GB2371004A

GB2371004A - Eccentric sander with speed limiting device

Info

Publication number: GB2371004A
Application number: GB0126793A
Authority: GB
Inventors: Walter Barth; Steffen Tiede; Joerg Dehde
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2000-11-08
Filing date: 2001-11-07
Publication date: 2002-07-17
Anticipated expiration: 2021-11-07
Also published as: GB0126793D0; GB2371004B; DE10055246A1; JP2002192452A; JP4214197B2

Abstract

An eccentric sander has a housing 2, a drive shaft 3, a sanding plate 1 rotatably mounted on an eccentric journal 7 and a speed limiting device 10 having two engagement parts 4, 11, the first 4 connected to the housing and the second 11 formed on a sanding plate, the connection between the housing and the first engagement part preferably being in the form of a bearing 6 and the two engagement parts preferably engaging through meshing teeth 15, 16. The speed limiting device causes braking of the sanding plate when free-wheeling under no-load in order to prevent excessive rotational speed when the sander is removed from the work piece. The speed limiting device may be engaged and disengaged by a clutch.

Description

2371 004

Eccentric sander having a device for limiting the speed of the sanding plate Prior art

The invention starts out from an eccentric sander having the generic features of Claim 1.

Convenient eccentric sanders allow a choice between two working movements for different material-removal capacities or finishes. When free-wheeling, the axis of the sanding plate executes an eccentric movement, the axis being freely rotatable for particularly fine sanding. In contrast, during constrained driving the sanding-plate axis is additionally driven for a high material-removal capacity. This is effected in that a wheel which is fixed on the sanding-plate axis rolls positively or non-positively in an internal-geared wheel situated in a rotationally fixed manner in the housing, so that the outer sanding-plate points execute a hypotrochoidal movement counter to the direction of rotation of the eccentric.

Without further measures, there is the problem here that the sanding plate, when free-wheeling after being lifted from the workpiece, assumes the speed of the eccentric shaft. This leads to a very high orbital velocity at its outside diameter. When it is placed onto the workpiece, an unintentionally high material removal then results for a short time. In order to avoid this, the sanding plate when free-wheeling is braked to a lower speed. Such a device is described in EP O 573 916 B1. In this case, the

internal-geared wheel, which is rotatable relative to the housing, is in sliding contact, by its outer peripheral surface, with the inner surface of part of the housing in order to reduce the speed when free-wheeling. A concrete embodiment is, however, not described therein.

It is, however, generally known to use a felt brake, a brush brake, an eddy current brake or a friction ring as the friction device. The most frequent solution is to use a friction ring. However, a series of problems arise with a friction ring: both wear of the brake and of the plate occur, resulting in a costly repair. Moreover, the moment of friction is dispersed even in the new condition or due to wear, in the event of a quality defect being present.

This too results in a repair. The friction ring is furthermore an additional part, which results in a greater effort of assembly required for the part.

Moreover, the risk of a quality defect occurring in the form of an assembly error increases. Furthermore, the moment of friction is present continuously both when free-wheeling and under constrained driving.

A further solution for reducing the speed of the sanding plate when freewheeling uses an electronic speed control. This also involves disadvantages in terms of costs and assembly. Moreover, the handling for the customer is made more difficult.

Advantages of the invention An eccentric sander according to the invention, having the features of Claim 1, has the advantage in contrast that hardly any wear of the speed-limiting device occurs,

both the effort of assembly and the presence of assembly errors are minimal and the handling by the customer is simple. In the case of an eccentric sander according to the invention, the gearing for the constrained driving additionally takes on the function for the braked free-

wheeling. The first engagement part of the speed-limiting device is not continuously connected in a rotationally fixed manner to the housing, but is disengaged for free-

wheeling. The moment of friction between the first engagement part and the engagement-part bearing in the housing brakes the sanding plate, since the latter is in contact, via the second engagement part formed on it, with the first engagement part at least at one place and continuous force transmission takes place there. Since the direction or rotation of the sanding plate during constrained driving is opposite to the direction of rotation of the sanding plate when free-wheeling, a greater speed difference results. It is thus possible to achieve the required braking energy for the complete stoppage of the rotational movement of the sanding plate with a comparatively small moment of friction as compared with the design known hitherto. With a somewhat higher moment of friction, the sanding plate will even rotate in the opposite direction. For a good sanding result, however, the direction of rotation of the sanding plate when free-wheeling is unimportant, and all that is required is that the no-load speed and the braking moment are not too high. Since for the permissible no-load speed the ranges in both directions of rotation are available, a doubling of the tolerance is achieved. As a result, any

adjustments which may have to be made for the braking moment are minimized.

