GB2273900A

GB2273900A - Eccentric disc grinder having a grinding disc brake

Info

Publication number: GB2273900A
Application number: GB9326305A
Authority: GB
Inventors: Guenter Schaal; Guenther Berger; Stefan Heess; Mario Frank
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1993-01-02
Filing date: 1993-12-23
Publication date: 1994-07-06
Anticipated expiration: 2013-12-23
Also published as: ITMI931013U1; FR2700131B3; CH687963A5; ITMI931013V0; FR2700131A3; DE9300016U1; IT230602Y1; GB2273900B; GB9326305D0

Abstract

In an eccentric disc grinder (1) having a motor-driven shaft (11), which is rotatable relative to an eccentric shaft (16) carrying a grinding disc (6), and drives said eccentric shaft (16) in a gyratory movement eccentrically to the shaft (11) and in a rotary movement, the movement of said eccentric shaft being determined under change-over control by rolling drive means (24, 29) and/or brake means (25), of which one is a hollow rolling wheel (29) which is secured against rotation and is displaceable by operating mans (31, 13), the problem of providing accurate and convenient change-over is solved in that the operating means (31, 33) are disposed on the side of the eccentric disc grinder remote from the grinding disc (6), particularly next to the on/off switch (4) on the handle, within the radius of action of at least one finger of the operator's hand situated in the working position, so that the operating means (31,33) can be operated without a change of position of the operator's hand. <IMAGE>

Description

Eccentric disc grinder having a grinding disc brake Prior Art The starting point of the invention is an eccentric disc grinder according to the generic definition in Claim 1.

From DE-OS 36 25 535 an eccentric disc grinder of the generic type is known. The rotation of the motor is converted by means of an angle gear unit and a shaft carrying an eccentric pin into the working movement of the grinding disc. The working movement is composed of a rotary movement and a gyratory movement of the grinding disc and is produced in the following manner: the doubly mounted shaft carries an eccentric pin at its free end.

The eccentric pin is eccentric relative to the axis of the shaft. Concentrically to its own axis it carries a hollow eccentric shaft mounted in two rolling-contact bearings. The eccentric shaft is coupled to rotate with the grinding disc. When the shaft rotates, the eccentric pin also rotates and gyrates together with the eccentric shaft and the grinding disc with the abovementioned eccentricity about the axis of the shaft. The grinding disc and the eccentric shaft corotate because of the bearing friction between the eccentric pin and the eccentric shaft. If a braking moment greater than the torque resulting from the bearing friction acts on the grinding disc or on the eccentric shaft, the grinding disc and the eccentric shaft gyrate without rotating.

The grinding disc should rotate only if relatively considerable removal of material is intended.

Rotation is undesirable when the driven grinding disc is not in contact with the workpiece, that is to say when there is no braking moment and the speed of rotation of the grinding disc can rise uncontrolledly, or when the eccentric disc grinder is to be used virtually like an oscillating grinder with minimum abrasion power. Here there is a risk of the so-called overspeed effect, which in the known eccentric disc grinder is limited by a friction brake disposed near the grinding disc.

This brake is of simple design, but requires accurate adjustment to ensure uniform wear. It must be dustproof, since otherwise additional frictional losses may lead to unnecessary heating and reduced power of the eccentric disc grinder, or to the destruction of the brake. The brake can be switched over. Movement of the hollow rolling wheel, which is secured against rotation in the eccentric disc grinder, by turning an eccentric adjusting shaft provided with an eccentric part converts the braked rotation of the grinding disc into a forced rotation.

In the known eccentric disc grinder the operating means for adjusting the machining stage are disposed near the grinding disc but at a distance from the handle. To adjust the operating means the operator must direct his eyes away from the workpiece and sideways to the eccentric disc grinder to find the operating means and switch them over by a hand removed from the handle. This is complicated, disturbs the work procedure and can easily lead to operating errors. Moreover, rotary knobs are provided for adjusting the machining stage. To turn them, the operator needs at least two fingers of his hand at the same time.

Advantages of the Invention The eccentric disc grinder according to the invention having the characterizing features of Claim 1 has the advantage in relation thereto that the mechanical friction brake having a uniform adjustment path can be especially accurately controlled, while its handling is particularly convenient and simple. During operation of the eccentric disc grinder, machining stages can thus be switched over by a single finger of the operator's hand without a hand having to be removed from the handle and without the eyes having to be directed away from the workpiece being machined.

