TRIBOLOGY: THE SCIENCE OF COMBATTING WEAR

By William A Glaeser (Member, STLE), Richard C Erickson (Member, STLE), Keith F Dufrane (Member, STLE) and

Jerrold W Kannel Battelle Columbus, Ohio

Reprinted with permission of Dr Sheldon R Simon, Manager, Battelle Technical Inputs to Planning Program, Battelle,

Columbus, Ohio

PART 7 – ELASTOHYDRODYNAMIC LUBRICATION (EHL)

Where EHL is needed

Machine elements with high pressure concentrated contacts – e.g. a ball-race contact in a ball bearing – often perform

best when designed and lubricated according to a specialized version of hydrodynamics called elastohydrodynamics.

So common and so successful are these “EHL elements” that they are often taken for granted, and yet they play critical

roles in nearly all technological areas requiring rolling or sliding contacts. These elements include, for example,

• Rolling element bearings - both roller and ball bearings form EHL contacts as their surfaces interface with

the rate. (Fig. 1).

• Gears – gear teeth form an EHL contact as they mesh during gear rotation (Fig. 2). Gear tooth contact

pressures tend to be very high. Even worm gears may operate by what is called micro-EHL.

• Cams and cam-followers – the roller or slider interface – are a very significant type of EHL contact (Fig. 3).

• Traction drives - many traction drives contain rolling/sliding EHL contacts with very high pressures. Here,

paradoxically, the objective is to simultaneously develop both adequate EHL films and high traction levels

(Fig. 4). Traction is defined as the tangential stress transmitted across the interface of a rolling contact.

Fig. 1 – Roller bearing and ball bearing

Gears

Fig. 2 – Gears.

Fig. 3 – Cam and cam follower

Traction drive

Fig. 4 – Traction drive

Advantages and Disadvantages of EHL

EHL devices are particularly advantageous when they are used for conditions requiring;

• Low-friction over a range of speeds,

• Meagre lubricant supply and minimal friction – e.g., mist lubrication or greased wheel bearings,

• Little monitoring or maintenance – e.g., in space vehicles.

However they also have certain disadvantages:

• They tend to be expensive,

• They occupy more space than do hydrodynamic bearings (See Fig. 5).

• They are susceptible to fatigue failures more readily than hydrodynamic bearings.

Fig: 5 – Size comparison between a hydrodynamic bearing and a ball bearing

How and when EHL works Though EHL elements have long been used in modern technology, only in recent times has any real understanding

evolved concerning their lubrication process.

For many years, it was generally believed that metal-to-metal contact occurred between moving members. Only since

1959 has the formation of lubricating films between interfacing surfaces been verified and measured. By now,

quantitative techniques based on EHL theory have progressed to the extent that film thickness under operating

conditions ca be predicted and related to service life of EHL components.

According to the general hydrodynamic theory, a thin film of lubricant wedges itself between the would-be contacting

surfaces (such as a ball and race), thus, inhibiting metal-to-metal contact. But in EHL devices, the contact pressures

are so high – literally hundreds of thousands of psi – that formation of this wedge is extremely difficult.

Hydrodynamic lubrication of these devices is a result of three fortuitous factors;

• The viscosity of a lubricant increases – by orders of magnitude – as lubricant pressure rises,

• Contacting surfaces are typically very smooth, facilitating lubricant distribution,

• The interface region elastically deforms under contact pressure, forming enlarged oil film areas.

By incorporating these factors into lubrication theory, the elastohydrodynamic concept (hydrodynamic lubrication

coupled with elastic deflection) was postulated. The term “elastohydrodynamic” lubrication is applied to

hydrodynamically lubricated rolling contact elements that operate at high loads, causing extremely high interfacial

pressures and significant elastic deflection – or deformation – of the structural material. As noted above, this

deformation creates larger oil film areas, and, therefore greater load carrying ability.

