DEVELOPMENT AND DESIGN OF HYDRAULIC-BRAKE UNITS
The principles of hydraulics have long been known and the use of a liquid for transmitting power has proved safe and reliable in many applications, notably in the operation of passenger elevators. Hence it was natural to make use of these principles in a device for controlling an automobile under traffic conditions that demand an efficient and dependable braking mechanism. The ideal of equalized braking-effort is sought but variation in the coefficient of friction between brake-bands and brake-drums and between tires and road introduces complications, so we must be content for the present with the nearest possible approach to equalized pressure at the brake-bands. In the hydraulic system, pressure is transmitted equally throughout the liquid and to the levers that actuate the brake-bands. These levers are also designed to transmit the pressure equally to the brake-bands on all four wheels.
The author describes briefly the general construction of the system but dwells more particularly upon the development of the major elements and the difficulties overcome in the search for entirely satisfactory materials. A slip-joint connection between the brake-pedal lever and the piston in the master cylinder was adopted to avoid drawing air into the system and producing soft pedal-action due to compressibility of the air. For the same reason a positive hand-operated plunger-pump with a needle valve is used for refilling the master cylinder from the supply reservoir. It was necessary to develop a flexible hose connection from the rigid tubes on the frame to the wheel cylinders that would be non-collapsible and also non-expansible under internal pressure. The solution of the problem is described.
Prolonged search for a material that would resist temperatures of more than 200 deg. fahr. and that would otherwise be suitable for use in the cups that provide the seal for the pistons in their cylinders finally resulted in the adoption of a special rubber composition. The liquid used in the system must remain fluid at temperatures well below zero, must not corrode the copper tubes or the cast-iron cylinders, must not attack the rubber cups and hose nor become gummy. That now used with satisfaction is a solution of 50 per cent castor oil and 50 per cent alcohol, neutralized with potassium hydroxide.
In a four-wheel-brake system, equalized pressure applied to all wheels is believed to give the maximum safe braking-effort. When the brakes are applied the center of gravity of the car's mass moves forward and often as much as 50 per cent of the weight is transferred to the front wheels; consequently equal braking-effort on the front and rear wheels assists in stopping all wheels at the same time. Yet the likelihood of locking the front wheels is slight. In simultaneously making a turn and braking, with equalized pressure, the shifting of the center of gravity toward the front and toward the outside of the turn results in locking the inside rear-wheel first, then in succession the outside rear-wheel, the inside front-wheel, and finally the outside front-wheel, thus automatically giving maximum braking-effort on the turn as well as on the straightaway.