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).
Technical explanation of how lead incorporated into brake linings improves braking performance by regulating friction and reducing wear.
| Identifier | ExFiles\Box 146\2\ scan0083 | |
| Date | 25th January 1937 guessed | |
| -2- moving surfaces, or the coefficient of Kinetic friction. Now hitherto, from the old wooden brake shoes to the present-day actual linings with asbestos and brass base, the wood, and especially the metals in contact assured a coefficient of static friction higher than the coefficient of kinetic friction. From which we had rough braking at low speeds and a less powerful operation at higher speeds. Conversely, continuing the process of slowing-up, the rather high static coefficient tends to lock the brakes or to make them skid at the end of braking. What had to be obtained was a predominance of the coefficient of kinetic friction, giving maximum power at high rolling speeds and allowing, per contra, an application of the brakes which would be smooth and progressive. Lead has solved this problem. The Lead--and this is its essential function--decreases the static and increases the kinetic coefficient of friction. Incorporated in the brake linings it regulates their action and makes them smoother in application. The action of the lead on the drum material is particularly successful. Under the action of friction the lead with which the lining is impregnated, coats the metallic surface of the drum with a very thin, but very adhesive film, which stops up the pores, fills in cracks and gives to the metal a dull gloss similar to sheet iron rubbed with black-lead. The first point: The coefficient of friction is thus regulated. It no longer is subject to abrupt variations, the increases being still more dangerous than the decreases, because these are the first things which promote vibrations, skids, locked wheels and fatigue in the working parts. It is moreover, curious to see how very small amounts will act. It is almost Homeopathic ( the system is analogous to inoculation, apparently). In the laboratory the influence of the lead is felt from the first turns of the flywheel. But, during the test, briskly rub the flywheel with a clean cloth: you thus remove, in a few seconds the film of lead left on the flywheel by the tested lining. Immediately the needle indicating the coefficient of friction swings about. Take the cloth away; the efficiency of the lead is restored as rapidly as it was taken away: the needle steadies again. For all that, the white cloth is hardly tinted with grey, which shows that the film of lead which suffices to regulate the braking, is slight. The second point is: the lead decreases, and even checks altogether, metallic wear in the drum. With the present linings, a grain of sand, a minute chip of steel, embedded in the lining, begins to attack the drum: a minute piece of steel is formed; friction and heat work it, hardening it and making it a cutting tool; at each revolution it digs out more small pieces of metal which the heat welds into an ever-increasing grain of steel. Now lead itself reacts unfavourably towards this welding process. Trade Welders call lead 'Welding Poison'-- it is one of the foreign bodies which will render a welded joint fragile. | ||