The speed-limiting device is preferably designed as an internal-geared wheel. This provides a particularly simple and low-cost option for realizing the speed-

limiting device.

Furthermore, it is advantageous if the two engagement parts are connected to each other positively, in particular in the form of teeth, and/or frictionally.

This ensures a simple and reliable method of force transmission between the first engagement part and the second engagement part and thus between the sanding plate and the housing. It is particularly advantageous if the connection between the two engagement parts is in point or line form.

It is particularly advantageous, if in addition to the bearing friction in the engagement-part bearing an opposed moment of friction is applied, in particular by friction with a solid and/or a fluid. The location at which the opposed moment of friction is introduced must move relative to the drive shaft in the opposite direction. The greater the difference, the slower the speed of the sanding plate will be, until finally the rotary plate stops. For example, the opposed moment of friction may be introduced at the housing or the gearing steps. Furthermore, it is advantageous if the speed-limiting device can be engaged and disengaged by means of a clutch, in particular acts only during the free-wheeling

of the sanding plate. As a result, the user of the eccentric sander can decide himself whether he desires a reduction of the speed of the rotary plate or not. When free-wheeling, this is desirable because of the above-

mentioned disadvantages of the prior art, but is of

rather lesser importance for the constrained driving.

Furthermore, it is advantageous if the clutch is movable resiliently against the speed-limiting device. As a result, it is easy to engage the clutch even at a standstill. It is particularly advantageous if the clutch and the speed-limiting device lock into place one in the other in the event of engagement. A relative movement between the fixed and movable clutch part is thus brought about as soon as a movement is imparted to the sanding plate or the eccentric sander is switched on.

Further advantageous refinements of the invention are the subject of the subclaims.

Drawings An exemplary embodiment of the invention is explained in more detail in the description below with reference to

the associated drawing.

The single figure shows a schematic section through a sanding plate of an eccentric sander having a speed-

limiting device according to the invention.

The figure shows part of an eccentric sander having a device 10 for limiting the speed of a sanding plate 1. In a housing 2 of the eccentric sander, a drive shaft 3 is driven rotationally about a drive axis 8 spatially fixed with respect to the housing 2. The drive of the drive shaft 3 is known and is not important to the invention, so that a more detailed description of it is dispensed

with. Formed at the output-side end of the drive shaft 3, at a distance from the drive axis 8, is an eccentric axis 9 which is parallel thereto and is embodied by an eccentric journal 7. The sanding plate 1 is mounted rotatably about the eccentric journal 7 via a plate bearing 5. As a result, the sanding plate 1 executes an eccentric movement about the drive shaft 3 when the drive shaft 3 is driven. Formed on the sanding plate 1 concentrically about the eccentric axis 9 is a second engagement part. This second engagement part 11 protrudes, in the direction of the drive side of the drive shaft 3, from the surface of the sanding plate 1 facing away from the working surface thereof. The drive axis 8 of the drive shaft 3 is mounted in the housing 2 in such a way that the latter is formed concentrically about the drive axis 8 in the illustrated region. A connection exists between the housing 2 and the sanding plate 1. For this purpose, an engagement-part bearing 6 is inserted in the housing 2 concentrically about the drive axis 8 and is acted upon, at its surface directed towards the drive axis 8, by an engagement part 4. The engagement part 4 can thus be moved rotationally about the drive axis 8 within the housing 2 via the engagement-

part bearing 6. On account of the bearing friction between housing 2, engagement-part bearing 6 and engagement part 4, rotation of the engagement part 4 is

braked. The engagement part 4 is designed as a internal-

geared wheel, its inner surface and its outer surface being formed concentrically with the drive axis 8. At its end remote from the drive, that is facing the sanding plate 1, the first engagement part 4 has an internally toothed ring 15. This internally toothed ring 15 engages along a line of engagement 14 in an externally toothed ring 16 on the second engagement part 11.

