH GMBH

Electric motor

Background art

The invention proceeds from an electric motor according to the preamble of claim 1.

In electric motors, especially in so-called series- or universal motors, transfer of the motor current is effected by means of carbon brushes to a rotating commutator of an armature. To achieve a good transfer of current from the carbon brush to the commutator, brush bodies having contact faces adapted to the commutator radius are used. Usually, the radius of the contact faces is selected 10-20% larger than the radius of the commutator in order to achieve the best possible run-in behaviour of the carbon brushes in that the current transfer during the run-in phase is effected over as large a surface as possible. The carbon brushes in said case graze over segments of the commutator, wherein juxtaposed segments spaced apart by means of slots form the periphery of the commutator.

Advantages of the invention The invention proceeds from an electric motor, in particular for hand tool machines, having at least one electrically conductive brush body for transferring an electric current to a commutator, which is rotatable about an axis of rotation, wherein the brush body lies with a contact face, which is adapted to a corresponding contact face of the commutator, against the contact face of the commutator.

It is proposed that the contact face of the brush body in contact position relative to the stationary commutator is at least at an, in relation to the direction of rotation, front edge spaced apart from the corresponding contact face of the commutator. The front edge of the brush body is spaced apart from the corresponding contact face of the commutator to such an extent in a radially outward direction that, when the brush body is tilted forward in direction of rotation, the commutator may slide under the contact face. The spacing of the front edge substantially compensates a tilting of the brush body as the commutator rotates. The contact face is then adapted to the geometric proportions in the rotating state and not in the stationary state of the commutator. It is therefore possible to prevent the tilting of the brush body from leading to a gap in the rear region of the contact face and hence reducing the contact area, which in the stationary state of the commutator is approximately midway between commutator and brush body, to a narrow region close to the front edge.

The danger, that the front edge will strike against irregularities, e.g. a segment pitch, on the periphery of the commutator and the brush body will even lift off, is moreover reduced.

The spacing to be observed between the edge and the corresponding contact face arises from the geometric dimensions of a brush holder and the brush body as well as of a desired clearance between these. For each set of given conditions, the person skilled in the art will optimize the spacing in the way that seems meaningful to him. Already during the run-in phase, while the the integrally moulded brush body is adapting to the surface of the commutator, almost the entire contact face of the brush body facing the commutator is available as a contact area, even if, as the commutator rotates, the brush body is carried along in direction of rotation and tilts owing to tolerances and air gaps in a brush holder. A manufacturing tolerance of brush body and brush holder leads in principle to an air gap, which simultaneously causes the tilting of the brush body during rotation of the commutator and, as mentioned above, in conventional brush bodies leads to a dramatically reduced contact area. The air gap, on the other hand, is perfectly desirable for easy movement of the carbon brush, or alternatively of a plurality of carbon brushes along the periphery of the commutator, even under the influence of dust. This is important above all when the electric motor is used in hand tool machines, in which the intake of cooling air draws large quantities of dust into the interior and therefore also into the region of the brush holder. The tilting of the brush body is utilized in order to achieve as large a contact area as possible by means of the brush geometry modified in accordance with the invention. By virtue of the new geometry of the brush body even an enlarged air gap between brush body and brush holder is possible, this being particularly advantageous given operation in dust-laden air. Manufacturing tolerances of the brush holder and brush body may moreover be increased, thereby reducing the cost of manufacture.

The large surface reduces the current density in the contact area and the thermal stress of the commutator.

Otherwise, the commutator might be damaged prematurely by a locally effective excessive current density in the run-in phase, and so its useful life might be reduced by operation under load. The substantially full-surface abutment between brush body and commutator moreover prevents the chattering, and hence a lifting-off, of the brush body, even when segments on the peripheral surface of the commutator have a segment pitch, i.e. different heights.

Whereas with conventional brush bodies in high-speed universal motors a segment pitch of only 2 um may already lead to a permanent lifting-off of the carbon brushes, with the construction according to the invention this effect is substantially avoided. The front edge of the brush body lying, in relation to the direction of rotation, in front of the axis of rotation of the commutator is prevented from hooking into the slots between the individual segments.

