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Q. How are circulating fans driven?

A. Air circulating fans may be driven by positive or flexible means depending upon the preferences of the designer. Fans have been operated by positive gearing, such as bevel, spur and spiral gears, or silent chains and by flexible gearing, such as round steel wire or leather belts and flat and V leather belt. When positive gearing is employed in connection with the usual form of built-up fan it is customary to interpose some form of slipping drive between the fan hub and the gearing in order to relieve the fan of any sudden jerks which might result in throwing off a fan blade when the motor is started or stopped suddenly or when accelerated too quickly. When flexible driving means such as belts are used some form of automatic belt tension regulator is included in the design in order to keep the belt tight and insure positive drive. The flat belt is widely used on account of its simplicity and cheapness, while a V belt is utilized by some designers because it will furnish more positive drive than the flat belt and its tension does not have to be so carefully regulated. A V belt will drive when loose, whereas the flat belt is apt to slip under the same conditions.

LESSON FIFTEEN

USE OF CLUTCH AND VARIOUS TYPES DESCRIBED

Q. What is the clutch and where is it generally placed?

A. The clutch is used to transmit power from the engine to the gearset and is a form of drive coupling that may be released when it is desired to allow the power generating and energy transmitting members to run independently of each other. A typical power plant, clutch, and gearset assembly is outlined at Fig. 143 with all parts clearly indicated. The power generated by the explosion of the gas against the pistons of the motor imparts a rotary movement to the crankshaft, and from thence to the flywheel which is rigidly connected thereto. The motor flywheel serves as one member of the clutch and is machined with a taper into which the cone or male member of the clutch fits. The cone is held in frictional contact with the flywheel rim by a spring and when the parts are in the positions indicated practically the full power delivered to the engine crankshaft will be transmitted through the clutch cone to the clutch shaft which is joined to the main shaft of the gearset by a positive drive coupling. If the cone is pulled away from the flywheel rim by means of the clutch release pedal provided for the purpose, the engine crankshaft may run independently of the gearset.

Q. Why is a clutch needed with a gas engine and not with a steam or electric motor?

A. A steam or electric motor may be started under load because the real motive power, which is steam or electricity, as the case may be is under pressure in a suitable container and this working medium may be directed to the power plant and sets it in motion against its load. In order to get a start it is possible to supply an excess of

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power if this is necessary. In a gas engine the power is generated by explosions occurring directly in the cylinders and it is not possible to start an engine of this kind under full load. In order to start the engine, it should be free from the retarding influence of the car, and for this reason, it is necessary to interpose the clutch between the source of power and the point of application to enable the engine to be started independently of the vehicle it propels. Then again it would be inconvenient to be constantly stopping the motor every time one desired to stop the car, so the clutch enables one to stop the car and the motor can be kept in operation until such time it is desired to start again, when the clutch may be re-engaged and the motor power again directed to the traction members.

Q. What is the principle of clutch action?

A. Most clutches operate because of frictional adhesion of various substances under pressure. In some forms the power is transmitted by the actual mechanical interlocking of two members, one which has projecting points and the other which has depressions made to receive the projections of the opposing member. These clutches are known as positive clutches, while those which depend on frictional adhesion are termed "friction clutches." The essential parts of all clutches are the same, consisting of a driving member attached to the source of power, a driven member attached to the power transmission member and suitable means to hold this into engagement when it is desired to transmit the power and separate them when it is necessary for the engine and gearset to run independently of each other.

Q. What are the principal types of friction clutches?

A.

Friction clutches may be divided into four main classes: the cone, plate, multiple-disc and band forms.

Q. Describe construction and name important parts of cone clutch.

A. A typical cone-clutch assembly with the main parts clearly indicated is outlined at Fig. 144 and an optional arrangement of parts sometimes used is depicted at Fig. 145. The driving member of the clutch is formed by the motor flywheel, while the driven member is a cone that is held in contact with the tapering inner

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