Friction and wear

Friction and wear

CONFERENCE Melton Mowbray, FRICTION England REPORTS 30-31st Janua7y 1968 AND WEAR There must be more conferences like this if design engineers a...

306KB Sizes 11 Downloads 104 Views

CONFERENCE Melton

Mowbray,

FRICTION

England

REPORTS 30-31st Janua7y 1968

AND WEAR

There must be more conferences like this if design engineers are really going to learn to ‘think tribology. ’ The organizers asked seven expert lecturers to present their material from the designer’s viewpoint and to unashamedly instruct the audience. Certainly they instructed-most lectures evoked at least a dozen questions-and a lot of the material was more easily assimilated than is usual at conferences. At the same time, it would be hard to accuse the speakers of being superficial or too academic. Most pleasing of all, 130 people attended, the bulk of them. designers of one kind or another. Rather than precis the papers, this report records a number of topics that keptarising in discussion and cut across several papers. A list of papers appears at the end. Film thickness, bearings

and

load, filtration gears

and surface finish in plain

.

The fluid film in a bearing can be liquid or gas and to carry the load must either be pressurized from an external source (hydrostatic bearing) or generate the pressure by relative movement of the surfaces (hydrodynamic). Generally speaking the permissible load on a hydrostatic bearing depends on the supply pressure. Hydrodynamic bearings on the other hand can carry increasing loads at higher speeds as long as enough lubricant can be supplied to keep the film intact. To do this the lubricant may have to be fed under pressure or if this is inconvenient, the bearing may have to be supplied from a sump or an oil-soaked pad. Both methods supply less lubricant per second than a pressure system and therefore reduce the permissible‘load. They also have the unfortunate characteristic of supplying less lubricant as the shaft rotates faster, which further reduces the load. To increase the loads, there is now much interest in hybrid bearings (between hydrostatic and hydrodynamic) as a means of carrying larger loads on a given thickness of film. Hybrids are not to be confused with mixed lubrication, also of current interest, which is intermediate between a fluid film and gross surface contact. However, hybrid and hydrostatic bearings require expensive pumps for pressurization so it pays the designer to think first of hydrodynamic ones. Although a larger area of oil film allows a bearing to carry a greater load, it also increases the power losses due to friction. A thin film also gives more efficient operation and it might be wise to calculate the film thickness and then specify the surface finish. Equally important, said Mr D. G. Scott, was the method of obtaining the surface finish. Speaking on rolling contact bearings, he explained that electric furnace steels have brittle inolusions that tend to crack and that in vacuum steels the inclusions are smaller and spread the load more evenly. He also asked for more measurements of stress in service to replace the calculated stresses that designers chiefly use. He had known a calculated load of 30Otons turn out to be a stress of 1700tons over a small area. Filters always pass some debris, however fine, but in practice a few of the particles of debris can be larger than the film is thick because of the ability of the whitemetals to embed hard particles and to survive momentary metal-tometal contact by wiping metal onto the journal. A filter tends tp pass the lighter pieces of debris and retain the heavy and fortunately it is the light debris that breaks down more quickly. Gears, being much harder than bearings, wffl not absorb material and will wear if the debris or the surface asperities exceed the minimum oil’film thickness. Filter size must therefore be carefully related to film thickness especially with extra hard surfaces and a long running-in period. However, one speaker, Mr H. J. Watson, reported experiments which showed that the critical temperature rather than the cleanliness of the lubricant determined the onset of scuffing. 184

TFUHOLOGY

August 1968

He also postulated a critical number of asperities per unit area-as yet unknown-but said that careful running-in could do much to prevent scuffing. Temperature could be CalCulated from a formula originally derived in 1937 and updated 20 years later: at least one delegate challenged this formula from his own experience of EN36 case hardened gears that scuffed at 50” below the theoretical scuffing threshold. The formula did not take into account the changes in load caused by distortion, Mr Watson suggested. By contrast there was reported an instance of EN34 gear teeth working in bulk oil at 200°C. Mr Scott suggested that small smooth surface protruberances were carrying the load. These areas would be metallurgically different from the bulk of the tooth and would survive at up to 800°C. Mr Watson said that, by and large, case hardened gears performed best but had to be ground to conformity and were therefore expensive. Solid lubricants Several delegates asked about the temperature at which solid lubricants could be used. The consensus of opinion was that up to 10% of graphite could usefully be added to a roller bearing though it was still debatable whether at low temperatures the bearing lasted longer as a result. Graphites were not recommended below 8O”C, but above 100°C there was some evidence of a longer life but also of greater clogging. One speaker, Mr D. G. Hjertzen reported limited experiments with ptfe, which has been found suitable for rolling bearings at small loads and high speeds. At least one German university was continuing the experiments. He thought it possible that small thrust washers of a lead/ptfe mixture might be developed which could compete with conventional thrust races in five years.or so. Another delegate reported successful use of ptfe on solid cages at up to 300°C provided the loads were small. Oil circulation

