Multiple disc and plate clutches are based on the same principle as the cone clutch, but constitute in a sense extreme opposites in design. This type offers several advantages not found in the others, being the most compact. The required frictional surface is obtained by a multiplicity of small surfaces, in preference to two large ones, as in the case of the cone and plate types.
A disc clutch consists of two sets of discs, one set being termed the driving discs and the other the driven discs. The driving discs are generally provided with key slots on their outer circumference, which fit over hardened steel keys riveted to the inside circumference of the housing bolted to the flywheel. The driven discs are also provided with key slots, but these are placed on their inner circumference, which fit over keys riveted to a housing attached to the driven shaft. It is general practice to use one more driving disc than there are driven discs, so that the two end discs may be of the same kind. The driving set is driven by the engine, while the remaining set is attached to a continuation of the transmission shaft. In some cases small flat springs are used to keep the discs apart under conditions where it is desired to render the clutch inoperative, that is, when the spring pressure is removed from them.
It is usual practice to enclose a clutch of this kind in an oil-tight case, which insures that the members will operate in a constant bath of oil, meaning long life of the frictional surface as well as gradual engagement. Owing to its comparatively small diameter, the inertia is not very great and gear shifting is somewhat easier than with the cone clutch. The spring pressure is great enough, so that when engagement is made the oil will be squeezed from between the plates and the frictional surfaces brought into contact. As the oil is gradually squeezed out, and as there will be a certain amount of slippage as long as any considerable amount of lubricant remains, the power will be applied gradually.
A multiple-disc clutch, which is extensively used in commercial cars in this country and abroad, is the Hele-Shaw clutch, illustrated in Fig. 72. The discs are made from steel and bronze, with V-groove corrugations. Only the walls of these grooves come in contact and the remaining portions of the disc serve to radiate the heat engendered during slippage. To permit the oil to enter and escape freely, these discs have small holes drilled in the inner walls of the grooves near the peak. The action obtained by these grooves is a wedge action similar to the cone clutch. This also illustrates a design in which pressed steel is used wherever it is possible to do so. The clutch is of the universal type, having an external and internal gear type of universal, mounted inside of the clutch. A clutch brake is also provided to facilitate gear shifting, this brake being mounted upon the drive shaft and adjustable for wear.
Fig. 73 is a type of multiple-disc clutch used in connection with unit power plants. It is built onto an extension of the transmission shaft, one end of which is supported by a ball bearing in the flywheel. The frictional surfaces consist of saw-steel driving discs and Raybestos-line, steel-driven discs. The construction is similar to those described above, excepting that two large springs are used, one being placed around the other and retained by three bolts, which provide adjustment for the spring tension.
The housing or driving member bolted to the flywheel has teeth cut on its inner periphery into which the plates fit instead of keys and the plates have teeth instead of key slots.
Fig. 71. Indirect Cone Type.
Fig. 72. Helle Shaw. Multiple-Disc Clutch. Universal Type.
Fig. 73. Type of Multiple-Disc Clutch used in Unit Power-Plants.
The dry-plate clutch is similar to the multiple-disc type; however, the discs are of a much larger diameter and but three or five plates are necessary. The driving discs are either Raybestos rings or bronze plates with cork inserts, while the driven discs are made of steel. These clutches are not designed to run in oil, but are liable to wear because of the amount of contact surface provided.
A popular three-plate clutch of conventional design is shown in Fig. 74. The face of the flywheel forms one surface, while a moving member forms the other. Between them are two discs of friction material and a floating member which is keyed to the driving shaft. Pressure is obtained by coiled spring acting on a sleeve which actuates the toggle mechanism operating the movable member. This toggle mechanism is supported by a disc which is bolted to the flange. This flange has two slots through which the bolts pass and this together with taper surfaces on the movable member form a means adjustment for wear of the friction discs.
In Fig. 81 is shown a simple plate clutch for low power delivery cars. This clutch is almost entirely of pressed steel and utilized the flywheel as a driving member. Two additional driving plates are used and these are supported by three hardened steel pins fastened to the flywheel, which also carry the springs. The driven member consists of a spur gear which supports the driven plates that have teeth cut on their inner periphery.
The Hilliard clutch (Fig. 75) is another excellent example of the plate type. Spinning of the rotating members is prevented by a combination of quick acting release, which does not permit of a drag while releasing and further by having the parts so light that rotation does not continue long.
Fig. 74. Bore & Beck Dry Plate Clutch.
Fig. 75. Hilliard Clutch with Double Annular Ball Bearing and Enclosed Spring.