Other advantageous developments of the invention can be seen in the claims following Claim 1.

Drawing One exemplary embodiment of the invention is explained more fully in the following description with reference to the appertaining drawing.

Figure 1 is a sectional representation of the exemplary embodiment of an eccentric disc grinder, and Figures 2 and 3 are schematic sectional representations of the machining stage adjustment mechanism of the eccentric disc grinder.

Description of the Exemplary Embodiment The eccentric disc grinder shown in Figure 1 has a motor casing 2 on which are disposed an electric connection cable 3 and an on/off switch 4. An angle drive casing 5 flanged to the motor casing 2 contains an angle drive 7 operatively connected to a grinding disc 6. The angle drive 7 consists of a small bevel gear 8 which is mounted on a motor shaft (not given a reference numeral) and transmits the movement of the motor to a large bevel gear 9. The bevel gear 9 concentrically embraces a shaft 11 which is fixed for rotation with it and which is rotatable about an axis of rotation 10 and is in the form of a hollow shaft. In the angle drive casing 5 the shaft 11 is mounted in a bearing 12 remote from the grinding disc and in a bearing 13 facing the grinding disc.At its bottom end the shaft 11 carries a balancing weight 14 compensating for the unbalance resulting from the eccentric movement of the grinding disc 6. The shaft 11 has a stepped through bore 15 eccentric to the axis of rotation 10. In the bore 15 an eccentric shaft 16 having a longitudinal axis 17 is mounted concentrically. The latter runs parallel to and at a distance from the axis of rotation 10 with the eccentricity "e". The eccentric shaft 16 is mounted at the side remote from the grinding disc in a bearing 18 which is in the form of a fixed bearing, and at the side facing the grinding disc is mounted in a bearing 19 which is in the form of a sealed needle bearing taking radial forces and having a cover sleeve (not shown). At its bottom end the eccentric shaft 16 carries the grinding disc 6.

At its side remote from the grinding disc 6 the eccentric shaft 16 carries in a recess 20 a circlip 21, against which are supported, following one another coaxially, a supporting ring 22, a bottom cup spring 23, a spur gear 24, an annular disc 25 provided with spur toothing (not given a reference numeral), a mating disc 26, a prestressed upper cup spring 27 and finally, as axial securing means, a screwed-on nut 28.

The nut 28 may be replaced for example by an additional circlip and the cup spring 23 by a spiral compression spring, in which case the cup spring 27 can be dispensed with.

The spur gear 24 and the mating disc 26 are disposed on the eccentric shaft 16 in such a manner as to be secured against rotation but adjustable by axial displacement. The annular disc 25 is twistable after the style of a loose wheel in relation to the eccentric shaft 16. It is supported with a braking action between the spur gear 24 and the mating disc 26. The spur gear 24 may have a smaller number of teeth than the annular disc 25.

A hollow rolling wheel 29 having two identical toothings 30 spaced axially apart is disposed concentrically to the axis 10. The two toothings 30 are so disposed that their pitch circles are tangent to those of the toothings (not given reference numerals) of the spur gear 24 and annular disc 25. The axial distance between the two toothings 30 is shorter by one tooth length of the toothings 30 than that between the spur gear 24 and the annular disc 25. Consequently, at any one time only either the spur gear 24 or the annular disc 25 can be radially supported or roll on one of the toothings 30.

The hollow rolling wheel 29 is disposed in the angle drive casing 5 with limited axial displaceability and twistability, so that it can be alternately disengaged from the spur gear 24 or the annular disc 25.

The adjusting means for the axial displacement of the hollow rolling wheel 29 consists of an adjusting shaft 31 coupled to the hollow rolling wheel 29. Said adjusting shaft carries an operating part 32 which can be swivelled from the outside. The adjusting shaft 31 is sealingly guided in a circular bore 33 in the wall of the angle drive casing 5 and is enclosed in a dustproof bellows 34 fastened on the angle drive casing 5. The adjusting shaft 31 carries a clamping or holding device (not shown), by which the swivelled position of the operating part 32 relative to the angle drive casing 5 can be fixed in the end positions. On its end facing the hollow rolling wheel 29 the adjusting shaft 31 carries a pinion 37.The latter engages in a toothed rack 38 obliquely disposed like a collar on the external cylindrical surface of the hollow rolling wheel 29, in the region of the latter which faces the grinding disc 6. At its end remote from the angle drive casing 5 the adjusting shaft 31 is secured against axial displacement by means of a securing ring 39.