and temperature

On the question of cooling one unusual piece of knowledge emerged. Enough oil must circulate if the oil is to cool as well as lubricate, but it appears that the way to cool an over hot bearing is not always to increase the flow. Raising the pump pressure also raises the shear stresses in the lubricant and may cause it to break down earlier. Churning is more or less constant over a wide range of throughput so the possible solutions to overheating are to take the flow right to its upper limit or to reduce it drastically. Much better however if the designer specified a high speed tool steel in the first place, and dispensed with expensive high pressure pumps. Alternatively, the designer should try to make use of the cooling liquid as the lubricant, said Mr M. J. Neale. Only if this was impossible should he introduce a second liquid as lubricant, although clearly this was what usually happened. The logical approach was to lubricate a water-cooled mechanism with water. There was a limit to the amount of heat that could be dissipated by a given bearing, set by the oil film thickness and the heat dissipating qualities of the housing. Mr Neale also made a plea for new thought on filtration (could not some of the debris be diverted rather than forced through a filter) apart from presenting some thought-provoking diagrams to help the designer in choosing the type of bearing in the first instance (gas bearing vs plain vs rolling etc). Life of bearings On the average life of a bearing Mr Hjertzen emphasized the idea of a 90% life, ie the life that 90% of the bearings attain. Designers must grasp the statistical nature of bearing life (just as of human life) and should consult the manufacturer at an early stage in their design. Several speakers made the point that any statistical analysis was useless unless based on a complete life history. Proper analysis, however, yielded results with a smaller scatter than full-scale tests. Paradoxically a good batch of bearings often had a greater variation in life than a bad batch. Emulsion lubricants were said

by some to reduce the life of a bearing to as liffle as 48th but Mr Scott said that modern emulsions were a8 good a8 mineral oils and sometimes better. But exactly what a modern emulsion is, nobody said. Diagnosis

of incipient

failure

Four main method8 of diagnosis were mentioned Mr D. G. Hjertsen valued noise analysis and vibration analysis for rolling-contact bearings, as a mean8 of predicting failure and of detecting bad fitting. The noise intensity al80 relates to the diametric clearance within the bearing. Two other delegates reported neutron activation analysis of oil samples a8 being accurate to 0.2 parts per million and the spectrographic analysis of oils. In all techniques, the important thing was to correlate the reading8 with bearing failures and to look for change8 in rate of wear and for a pattern of failure. Conference

papers

Jost, H. Peter ‘Tribology-the science and technology of interacting surfaces-the designer’s role. ’ (Importance and definition of tribology. The committee on tribology and the tribology centres. Bad tribology; examples and cost. Materials, lubrication planning. E515m.j Neale, M. J. ‘Tribology and design of bearings and bearing surfaces. ’ (Types of bearing: rubbing, fluid film, rolling/sliding, magnetic/electrostatic, flexible. Load/speed curve for each type. Journal bearingsgraph of speed against load. Rubbing, pin and disc, bearings-graph of speed against bearing pressure.) Hjertzen, D. G. ‘Tribology and selection and installation of rolling bearings. ’ (Boundary dimensions, clearances, tolerances, type numbers and symbols. Carrying capacity, fife.

Materials, cages, lubrication, noise, mounting,

transport.)

4

7

Watson, H. J. ‘Tribology and the lubrication of gears.’ (Spur, helical and worm gears. Hybrid gears, worn gears. Dip and spray lubrication and quantity of oil. Environment and methods of choosing lubricants.)

6

Jagger,

(Housing, lubricant-whitemetals, copper base alloys, aluminium base alloys. Causes of damage. Bad design, assembly, manufacture and use.)

