CHAPTER X

SCHEMES FOR POWER TRANSMISSION

Since an internal combustion engine cannot develop power until it is running at a certain speed it will not start on full load like a steam engine.

It is evident, then, that some sort of a device must be used to throw the load on gradually after the engine is running at its full working speed. There are three general schemes used for this purpose, and these are (1) the tight and loose pulley, (2) friction wheels and (3) clutches.

The Tight and Loose Pulley.-The simplest but far from being the best way to connect an engine with one or more machines is to put a tight and a loose pulley, see Fig. 70, on the machine and belt it either to the pulley of the engine direct or to a pulley on a line shaft which is driven by the engine.

When this arrangement is used, before you stop the engine you shift the belt onto the loose pulley. Now after you start the engine and it has gathered momentum enough you can gradually shift the belt from the loose pulley onto the tight pulley that is keyed on the driven shaft of the machine.

The Use of Friction Wheels.-Wheels in Alignment.-A simple and effective way for gradually transmitting the power of a small engine to a machine is by means of friction wheels, and, further, in some types of these wheels the speed of the ma chine can be varied within wide limits as the exigencies of the work may require.

FIG. 70. THE BELT DRIVE. A TIGHT AND LOOSE PULLEY

All through this book I have preached the doctrine of lubrication, as this is the last step in reducing friction to a minimum in machinery, but while friction is a thing to be gotten rid of in engines it can be utilized to advantage when it is applied in the right way and one of these is in friction wheels.

In its simplest form a friction wheel transmission consists of two wheels whose outer faces, or peripheries, as they are called, set in the same plane, and which can be pressed together with more or less force,

as shown at A in Fig. 71. The wheel which is connected with the crankshaft of the engine is called the driving wheel and the wheel fixed to the line or propeller shaft which it drives is called the driven wheel, and it is due to the friction, or bite as it is called, between their surfaces when in contact with

A

B DRIVEN WHEEL

FIG. 71. FRICTION WHEEL DRIVES

A, Friction Drive with Wheels in Alignment. B, Friction Gear Drive. C, Friction Drive with Wheels at Right Angles.

each other that the first wheel makes the second one revolve.

To get the best tractive effort by means of this combination one wheel should have a smooth iron face and the other should be covered with leather or rubber. To make friction wheels of this type bite still better the faces of both wheels are often grooved, as shown at B, when they are called friction gears.

Although in friction wheels there is but a very small point of contact between them, yet it takes only a slight pressure to bring them together so that the driving wheel will transmit nearly all of its power

to the driven wheel when the latter has been speeded up and, consequently, there will be very little slippage, but, of course, the action is not positive.

Wheels at Right Angles.-It is possible to do many things with friction wheels when they set at right angles to each other which could not be done with belted wheels or with toothed gears, as cogwheels are sometimes called.

As an illustration, suppose you want to change the direction of the rotative power of the crankshaft of your engine at right angles with it, as shown at C, then all you have to do is to use a driving disk or wheel the side, or flat face of which is covered with leather and fix another wheel, or pulley on a shaft at right angles to and so that it will press on it.

Now, when the engine runs and you apply pressure to the friction wheels the power will be transmitted from the driving wheel to the driven wheel and the direction of the rotation will be changed 90 degrees; moreover, if you will fix the driven pulley on its shaft so that it can slide on and yet turn it and then move it from the center of the driving wheel to its circumference, the speed of the driven wheel will be raised from 0 + to maximum.

1

'This is done by using a feather key to secure the wheeï and the shaft together. That is the shaft and the inside of the hub of the wheel have shots, called keyways, cut lengthwise in them; a straight key is fixed in the keyway, of the wheel and when the latter is slipped over the shaft it makes a sliding fit in the keyway of it.

Further, if you shift the driven wheel across the center of the driving wheel from one side to the other the direction of rotation of the driven wheel will be reversed. This is the kind of a power transmission device that is installed in the Metz motor car for connecting the propeller shaft with the crankshaft of the engine, and by means of it the usual clutch arrangement and transmission gears are done

FIG. 71-D AND E. FRICTION WHEEL DRIVES

D, Friction Drive with Beveled Driven Wheel. E, Friction Drive with Driven Shafts in Alignment.

away with and a variable change of speed is gained.

Finally, you can bevel the driven wheel, as shown at D, and in this way change its angle of rotation in relation to the rotation of the crankshaft, and make it revolve at whatever angle it may suit your purpose the best.

But setting another wheel on the opposite side of the driven pulley, as shown at E, with its face parallel with the face of the driving wheel and with its shaft in a line with the crankshaft, then the latter