As a result, when the sanding plate 1 is free-wheeling, the rotation of the sanding plate 1 about the eccentric axis 9 is braked, since friction exists between the second engagement part 11 and the housing 2. The friction in the engagement-part bearing 6 is so great here that the sanding plate 1 is powerfully braked. This prevents the sanding plate 1, when free-wheeling, from being accelerated in such a manner that it has the same high rotational speed as the drive shaft 3. In addition to this sliding friction, it is also possible for the friction in the engagementpart bearing 6 to be so high that the two engagement parts 4, 11 and thus also the sanding plate 1 stop completely. What is then involved is static friction. As a result, it is ensured that the eccentric sander, when freewheeling after being lifted from the material to be worked, does not rotate at a high speed, if at all. This precludes the possibility of excessive material removal when the eccentric sander is placed onto the surface to be worked, as would be the case if the sanding plate 1 were to move at high speed.

A sliding or rolling bearing is used as the engagement-

part bearing 6. This is sufficient, in conjunction with the shearing and churning work of the lubricant, to

reduce the speed of the sanding plate 1 to the required degree. Other bearings are also conceivable just as well.

By designing the first engagement part 4 as an internal-

geared wheel, the heat from the rotational energy of the sanding plate 1 which is to be dissipated by friction is produced directly at the inner wall of the housing 2 and can be carried away effectively from there. This prevents an excessive temperature gradient from being produced inside the gearing.

A further refinement (not shown) of an inventive device 10 for limiting the speed of the sanding plate 1 comprises engaging and disengaging arresting of the first engagement part 4, designed as an internal-geared wheel.

The arresting can be produced both non-positively and positively at the periphery or at one of the two end faces of the first engagement part 4 in each case. In the case of a non-positive clutch, by limiting the moment an overload protection for the eccentric sander can be provided. One possible embodiment of a non-positive clutch can reduce the moment of friction at intermediate positions for the constrained driving, with the result that the constrained speed is reduced. Less aggressive sanding and a more favourable finish thus result. It is also conceivable to achieve positive arresting by means of the clutch. In order to enable simple engagement of the clutch even at a standstill, the clutch elements are moved resiliently. Even if one tooth strikes another tooth in this case, the actuating means can be brought into the desired end position and the clutch halves lock into place as soon as the sanding plate 1 moves or the eccentric sander is switched on, since relative movement

between the fixed and movable clutch part thus takes place.

Claims

Claims 1. Eccentric sander with a sanding plate (1) and a housing (2), in

which a device (10) for limiting the speed of the sanding plate (1) is arranged, with a drive shaft (3) driven rotationally about a drive axis (8), with an eccentric journal (7), the sanding plate (1) being mounted rotatably about the eccentric journal (7), characterized in that the speed-limiting device (10) has a first engagement part (4), which is arranged, in particular concentrically, about the drive axis (8) and is connected to the housing (2), preferably via a first engagement-part bearing (6), and a second engagement part (11) is formed on the sanding plate (1), the first engagement part (4) and the second engagement part (11) being in contact with each other at least at one place (14), with continuous force transmission taking place.
2. Eccentric sander according to Claim 1, characterized in that the speedlimiting device (10) is designed as an internal-geared wheel (13).
3. Eccentric sander according to one of the preceding claims, characterized in that the two engagement parts (4, 11) are connected to each other positively, in particular in the form of teeth, and/or frictionally.
4. Eccentric sander according to one of the preceding claims, characterized in that the connection between the two engagement parts (4, 11) is in point or line form.
5. Eccentric sander according to one of the preceding claims, characterized in that in addition to the bearing

friction in the engagement-part bearing (6) an opposed moment of friction is applied, in particular by friction with a solid and/or a fluid.
6. Eccentric sander according to one of the preceding claims, characterized in that the speed-limiting device (10) can be engaged and disengaged by means of a clutch, in particular acts only during the freewheeling of the sanding plate (1).
7. Eccentric sander according to Claim 6, characterized in that the clutch acts on the speed-limiting device (10) at a plurality of points, in particular along the entire periphery.
8. Eccentric sander according to one of Claims 6 and 7, characterized in that the clutch is movable resiliently against the speed-limiting device (10).
9. Eccentric sander according to one of Claims 6 to 8, characterized in that the clutch and the speed-limiting device (10) lock into place one in the other in the event of engagement.
10. Eccentric sander according to one of Claims 6 to 9, characterized in that the clutch and the speed-limiting device (10) act at the periphery or end face of the latter.
11. Eccentric sander substantially as herein described with reference to the drawing.