Such hooking might lead to spark discharge and brush sparking. Edge wear and/or erosion at the segment edges is avoided. The radio interference suppression behaviour and the life of the electric motor are improved.

When the contact face is spaced apart at the rear and the front edge from the corresponding contact face and touches the commutator with a contact area within the contact face, it is possible to optimize the available contact face of the brush body for a commutator that is rotatable in both directions of rotation.

When a centre line of an imaginary cylinder, of which the contact face forms a peripheral segment, extends at a distance from the axis of rotation, the brush body may be l produced with a geometry that is easy to manufacture. In particular, the centre line extends at a given distance parallel to the axis of rotation. The cylinder preferably has a circular cross section. Alternatively, the cylinder may have an ellipsoidal cross section or another cross section that seems meaningful to the person skilled in the art.

When the brush body has parallel side faces, between which a centre plane of the brush body lies, wherein the centre plane cuts the contact face and the axis of rotation lies in the centre plane, the brush body has a shape, which is easy to manufacture and with which an adaptation of the contact face to the commutator may be effected with a high degree of reliability owing to the clear geometric proportions.

In a preferred manner, the axis of rotation and the centre line lie in a plane at right angles to the centre plane of the brush body, thereby simplifying the geometric proportions. An adaptation of the contact face of the brush body may be effected in a defined manner.

When in the case of a commutator rotatable, when viewed in an operating direction, in clockwise direction the centre line lies to the left of the centre plane, an allowance for a tilting of the brush body counter to the direction of rotation may be made in advance during manufacture of the contact face. The operating direction is defined e.g. by an insertion tool in a preferred hand tool machine; an operator looks in operating direction at the insertion tool. A tilting of the brush body in the case of a clockwise-rotating commutator assists a large-area contact of the contact face with the commutator. Increased manufacturing tolerances may further intensify this effect.

When in the case of a commutator rotatable, when viewed in an operating direction, in anticlockwise direction the centre line lies to the right of the centre plane, allowance for a corresponding tilting of the brush body may be made in advance during manufacture of the contact face.

A tilting of the brush body in the case of an anticlockwise-rotating commutator assists a large-area contact of the contact face with the commutator. Increased manufacturing tolerances may further intensify this effect.

When in the case of a commutator rotatable in both directions the contact face of the brush body is formed by two juxtaposed peripheral segments of two juxtaposed imaginary cylinders extending parallel to the axis of rotation, it is possible to optimize the available contact face during rotation of the commutator for both directions of rotation. It may be guaranteed that in both directions of rotation through tilting of the brush body a large contact area is available.

The centre lines of the two cylinders, of which peripheral segments form the contact face, preferably lie on different sides of the centre plane. It is further preferred that the centre lines lie at the same distance from the centre plane. A symmetrical arrangement facilitates purposeful machining of the contact face of the brush body.

When the brush body has at least one chamfered edge, this provides even better protection against a hooking of the edge into the segments in direction of rotation. Usually, on the brush body there is no, or only an undefined chamfer in the direction of rotation of the commutator. The hooking of the brush body into the segment would then be promoted, especially in the case of segment pitches. A single chamfer, here too, leads to improved run-in behaviour. When the brush body has two chamfered edges, hooking into the segments may be prevented in both directions of rotation.

When the chamfer angle lies in the region of 5 to 35 , preferably in the region of 10 to 30 , it is possible to ensure that even large segment pitches do not strike during rotation of the commutator against the edge of the brush body.

When the chamfer has a width in the region of 0.2 to 2 mm, preferably 0.5 to 1.5 mm, a disturbance caused by segment pitches is further reduced. Such striking would lead, as described above, to edge wear at the segments as well as to chattering and lifting-off of the brush body. By means of the chamfer a component of force leading to lifting-off of the brush body is reduced and so quiet, close-lying operation of the brush body is achieved. Larger or smaller chamfers are also possible, wherein the person skilled in the art will select a meaningful optimum between possible manufacturing problems in the case of smaller chamfers and an unnecessarily reduced contact area in the case of larger chamfers.

