consideration of the amount of wear likely to occur, and the allowable pressure per square inch of surface should not be greater than 50 lbs., whence we have for the width of the cone surface W

In the case of clutches of the expanding-ring type, all

FIG. 27.

the calculations are essentially as above, but the arrangement of the levers requires consideration. Usually the ring is expanded by screws. In the majority of cases the screws are right- and left-handed, as shown diagrammatically in Fig. 27. The clutch seen in Fig. 28 (Benz-Parsifal clutch) is an example of the employment of single screws. The mechanical advantage or gain in power from the employment of the levers and screws can be found from

=

in which A = the gain in power, s = the pitch of the screw in inches, and the length of the lever in inches. This formula is applicable to clutches in which the screws are single as to the pitch, as Fig. 28. For those in which

the screws are of equal but opposite pitch (Fig. 27) the expression should be halved, and so becomes

The last two formulæ are for clutches having right- and left-handed screws; for those in which only one screw or

FIG. 29.

two screws of the same pitch (Fig. 28) are in question these formulæ become

It will be found that clutches designed from the foregoing considerations will be somewhat larger for a given power than is usually the case in an automobile, but the writer is convinced that the results obtained by more liberal clutch dimensions fully justify the increase in size. Automobile clutches are subjected to a lot of wear from the frequency with which they are put in and

out of engagement, and this fact has received due consideration in the formulæ.

In order that the clutch shall take up its load gently and start the car without shock, it is the practice to place springs under the leather of the one cone, in places, to make the engagement gradual, and thus, by allowing a

certain amount of initial slip, render the starting easier. One arrangement of these springs is seen in the clutch shown in Fig. 29. Openings are made through the metallic portion of the cone through which bent sheet-steel pieces project. These tend to force the leather cover of the cone

FIG. 31,

outwards by the pressure exerted by the small helical springs shown on either side of the opening.

The majority of clutches are designed so that the resultant of the axial pressure due to the spring is contained within the clutch itself, and has no effect upon the