tric motors, which derive their power from stored energy, are capable of operating at an overload and while their speed of rotation may be reduced it is possible to obtain the rated horse-power by admitting more steam to the cylinder or more current from the storage battery to the windings of the motor. Either the steam engine or the electric motor is capable of being reversed; by changing the direction of flow of the energizing agent in the case of the electric motor, by a simple shifting of valves in the steam engine. This flexibility of steam and electric power enables the manufacturer of these cars to dispense with change speed or reverse gearing.

Fig. 154.-The Lambert Friction Disc Combined Clutching and Speed Changing Mechanism.

Q. What is the simplest method of obtaining speed reduction?

A. The simplest method of obtaining speed changes and one that will provide a large range of speed variations is by using friction discs or, as they are sometimes called, friction gearing.

Q. What is friction gearing?

A. Friction gearing in its simplest form consists of bringing two wheels into frictional contact with each other and holding them in

contact with sufficient pressure so the driving wheel will cause the member against which it bears to rotate as well. The simplest form of friction gear is the face friction drive.

Q. Describe parts of face friction gearing.

A. A typical face friction gear that has given good results in practice is shown at Fig. 154 and all parts are clearly depicted in the illustration. The engine drives an aluminum face driving member with its face at right angles to the engine crankshaft. The driven member is a fiber faced wheel carried by a cross shaft which extends across the frame and which is carried in suitable bearings attached to the frame sides. This countershaft carries the driving sprocket and is parallel to the face of the aluminum driving member. The fiber face driven wheel can be moved back and forth across the face of the driving member by suitable leverage connected with the control handle. The direction of countershaft rotation and its speed relative to the engine crankshaft is determined by the position of the fiber face driven wheel relative to the center of the driving member.

Q. How is clutching effect obtained?

A. The aluminum face driving member is capable of being brought into contact with the driven wheel or moved away from it at the will of the operator. The amount of pressure between the two members may be varied by a pedal. When the discs are brought into contact the drive is taken up and power is transmitted from one member to the other and as soon as the discs are separated the drive will be interrupted. The operator can control the amount of pressure and consequently the amount of drive to a nicety and consequently a gradual power application may be obtained.

Q. How is the car reversed?

A. The process of reversing is easily accomplished by pushing the fiber face driven wheel over to one side of center of the aluminum face driving member. As indicated in Fig. 154 the driven wheel is placed to the left of center, which is a position giving forward drive. If moved toward the right of center or toward the driving chain sprocket the countershaft will revolve in an opposite direction to the way it rotates when the driven disc is in forward drive

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