Spring 2000

Eureka on Campus sponsor feature

Better bearings make maintenance obsolete

Maintenance-free bearings that need no relubrication on drive shafts for truck and off-road applications boast a service life of 1 million kilometres

SKF's successful introduction of thin-walled universal joint bearings in the early 1980s for propeller shafts for truck and off-road applications can be gauged by the fact that about 30 million such bearings have been produced.

Today, however, a newer design of maintenance-free universal joint bearing guarantees a service life of over one million kilometres. The company intends to offer a range of eight bearing sizes with outside diameters of 48 mm to 68 mm to cover the medium truck to heaviest dump truck market.

The new bearings are of the same basic design as their predecessors, but incorporate certain important developments. The bearings comprise thin-walled drawn cups with ground outside diameters coupled with a significant number of large diameter rollers. They have a special roller profile to guarantee optimum load-carrying performance. The cup base shape gives 40 per cent higher strength and good axial damping - no base fractures even under 'W' bending conditions.

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The new design has low wall-thickness variation and raceways with a high degree of roundness. A cuff seal of polyurethane and a double lip nitrile butadiene (NBR) seal prevents the entry of dirt and water and the leakage of grease. An integral plastic washer with lubricant grooves and waffle pattern optimises lubrication conditions. In addition, cup springs prevent false brinelling and retain the rollers before the joints are assembled.

The bearings have been designed to fulfil a number of requirements, including effective protection against the entry of dirt and water, sealing against grease leakage and the avoidance of relubrication channels in the 'crosses' of the universal joints. In addition, SKF had environmental concerns and sought to eliminate relubrication, which presents environmental hazards. It is estimated that each year European trucks waste more than 1,000 tonnes of grease and pollute about 2,000 million cubic metres of water.

The design improvements are concentrated mainly on the garter seal, spring-loaded garter seal and a special grease with an optimised filling volume. All steel parts of the garter seal are coated with nitrile rubber to avoid metallic contact and to improve sealing performance. The spring-loaded garter seal ensures a defined contact pressure of the seal lip on the journal of the cross. This garter seal is protected by the cuff seal, which is also made of nitrile rubber and includes a steel insert.

The specific properties of the nitrile rubber mix give a permanent tight fit on the cross even under extreme conditions. Considerably improved contact geometry at the outside diameter reliably prevents the introduction of dirt and water.

In the event that the cuff seal becomes damaged, the design ensures that should dirt particles enter the bearing, they are transported to the free space between the garter seal and cuff seal by centrifugal force and are deposited there.

A carefully defined grease quantity in the bearings prevents dry running of the seal lip and also avoids the generation of unacceptably high internal pressure during mounting. The particular grease used guarantees a service life of more than one million kilometres.

The enhanced design has produced other benefits, including high precision, excellent roller guidance, a high degree of roundness and optimal load-carrying performance.

The long service life is due mainly to the profiled rollers and high load-carrying capacity. The cuff seal with its steel insert offers high stiffness and damage-free operation. The use of a high elasticity spring-loaded seal also allows re-greasing. This means that maintenance-free universal joint bearings can also be used with crosses having relubrication channels, if required. Finally, there is no possibility of false brinelling or edge loading and both inner and outer snap ring positions are possible. The bearings can operate over a temperature range of –250C to 1500C and for short periods at temperatures up to 1800C.

Get your skates on

Jürg Schläfli of Switzerland has set the 1998 world record for high-speed inline skating while being pulled by a motorcycle. Pulled by a 1100 cc Yamaha motorcycle, Schläfli reached a top speed of 233.76 kmh. Schläfli's inline skates are equipped with special SKF deep groove ball bearings, 608-ZRZ/VK251. His next goal? To clock 320kmh while being pulled by a car. That would beat the current record of 307 kmh.

Staying on the rails

A patented Anti Creep System (ACS) taken out by SKF’s Linear Motion Division eliminates ‘cage-creep’, which is common in all precision linear rail guides.

Cage-creep is the term given to the action of the cage as it progressively edges along the vee of the guideway caused by uneven pre-loading and poor rail alignment. The problem tends to be exaggerated, due to the effect of gravity, when the guide is mounted vertically.

When cage-creep occurs, and the cage housing containing the rolling elements contacts the end stops of the guideway, the elements begin to skid rather that roll thus reducing the effective working operation and life of rail guide assembly.

With SKF ACS, a plastic moulding that forms the cage is further utilised to enclose a small plastic sprocket in its centre. Corresponding tiny notches are machined in the bottom of each of the guideway vees in order that they mesh with the sprocket. This arrangement creates, in effect, a low-duty rack and pinion drive as the guideway moves generating a synchronised movement which optimises the cage position throughout the stroke of the rail.

ACS has been developed around the SKF modular range of LWRE precision linear guideways to provide the option of direct interchangeability within the three standard cross-section dimensions of 18 mm by 8 mm, 31 mm by 15 mm and 44 mm by 22 mm.

The LWRE Cross Roller Series has already achieved considerable application success due to its design capability to ensure full roller contact with the respective vee guideway. This provides a far more rigid operation when compared to the 40 per cent roller to vee contact ratio employed by traditional designs.

This high roller contact enables a 100 per cent increase in stiffness with a five-fold increase in load carrying capacity when compared to a traditional linear rail design while maintaining the same cross-sectional dimensions. These higher performance characteristics not only ensure better performance with increased wear resistance, they provide the designer with the choice to upgrade the specification and capability or downsize the assembly to a more compact unit.

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