When the radius of the peripheral segments forming the contact face is at least 10% larger than the radius of the corresponding contact face of the commutator, the result is an advantageous geometric ratio of the contours of the contact faces of the brush body and the commutator to one another.

A hand tool machine having the electric motor according to the invention is notable for improved properties in terms of brush wear and for fewer problems with regard to radio interference suppression.

Drawings Further advantages arise from the following description of drawings. In the drawings three embodiments of the invention are illustrated. The drawings, the description and the claims contain numerous features in combination.

The person skilled in the art will advantageously consider the features also individually and combine them into meaningful further combinations.

The drawings show: Fig. 1 a section through a conventional brush body, Fig. 2 a cross section of a brush body according to Fig. 1 in contact with segments of a commutator, Fig. 3 a section through a preferred brush body, Fig. 4 a section through a preferred brush body having two peripheral segments, Fig. 5 a cross section of a preferred brush body in contact with an anticlockwise-rotating commutator, Fig. 6 a cross section of a preferred brush body in contact with a clockwise-rotating commutator, Fig. 7 a cross section of a preferred brush body having a chamfered edge.

Description of the embodiments

Fig. 1 shows the geometric proportions of a conventional brush body 10 as a section through the brush body 10, which has a curved contact face 16 for abutment against a non- illustrated commutator. The cutting plane is at right angles to a non- illustrated axis of rotation of the commutator. The apparatus has an axial height at right angles to the drawing plane, so that circles or circular lines represent cylinders or cylinder surfaces. The brush body 10 is shaped substantially as a cuboid with parallel side faces, wherein the contact face 16 is integrally moulded onto a corresponding contact face at the periphery of the commutator and forms a peripheral segment of an imaginary cylinder having the radius R2 about an axis of rotation 46 of the commutator. A centre plane 24 extends in the middle between the parallel side faces 54, 56 of the brush body 10. The contact face 16 is symmetrical to the centre plane 24. The axis of rotation 46 lies in the centre plane 24 of the brush body 10. When the electric motor rotates, especially in a run-in phase, the brush body tilts and lies only with a narrow region close to the front edge in abutment against the commutator. The brush body 10 is electrically conductive and is made preferably of graphite.

Fig. 2 shows a cross section through a commutator 12 with a conventional brush body 10 according to Fig. 1. The brush body 10 is held in a holder 44 and in contact with the commutator 12, which at its corresponding contact face 14 has a plurality of segments 40, of which for the sake of clarity only one is numbered. The contact face 14 has a radius R1. The contact face 16 of the brush body 10 is wider than the individual segments 40 and projects beyond both sides of a segment 40 to such an extent that the adjacent segments 40 on the left and right are partially overlapped. The centre line of a cylinder having the radius R2, a peripheral segment of which coincides with the contact face 16, coincides with the axis of rotation 46 of the commutator 12; both therefore lie in the centre plane 24, which extends midway between the two side faces (54, 56) through the middle of the brush body 10.

For this reason, in the inoperative position of the commutator 12 the contact face 16 rests centrally and flat against the contact face 14 at the periphery of the commutator 12. Owing to an air gap between the brush body 10 and the holder 44, as soon as the electric motor starts to rotate the brush body 10 aligns itself in one direction and tilts as far as the air gap between brush body 10 and holder 44 allows. A gap is therefore formed between the contact face 16 of the brush body 10 and the contact face 14 of the commutator 12 in direction of rotation at the rear edge.

