Load-speed Limits of Oil-retaining Bronze Bearings
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Specific. Capacities in Terms of Loads and Speeds for Various Bore Sizes of Bearings Produced by the Bound Brook Concern I N connection with the use 'of the special " Comp° " self-lubricating oil-retaining bronze bearings, of particular value for motor engineering, much interest attaches to the subject of the maximum loads and speeds of revolution that are permissible.
It will he remembered that these bearings are a product of Bound Brook Bearings (G.B.), Ltd., Witton, Birmingham, which-concern is associated with Hardy Spicer and Co., Ltd. A typical-composition is approximately 88 per cent. copper, 9.7 per cent, tin, 1.4 per cent, graphite, and 0.9 per cent. other materials, and they are made by die-pressing the pure electrolytic metals in finely divided state. Consequently, the bronze is porous because of the micro-spaces between the particles, and up to 30 per cent, by volume of oil can be absorbed, although the metal is almostaas hard as if cast, and 'has a 'crushing strength for example of about 70,000 lb. per sq. in.
When the pressure or temperature varies in the bearing, the oil in the metal exudes, giving highly efficient lubrication for a very long period, bf ten as long as the life of the bearing itself. Under extreme pressure, however, the oil is squeezed out, and consequently there is theoretically a maximum combination of bearing pressure and shaft velocity above which, the bearings will not function because
of the ,lack of oil. This, however, obtains only under purely abnormal circumstances and if the limit needs to heexceeded ordinary lubrication can be employed; that is by the external .addition of oil.
The maker has prepared a chart for the determination of permissible loads and speeds for various shaft diameters, based upon an empirical forrfiula determined as the result of long experience, namely, PV=50,000 where P= the load or pressure in lb. per sq. in. on the actual bearing area and V= shaft velocity in ft. per min.
These curves relate to seven standard shafts between the sizes 1-In. and 2-in, diameter, and, in general, as already indicated, if the pressure on the bearing surface multiplied by the velocity exceeds 20,000, additional lubrication must be given. Obviously there are very few bearings in which such extremely severe conditions exist.
With low pressure and small shafts, speeds up to 25,000 r.p.m. can be used, and there is quite a number of installations running in which such speeds are normal. Also it will be noticed that with low-speed .shafts enormous bearing pressures can be taken without difficulty, such as 2,000 lb. per sq. in. with a shaft velocity of 25 ft. per min. When the shaft velocity is 50-100 ft. per min. a high bearing pressure, up to 500 lb. per sq. in., is allowed and so on,* whilst the chart covers limits of up to 800 lb. per sq. in. bearing pressure and 15,000 r.p.m.
It will be understood, of course, that such a chart relates to normal average 'good conditions as regards heat dissipation from the bearing, surrounding atmospheric temperature and normal dust content in the air.
In general, these oil-retaining bronze bearings will carry a load equal to well-lubricated cast bronze, and such bearings can be obtained with a length of times the diameter, whilst hundreds of standard intermediate sizes are available, all made to extremely accurate dimensions, as indicated in a previous article.