Wiping the slate clean

Bearings are often the forgotten part of mechanical systems. With all the attention lavished on energy-saving drives, intelligent control systems and space-age sensors, the literal nuts and bolts — the actual mechanical bits and pieces that keep the moving parts of the system moving and the static parts still — can be relegated to the background. Until they fail, of course, at which point gears clash, axles buckle and the engineering manager starts rending his clothes.

Power cannot be transmitted to a moving part without a bearing to make everything run smoothly; and when your stock in trade is grit, rock and clay, keeping bearings running becomes even more important. At Belgium's Beez quarry, on the banks of the River Meuze, there is a constant battle between the nature of a quarry — to be dirty — and the need to provide its limestone products in a clean state.

The Beez quarry is 80m deep and covers more than 85 hectares. It produces high-quality limestone. About 1.7 million tonnes of rock are blasted out of the quarry every year, to be used in applications as varied as the dykes that keep the North Sea out of the Netherlands, to toothpaste and chicken feed.

The civil engineering industry is one of the quarry's most important clients. 'The limestone we extract here is particularly suitable for making concrete,' said quarry director Jean-Michel van Peteghem of Sagrex, the company that owns and runs Beez. 'But there is a lot of clay here, and if you do not wash it off, it makes the concrete weaker and less resistant.'

After the limestone is dynamited out of the rock face, it is crushed into manageable chunks and then sent to two huge rotary screen washers. The cylindrical washers have a 3m diameter, are 10m long and turn constantly. They sit on two rows of nine tyre-shod wheels linked by couplings, one set of which is driven by a 132kW motor and a reduction gear. The aggregate of limestone pieces, mixed with water, is fed in at one end, and the rotation of the cylinders clashes the rocks together so that they rub the clay off each other.

It is an arduous duty, and when the washers were first installed, the bearings on which the wheels turn failed regularly, often lasting only three months. Every time a bearing failed, the whole system had to be shut down for eight hours, as the axle had to be removed and taken to a workshop to be fixed.

In a bid to make the system more reliable, Sagrex approached SKF about an improved set of bearings, initially using a non-driven pair of wheels. After looking at the mechanism and the way it operated, SKF came up with a set of solutions for both the driven- and non-driven sets of wheels.

Jurgen Matthieu, business manager of reliability systems engineering at SKF Belgium, said: 'We designed new axles able to fit standard SKF bearings, making parts interchangeable and readily available.' These axles sit in split bearing housings, so a pair of tyres can be lifted out without having to dismount the bearings and housings first.

The bearings are also of a smaller external diameter than the previous set, but are capable of carrying heavier weights; this means that the wheels can be moved over the bearings without having to dismount them. Moisture-resistant seals keep the clay-laden water away from the moving parts, and efficient lubrication keeps everything running smoothly.

One problem the washers experienced was uneven wear on the axles, owing to only one of the nine being driven. Matthieu said: 'The couplings were not connected along the whole length of the washer, and the drive only came from one set of tyres, which led to this uneven wear. This meant that the driven axles had to be repaired or replaced regularly, and that meant even more downtime.'

He added: 'There were also provision problems, because all the parts were different sizes.'

Once again, SKF redesigned the axles so that a standard set was used, and installed a gear coupling to make sure that the weight of the washer was distributed equally across all the tyres and all the wheels were driven. The entire system was aligned using laser equipment.

The result was a halving of downtime, much to van Peteghem's satisfaction. 'We are extremely happy with the solution,' he said.