R. G. GRISWOLD

The object of a friction clutch is to provide a ready means of throwing a machine or other mechanical device either into or out of engagement with the source of power. The requirements for the general run of clutches are sufficient area of contact surface to cause the two engaging parts to hold without undue pressure ; ability to engage slowly and without taking hold suddenly, depending upon the pressure used, and sufficient strength to transmit the full power required without slip.

The number of designs is legion, and many of them are exceedingly clever, but the simpler a clutch may be made generally the better. The fewer parts used, consistent with the work required, the fewer joints there are to wear loose and rattle. Perhaps the simplest design is that of the cone fitting into a similar cone, either piece being lined with leather or other similar material, or finished contact surface being left so as to work metal to metal.

But with this design of clutch, which is used to a great extent on automobiles, it is necessary to make it of very large diameter in order that it may hold the load. The cone is generally forced into engagement and held there by a spiral spring, the pressure of the foot on a lever releasing the cone and pushing it back clear of the seat. These clutches depend for their successful action upon the condition of the leather or other lining material, and the presence of oil on the surface lessens the friction very considerably. Then, if the clutch is thrown in slowly, the swiftly revolving surface generates sufficient heat to burn the lining, or at least to glaze it and render it unfit for service. Where it does act well, however, it is a very satisfactory device, but can be very easily injured.

With a clutch operating metal to metal, the surfaces in contact may be coated with a film of oil or other lubricant which will prevent their cutting during the periods that slipping does occur, as it always does

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when the clutch is thrown in. But the greatest feature, probably, of any metallic contact-surface clutch is the positive action that may be secured with greater ease than on conical types held in by the action of a spring. For instance, the clutch about to be described can be expanded with such pressure that the keys in the shaft can be easily sheared without slipping the two members, and some recent experments with this particular clutch positively determined its excellence.

A shaft was fitted in a lathe carrying one of the members. In the clutch was fastened the driving member. The back gears were thrown in and the belt put on the largest cone, giving a tremendous pull. The lathe was started and tests made to determine the holding power of the clutch. As the sliding cam was pushed in with a tool held in one hand, the friction began to increase with a perceptible slowing down of the spindle, and when the cam was forced entirely home the lathe was stopped and the belt slipped off the pulley. The test was especially severe as the speed of rotation was very low, not more than ten or twelve turns per minute, while the clutch is designed to transmit about twenty-horse power at 1,000 revolutions per minute. One feature was very notice-able, and that was the readily controlled grip. In a great many clutches, it is difficult to throw them in easily as they take a full grip or none. As the cam was thrown in the expanding ring gradually took hold and the slipping was smooth and without cutting, owing to the lubricated surfaces. Very little pressure was required to operate the cam, and the pressure of a finger would make the clutch grip, while very little more served to lock it fast.

The driving member is a plain drum of cast iron, turned on the inside and outside peripheral surfaces. It is keyed to the engine or other driving shaft by a 5-16 in. key two inches in length. It is also further secured from slipping by the 1/4 in. set screw shown.

The expanding member is also of cast iron, and consists essentially of a ring of metal carried by a spider. This spider is also keyed to a shaft, and this shaft is provided with a 1/2 in. journal running in a bearing in the driving shaft. This serves to center the inside ring which has about 1-64 in. clearance. As the clutch is generally engaged, leaving very little time when the engine is running idly, there is little work for this bearing to do, hence little chance for wear or cutting. It is, however, provided with a Babbitt metal lining, and the hole is continued to a sufficient depth to allow a lump of some bearing grease to be put in before assembling. Should the bearing begin to heat from any cause, this grease will melt and lubricate the surfaces. An oil hole is also drilled and provided with a screw plug for ordinary oiling.

This ring is entirely finished before cutting apart at the points over the levers, which is accomplished with a hack saw. The pins upon which the levers turn should be hardened and are held in place simply by a small split-pin. The levers are simple forgings of steel. It would be unwise to use cast. iron for this purpose, owing to its comparatively low tensile strength. The forgings are very simple, however, and can be easily made by any blacksmith. They are provided with hardened set screws and jamb-nut for adjustment ; the end of the set screw is rounded and polished where it bears on the fingers of the cam.

The cam has been provided with exceptionally large thrust surfaces to take care of wear. This is necessitated by the high speed at which the clutch runs. It was primarily designed to be held in by a spring and released by lever pressure, but the reverse may be just as easily done. The throw of the cam is 1 1/4 in. and a small flat is left on each finger to form a positive rest for the lever in case the cam is not spring-actuated. Each lever is attached to a stiff spring, which overcomes the centrifugal action. Were these not provided it is possible that some difficulty might be experienced in releasing the clutch at high speed owing to these levers flying and remaining in that position.