A short consideration of the band style of clutch shows that this does not differ radically from the ordinary band brake, either in construction, application, or actual working. The difference in the two lies in the fact that the band, as a clutch, is designed to transmit power with as little loss as possible, while the band as a brake is designed to absorb the forward energy of a moving vehicle (equal in the last analysis to power) in the shortest possible space of time, i.e., to waste as much power as possible.
Fig. 48 shows the form of band clutch used on the Mors (French) cars. In this form, the band is in two parts, and a rocker arm moved by a sliding cone operates the free ends of the two bands, which thus contract upon the clutch drum. Fig. 49 is the clutch which was used on Gaeth cars, made in Cleveland, Ohio. While not differing radically from the Mors, this has the two parts or sections of the band united at the bottom and two operating levers are pivoted at the top, where a single conical-shaped cam moves both outward and tightens the bands on the drum.
Fig. 48. Man (French) Contracting Band Clutch.
Fig. 49. Typical Contracting-Band Clutch.
The usual place in which the band clutch is found is in connection with a planetary transmission. There the band is always used, and there it reaches its simplest form, that of the plain band wrapped around the drum. One end is fixed and the other attached to the braking, or more correctly the clutching lever. A plain pull on this effects the clutching action. A more modern and more efficient form has one end of the band attached to one extremity of the clutching lever, while the other end of the band is fastened to the middle of this lever. The clutching pull comes upon the upper extremity of the lever. Then the band might be said to aid in clutching itself, i.e., a scissors action is obtained, and the required pull is lessened.
This principle is used in the Ford car, the planetary transmission being located just forward of the contracting bands and clutch disks. This is of particular interest as Ford is now the only American maker using the planetary form of transmission, all other makers, even of very low-priced machines - some below the Ford price - having gone to the selective sliding gear form.
When the band is used as a brake the pull necessary to stop the car is p = fw(D/d) in which f is the coefficient of friction, w the weight to be stopped, D the diameter of the road wheels, and d the diameter of the brake drum. Now the ratio of the diameter of the road wheels to that of the drum is but the ratio of the work arm to the power arm, so when the band is used as a clutch, the ratio of the radius of the two arms may be substituted. The power arm is taken as unity and the work arm as the radius of the clutch drum. Since this divided by 1 remains the same, it may be substituted in the formula above. So, too, with the weight, in place of this must be substituted the power to be transmitted, which is the equivalent of the weight in the other case. The formula then becomes p'=fPr.
Owing to the winding action of the band, the pull p' will be less than the pull p, by an amount which varies as the portion of a circle or number of degrees encircled by it. Then taking 0 as the number of degrees logp = .434 f θ logp' from which the value of p' may be evaluated in terms of p and substituted in the previous formula.