Practically all modern clutches are operated by means of a special pedal, moved by the left foot. This is connected by means of rods and levers to the internal member, which compresses the clutch spring or springs, thus allowing the clutch members to separate. In this way the clutch is thrown out. To throw it back in, the foot pressure is removed from the pedal, the springs again exerting pressure and forcing the parts together, thus allowing them to take hold. There was a time when a considerable number of cars had the clutch so constructed that the pedal held it in and the springs threw it out, just the reverse of the present plan. This, however, is no longer used, as it necessitated maintaining a constant pressure on the pedal while driving, and after a long ride this became very fatiguing.
The Dorris clutch made by the Dorris Motor Car Company, St. Louis, Missouri, Fig. 65, is a new arrangement of the clutch pedal, and its operation is such that the clutch is released or thrown out with very light pressure on the pedal. Pressure on the pedal A is transmitted by the shorter lever arm B, thus greatly increasing the leverage. This pressure is transmitted to lever C and through it to lever D, these two being hung on the frame across member E. As C is much longer than D, there is another multiplying action here. This does not act directly upon the clutch, but upon the upper end of the clutch shifter F, which is attached to the clutch at G and pivoted at its lower end H - here again in a multiplying action. The net result of these three multiplications is a combination which will release the strongest and stiffest clutch with a very slight pressure of the foot.
As has been previously pointed out, some clutches run in oil, while others run dry. The former type must be kept filled with lubricant at all times, the general plan in such a case being to provide a lead from the engine oiler when the clutch case is separated from the engine case, or a connecting means when the two are in one case. In addition to the actual clutching members, there is practically always a sliding member, which must have lubricant of some form, while the thrust bearings to take the thrust of the clutch springs must be cared for. Generally, these two cases are cared for by a pair of grease cups, these being visible in Figs. 44 and 50. The operating rods are lubricated usually by means of small oil holes, either drilled directly into the part or covered with a small oil cup. In those cases in which the clutch runs in oil, it will be noted that a filling plug is provided, by means of which additional lubricant can be poured into the casing. For this, refer to Figs. 47, 51, 56, and 60.
Fig. 65. Multiplying Lever of Dorris Clutch to Make Pedal Pressure Light.
Courtesy, of Dorris Motor Car Company. St. Louis, Missouri.
The need for bearings in a clutch depends somewhat upon its nature and location, but regardless of these a thrust bearing is needed for the clutch spring. To explain this briefly, it is known that action and reaction are equal, and opposite in direction. For this reason, when a clutch spring presses the disks or parts together with a force of, say, 100 pounds, there is exerted in the opposite direction this same force of 100 pounds. In order to have something for this to work against, a bearing is used, and since it takes up this spring thrust, it is called a thrust bearing. Not all bearings are fitted to take thrust, the majority of them being designed for radial loads only. For this reason a special design is needed.
When the clutch is incorporated in the flywheel, there are needed, generally, two additional bearings, one for the end of the crankshaft and another for the transmission or driven shaft. This will be noted in Figs. 42, 51, 52, 59, 60, and 55, although the last-named does not have the clutch combined with the engine, but rather with the transmission. In the majority of cases, it will be found that a means of fastening the end of one shaft has been worked out so as to eliminate one bearing. This accounts for the large number which show but two - the thrust and one other. In looking back over these, it will be noticed also that practically all the bearings are of the plain ball form. This is due in large part to the fact that these take up the least room for the load carried, both in diameter and width, a contributing reason being the fact that in many cases one of the shafts or parts can be formed to take the place of either the inner or outer ball race.
In general, adjusting a clutch is not a difficult task, there being but two possible sources of adjustment; the throw or movement of the operating pedal or lever, and the tension of the spring. Generally, an adjustment is provided for each. When the fullest possible throw of the pedal does not disengage the clutch, an adjustment is required to give a greater throw. If the throw is correct, but the clutch takes hold too quickly and vigorously, then the spring pressure can be lessened somewhat to soften down this action. On the other hand, when dropped-in quickly, if it takes hold slowly, more spring pressure is needed, which is obtained by tightening.
Clutches are made accessible in two ways: by their location on the car, and by the relative ease with which they can be removed. Accessibility as to location is less in the various combinations, such as in the unit power plant, housed within the flywheel, or combined with the transmission. Ease of removal is determined by the number and location of the joints (usually universal) used with the clutch. Sometimes, one on each side makes removal easy, but generally, a single universal makes much work.