Fig. 3 shows a preferred construction of a brush body 10 according to the invention in section. In the drawings, identical parts or parts corresponding to one another are provided with identical reference characters. The centre plane 24 of the brush body 10, indicated here by a dash-dot straight line, no longer coincides with the centre line 18 of the imaginary cylinder 20, which is indicated here in the drawing by a circle of dots and of which a peripheral segment 22 is formed by the contact face 16. Rather, the centre line 18 is spaced apart by a distance A from the centre plane 24. When the commutator 12 is stationary and the brush body 10 is not tilted, the edges 58, 60 of the brush body 10 are therefore situated at a different height, in relation to an imaginary line perpendicular to the centre plane 24. This also means a different distance of the edges 58, 60 from the stationary commutator 12 (Fig. 2). The centre line 18 lies to the right of the centre plane 24, if the electric motor rotates leftwards in an anticlockwise direction. In this case, the edge 58 of the brush body 10, viewed in the direction of rotation, is at the front and further remote from the rotating commutator 12 than the opposite edge 60, which viewed in the direction of rotation is at the rear. The intended direction of rotation of the stationary commutator 12 is indicated in the drawing by a dashed line.

When, as the electric motor starts to rotate in anticlockwise direction, the brush body 10 tilts to the right, segments 40 provided at the periphery of the commutator 12 do not strike against the front edge 58 but slide under this and come into contact with the contact face 16. During rotation of the commutator 12, the brush body 10 tilts in the holder so that the front edge 58, which lies at a distance in the stationary state, then abuts against the contact face 14 of the commutator 12 and the rear edge 60 practically no longer lifts off. In the opposite direction of rotation of the electric motor in clockwise direction, then, given a corresponding brush body 10, the centre line 18 would lie to the left of the centre plane 24. The front edge 58 lying further remote from the commutator 12 would accordingly then be on the left side of the brush body 10; the brush body 10 would tilt to the left as a result of rotation of the commutator 12.

A preferred construction of a brush body 10 for a commutator 12 that is rotatable in both directions is shown in Figure 4. The contact face 16 of the brush body 10 is formed by two juxtaposed peripheral segments 28, 30, which belong to two juxtaposed cylinders having the centre lines 32, 34. Preferably, both cylinders have the same radius. The centre lines 32, 34 of the two cylinders having the peripheral segments 28, 30 lie on different sides of the centre plane 24 and are preferably identically spaced at a distance A from the centre plane 24. In the stationary state of the commutator 12, both edges 36, 38 are now disposed at a distance from the commutator 12.

Admittedly, when the commutator 12 rotates, in each case the rear edge 36 or 38 lies slightly apart, but during a run-in phase of the brush bodies 10 during clockwise and anticlockwise rotation there are still far greater contact areas achieved than with conventional brush bodies 10.

Fig. 5 and Fig. 6 show the proportions, in the case of the commutator 12 rotatable in both directions, for a brush body 10 according to Fig. 4, in which the contact face 16 is formed by two peripheral segments 32, 34 of two cylinders. The centre plane 24 is indicated in each case by a dashed line in the non-tilted state and in the tilted state. In the stationary state, both edges 36, 38 are spaced apart from the commutator 12. Depending on the direction of rotation, sometimes the one and sometimes the other edge 36 or 38 is the front edge and abuts against the corresponding contact face 14.

In Fig. 5, the commutator 12 rotates to the left in anticlockwise direction, as indicated by an arrow. Owing to the air gap 48 between the brush body 10 and a brush holder 44, the brush body 10 may tilt to the right when the commutator 12 rotates to the left. Owing to the modified geometry of the brush body 10, the segments 40 do not strike against the, in this direction of rotation, front edge 36 of the brush body 10 and the contact face 16 lies with a contact area 52 flat against the segments 40. The rear edge 38 lies at a distance because the brush body 10 is designed for both directions of rotation.

Fig. 6 shows the situation in the case of a commutator 12 that rotates to the right in clockwise direction. The brush body 10 may tilt to the left in the holder 44 without the segments 40 striking against the now front edge 38 of the brush body 10. Here too, the contact area 52 is large even though the now rear edge 36 lies slightly apart from the corresponding contact face 14.

Fig. 7 shows a preferred development of a brush body 10 having a chamfered edge 62, wherein a chamfer 50 having a width L in the region of preferably 0.5 - 1.5 mm from the edge 62 is provided. The chamfer 50 forms a chamfer angle or in the region of preferably 10 to 30 . The brush body 10 advantageously has two chamfered edges 62 when the electric motor is rotatable in both directions of rotation.

