Flexible couplings cut vibration at one of the world's biggest grinding mills




Renold Hi-Tec Couplings has worked successfully with a mining company in Australia to reduce a problem it was experiencing with extreme vibration in the drive train of one of the biggest grinding mills in the world. The autogenous mill for grinding nickel ore is powered by two seven megawatt electric motors that drive a 9.6 metre diameter shell with a 12.1 metre diameter girth ring. The mill grinds the nickel ore within a rotating drum that propels material in a cascading motion causing impact breakage of larger rocks and compressive grinding of smaller particles.

Excessive vibration in any machinery can cause all sorts of problems but in particular it can result in damage to the machinery being driven as well as to the motor and the components of the power transmission system. Left unchecked vibration can reduce the life of a machine very quickly by causing premature fatigue failure, particularly if the vibratory frequencies approach the natural frequency of the equipment.

Renold Hi-Tec's engineers analysed the levels of vibration and recommended the use of two PM600 rubber-in-compression couplings with specially selected rubber blocks. The two couplings, located between two reduction gearboxes and the girth ring pinions, deliver 200KNm at a speed of 170rpm and have completely resolved the problem with vibration.

Rubber-in-compression couplings are comprised of two round, metal sections fitting one inside the other with what looks like the paddles of a paddle steamer projecting inwards from the outer section and outwards from the inner. Rubber blocks are placed in the spaces in-between the paddles, and, as the outer section is turned by the motor, it drives the inner section through the rubber blocks. As this happens the rubber is compressed, hence the term 'rubber-in-compression.' When rubber is loaded in compression it behaves as an incompressible fluid and is inherently much stronger than alternative technology and is therefore well suited to arduous, heavy-duty applications.

The PM range of couplings is designed for applications up to 6,000KNm and the materials used in their construction make them particularly suited to applications where severe shock loads and vibration could cause problems.