At the opposite side of the hollow rolling shaft 29 to that where the adjusting shaft 31 is disposed an oblique slot having oblique faces 35 can be seen in cross-section; a pin 36 fastened to the angle drive casing engages in said pin. Additional oblique faces and pins are not shown in the drawing. The inclination of the oblique faces 35 corresponds to the inclination of the toothed rack 38.

When the motor (not given a reference numeral) of the eccentric disc grinder 1 is switched on by means of the on/off switch 4, the bevel gears 8 and 9 rotate. The bevel gear 9 rotates, conjointly with the shaft 11 and the balancing weight 14, about the axis of rotation 10.

The shaft 11 drives the eccentric shaft 16 and the grinding disc 6 by means of the fixed bearings 18; 19.

Said eccentric shaft and grinding disc gyrate about the axis of rotation 10 with the eccentricity "e" and rotate, because of the friction in the bearings 18, 19, about their axis 17.

The movement of the eccentric shaft 16 is followed by the spur gear 24, the annular disc 25 and the mating disc 26, the spur gear 24 being out of engagement and the annular disc 25 meshing with the hollow rolling wheel 29 in the position shown in Figure 1.

When the hollow rolling wheel 29 is turned through the turning of the adjusting shaft 31, it is moved axially to the extent of the pitch of the oblique face 35, so that the toothed rack 38 and the pinion 37 mesh with one another. The axial displacement of the hollow rolling wheel 29 brings about the change of the machining stage between coarse and fine machining in the following manner: if the lower of the two toothings 30 is coupled to the spur gear 24, the latter rolls on the hollow rolling wheel 29 and subjects the grinding disc to a rotation stepping down the speed of rotation of the shaft 11 to a speed which is lower than the eccentric rotational speed of the grinding disc and which is superimposed on the eccentric movement. This position corresponds to the coarse machining stage with considerable removal of material.

When the upper toothing 30 is coupled to the annular disc 25, as shown in Figure 1 the latter can oscillate along the toothing 30 practically without rotation, depending on the difference in the numbers of teeth. At the same time the annular disc 25 holds the eccentric shaft 16 fast by a braking action to such an extent that the torque can be neutralized through the friction of the bearings 18, 19. Depending on the prestressing of the brake device by means of the nut 28, the grinding disc 6 can thus be adjusted to rotate at a higher or lower speed without the risk of overspeed. This position corresponds to the fine machining stage with little removal of material.

In consequence of the prestressing by means of the cup springs 23, 27, the aforesaid parts are supported against one another with a determined axial force. This leads to a predetermined friction on the surfaces supported against one another and moving in relation to each other. The friction is variably and individually adjustable in accordance with customers' wishes.

Since the bearing friction in the fixed bearing 18 increases with the application of the grinding disc against the workpiece and in dependence on the pressure exerted by the operator, the grinding disc corotates only when it is applied against the workpiece.

Figures 2 and 3 show the schematic representation of an angle drive casing 50, which in principle coincides with that shown in Figure 1. A hollow rolling wheel 52 is guided for axial displacement and rotation on the inner wall 51 of the casing 50. The hollow rolling wheel 52 carries on its outer periphery a toothed rack 53 disposed obliquely, like an inclined plane, or curved in the peripheral direction.

A pinion 54 meshes with the toothed rack 53 and is fixed on an adjusting shaft 55 for rotation with the latter, said adjusting shaft being sealingly mounted for rotation in a bore 56 in the angle drive casing 50 and being able to be turned by means of an operating part 57.

Depending on the direction in which the operating part 57 is swivelled, the hollow rolling wheel 52 is displaced axially in the direction of or away from the grinding disc. The pinion 54 is disposed at a distance from the periphery of the hollow rolling wheel 52, so that when the latter is displaced axially no disturbing friction with the pinion 54 comes into action. When the operating part 57 is swivelled, the oblique faces 58, 59 slide onto the pins 60, 61 to impart to the hollow rolling wheel 52 an accurately adjustable axial displacement which is necessary for changing over the machining stages. With a swivelling angle of the operating part 57 of 180 , the hollow rolling wheel 52 is adjustable to the extent of its maximum axial displacement path from one switch position to the other.

