What Is Tribology ?
Sandy Polak discusses the many facets of tribology and why it is a key to improved reliability.
Tribology – A Key To Improved Reliability
Machinery and mechanical equipment can break down for a large variety of reasons, and frequently the tribological components, i.e. bearings, gears, seals and couplings are the source of the problem.
The tribological components in a machine can be compared with the fuses in electrical equipment; when anything goes wrong, they melt! Unfortunately, unlike electrical fuses, the bearings and gears are not quick and easy to repair, nor, by melting, do they necessarily protect the rest of the machine. For example, a failed bearing can result in complete destruction of a whole gearbox.
Continuing the analogy, merely replacing a fuse is not likely to cure an electrical problem. Similarly, with tribological components, when they fail it is tempting to merely repair or replace the component, but if the underlying cause of the failure was not the component itself then the problem is will recur.
The reason why tribological components tend to fail before other parts of the machine can be readily understood by thinking about their function. A bearing is required to carry a substantial load, while allowing rotary motion with very low friction. Typical friction coefficients for continuously rotating bearings are 0.01 or less. It does not require very much to go wrong to cause this friction coefficient to rise by a factor of 10 or more. When this happens, there is suddenly a large amount of heat generated in the bearing, leading to melting and possible ignition of surrounding materials. The cause may be something quite small: dirt or water in the oil; external shock loads and vibration; or thermal distortions during warm-up, are just some possibilities.
This article uses examples relevant to the rail industry to illustrate the advantages to be gained from a full investigation of machinery failures, using tribological knowledge and understanding to establish the causes. It is frequently possible to improve reliability substantially by quite simple changes, once the real cause of the mechanism of the failure is understood.
The following topics will be covered:bearings in gearboxes and motors; axle bearings; and various engine components.
Tribology Within EMU Gearboxes
On modern EMUs (electrical multiple units) high speed traction motors are often used, resulting in gearbox input shaft speeds in excess of 6000 rpm. When bearings at these speeds fail, the resulting damage to the whole gearbox tends to be so severe that it is often difficult to determine the sequence of failure, and almost impossible to confirm the underlying cause or triggering event.
Investigation of failures on various EMU traction gearboxes has revealed various features, all of which contribute to unreliability.
The input shaft bearings, in common with all the gears and bearings, are usually splash lubricated. However, at maximum vehicle speed, the input shaft bearings are often above the normally recommended speeds for splash lubrication, so any deficiency in lubrication, or any deterioration of the bearing caused by debris in the oil, can lead to excessive heat generation and rapid failure.
Some types of EMU gearbox have a hollow input shaft, to allow the input driveshaft to pass through. This results in relatively poor heat dissipation from the shaft, and while not normally a problem, during high speed running straight from a cold start, the hollow shaft may heat up considerably faster than the casing, which can reduce or eliminate the internal clearance in the bearing. Failures of this kind tend to happen early in the life of the equipment, but can also happen later if the heat generation within the bearing increases for any reason.
The gearbox output gear is usually press-fitted to the vehicle axle and, as the wheels form part of the power circuit it is difficult to isolate the gearbox completely from electrical currents. Any faults on axle earthing brushes can Examples of tribological components include bearings, gears, seals, piston rings, valves, wheels. lead to occasional stray currents passing through the gearbox, and the high speed input shaft bearings are likely to be the most sensitive to electrical erosion damage. A carefully designed earthing system does, however, almost eliminate the risk of damage.
Tribology of Mono-Motor Drive Gears
Mono-motor drives are used on many light rail vehicles, where one motor drives two axles via two bevel gearboxes. On one particular design of vehicle, persistent failures of the bevel gear teeth were occurring.
Investigation revealed that the reason was slight differences in wheel diameter between the two axles. This meant that one axle wanted to rotate faster than the other, but since they were connected by virtue of being driven by the same motor, a locked-in torque several times greater than the nominal motor torque was generated in the gear-boxes.
The wheels started out at the same diameter, but as wear occurred, the diameters varied. Analysis of the friction forces showed that the wear was unstable; whichever wheelset was the first to reduce in size then slipped more and wore faster.
The only real solution was stronger gears, or a redesigned drive system incorporating a differential. However, as the vehicles were being used in a relatively flat city, a pragmatic solution of disconnecting one axle from the drive was adopted. The traction and braking performance were still adequate for the duty.
Axle Bearing Tribology
Most modern rail vehicles now use roller bearings, which are extremely reliable, as indeed they have to be to ensure safety. The speeds are low compared to those of the gearbox input shafts described above, and so rapid catastrophic seizure is a very rare event.
