From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article from 'The Autocar' magazine discussing the principles of bearing lubrication and potential failure points.
| Identifier | ExFiles\Box 76\4\ scan0123 | |
| Date | 12th August 1911 | |
| THE AUTOCAR, August 12th, 1911. 291 bearing, we must obviously introduce the oil at the point of minimum pressure, otherwise if an oilhole be drilled where the maximum load occurs the effect will be that instead of oil being introduced at this point it will simply be extruded, due to the very high pressures generated, which may easily be up to 200 lb. per square inch. When a bearing is designed, the fundamental figure to be borne in mind is the loading per unit area upon the brass, the area being reckoned as the product of the diameter of the shaft and the length of the brass. This area can therefore be obtained by either making the bearing of certain length when its diameter is fixed or of any reasonable diameter when the available length is limited. The load supported for any given speed must not exceed the limit at which the oil film is broken through, and this depends to a certain extent upon the nature of the lubricant. Any book on machine design will give the standard permissible loading for an ordinary bearing, but as motor car practice departs so widely from ordinary engineering theory on account of the higher quality of materials generally employed to obtain lightness, and the special care taken in the matter of lubrication, textbook figures will not generally hold good for such work. To the ordinary motorist owning a high-class or well-designed car the design of the bearing must be taken for granted, but the nature and method of application of the lubricant is often under his control. Let us therefore investigate one or two points of considerable importance in the maintenance and life of a plain bearing. The failure of any lubricating system with the consequent result of seized or fired bearings is generally due to one of the following causes: (a) Insufficient supply of the lubricant, due, in a forced system, either to the choking up of an oilway or the failure of the oil stream to reach those bearings which are furthest remote from the pump. This failure may be due to a too free means of efflux for the oil from those bearings which are situated nearest to the pump and generally at points of maximum pressure on the oil line. (b) Loss of nature in the oil itself, or a reduction of its viscosity caused either by excessive heating or by inherent defects in the oil itself; this can only be guarded against by care in the choice of the oil, but the viscosity of the lubricant when cold is not by any means an indication of this same property when the oil is heated up to its working temperature. (c) Unsuitably placed admission devices for the oil supply, as, for instance, the oil holes entering the bearings at points of high pressure or oil grooves cut in such positions that they do not tend to distribute the oil properly over the loaded surface of the bearing itself. There is a considerable diversity of opinion as to the best position and shape of oil grooves upon a bearing, and as the oil generally enters the bearing at some point near its centre these grooves should be so arranged that the oil is distributed across a region of low pressure. Preferably several such grooves should be so cut that the oil is distributed over an appreciable portion of the circumference as well, so that as the point of minimum pressure moves around the bearing under the variation of working conditions an adequate oil supply may be insured at the point of minimum pressure. Transverse grooves of this kind should be connected by a circumferential groove which should be staggered to such an extent that local wear is not set up on the shaft journal. When forced lubrication is employed the oil grooves are necessarily much more simple than when a purely splash system is relied upon, and, in addition, the load, which can be carried by a bearing served with forced lubrication, can be much higher than in the other system per unit of area. (d) OVERLOADING.—The load which a bearing will carry depends upon several factors, the principal of which are the time of duration of pressure upon it, the facilities for cooling, the method of oil supply, the materials in contact, and the nature of the lubricant. When a bearing is continuously loaded, as in a crankshaft bearing, the intensity of the load must be less than when intermittent loading only is borne, because the time question is an important consideration. It is obvious that if a film of oil be interposed between two metallic surfaces and a pressure comes upon those surfaces, the oil will be squeezed out, if the intensity of pressure be great enough. Naturally a certain amount of time elapses before the whole of the oil is extruded, and if the time interval be short enough, a load which would be fatal when continuously applied could be easily carried when its duration is only intermittent and for very short periods. For this reason such bearings as are fitted at the ends of connecting rods will carry safely a far higher load per unit area than the continuously loaded crankshaft bearings. In the former case the oil has not time to squeeze out entirely from between the surfaces in contact during the time of application of the load. (To be continued.) (upside down text at bottom) R.R. 235A (150 H) S.A.A. 44 S-12-14) G.{Mr Griffiths - Chief Accountant / Mr Gnapp} 0855. | ||