In Figure 2 can be seen the toothings 62, 63 which can each be coupled to a braking means arranged on the eccentric shaft or to a rolling drive means. An O-ring 64 is disposed as sealing means between the adjusting shaft 55 and the bore 56.

In a view of the exemplary embodiment according to Figure 2 which is turned by 900, Figure 3 shows by means of a curved arrow 65 the possible directions of rotation of the pinion 54, and thus the swivelling direction of the operating part 57, while a straight arrow 66 shows the possible axial displacement of the hollow rolling wheel 52.

In an exemplary embodiment which is not illustrated a friction wheel drive or an eccentric pin, by which the axial displacement of the hollow rolling wheel is brought about, can be provided instead of a pinion and a toothed rack.

In another exemplary embodiment of the invention which is not illustrated a lever can be disposed at the end of the adjusting shaft instead of a pinion and be supported in crank fashion between cams on the outer periphery of the hollow gear, the change-over of the gearing being effected with its aid, as is known in hand drills and drilling hammers.

Claims

1. Eccentric disc grinder (1) having a casing (2; 5) which is provided with an on/off switch (4) and in which a motor-driven shaft (11), which is rotatable relative to an eccentric shaft (16) fixed for rotation with a grinding disc (6), drives said eccentric shaft (16) in a gyratory movement eccentrically to the axis (10) of the shaft (11) and in a rotary movement, the movement of said eccentric shaft being determined under change-over control by rolling drive means (24, 29) and/or brake means (25, 29), of which one is a hollow rolling wheel (29) which is disposed in the casing (2; 5) for displacement with the aid of guide means (35, 36) and of operating means (31, 33), characterized in that the operating means (31, 33) are disposed on the side of the eccentric disc grinder (1) remote from the grinding disc (6), particularly next to the on/off switch (4), within the radius of action of at least one finger of the operator's hand situated in the working position, particularly the hand associated with the on/off switch (4).

2. Eccentric disc grinder according to Claim 1, characterized in that the rolling drive means (24, 29) and/or brake means (25, 29) and also the guide means (35, 36) are disposed on that end of the shaft (11) which is remote from the grinding disc (6).

3. Eccentric disc grinder according to Claim 1 or 2, characterized in that the guide means (35, 36; 58, 59, 60, 61) used comprise at least one pin (36; 60, 61) or the like which is fixed to the casing and on which the hollow rolling wheel (29, 52) is supported by at least one, in particular external, oblique face (35; 58, 59), so that it is displaced axially as soon as it is turned, and in that the operating means (31, 33; 55, 57) for turning the hollow rolling wheel (29; 52) has at least one adjusting shaft (31; 55) passing through the casing (2, 5; 50).

4. Eccentric disc grinder according to Claim 3, characterized in that on a swivelling path of 1800 of the operating means (31, 33; 55, 57) the hollow rolling wheel (29; 52) is turned by means of the oblique face (35; 58, 59) and thereby can be adjusted to the extent of its axial displacement path from one switch position to the other.

5. Eccentric disc grinder according to Claim 4, characterized in that the adjusting shaft (31; 55) is coupled to the hollow rolling wheel (29; 52) by means of a pinion (37; 54).

6. Eccentric disc grinder according to one of Claims 1 to 5, characterized in that the hollow rolling wheel (29; 52) carries a toothing (38; 53) which is disposed, obliquely to its axis, on its external cylindrical surface (40) and with which the pinion (37; 54) meshes.

7. Eccentric disc grinder according to Claim 6 or 7, characterized in that the toothing (38) is disposed on a toothed rack (38; 53) which in particular is curved in the longitudinal direction and which is parallel to the oblique face (35; 58, 59).

8. Eccentric disc grinder according to one of the preceding claims, characterized in that the shaft (11) is in the form of a hollow shaft which is open at both ends and in which the eccentric shaft (16) is disposed to project at both ends.

9. Eccentric disc grinder according to one of Claims 3 to 8, characterized in that the oblique face (35; 58, 59) is disposed on an end face of the hollow rolling wheel (29; 52).

10. Eccentric disc grinder according to one of the preceding claims, characterized in that the adjusting shaft (31) is enclosed in a dustproof sleeve (34), particularly a bellows.

11. Eccentric disc grinder according to one of the preceding claims, characterized in that at least one brake means (25) is in the form of a disc which retardably follows the movement of the grinding disc (6) and which is operative in one of the switch positions of the operating means (31).

12. An eccentric disc grinder substantially as herein described with reference to the accompanying drawings.