that the projections of one do not fit into the other, but are kept apart by a film or layer of the lubricant, the laws of Morin are not even approximately true. The study of the friction of lubricated surfaces, then, may be divided into two parts: (1) the study of poorly lubricated bearings, and (2) the study of well-lubricated bearings, the friction of which varies from to that of dry or poorly lubricated bearings.

Since the friction of poorly lubricated bearings is about the same as that of dry surfaces, we shall consider that the laws of Morin hold, and shall confine our attention to the friction of well-lubricated bearings. If the lubricant is an oil, the friction of the bearing is no longer due to one surface rubbing over the other, but to the friction between the bearing and the oil, and to the internal friction of the oil. That is, the oil adheres to the two surfaces and its own particles attract each other, and the motion of one of the surfaces with respect to the other changes the positions of the oil particles. It is to be expected then that the friction of an oiled bearing will depend upon the viscosity of the oil, upon the thickness of the layer interposed between the surfaces, and upon the velocity and form of the bearing.

The coefficient of friction is no longer constant, but varies with the temperature, velocity, and pressure. The variation of the coefficient of friction of a paraffine oil with temperature is shown in Fig. 170 when the pressure on the bearing is 33 lb. per square inch and a velocity of rubbing of 296 ft. per minute. It is seen that the coefficient of friction decreases with increase of temperature until a temperature of 80° F. is reached, when it increases rapidly.

This means that above this temperature the oil is so thin that it is squeezed out of the bearing, and the conditions of dry bearing are approached. The temperature at which oils show an increasing coefficient of friction is dif

ferent for different oils, even at the same pressure and velocity. The curve in Fig. 170, however, may be regarded as typical of all oils when the pressure and velocity

are constant.

the relation between the coefficient of friction and temperature for a sperm oil in steel bearings when the velocity of rubbing is 30 ft. per minute.

PRESSURE, LB. TEMPERATURE, COEFFICIENT PRESSURE, LB. TEMPERATURE, COEFFICIENT
PER SQ. IN. DEGREES F. OF FRICTION
DEGREES F. OF FRICTION

PER SQ. IN.

It is seen that for a pressure of 200 lb. per square inch as the temperature increases from 90° F. the coefficient increases, indicating that the temperature of 90°, for the given pressure and velocity, was above the temperature at which the oil became so thin as to be squeezed out and the bearing to approach the condition of a dry bearing. For a constant temperature 110° F. and 90° F. the coefficient is seen to decrease with increase of pressure up to a certain point and then to increase. This is a typical behavior of oils when the temperature is constant and the pressure

varies.

At speeds exceeding 100 ft. per minute, the same authority found "that the heating of the bearings within the above range of temperatures decreases the resistance due to friction, rapidly at first and then slowly, and gradually a temperature is reached at which increase

takes place and progresses at a rapidly accelerating

rate."

The relation between the coefficients of rest and of motion as determined by Professor Thurston for three oils is given below. The journals were cast iron, in steel boxes; velocity of rubbing 150 ft. per minute and a temperature 115° F.

It is seen that the coefficient of friction at starting is much greater than at stopping, and that these are both much greater than the value at a speed of 150 ft. per

minute.

For an intermittent feed such as is given by one oil hole, without a cup, oiled occasionally, Professor Thurston found for steel shaft in bronze bearings, with a speed of rubbing of 720 ft. per minute, the following coefficients

of friction:

The results show that the coefficient decreases with the pressure within the range reported, but that the results are considerably higher than those for well-lubricated bearings. He also found in connection with the same tests that with continuous lubrication sperm oil gave the following coefficients:

The results of tests of the friction of well-lubricated bearings are summarized by Goodman (Engineering News, April 7 and 14, 1888) as follows:

(a) The coefficient of friction of well-lubricated surfaces is from to that of dry or poorly lubricated surfaces.

(b) The coefficient of friction for moderate pressures and speeds varies approximately inversely as the normal pressure; the frictional resistance varies as the area in contact, the normal pressure remaining the same.

(c) For low speeds the coefficient of friction is abnormally high, but as the speed of rubbing increases from about 10 to 100 ft. per minute, the coefficient of friction diminishes, and again rises when that speed is exceeded, varying approximately as the square root of the speed.