The G.M.C. construction, Fig. 150, depicts a type employing concentric drums. The internal brake is of the conventional shoe type, fabric-lined, expanded by a cam and operating on the inner drum. The external brake operates on the outer drum, being of the band type and contracted by a double-armed lever. This double-armed lever is formed integral with the operating shaft and is slotted to receive the band bracket and the clevis rod, which carries the releasing spring and also forms the adjustment.
Fig. 156. G.M.C. Concentric Brakes.
The Timken Detroit Axle Company has recently introduced a new type of brake (Fig. 157) on its worm-drive axles, which is termed a "duplex brake." These brakes consist of four fabric-lined shoes, located in such a manner that the pair of brakes takes up but little more room than a single brake of ordinary shoe type. These four shoes are equally spaced on the inner circumference of the brake drum and each pair located diametrically opposite are expanded by cams and supported by the brake spider and large pins. One pair of shoes has single arms, which extend to the cam and its support and these pass between two arms of the other pair of shoes. These shoes are made somewhat larger in width, to obtain the proper brake area and are held longitudinally, by means of hardened steel washers on the operating shaft and by the brake spider.
Any one of the brakes described above, with the exception of the Pierce and Knox, may be applied to a chain- or shaft-driven vehicle in any position mentioned.
Another type worth mentioning is the Saurer brake, being an air brake worked by the the throttle lever. For a quarter of a circle this lever controls the throttle, but beyond this position, through a device incorporated in the carburetor, it causes the motor to operate on the two-cycle principle, compressing air in the cylinders to an extent which enables the car to be controlled on a 20 per cent, grade without the need of a brake.
In applying any type of brake, it must be held from rotating and this strain is generally taken by the brake spider which rides free on the axle or jackshaft and transmits this strain to the radius rod or frame on the chain-driven vehicles and to the axle housing on shaft-driven vehicles.
Mounting the service brake on the jackshaft or propeller shaft places these brakes where they are well protected and, as they are of the high-speed type, the reduction through the chains or gearing makes them more powerful. This presents a disadvantage, in throwing the entire braking strain on the chains or drive shaft and differential and thus shortening their lives. Should the chains or drive shaft break on a bad hill, and the emergency brakes be out of commission, serious damage would result. Although this may rarely occur, some makers have protected their vehicles against such accidents, by placing both sets of brakes in the rear wheels, thus placing the retarding force as near as possible to the point where the momentum of the vehicle is checked. In a sense, this argument is true, especially on the heavier vehicles, for the fewer elements there are between the tire and brake, the fewer are subjected to stress and the fewer are the chances of failure.
Brake adjustments are also receiving considerable study and, while in some cases they are almost hidden, in others they are very accessible. This adjustment may either be incorporated in the brake linkage or in the brake. The tendency seems to be toward locating it in the brake in such a manner that it is accessible without removing the wheel.
Brake equalizers are quite common on commercial cars, the well-known whippletree type being quite popular.
When a single brake is applied on the propeller shaft, the differential takes care of the distribution of force to the two wheels equally, but this kind of compensation has a disadvantage, in that, if the adhesion of the two wheels is greatly different, that with the slightest grip on the road may actually cause it to rotate backwards. Still, it is only on very rare occasions that this re-verse motion occurs and it is not, therefore, a cause of much added tire wear.
If the braking forces are not equalized, the task of adjusting the brakes is much more difficult than it need be. On one vehicle, the whippletree equalizer is replaced by a diminutive differential gear, providing a smoother action and a much larger range of equalization.
Fig. 157. Timken Duplex Brake.