High speed grease technology "engineers out" the cost and complexity of oil lubrication

Citation

Morris, T. (2001), "High speed grease technology "engineers out" the cost and complexity of oil lubrication", Aircraft Engineering and Aerospace Technology, Vol. 73 No. 6. https://doi.org/10.1108/aeat.2001.12773faf.001

High speed grease technology "engineers out" the cost and complexity of oil lubrication

High speed grease technology "engineers out" the cost and complexity of oil lubrication

Keywords: FAG/Barden, Lubrication, Greases, High speed

Introduction

This article reviews the developments in high speed greases, outlines considerations for their selection and describes a new generation high speed grease with unique "run-in" qualities.

The convenience, effectiveness, low cost and simplicity of grease has meant that it has always been the most frequently employed lubrication method for rolling bearings. Over recent years the advantages which grease provides have closely matched the demands emanating from the ultra competitive global manufacturing environment. This has resulted in further developments of the lubricating medium to allow its use in applications, notably high speed and high temperature, where oil lubrication would once have been the only option.

Traditionally, high speed = oil, low speed = grease

Traditionally, high speed bearing applications were classed as those above 1,000,000ndm (ndm being a speed factor derived from = the mean diameter of the bearing in mm speed in rpm). In such applications oil lubrication was generally recommended as, at the high speeds involved, the thickeners used in greases to hold the oil suffered degradation due to excessive churning. This problem was not usually in evidence below 1,000,000ndm, enabling greases to be used successfully to give an appropriate system life without the requirement for expensive, and often complex, oil delivery systems.

What has changed the above situation is the appearance of a new generation of "ultra high speed" greases, rated at speeds greater than 1,000,000ndm. These have revolutionised the lubrication of rolling bearings in the machine tool, textile and aerospace industries, allowing a dramatic increase in bearing rotational speed through one single application of lubricant.

Considerations when choosing high speed greases

High speed greases are now used widely. However, their increasing use should not mask the fact that they have to be chosen carefully, matching the characteristics of the grease to the demands of the application.

When considering the choice of rolling bearing grease for high speed applications it is important first to categorise the "speed factor" of the bearing. This is simply done by ascertaining the mean diameter of the bearing in millimetres and multiplying this figure by the bearing rotational speed in rpm. The importance of this calculation is that it is a useful indicator in enabling selection of the optimum base oil viscosity for a particular bearing, as well as being a major consideration in determining grease fill quantity.

Viscosity and its effect on bearing speed factor n.dm

In general it can be assumed that, the lower the base oil viscosity of the grease, the higher the speed capability will be in respect of the attainable n.dm speed factor. On a cautious note, however, it should be realised that not all low viscosity greases can be selected to fulfil long term or lifetime lubrication of high speed rolling bearings. Only specially formulated greases incorporating special soap types with high mechanical stability and low oil separation qualify for this special application field.

As can be seen from Table I, high viscosity based greases are generally selected for rolling bearings subject to slow speed and high load conditions. Such bearings typically operate under boundary or mixed friction lubrication regimes where metal-to-metal contact within the bearing is likely to occur.

Higher speed bearings lubricated with low viscosity greases operate under elastohydrodynamic lubrication conditions with practically no metal-to-metal contact occurring within the bearing friction zones.

Correct selection essential to ensure optimum wear life and performance

Selection of the correct lubricant viscosity for high speed rolling bearings is very important in order to ensure optimum wear protection and attainment of the desired operating safety margin. Choosing a lubricating grease with too high an oil viscosity may result in excessive heat generation, high power losses and reduced grease life. In addition, overlubrication of a high-speed bearing can also prove disastrous with overheating and probable operational failures.

New generation high speed grease – FAG-Arcanol L75

The crucial role of a grease in providing stable bearing operating conditions at high speeds and temperatures has been underlined recently with FAG-Arcanol L75, a new generation product developed by FAG/Barden in conjunction with a major lubricant manufacturer.

Favourable viscosity/temperature behaviour maintains load carrying capacity

FAG-Arcanol L75 is one of the new generation of polyurea based greases. It has been developed for lifetime lubrication of sealed super precision ball bearings (steel bailed or hybrids) at high speeds of up to 2,5000,000ndm. The advantage of using polyurea is that it can be employed as a thickener with a number of different types of oil – mineral or synthetic. In this capacity it helps to overcome the limitations of the oil at higher temperatures and also ensures a more structurally stable mixture. In the case of FAGArcanol L75 the base oil is fully synthetic. The special feature of this base oil is its favourable viscosity-temperature (V-T) behaviour. V-T is a measure of the change in base oil viscosity in relation to temperature. If the oil viscosity changes only slightly with changing temperature, as is the case with FAG-Arcanoi L75, this is favourable V-T behaviour. The operational benefit of this to users is that the load carrying capacity of the lubricating film inside the bearing is sustained over a wider range of operating temperatures.

Critical "run-in" period reduced by 50 per cent

Another unique quality of FAG-Arcanol L75 is its excellent "run-in" behaviour. Run-in is the ability of a lubricant to reach a steady operating state in as short a time as possible. The polyurea based L75 does this in less time than that of FAG/Barden's well proven FAG-Arcanol L74, the barium-based grease that it is replacing.

Importantly, the temperature of L75 during the critical run-in period is typically half that of L74 grease when tested under the same conditions. What these results reveal is that in the critical phase of grease distribution with L75 the damaging effects of thermal stress conditions and lubricant degradation are clearly eased and a significantly higher degree of operational reliability is achieved as a result.

Further details are available from The Barden Corporation. Tel: +44 (0)1752 735555; Fax: +44 (0)752 733481.

Trevor MorrisFAG/Barden, Plymouth, UK