Bearings That Need No Maintenance
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ALTHOUGH it will not emerge for some years, a completely maintenance-free commercial vehicle has been the aim of .designers for many years. Lubrication is possibly the main factor concerned.
For some years self-lubricated bearings have been applied on a limited scale to the automotive field, but the lubricant-free bearing can offer great advantages over a conventional self-lubricated type.
A bearing devoid entirely of lubricant can be used where the presence of any conventional lubricant is undesirable, impracticable or unreliable, and where temperatures are either too high or too low for effective oils and greases. The Glacier DU is a completely dry bearing making use of polytetrafluoroethylene, and is derived from an earlier type of bearing developed in 1954, by the Glacier Metal Co., Ltd., Alperton, Middx.
Polytetrafiuoroethylene is a fluorocar: bon plastic with a unique combination of properties which include a low coefficient of friction, wide service temperature range, chemical inertness, and immunity to solvents and water. On the other hand, it ha `s poor mechanical strength and high
c22 thermal expansion associated with low thermal conductivity. It is also costly.
In the Glacier application these disadvantages are overcome by mixing the p.t.f.e. with a fine lead powder and forcing the mixture into a porous tin-bronze layer. This is laid on a tin-plated steel backing strip. On top of the impregnated bronze interlayer an overlay coating of p.t.f.e.-lead mixture is applied.
This overlay gradually disappears, because, when the bearing is used, the surface film of the p.t.f.e. mixture becomes attenuated and momentarily the bronze particles rub against the metal of the rotating or oscillating item being carried by the bearing. The heat generated causes the p.t.f.e. mixture to expand and extrude from the porous interlayer, immediately smearing over and healing the bearing surface. Similarly, the heat generated causes the bearing material itself to expand and it is when this 'expansion is substantially greater than that of the bearing housing— as is the case with plain plastics materials —that the bore of the bearing will close in on the shaft and seizure results.
Interference Fit Maintained
The DU bearing, however, has the thermal conductivity of an oil-impregnated bearing, or about 60 per cent, that of solid bronze. The thermal-expansion coefficient lies midway between that of steel or cast iron and aluminium or brot4e, so that journal bearings will not lose interference fit in any normal type of housing.
It is claimed that a DU bearing can be used satisfactorily at all temperatures between minus 200° C. and plus 280° C. and that the performance varies little over this range. Rapid fluctuations of temperature have no effect on it.
The bearing is also generally more tolerant of dirt-laden atmospheres than are oilor grease-lubricated units and can operate satisfactorily when exposed to abrasive dusts. Nevertheless, sealing is advisable to guard against jamming of the bearing. .
In addition to reducing friction, a normal lubricant helps to protect from corrosion the.item carried in the bearing. Because there is no conventional lubricant in a DU bearing, a steel item would be susceptible to corrosion unless protected. There is no reason, however, why a conventional lubricant cannot be used in conjunction with a DU bearing purely
to resist corrosion. Indeed, in certain cases lubricant can improve the performance materially by carrying heat away from thc bearing surface.
Protection Against Corrosion To guard against mildly corrosive surroundings the back and edges of the bearing are tin-plated. but if liable to be exposed to seriously corrosive liquids these parts may be further protected from attack by electroplating with lead or other corrosion-resistant metals. Standard DU bearings, however, are suitable for use in pumps and metering devices dealing with petrol, paraffin and other industrial solvents without the need for". additional protective treatment.
For normal applications where there is a substantial degree of movement, a design limit.of 4 tons per sq. in. is recommended fora DU .bearing, but the yield strength of the bearing in compression is a minimum of 20 tons per sq. in., and loads apprbaching this figure can be imposed if there is little movement.
Although it is six years since Glacier produced their first dry' bearing—which was known as the DP and was suitable for, only light loadings--the more successful DU type is only slowly being adopted by Commercial-vehicle manufacturers. The largest users are A.B. Scania-Vabis. who employ DU bearings for the thrust washers in king-pin assemblies, front and rear brake-camshaft bearings, brake-shoe anchor-bolt bushes, and various controllever and pedal bushes.
Well-known Users
In Britain. A.E.C., Ltd., fit DU bearings in the king-pins of their Bridgemaster buses. Albion Motors, Ltd.. and Commer Cars, Ltd., use DU thrust washers in kingpin assemblies. Bristol Commercial Vehicles, Ltd., employ bushes of this type for brake-shoe anchor pins. Dennis Bros., Ltd., use them on brakeand clutch-pedal cross-shafts and Fodens, Ltd., fit them in accelerator linkages.
Both Guy Motors, Ltd., and Transport Vehicles (Daimler), Ltd., employ DU bushes and thrust washers for king-pins and further bushes are incorporated in Guy accelerator linkages and the brakeand clutch-pedal assemblies. Leyland Motors, Ltd., fit such bushes to the levers of their centrifugal clutch.
DU bushes are available in bore sizes up to 2 in. diameter and thrust washers are offered in thicknesses of up to 0.091 in. and outside diameters of 3+ in. Strips 4 in. wide and 18 in. long of up to 0.091 in. thickness are also available. In all cases the p,t.f.e. used is supplied by Imperial Chemical Industries, Ltd., under the trade name Fluon.