9

Wyllie, D. ‘Tribology and lubricant selection and application.’ (Viscosity, load-carrying capacity, life of an oil. Internal combustion engines. Synthetic lubricants. Grease. Solid lubricants. Gases.)

Dowson, D. ‘Tribology and the design of fluid film bearings.’ (Classification of fluid film bearings. Design procedures. Hydrodynamic, and hydrostatic journal and thrust bearings, steadily loaded. Gas bearings, self acting and externally pressurized.)

10

Midgley, J. W. ‘Selection and design of abrasion resistant surfaces. (Types of abrasive wear. fnfluencing factors. Relative hardness and particle size and shape of abrasive. Hot hardness. Elastic modulus. Wear resistant coatings and methods of hardening.)

Scott, D. G. ‘Tribology

THE SECOND INTERNATIONAL LUBRICATION

SYMPOSIUM

Avoidable failure. Material properties, lubricant, rolling contact,

failure.)

5

Holligan,

P. T. ‘Tribology

6

ON GAS

The Symposium was the outcome of cooperative effort between the US Office of Naval Research, which handles most gas lubrication projects sponsored by the US Government, and the American Society of Mechanical Engineer8 operating on the civil side. The ASME is by no means oil-bound in its outlook, and although it still denote8 lubrication by this term, it devotes substantial proportions of it8 own deliberations to the compressible modes. In the keynote address, Dr B. Sternlicht (Machine Tool Institute, Latham) emphasised the multi-disciplinary approach now made to gas lubrication and pointed out that over twenty disciplines were involved in the present symposium.

bearings

and plain

bearing materials and their failure.’

It is perhaps ironic that Las Vegas, home of the wheels of chance should be the selected convention venue for scientific deliberation8 on bearings. At the recent Second International Symposium on Ga8 Lubrication there papers were presented covering most classes and permutations of gas bearing and representing the state of the science as it had developed since the first symposium in 1959.

acthg

and rolling

contact bearings and their failure. ’ (Surface deterioration limits, life.

Las Vegas, LISA June 1968

Self

Enough has already been written about the potential benefit8 of good tribopractice. Mr Jest’s paper contains several example8 from his own experience and the discussion period8 brought many more to light. Whether the failure8 are due to bad tribopractice, bad engineering design or bad metallurgy hardly matters. The lessons are the same. The last word must be with Mr D. G. Scott. Asked to judge the old argument of lubricant vs material a8 the cause of a failure, he took one bland look at the disputants, who were already a little ruffled. ‘This failure’ he said, and the audience leaned forward expectantly, ‘was obviously due to lack of tribological knowledge. ’ The conference was organized by the Production Engineering Research Association and 130 delegates attended. M.I. Dawes

(aerodynamic)

The state of the art in this class was surveyed by Professor D. D. Fuller (Columbia University), who included cylindrical, plain and grooved journal bearings, pivoted pad types, compliant surfaces and various thrust bearing8 in his review.

E. T. ‘Tribology

and seals.

Subsequent papers in this session dealt with theory, design and mathematical analysis of the self-acting class. In the concluding paper of this session, Dr H. Marsh (University of Cambridge), whose work on gas bearing stability is already internationally known, discussed the effects on stability of a non-circular member in the presence of bearing flexibility. Instrument Moving

applications

into

the field

of instruments,

Mr

W. G. Denhard

(Massachusetts Institute of Technology), in a joint paper with Dr C. Pan (MTI), discussed applications of self-acting bearings to gyro-spin axes, of pressurised bearings to gyro-support

axes

and in addition

included

the relatively

new

squeeze

film bearing. The use of gas bearings in gyro8 has become a matter of vital significance in Britain, particularly to the Navy Department. Progress in this field was reviewed by Mr A. G. Patterson (Admiralty Compass Observatory, Slough). Design of externally pressurised bearings (aerostatic) This class of bearing, perhaps the most widely known, was reviewed by Dr W. A. Gross (Ampex), who listed the many permutation8 of this group. Various subsequent authors dealt with some of the theoretical analyses and concepts, including the Soviet authors L. G. Stepanyants, N. D. Zablyotsky and I. D. Sipenkov (Leningrad Polytechnic Institute) who put forward a method of theoretical investigation. Process fluid bearings including steam are actively being investigated in Britain and the U.S.A. In the final papers of TRIBOLOGY

August 1968

185