These conditions also cause a somewhat different lubricating process to occur. As a typical lubricating oil enters the

contact zone between moving elements, its pressure rises sharply. This pressure rise, in turn, increases oil viscosity,

which further increases pressure. Once the lubricant is in the contact zone, the pressure is sufficient to deform or

flatten elements and to create an oil film with 106 10

7 centipoise viscosity, which is approaching solid like behaviour.

(Fig. 6).

Fig. 6 – Surface deformation during EHL Contact

Under ideal conditions, the asperities, or surface rough spots, harmlessly puncture into (but not through) this viscous

layer as they move through the contact zone. Due to this absence of metallic contact, no wear of the bearing elements

occurs. Also bearing fatigue life due to localized pressure spikes is greatly increased.

How EHL effects bearing performance EHL effects bearing performance in several crucial ways:

• Bearing life

• Friction

• Cage dynamics

• Edge stresses

• Temperature.

Bearing life

Elastohydrodynamic lubrication has a profound effect on bearing life. This is demonstrated in Fig. 7, which shows

how bearing life is influenced by the ratio of EHL film thickness to surface roughness. For thick oil films, very long

life can be expected. Conversely, if films are extremely thin, bearing life is greatly reduced. Thus, the critical

parameter, lambda, λ, is the ratio of lubricant film thickness to surface roughness.

Note than when λ is large – in the 5 to 10 range – the life factor is nearly 3.5. Conversely, when the value of λ is less

than 1, the life factor is about 0.5 – a difference in service life of 700 percent! A good rule for bearing life is to

maintain thick EHL films.

Fig. 7 – h, EHL film thickness

surface roughness ratio.

Friction EHL film thickness effects friction in interface regions. In a traction drive, for example, the EHL film must yield high

friction with very little slip, since slip may represent lost energy. As a general rule, the thinner the oil film, the higher

the shear rate (sliding speed divided by film thickness), and, for a given oil viscosity, the thinner the film the higher the

friction. However, lubricant performance factors – such as viscosity and pressure – which cause thick films to form,

can also create frictional losses at the interface. For traction drives, then, lubricants should be selected to yield

sufficient film thickness and simultaneously to achieve proper friction.

But because most EHL lubrication technology was originally developed in order to reduce friction, the technology for

designing systems to achieve “proper” friction is still in an early developmental stage.

Temperature Formation of an EHL film in bearings is always accompanied by heat generation. Often, the temperature does not rise

enough to effect performance. However, in some cases, the temperature rise is sufficient to significantly reduce

viscosity and degrade bearing performance, especially in highly loaded bearings.

Cage dynamics One of the least understood parts in a rolling element bearing is the cage, which controls the ball-to-ball, or roller-to-

roller, spacings. At the rolling element – cage contacts, pure sliding occurs. The cage if unconstrained can be the

source of serious problems. If, for example, the cage is too tight, then the cage may fatigue. Conversely, when the

cage is very loose, it may undergo complex dynamic motions which may cause erratic torque, noise, and even failure.

Torque problems are especially serious in a gyroscope bearing. The elastohydrodynamic process effects cage

dynamics enormously, as illustrated in the diagram in Fig. 8.

. Fig. 8 – Elements hit the cage

When a ball slides against a cage, the cage either gains or loses momentum. It loses momentum when the ball slides on

the race sufficiently to absorb some of the impact: it gains momentum from the ball motion during the momentary

frictional coupling between ball and cage. This net gain or loss of momentum then determines whether a bearing cage

is stable or unstable. EHL analyses – based on instantaneous friction at the ball-race interface – can be used to predict

cage behavior and to design more stable bearings.

Edge stresses

Failure of roller bearings or gears often occurs near the edge of the contacts. In roller bearings, the stressers are

variable across the roller and can only be predicted with computer models. Such models show the stresses to be very

large near the edges. These large edge stresses cause the lubricant film to thin and result in this edge-initiated failure.

For these applications, either the roller must be designed to eliminate these thin films to compensate for this potential

problem, or very thick EHL films are needed to eliminate edge contact.

Editor’s Note: Elastohydrodynamic lubrication to be continued in Part 8.