This method has the additional advantage that the central part within which the clutch is housed is very small in diameter, so that the portion of the flywheel between the rim and the clutch housing may be made in the form of fan spokes, thus converting it into a fan and serving to cool the motor better.
In this type, the spring presses the member P forward, jamming one-half the disks against the other half, this jamming action transmitting the power. To throw out the clutch, the lever L moves backward and pulls with it the casing M, this being connected to the member P draws the latter out and away from the disks. The natural spring of the latter then asserts itself, and they free themselves.
As the various examples of disk clutch shown would indicate, the designer has had his choice between a few large disks and a large number of small ones. If he chose the former, the clutch could be housed within the flywheel, but this makes it inaccessible, although saving length. If he chose the latter, the clutch could not be kept within the flywheel length, a separate clutch housing being a necessity, but the clutch could be made accessible and flywheel fan blades could be used.
Another example of the plain metal-to-metal disk clutch is shown in Fig. 55. In this case also the clutch is not housed in the flywheel, as in most of the preceding examples of this form of clutch, but in the forward end of the transmission case. That is, instead of motor and clutch forming a unit, the latter is a unit with the transmission. It is claimed that this position makes it more accessible, since it brings the clutch directly under the floor boards of the driver's compartment, and that better lubrication is another result. The latter is effected through communication with the gear part of the case, which is always filled with lubricant.
In the figure it will be noted that there are 13 driven disks, with keyways, which hold them to the driven drum. Note that the latter is held to its shaft by means of a pair of large set screws. The clutching springs are of small diameter and size, spaced equally around the periphery of the disks, each being enclosed in a small and thin metal casing. Attention is called also to the universal joint shown, this forming the rear end of the driving connection with the flywheel, which will be referred to later. These disks are perfectly flat, stamped out of sheet steel with the proper keyways for internal or external holdings.
Fig. 56. Argyll (Scotch) Disk Clutch.
Differing from the foregoing is the clutch used on the Scotch Argyll cars, Fig. 56. In this the disks have been made larger in diameter and smaller in number. Moreover, no use of the flywheel as a fan has been attempted.
The more modern disk clutch has two sets of sheet metal disks, one of these being faced ■ on one side or both with a special material. Without a single exception, all the disk clutches shown have had plain disks against plain disks. This makes a simple and fairly inexpensive construction, but as has been found out recently, one that is not very efficient. Thus, the most recent tests have shown that metal against metal gives a coefficient of friction of but .15, which is reduced to .07 when the surfaces become oily or greasy. With one of these contacting faces lined with leather, this rises to .23 when dry, and .15 when oiled. Again if fiber is used for the facing, the coefficient becomes, respectively, .27 and .10, while with cork or cork and leather, it becomes, respectively, .35 and .32. Here then is a very apparent reason for (1) facing the clutch disks, and (2) running them dry.
By going over these figures, it will be noted that disks with almost any form of facing will show an increase in efficiency over the same disks without facing, varying from 60 up to almost 300 per cent. Again, any form of disk clutch, faced or otherwise, will show a much higher coefficient dry than oiled, and thus, a greater efficiency. These two facts point out the obvious reasons for the modern tendency toward the multiple-disk clutch, faced and running dry.
Fig. 67. Multiple Disk Clutch Used on Cadillac Cms.
Courtesy of Cadillac Motor Car Company, Detroit. Michigan.
To present an example of the faced type, Fig. 57 shows the multiple-disk clutch of the eight-cylinder V-type Cadillac. In this, the eight driving disks can be seen, with the facing on each side of each one. This facing is of wire-mesh asbestos, and between each pair of disks comes a plain driven disk, so that it has a facing of the asbestos against each side of the metal which it grips. The six keys which hold and drive the outer disks can be seen on the inside of its housing, while the slots into which these project can be seen on the periphery of the disks. By examining the group closely, the driven plain disks can be seen between each pair of the drivers. Fig. 5S shows the pedals and the exterior of the clutch case, where it bolts up to the engine. This indicates how a unit power plant simplifies the control group, and eliminates parts.
The clutch on the Locomobile cars, shown in section in Fig. 59, is very much like the Cadillac just shown, except for this novel feature, that the fabric facings are not attached either to the driving or to the driven disks, but float between them. This fabric, usually a woven asbestos material with a central core of interwoven metal wires, instead of being attached to both sides of every other disk or to one side of every disk, is not attached at all. The rings for the fabric disks are made up in the form of annular rings, have the same inner diameter as the inside of the driving disks, and the same outside size as the driven disks; consequently assembling one of these clutches is simply a question of piling first a driven disk, then a fabric, then a driving disk, and so on.
Because of the fact that the fabric rings are not united to either of the metal disks, they free themselves with remarkable rapidity so that either on engagement or on declutching the action is very quick.
Fig. 58. Housing and Foot Pedals on the Cadillao Car.
To increase the power transmitted by a clutch of given size, either the number of plates must be increased or the form of the surface changed. The latter method was followed on the clutch of the French car, Ours. The disks of this unusual clutch had a perfectly flat outer portion, and a conical inner portion, only the latter taking part in the transmission of power. In this disk form, then, we have the advantage of the disk economy of space, together with the advantages of the cone clutch, and the additive gain of running in a bath of oil.
Another form utilizing this principle, and one that is more widely used, is that known as the Hele-Shaw, so named from its inventor, the famous English scientist, Dr. H. S. Hele-Shaw. This is essentially a flat disk, as shown at A, Fig. 60, with a ridge B at about the middle of the friction surface, this ridge consisting of a portion of the surface, which has been obtruded during the stamping process in such a way as to leave the surface of the ridge in the form of an angle of small size. The angle used is 35 degrees, and this value has been determined upon experimentally as the best. Fig. 60 shows a cross section through an assembled clutch, which reveals the clutch angle very plainly. In use, the ridges nest one on top of the other; and in the extreme act of clutching, not only the flat surfaces but both sides of the ridge are in contact with the next plate. Thus, not only is the surface for a given diameter increased, but the wedge shape is also taken advantage of. Smaller views of the single disks and of the complete clutch, disassembled to make plain its simplicity, are shown in Fig. 61 and Fig. 62.
A brief mention of the method pursued in the design of flat disk clutches will not be out of place. Consider the disk to be used as an annular ring having an internal radius r1, and external radius r2. If / is the coefficient of friction, n the number of disks, and p the specific pressure normal to the friction surfaces as distinguished from the spring pressure, then by certain theoretical considerations involving integral calculus, it is found that the moment M of the clutch around the center will be as follows:
M, total = pfπ(n-1) (r23- r13) in foot-pounds.
In use, the factor p is found first, by figuring the area of the whole surface and dividing the spring pressure by it thus area=π(r22 - r12) and the specific pressure is spring pressure P = (r22-r12)
Knowing the material, the coefficient of friction is known, and therefore everything is known but the number of plates and their size. By trial the size may be selected, from which the number is easily figured, using the above formulas. Care should be taken in their use to add 1 when the number of plates comes out with a decimal or fractional quantity. In the use of a formula like the preceding one, it is always assumed that the power is known at some specific speed. This being the case, the total torque, which is divided by the total moment to find the number of the plates to use, is T, total = hpX33,000 2πXspeed.