Rotary systems, including angle grinders, are often in a state of imbalance that leads to vibration of the machine. Operators using vibrating machines can develop a condition known as"vibration white finger"as the fingers, and particularly the nerve endings, are damaged by this vibration. Systems have therefore been developed to reduce this vibration. Typically such systems involve providing an annular trackway concentric with the rotation axis of the rotary system and providing freely rolling bodies in this trackway. When the rotary system is in operation the rolling bodies will automatically find their appropriate angular position in the trackway to compensate for the inherent imbalance of the system. Such auto-balancing devices have proved to reduce the vibration in a rotary system.

It is essential that an auto-balancing device is properly centred around the axis of rotation to ensure maximum balancing effect and thus maximum vibration reduction. To achieve correct centring of the device, auto-balancing devices are built into rotary devices. However many such devices do not incorporate these auto- balancers.

The object of the present invention is to provide an auto-balancing device suitable for use as a retro-fit on a rotary device.

According to the present invention there is provided an auto-balancing unit for an angle grinder or similar rotary device including a threaded shaft on the central axis of the device for receiving a grinding wheel or the like, and securing the grinding wheel to the grinder, the auto-balancing unit comprising :- a body having an annular trackway open from one side, for receiving and movement of spherical weights, the open edge of the trackway being formed to accept a lid; and a central aperture;

a lid sized to close the body; and a clamping flange being internally threaded for securing to the threaded shaft on the central axis of angle grinder, for clamping a grinding wheel or the like and having an extension being sized to be a tight fit with the central aperture of the body.

In use a number of spherical weights, typically ball bearings, are placed into the body of the auto-balancer. It has been found that five weights achieve good results, although other numbers of weights can be used. A small quantity of oil or grease is added to the weights and the body is sealed with an annular seal.

In a standard angle grinder or the like a grinding wheel is normally secured to the machine shaft between two mounting flanges which fit onto or over the machines central exposed drive shaft.

The flange furthest from the main body of the machine has a threaded aperture. The flange closest to the main body has a loose fit aperture. The auto- balancing unit of the present invention is designed to replace the flange closest to the body of the machine and perform the fame function as this flange in securing the wheel to the machine.

The aperture of the auto-balancing unit is placed over the threaded machine shaft. The aperture of the grinding wheel is then fitted over the threaded machine shaft and then the grinding wheel is secured to the machine by tightening the threaded aperture onto the threaded machine shaft.

Due to a designed fit condition between a specific aperture in the auto- balancing unit and an extension to the threaded flange of the auto-balancing unit is fully centred.

Preferably the auto-balancing device will be made of metal, but any other substance can be used.

To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of an anti-vibration unit according to the present invention; Figure 2 is a top view of the anti-vibration unit of Figure 1; Figure 3 is a bottom view of the anti-vibration unit; and Figure 4 is the anti-vibration unit of Figure 1 for securing to an angle grinder or the like.

Referring to Figures 1,2 and 3 the anti-vibration unit 1 thereshown comprises a circular body 2, having an annular trackway 4 machined into the body 2 and a central aperture 5. The trackway 4 is sized to allow a number of ball-bearings 6 to roll around the trackway in use.

A lid 10 for the body is also provided in the form of a disc 12 having a central aperture. In use a small amount of oil or grease is placed in the trackway 4. The ball bearings, usually 5 or 7, are placed in the trackway 4 and the lid 10 is placed on top to seal the body 4.

The central aperture 5 of the anti-vibration unit 1 is of a different size and shape on one face 15 of the unit from that of the other face 16. A lip 17 is provided in the aperture between the sections of differing size 15,16.

A modified securing flange 24 is also provided to secure the device 1 to an angle grinder or the like. The flange 24 is threaded internally 26 for connection to a central shaft of the angle grinder 30. The flange 24 also has extensions 28 sized as a tight fit into one of the diameters 15 of the central aperture 5 of the unit. This ensures that the unit is fully centred on the central axis of the angle grinder or the like. The flange is reversible to allow for different sizes of grinding wheel 18.

Referring now to Figure 4, in use the anti-vibration unit 1 is fitted over a central shaft 30 of an angle grinder or the like (not shown). The shaft is provided with machined flats 32 and the opening 15 of the central aperture 5 is sized and shaped to

fit the machined flats 32 thus ensuring that the anti-vibration unit rotates at the same speed as the machine shaft.

The grinding wheel 18 is then fitted over the machine shaft 30 and onto the anti-vibration unit 1. The securing flange is then fitted onto the machine shaft, the thread 26 on the flange engaging with a threaded end 34 of the machine shaft. The opening 16 on the other face of the central aperture 5 of the unit 1 is sized for receiving the extension 28 of the flange 24 as a tight fit ensuring centralisation of the anti-vibration unit 1.