The electric motor is preferably used in hand tool machines that are operated by a universal motor.

Reference characters O brush body 52 contact area commutator 54 side face 14 corresponding contact face 56 side face 16 contact face 58 edge 18 centre line 60 edge cylinder 62 edge 22 peripheral segment R2 radius 24 centre plane R1 radius 26 extension a chamfer angle peripheral segment A distance peripheral segment L width 32 centre line 34 centre line 36 edge 33 edge segment 42 segment slot 44 holder 46 axis of rotation 48 air gap chamfer

Claims

Claims 1. Electric motor, in particular for hand tool machines, having at

least one electrically conductive brush body (10) for transferring an electric current to a commutator (12), which is rotatable about an axis of rotation (46), wherein the brush body (10) lies with a contact face (16), which is adapted to a corresponding contact face (14) of the commutator (12), against the contact face (14) of the commutator (12), characterized in that the contact face (16) of the brush body (10) in contact position relative to the stationary commutator (12) is at least at an, in relation to the direction of rotation, front edge (36, 38, 58, 62) spaced apart from the corresponding contact face (14) of the commutator (12).

2. Electric motor according to claim 1, characterized in that the contact face (16) at the rear and the front edge (36, 38, 58, 62) is spaced apart from the corresponding contact face (14) and touches the commutator (12) with a contact area within the contact face (16).

3. Electric motor according to claim 1 or 2, characterized in that a centre line (18) of an imaginary cylinder (20), of which the contact face (16) forms a peripheral segment (22), extends at a distance from the axis of rotation (46).

4. Electric motor according to one of the preceding claims, characterized in that the brush body (10) has parallel side faces (54, 56), between which a centre plane (24) of the brush body (10) lies, wherein the centre plane (24) cuts the contact face (16) and the axis of rotation (46) lies in the centre plane (24).

5. Electric motor according to claim 4, characterized in that the axis of rotation (46) and the centre line (18) lie in a plane at right angles to a centre plane (24) of the brush body (10).

6. Electric motor according to one of claims 3 to 5, characterized in that, given a commutator (12) that is rotatable, viewed in an operating direction, in a clockwise direction, the centre line (18) lies to the left of the centre plane (24).

7. Electric motor according to one of claims 3 to 6, characterized in that, given a commutator (12) that is rotatable, viewed in an operating direction, in an anticlockwise direction, the centre line (18) lies to the right of the centre plane (24).

8. Electric motor according to one of claims 3 to 7, characterized in that, given a commutator (12) that is rotatable in both directions, the contact face (16) of the brush body (10) is formed by two juxtaposed peripheral segments (28, 30) of two juxtaposed 1 ' imaginary cylinders (20) extending parallel to the axis of rotation (46).

9. Electric motor according to claim 8, characterized in that the centre lines (32, 34) of the two cylinders (20), of which peripheral segments (28, 30) form the contact face (16), lie on different sides of the centre plane (24).

10. Electric motor according to claim 9, characterized in that the centre lines (32, 34) are spaced at an identical distance from the centre plane (24).

11. Electric motor according to one of the preceding claims 3 to 10, characterized in that the cylinder (20) has a circular cross section.

12. Electric motor according to one of the preceding claims, characterized in that the brush body (10) has at least one chamfered edge (62).

14. Electric motor according to claim 12 or 13, characterized in that the chamfer angle (a) lies in the region of 5 to 35 .

15. Electric motor according to one of claims 12 to 14, characterized in that the chamfer (50) has a width in the region of 0.2 to 2 mm.

16. Electric motor according to one of the preceding claims, characterized in that the radius of the peripheral segments (22, 28, 30), which form the contact face (16), is at least 10% greater than the l radius of the corresponding contact face (14) of the commutator (12).

17. An electric motor substantially as herein described with reference to the accompanying drawings.

18. Hand tool machine having an electric motor according to one of the preceding claims.