Hydraulics (1936)

Over the brink of Niagara Falls, a million tons of water pour, and 212 feet below, the mighty torrent falls against the blades of huge turbines, creating horsepower to light 275 cities, towns, and villages, and run the motors of thousands of factories. This ability of liquids to transmit power comes from one important natural law. You can't push a liquid together or compress it into a smaller space that it already occupies. If you take a piece of ordinary pipe and fill it with liquid and put a plunger at both ends, any blow on one plunger sends the other plunger flying out. Liquids can't be compressed, so here's what happens inside. The power of your blow is transferred to the other plunger by hydraulic transfer of power. This is the simplest kind of hydraulic system. Bending the pipe doesn't affect this transfer of power at all. It goes around corners just the same. When one plunger is pushed down an inch, the other end comes up an inch, and a pound of pressure exerted at one end is transmitted to the other. Suppose we have four pistons at one end and only one at the other. Now when the single piston travels four inches, each of the four at the other end travels one inch. A large piston of the same area behaves the same as four smaller pistons. Moving the small piston down four inches carries the large piston up only one inch, but with four times the force applied. This is the principle on which commercial and industrial hydraulic systems are based. Down in Dixie, billions of pounds of cotton are bailed up every year by the power of hydraulic presses. With Uncle Sam's long-range guns, not only are power and reliability needed, but minute precision is required. Precision that aims these mammoth guns within limits of one one-thousandth of an inch hydraulically. The hydraulic stage works by the same method and times its cue right on the nose with the reliability of an old-time actor. This model is a hydraulic system which shows how the hydraulic press multiplies power. This is the pump. This tube holds the liquid through which the power is transmitted, and these four cylinders at the other end receive the power. The movement of the large piston compresses or crushes anything put in the press. The hydraulic stage operates with the same reliable elements. The accurate hydraulic transmission screw in the Coast Defense guns is more complicated, but in principle it is the same as the lift and the press. In any hydraulic system, regardless of how many cylinders are attached to a master cylinder, each of the small cylinders receives the same amount of pressure and moves the same distance as the others. This principle is used in the latest application of hydraulic transfer of power, stopping the modern motor car. The master cylinder is placed under the brake pedal, and the power cylinder is put in the brake of each wheel. Now, when you press down on the brake pedal, each of the four power pistons is forced an equal distance against the brake shoes, pressing them uniformly against the brake drum. The shoes are hinged in such a way that the entire lining contacts the drum throughout the life of the car. No grab at the top or bottom. Smooth, even, taking hold by the entire shoe surface. These are the brake shoes of a new car. Let's paint this pair black and examine the wear they take. Note the even grip on the shoes in the first 20 miles. Perfect contact at 5,000 miles, and after 20,000 miles, uniform wear and still good for many more. For durability, the linings must be hard, uniform, and unaffected by moisture. All else being equal, long life and effectiveness of the brake depend on the amount of material contacting the drum. Brake drums must be wide and large in diameter, smoothly machined on the inside. A car that takes 90 horsepower to go from a dead start to 60 miles per hour in 20 seconds can stop in four seconds, but it takes 450 horsepower in the brakes. The final safety and control of the car depends on the construction of the brake itself. It is the brake itself that stops the car, not the hydraulic control. Perfected hydraulic control transmits the power from the foot pedal to the brake itself and gives you the sure, quick-stopping safety of the modern automobile.