The expanding-band clutch finds favor among few. Like the contracting band, which is very similar to the band form of brake, the expanding band is much like the expanding type of brake, with the exception that the clutch is used to form the connection between two rotating parts. Viewed from the standpoint of pure engineering, the expanding band is little different from the cone type of clutch, granting that the angularity of the operating cam is the same as that of the cone.
Much depends upon how the band is expanded, the methods differing widely in practice. This is usually accomplished by means of screws, which may be either right-handed, or left-handed, or both. In one expanding clutch, the screws are single and right-handed. This superinduces a gain in the power required to clutch by the amount of 2π l.
A, gain in power = 8 in which l is the length of the lever in inches, and 8 the pitch of the screw in inches. If the screw is made double, i.e., one-half threaded right-hand and the other half threaded left-hand, then the expression should be halved. This latter case being more usual, it will be of interest to resolve the gain into the original formula for power, i.e.,
P= fFRxl 63,000s.
Another form is expanded by a double-threaded screw operated by a lever. This, in turn, is moved by a pair of sliding collars on the main clutch shaft, the clutch foot pedal moving these forward.
With its advent in 1904, the multiple-disk clutch has steadily grown in popularity until today it is looked upon as the most satisfactory solution of the difficult clutch problem. Designers who have once adopted it, seldom, if ever, go back to another form, while of the new cars coming out from time to time nearly three-fourths are equipped with some form of disk clutch.
Statistics for 1914 show that the disk form of clutch was easily the most popular type, and further comparisons with the previous year, and with the apparent tendencies for 1915, show that it is gaining more rapidly than any other type. Of 230 different chassis for 1914,119 were equipped with disk clutches, 97 with the cone, 9 with a contracting-band type, and but 5 with an expanding-band form. As the majority of the newer models have adopted this form also, while several of the others have changed, the relative figures for 1916 are estimated at about 94 disk, 81 cone, no contracting band, no expanding band, and 1 electric This would give the first-named approximately 54 per cent of the total. Two Forms of Same Make. This brings to mind the relative advantages of the two leaders, the cone and the disk. This is presented in a very striking manner in Figs. 50 and 51, which show the cone and disk clutches used interchangeably by the Austin Motor Company, Birmingham, England, in the 18-24-horsepower chassis. Note in these two that the cone requires a considerably greater length, for in that form it is necessary to make the flywheel with a sloping series of spokes in order to throw the cone farther forward, and thus make room for the greater length of spring. Note that the larger diameter of the cone and its inevitable flywheel action has resulted in the removal of considerable metal from the inside rim of the flywheel. Furthermore, on the disk form, note how the space between the compact disks and the rim of the flywheel is used as a fan, six blades being set in here in place of spokes, these assisting in cooling the water.
Fig. 50. Cone Clutch for the 18-24 Horsepowcr Austin (English) Car.
Fig.51. Disk Clutch for the 18-24 Austin for Comparison with Fig. 139.
While the cone is apparently more simple, it will be noted on closer inspection that the pedal and other operating mechanisms are all shown in the disk form, while none are shown in the cone type. If the two drawings were on a par, they would show little or no difference in this respect beyond the fact that the disks and their springs make some 40 or 50 parts, while the cone and its spring parts do not total much more than half a dozen.
On this cone form, special attention is directed to the method of applying the clutch lining in six sections, with a single bolt and clip above each. It is said that any one of these lining sections can be removed in a couple of minutes without touching any other parts. In both instances, note the braking surface provided to stop spinning when the clutch is removed. In the case of the cone, this is a single conical surface at the left of the figure, while on the disk it consists of a pair of wedge-shaped projections which enter a pair of similarly shaped grooves, this giving actually four braking surfaces. In the latter, too, attention is called to the simple adjusting means, a single large set screw, with a locknut, being so placed that when the throw of the pedal is not just right for engaging or disengaging, a turn of the screw allows of more or less movement, according to the needs.
These differ in number and shape of disks, method of clutching, material, and lubrication; but in principle all are alike. This, briefly stated, is that flat surfaces properly pressed together will transmit more power with less trouble than any other form. By multiplying the number of surfaces and making them infinitely thin, the power transmitted may be increased indefinitely. That this is not idle fancy is shown by a number of very successful installations of 1000 horsepower and over in marine service, and certainly no such power is required for an automobile.
The minimum number of plates in use is said to be three, but very often the construction of a three-plate clutch is such that one or two surfaces of other parts are utilized, making it a two- or even one-plate clutch in reality. Thus, in the Austin clutch, shown in Fig. 52, made by the Austin Automobile Company, Grand Rapids, Michigan, a copper disk G of some thickness is clamped by spring pressure between the flywheel A and a floating ring D. In reality, the copper disk transmits all the power, the flywheel being a necessity, and the floating disk only an accessory before the fact. It would, therefore, not be wrong to call this a one-plate clutch.
Fig. 52. Austin Clutch with Single Disk.
Austin Automobile Company, Grand Rapids, Michigan.