Bearings

5-2000


Technical Report

Plain speaking

Mark Fletcher helps engineers to negotiate the tricky question of where plain bearings might be used instead of roller bearings

Never before in the field of engineering has something so small had such a big impact on so much. We are, of course, talking about the humble bearing.

Say the word 'bearing' to an average Joe in the street and they will talk balls to you. But engineers know that there is a gamut of technologies to chose from – depending on your application. Few companies specialise in both roller element and plain bearings, so impartial advice – whether to use one or the other – is hard to come by.

One simple trip to Findlay's Design Selector website( www.designselector.co.uk ) shows that there are 201 companies in the UK under the generic banner of bearings. Of those, 96 supply roller bearings (these can be broken down into a further 20 sub categories), of which 40 are manufacturers. 'Plain bearings' includes 72 companies (which can be broken down to a further nine sub categories), of whom 40 are manufactures.

Plain bearings are often overlooked by designers. One manufacturer of plain bearings, Glacier Industrial Bearings, suggests a list of advantages: a higher load carrying capability; no risk of brinelling and the ability to support higher shock loads; the ability to handle small oscillating motion where a full revolution does not occur; and the fact that they are smaller and lighter than roller units. Some can operate at elevated temperature without lubricant. In general, plain bearings are non-magnetic, quieter than rollers and get cheaper as their size increases.

The company is quick to point out that the application will dictate the bearing used. An example is the Rolls-Royce Olympus engine (the loud bit under Concorde). In the aerospace version, high-performance rolling contact bearings are used; but when used as part of a gas turbine, the weight penalties of lubrication systems are not evident and plain bearings are used – offering a longer life.

Finding your bearings

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As a consultant specifically targeting bearings, Neale Consulting Engineers offers an impartial opinion as to what designers should consider.

First of all, says managing director Stephen Maw, consider the fundamentals of the different bearing types. There are three types of plain bearings: rubbing bearings exhibit rubbing contact and are limited by wear rates and friction; hydrodynamic bearings rely on the motion of the surfaces to generate a pressure in the oil film that separates them; and hydrostatic bearings rely on pressure exerted by an external source. Their load capacity is determined by fluid pressure and bearing area and not by speed.

Plain bearings use area support, so misalignment can give rise to edge contact. Soft materials are generally used against a hard surface – the hardness ratio between bearing and the hard surface (e.g. shaft) needs to be at least 1:3 (i.e. harder shaft). The soft material also embeds hard contamination particles.

Rolling element bearings are a bit trickier to define as there are so many variants, but they all exhibit the same basic operating principles. Ball bearings have excellent speed capabilities, but the Hertzian point contact limits the load capacity; roller bearings, on the other hand, have a greater load capacity but reduced speed capability. The hard surfaces are separated by a mechanism known as elasto-hydrodynamic lubrication, similar to hydrodynamic lubrication but the hard steel surfaces deform elastically to generate an oil film. Surface separation is typically 1-3 microns. Hard contamination can substantially reduce bearing fatigue life.

According to Maw: "The main constraints are load, speed, size and cost."

Any of these factors can dictate the suitability of a particular type of bearing. Plain bearings rely on the actual movement of the two constituent components to draw oil into the space between the surfaces and generate an oil pressure to balance the load. This effect leads to an increase in load capacity as the speed climbs. A roller bearing has a load capacity dictated by the surface stress, so exhibits a constant load capacity at low speeds. As speed increases, the size of the rolling bearing is determined by the fatigue life of the loaded surfaces. This means that load capacity reduces with speed (for a given bearing size).

Cost does pay a part in the choice of the designer. The usual factors take shape, namely the size of production runs, whether the bearing is a catalogue or bespoke item and the actual size of the unit itself. Unfortunately so many decisions based on cost should really be decided on performance. The pendulum does swing both ways – in certain applications, types and sizes plain or roller bearings will compete more favorably.

Maw also adds that bearing design can be more complicated with plain bearings but: "The designer needs to understand the relative merits of different rolling bearing advantages and disadvantages in obtaining a reliable bearing design".

Stephen Maw's seven-point plan to bearing selection

* Vertical or horizontal shaft? (lubrication of vertical shaft bearings is generally more difficult)

* Selection of rolling bearing close to their maximum catalogue ratings (load and speed) is likely to decrease reliability.

* A rule of thumb for rolling bearing selection is "The simplest design will be the most reliable." This means a ball bearing and a cylindrical roller should be the first choice. Only depart from this arrangement if load, speed, assembly or other constraints force you to.

* Watch out for thermal effects (thermal growth can cause a loss in bearing clearance and lead to failure). Both radial and axial growth need consideration.

* Designers should use the smallest rolling bearing that has adequate load capacity for the duty (L10 life is the way of calculating rolling bearing life – see manufacturer catalogues for technique). A larger bearing is more difficult to lubricate, even though it has greater fatigue life. Many failures are lubricant-related rather than load- related.

* The rolling bearing manufacturer catalogues are excellent in terms of the information presented. Pretty much all that the designer needs is in them.

* Use grease, if possible, for rolling bearings. Oil lubrication will add cost. If oil is used, splash lubrication first, then oil circulation. Oil mist is a specialist application.

Further information on bearing matters including selection, failure and FAQs can be found on the Neale Consulting Engineers website www.tribology.co.uk

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