TechArchive

Original article date: March 2000

It’s not often that we hear of a new concept in roller bearings, so we sent Tommy Miller to find out more

Roller bearings, one with a cage, one without, and yes, the rollers are hollow? This design offers several advantages compared with solid roller bearings and preloaded ball bearings, making the so-called Holo-Rol bearings suitable for many applications. In particular, high speed machine tool spindles will benefit from the radial stiffness, high speeds, tolerance of shock loads and smoother running. A satellite tracking telescope has also been equipped with the high-accuracy, smooth running Holo-Rol bearings.

Because the rollers are hollow, they can be preloaded easily, allowing the radial load to be shared – unlike conventional non-preloaded bearings where less than half of the rollers are loaded and the remainder are unloaded – and solid roller bearings are notoriously difficult to preload. An important requirement for machine tool bearings is for high radial stiffness, or low radial runout. Because the bearings have no running clearance, the radial stiffness is inherently better; a radial runout of just 0.00127mm is quoted for the Holo-Rol bearings, which is half the figure given for an ABEC-9 class bearing, the most accurate standard class of roller bearing. Furthermore, the preloaded hollow rollers also dampen radial vibrations, which helps to improve the accuracy and surface finish of parts produced on machine tools equipped with them. It is claimed that machine tool spindles running in hollow roller bearings experience vibration levels only 25% of those for ball bearings.

The performance of conventional roller bearings is usually limited because of the high temperatures that are generated and the need for adequate lubrication. But the Holo-Rol bearings generate lower centrifugal loads and the outer race therefore experiences lower stresses. The stresses at the surfaces of the races are also lower than usual because of the way that the rollers deform and therefore share more of the load than is the case for solid rollers.

Reduced slipping

A common problem with roller bearings operating at high speeds is that slip occurs between the rolling elements and the races. This is unlikely to happen with the hollow roller bearings because of the preload; the rollers are in forced contact with both races so are positively driven at all times. The lower mass (and inertia) also helps the rollers to accelerate rapidly without slipping.

While the new bearings clearly have their advantages, they also have their limitations. Continuing the comparison with conventional roller bearings, the hollow rollers have a lower load carrying capacity due to the lower radial strength. While this may not necessarily be a drawback for high speed machine tool spindles, it has to be borne in mind by the designer, especially in more unusual applications.

Because the rollers are preloaded, it might be expected that their outer surfaces experience a cyclical tension-compression loading and that this might lead to fatigue failures. However, the manufacturer says that the stresses are maintained well within the elastic limit and that such fatigue failures do not occur. Indeed, the bearing life is said to equal or exceed that of conventional bearings, and the hollow rollers might even be better equipped to survive contamination by dirt since the hollow rollers allow dirt to be washed away by the lubricant.

Because of the performance benefits, it might be expected that the hollow roller bearings would be significantly more expensive than either preloaded ball races or conventional roller bearings. But this is not the case, partly because of the ease with which the preload can be fine-tuned during the manufacturing process. Also, customers are not limited in their selection; versions with and without cages are available, some types have shoulders on the outer race to permit axial loads to be carried, and there are two series with double rows of rollers – one with a straight bore, the other with a tapered bore. While the hollow roller bearings have been designed to be interchangeable with ‘light’ and ‘extra light’ series ball bearings, other sizes and special configurations can be manufactured to order. Standard bore sizes range from 17mm to 140mm.

• RA Rodriguez

March 2000