IAVDUSTRIALA S D E S G I S E E R I N G CHEMISTRY
May, 1926
497
The Role of Graphite in Lubrication By F. L. Koethen ACHESON
GRAPHITE
CO
This paper describes tests on a Riehle machine to determine the amount of pressure required to rupture the fluid film in a 3-inch bearing rotating at constant speed, and measurements of the coefficients of friction of weighted sliders, in each case using oil with and without suspended graphite. Both methods of test showed that under conditions of ruptured film lubrication with some solid-to-solid contact, the presence of graphite substantially reduced the friction.
VBRICATIOK consists of two separate problems: (1) where conditions are such as to permit perfect fluid films to support the load, in which case viscosity is the controlling factor; and (2) where the fluid film has broken and actual rubbing contact takes place. Lubrication in this second case is accomplished by adsorbed substances of a semiplastic or solid nature which protect the surfaces and prevent clean metallic contact, which is the main cause of friction. I n this stage viscosity of the oil is of little importance, but the problem is a chemical one, different substances having varying individual power of adhering to the metallic surfaces and preventing metal-to-metal contact. Although this broken film stage may appear to be relatively unimportant, i t really affects operation seriously because of the damage done when the film does break, and also because many bearings in actual operation have one or more rings or streaks in which the film has broken. These streaks carry most of the load and produce most of the friction. The literature has contained many general statements of the value of graphite, but i t was considered worth Yhile to determine i t definitely in the light of the modern understanding of the problem. Various methods of determining friction were studied.
L
Friction by Riehle Tester
The balanced arm type of friction testing machine, such as the Riehle testing machine (Figure 1) and the Thurston, is greatly favored by practical men because it seems to approximate actual conditions very closely. It has been somewhat in disrepute lately because when operating on
SIAGARA FALLS, hT.Y.
with 3-inch steel shaft and cast iron bearing blocks was operated to constant conditions with a 300-viscosity motor oil; then the feed was changed to graphited oil and the increase in bearing pressure possible without breaking the film was determined. After running 70 hours with plain oil, the conditions were constant at 19 pounds torque and 100 pounds per square inch pressure. Speed was constant a t 196 feet per minute. Repeated trials a t greater pressure than 100 pounds per square inch had resulted in greatly increased friction and scoring of the surfaces. After running long enough to secure constant conditions again, the figures given represent the best possible with this oil and these conditions. Graphite was then introduced into some of the same oil as above and the bearing run long enough to form a graphited surface. Colloidal graphite (Oildag) 13 per cent was used for 2 hours, then 6 hours with 6.25 per cent graphite, and 8 hours with 0.25 per cent graphite. Conditions then
Figure 2-Friction
Slider Made from Bearing Balls
seemed again constant. The torque weight required to balance the arm was only reduced to 16, but an increase of bearing pressure up to 180 pounds per square inch was made before the torque exceeded the set figure of 19 pounds and the film commenced to break down. This indicates that the machine while running on plain oil gave results nearly those for internal friction of the oil and that after a graphite film had been deposited on the surface, although the oil still set the lower limit for the friction, the pressure on the bearing could be increased to 180 per cent of its original value before the transition began to take place into the stage of broken film operation. Friction by Sliding Weight Method
ue
G Figure I-Riehle
Friction T e s t i n g M a c h i n e
perfect film lubrication it registers the viscosity of the oil as a minimum frictional reading and it is impractical to operate it under broken film conditions. With proper discretion in certain special cases, however, comparative results of undoubted accuracy can be obtained. A Riehle machine
Success with the rotating type of friction testers depends on recognizing when the film breaks and i t is impractical to test further than to incipient break. Another method, measuring the force necessary to draw a slider over a plane surface, actually tests the surface properties of the bearing metals without appreciable influence by the viscosity of any oil present. Although it does not have the appearance of reproducing actual operating conditions as completely as a regular testing machine, it is really very reliable and satisfactory. A number of investigators have used metal buttons, plates, and lens-shaped sliders. The writer used a slider composed of three steel bearing balls ( l / 2 inch diameter)
lNDUS?'IZIAL B.VD & V ~ ~ I X E l 9 M N GCHE'MISTIZY !
498
claznped l i r d y t