Roller bearings are expected to fail eventually by the mechanism of fatigue pitting. Premature failure can occur, with a number of possible causes. The appearance of the failure may well be that of fatigue pitting, i.e. apparently identical to the appearance of a normal life-expired bearing.
There are various causes for premature fatigue. A common one is water contamination. The water has two effects; it causes corrosion of the surfaces, and it also disrupts or degrades the mechanism of lubrication. Rolling bearing lubrication is by elastohydrodynamic action (EHL or EHD lubrication), and the presence of water tends to inhibit the formation of the EHL film. Even 0.1% of water has a significant effect, and 1% water can reduce bearing life by a factor of 10. Vehicle washing procedures are thought to be a significant cause of water entering bearings.
Poor lubrication, or wear particles in the grease or oil, can also cause premature fatigue. If the bearing moves or creeps in its housing, the resulting fretting debris may enter the bearing and cause damage.
Shock loadings, and electrical currents can also cause damage and premature fatigue.
Traction Motor Bearing Tribology
Unlike gearbox bearings, traction motor bearings are usually grease lubricated, to avoid the complication of an oil system, and to avoid any risk of oil entering the motor.
Historically, grease lubrication has been used mainly for low speed bearings, where it is entirely appropriate and reliable. For high speed bearings, special greases have been developed to cope with the relatively high temperatures and high degree of mechanical working. Greasing instructions need to be carefully followed, and the bearing housing must have appropriate arrangements to expel excess grease. If a high speed bearing is over-packed with grease it will overheat.
Therefore, problems with traction motor bearings are quite likely to be caused by accidental use of incorrect grease, or over-long greasing intervals.
With traction motors there is clearly a risk of electrical current damage, either due to electrical faults, or due to the dynamo effect of asymmetric magnetisation on rotating shafts. Usually at least one of the motor bearings is of the insulated type to avoid this problem.
Engine Component Tribology
A diesel engine contains a multitude of tribological components: crankshaft bearings, pistons and cylinders, gears and chains, to name but a few. In rail traction duty, the engine operating conditions are unusually severe, in that the load frequently alternates between full power and idle. Stationary applications and marine engines tend to have much more steady loads, while road vehicles do not usually sustain full power for more than short bursts.
Therefore, certain problems can occur in rail traction duty which may not occur with the same engines in other applications. For example, cracking due to excessive thermal cycling of hot components (such as turbochargers, cylinder heads, exhaust manifolds).
If a diesel engine is correctly maintained, in particular changing the oil and filters regularly, then most of the components are highly reliable, suffering very little wear. The exception is the piston rings and cylinder liner which are inherently poorly lubricated, particularly around the piston top-dead-centre (TDC) position. These components will inevitably wear, and under certain circumstances the wear may be unacceptably rapid.
On one engine design, which used chromium plated steel cylinder liners for strength and wear resistance, unacceptable wear was traced to a combination of design and manufacturing process problems. The cylinder liners were honed to give an appropriate tribological surface finish, but the subsequent cleaning process was not well controlled, leaving substantial numbers of loose or almost-loose chromium particles. The piston rings were copper-plated as a running-in aid (a common feature), and the loose chromium particles embedded in the copper This resulted in unacceptable wear to both rings and cylinders. Once these aspects were changed, the life of the components increased dramatically.
Tribological Aspects of Bearings With Oscillating Movement
For bearings with continuous rotation, such as all the preceding examples, rolling element bearings are generally used. At high rotational speeds, such as engine bearings and turbocharger bearings, a better solution is to use plain bearings fed with oil under pressure.
However, there are many bearings in railway applications such as bogie pivots and various operating linkages and couplings, where the movement is one of oscillation, usually at relatively low speeds. Since the working components are generally made of steel a common solution is to allow them to work against each other with grease lubrication. However, similar materials are not really compatible in rubbing contact because they tend to pick up and seize together and, in the case of ferrous components, corrosion can also make matters worse. A better solution is to use a non-ferrous bush component such as leaded bronze which can give reduced wear, but still needs regular lubrication.
In recent years, a large number of non-metallic materials have become available which give low wear rates in rubbing contact with steel. They have the advantage of not requiring lubrication, but do require a non-corrodable hard mating surface, such as chromium plate or stainless steel. They offer major improvements in reliability because the wear rate is consistent and low, giving predictable wear lives with low friction.
Sandy Polak MA CEng MIMechE.