position and as a result the direction of rotation of the rear wheels would be reversed and the car would be driven backward.

Q. How are speed changes effected?

A. The aluminum face driving member and the fiber face driven wheel contacting with it are practically of the same size and when the driven member is moved to its extreme position at the left of the countershaft that member is turning at practically the same speed as the engine crankshaft. As the fiber face driven disc is moved toward the center of the driving member the speed of the countershaft decreases until center is reached where the shaft will not turn in either direction. The nearer the center of the disc the fibre faced wheel bears the greater the reduction of speed between the engine crankshaft and the countershaft. Assume, for example, that both driving and driven members were twenty-four inches in diameter. If the driven disc is brought to bear against the aluminum driving member at a point two inches from the center it would be just as though a four inch diameter wheel was employed to turn one twenty-four inches in diameter. The result would be that the engine shaft would turn six times as fast as the countershaft If the driven disc was moved so it contacted with a point on the driving member at a radius of four inches from center it would be the same as though the twenty-four inch driven disc was turned by a wheel eight inches in diameter and the countershaft would turn at one-third the speed of the engine crankshaft.

Q. What material is used for the driving member?

A. The driving member is usually a cast iron flywheel, which is faced with an aluminum alloy or copper-aluminum alloy plate attached to the driving member casting by means of countersunk head machine screws.

Q. What material is used for the driven member?

A. The driven member is usually a cast iron wheel provided with flanges between which a ring of straw board fiber is clamped, the metal portions of the wheel serving merely as a carrier for the fiber driving ring. Experiments have demonstrated that the combination of aluminum and strawboard fiber has a higher coefficient

of friction or greater adhesion and endurance than any other common materials.

Q. How long does a friction fiber ring last?

A. The life of a friction fiber ring depends upon the amount of pressure with which the driving and driven members are kept in contact. Under average conditions a fiber ring will last several thousand miles, but as they are cheap and easily renewed their

Fig. 155.-Friction Drive Set Utilizing Double Disc Principle.

replacement is not considered a disadvantage of sufficient moment to condemn that form of drive.

Q. What are the limitations of friction gearing?

A. As a rule, friction gearing will not transmit power as efficiently or as positively as the geared transmission on account of the loss of power due to slipping between the discs. The friction. disc type of transmission is not suitable for transmitting large power unless it is made very bulky and experience has demonstrated that

it is not as reliable for general application as toothed forms of gears.

Q. What advantages does friction gearing offer?

A. Friction gearing offers important advantages in that it is easily operated by the novice, is simple in construction and not liable to injury by careless operation. The face friction type offers an infinite number of drive ratios and may be so constructed as to give just as many ratios in one direction as in the other. As speed

Fig. 156.-Friction Driving Gearing of Bevel Face Type Giving Two Speeds Forward and a Reverse Ratio.

changing and clutching functions are combined in one device, the principles of operation may be easily understood by people with out previous mechanical experience.

Q. How can friction gearing be used for transmitting large amounts of power?

A. Friction gears have been devised that are capable of trans

mitting more power than the simple face friction gearing previously described by using more discs in order to obtain a greater driving surface. The form shown at Fig. 155 utilizes two driving discs A and B driven by the engine and two driven discs A and B attached to a countershaft divided in the center and supported by a bearing so that it virtually becomes two countershafts. Driven disc A which contacts with the driving disc A only turns sprocket A while driven disc B contacts with driving disc B only and drives sprocket B. The various speed ratios are obtained by moving the driven discs simultaneously, the principle of speed reduction being the same as with simple face gearing. Reverse motion is provided by auxiliary gearing.

The friction gearing shown at Fig. 156 is a form which uses beveled driving and driven members but this method of construction does not provide an infinitely variable number of speed reductions. The form shown gives only two forward speeds and a reverse motion. Three cones are mounted on the driven shaft, which is turned by the engine, each of these being actuated by independent means so that they may be brought into action with corresponding faces of the driven wheels A and B. When the low speed cone is brought into action with the larger beveled face the greatest reduction of speed is obtained. When this is released from contact and the high speed cone is pushed into engagement with the small diameter beveled faces the highest ratio of drive is provided. The reverse cone is brought in contact with the large diameter bevel surface and driven wheels A and B reverse the direction of motion of the drive sprockets which transmit power to the wheels by chain connection. It will be observed that driven wheel A is not attached to the sprocket as is driven wheel B but that the sprocket is turned by means of a pair of spur gears which reverse the motion of driven wheel A and cause both sprockets to turn in the same direction.

Q. Has friction drive ever been applied directly to rear axle? A. A form of friction drive in which the change speed and driving mechanism has been applied directly to a rear axle is shown at Fig. 157. The driving disc A in this case is the fiber faced member and slides on the squared shaft H which is driven from the engine