英文科學讀本 第六冊·Lesson 55 The Hydrostatic Press

Lesson 55 The Hydrostatic Press

When we were discussing water as a machine, I illustrated the water pressure by means of two tubes, a large and a small one. That will now help us considerably to a right understanding of the contrivance known as the hydrostatic press. The name is a long one, but it will be easy to understand if you remember that it is derived from two Greek words: hudor, meaning water, and stasis, meaning standing. It represents the power of standing water. The machine is also known as the hydraulic press. It consists essentially of a small force-pump, connected by means of a pipe with a large and strong cylinder, fitted with a piston. The pump itself, like all other force-pumps, consists of a barrel, fitted with a solid piston, and having in its floor a valve opening upwards. The piston-rod is moved up and down by a lever handle.

As the piston rises and exhausts the air, water rushes up the suction-pipe from the reservoir into the barrel. At the next descent of the piston, the water is pressed downwards, and effectually closes the valve in the floor of the barrel. But the water will not be compressed, and hence it finds its way out of the barrel by the only available channel—through the pipe, and so into the large cylinder.

The pipe itself, moreover, is provided with a valve, also opening outwards, and this prevents any return of water into the barrel of the pump. The consequence is that each stroke of the pump-handle forces more water into the large cylinder, and this water presses upwards upon the great cast-iron piston in it.

This upward pressure of the water causes the piston, or ram, as it is called, to rise vertically. The larger the surface of this piston, as compared with that of the little piston of the pump, the greater will be the force of the upward pressure.

The top of the ram is broad and flat, and above it is placed an immensely strong iron plate, supported on stout iron pillars.

The great use of the hydraulic press is to pack wool, cotton, hay, and other light but bulky articles into smaller space. If any of these objects be placed on the top of the ram, the pressure of the water will force them upwards against the iron plate, and so compress them into a much smaller bulk.

Bales of cotton and wool are, in this way, so closely compressed that they have to be bound with strong iron bands, to prevent them from expanding and bursting during the voyage. The object of the pressing is, of course, to enable the ship's hold to carry more cargo than it otherwise would.

The hydraulic press is also used in raising heavy bodies, such as immense masses of iron plates, and other metallic work used in building. Thousands of tons are raised in this way with comparative ease.

The most marvellous illustration of the powers of this machine is seen when immense ships are raised by its means bodily out of the water, and held suspended there, while the workmen examine and repair the under parts of their hulls.

Much use is also made of this machine in warehouses, hotels, workshops, and great public offices, for providing lifts, by which people and goods are easily and quickly carried from the very basement to the highest story of the building. Very few public buildings can now be found without their hydraulic lifts.

If you turn next to the picture of the press, you will see that the ram is smaller than the cylinder in which it works. When this machine was first invented it was of comparatively little use, because as the force, applied at the pump-handle, sent more water in, it caused a corresponding overflow at the top, between the ram and the sides of the cylinder, so that the force was lost.

An English engineer, Mr. Bramah, invented a clever contrivance for overcoming this difficulty. It is known as the Bramah collar. It is made of leather, and is cut in the form of a hollow ring or circle. The leather ring is first well soaked in oil to make it perfectly impervious to water, and then bent down on both sides. You will best understand the arrangement by glancing at the picture of the collar supposed to be cut in two. The cut edges resemble the letter U inverted. This leather ring, or collar, as it is called, fits close to the sides of the cylinder and the ram, and the water as it rises fills the hollow space.

As more water is sent by the pump into the cylinder, it presses with increasing force against the inner sides of the collar. This pressure holds the leather firmly to the sides of both the cylinder and the piston, and effectually closes the opening, so that there can be no overflow—none of the power is lost.

Since the invention of the collar the machine has been styled indiscriminately the hydrostatic, the hydraulic, and the Bramah press.