From what has been said of the brakes in discussing the final drive and the various methods of applying the power to the road wheels, it can readily be understood that there would be little uni-formity as regards brake construction and location, except that all states have laws which specify that vehicles must be equipped with two-brake systems - one system for ordinary service and one for emergency. In horse-drawn vehicles, the brakes are applied directly to the steel tires: however, since rubber is quite expensive, this method of brake application is not commercially possible; but it is effectively accomplished by securing to the wheels a metal drum, on which the friction meml>ers act.
Until recently more attention has been given to acceleration of the commercial car than to the retarding forces at the driver's disposal. This subject at present is receiving considerable study, which is evident by the variety of types and the numerous locations. The brakes are invariably applied to the rear wheels, as they present considerable advantage, supporting the greatest portion of the vehicle and load.
On vehicles employing the double side-chain drive, it has been considered good practice to place one set of brakes on the jack-shaft and one set in the rear wheels. There are various positions for this brake, either inside or outside of the frame. This type of brake can l>e made light and powerful, but possesses other disadvantages which have led some makers to place both brakes in the rear wheels. On shaft-driven vehicles, one brake can be mounted either at the front or rear end of the propeller shaft and one set in the wheels, or both sets may be placed in the wheel drums.
Rear-wheel brakes for either type of final drive may be divided into three general arrangements, viz., two internal brakes on the same drum, one internal and one external brake on the same drum and one external and one internal or two internal brakes operating on concentric drums.
There are two general types of brakes - the band and shoe types, and either may be made external contracting or internal expanding. The band type consists of a continuous steel band having a fabric fractional facing, while the shoe type may either be of cast iron with a high percentage of manganese, of phosphor bronze with cork inserts, or provided with a fabric facing.
When frictional facings are not used, the shoes are provided with diagonal grooves to prevent chattering and squeaking. Each type presents a variety of constructions, as regards anchorage, adjustments, and operating mechanism. In addition to the brakes acting on the rear wheels, the Saurer motor brake can also be mentioned.
The writer, in discussing this subject, will endeavor to cover the principal types in use and in conclusion outline their advantages and disadvantages, Considering first the chain-driven vehicles, we find two locations for the service brake, either outside or inside of the frame.
Fig. 149. Knox Jack Shaft Brake.
Fig. 149 serves to illustrate the jackshaft brake used on the Knox tractor. This is of the shoe type mounted outside of the frame and is anchored to the frame side rail by heavy brackets. Like all jackshaft brakes, this is of the highspeed type and equipped with two cast iron brake shoes, which are easily removed for renewal and mounted on large supports, while ample adjustment is provided through the rod connecting the two shoe supports with the operating lever. The brake drum is made from cast steel and is fourteen inches in diameter with four-inch face. It is attached to a hub which also carries the driving sprocket. Features worthy of attention in connection with this construction are the particular attention paid to the strength of the brake anchorage and the unusual provision for cooling presented by the twenty rile on the brake drum.
The rear-wheel brakes are perhaps the largest ever attempted and are 20 ins. in diameter with a 6 1/2-in. face. They consist of steel shoes lined with friction fabric and hydraulicallly operated. This hydraulic mechanism is of ingenious design, so that there is practically no difference in their operation from the ordinary hand brakes. A pump lever takes the place of the ordinary hand lever and a button for the release of the brake. To apply the brakes, the operator makes two or three full strokes with the handle forward and backward and the application of the brake will then be felt in the form of a resistance pulling the lever backwards, the same as with the hand lever. After the resistance is felt a good hard pull on the handle will lock the rear wheels. The release is accomplished by pushing the button on the top of the handle and pushing the handle forward beyond the stop normally interposed. Passing beyond this stop exposes a release part in the pump, which allows the liquid to flow back through the pump into the reservoir.
Most any of the following brake constructions can be applied to the jackshaft, while they may be placed either outside of the sprocket, between sprocket and frame and as mentioned previously inside of the frame.
Fig. 150. Kelley Internal Expanding Rear Wheel Brake.