Fig. 137.
Section of Claw Type Hydraulic Jack.
HYDRAULIC JACK.
A Lifting-Jack is a contrivance for raising great weights by force from below; also called a jack-screw. From its derivation from Jack, equivalent to lad or boy, has arisen its modern use as denoting a contrivance which is subject to rough usage. It is operated by a screw, whereas—a hydraulic jack is a jack or lifting apparatus operated by some liquid, usually oil, acting against a piston or plunger, the pressure on the liquid being produced by a force pump. The hydraulic jack consists of, 1, a cylinder; 2, a ram or plunger; and 3, a pump. One of these machines is shown and described in the Glossary, [page 24], another is illustrated by [Fig. 138]. The [Fig. 137] on the opposite page shows the inside view of [Fig. 138] but on a different scale. The names of the parts are particularly to be noted.
Fig. 138.
Movable hydraulic, or screw, jacks serve on numerous occasions most effectively for lifting or propping-up of less accessible parts. Eye-bolts and jack-bolts are arranged for, in all parts that are likely to be handled, to facilitate and accelerate the work in necessarily crowded quarters.
The base or foot is usually made of cast iron or cast steel and may be either round or square to suit requirements. The cylinder is bored from a seamless steel ingot and having a thread upon its lower end is screwed into the base.
The ram is also a tube of seamless steel having a thread at the top and is screwed into the head or cap which is made either of cast iron or cast steel. The lower end of the ram has a thread inside to receive the pump plug which contains the delivery valve, while upon its outside is placed the cup leather packing and the ram packing ring. The pump for operating the ram is from five-eighths to three-quarters of an inch in diameter depending upon the capacity of the jack, and has a plunger packed with a cup leather.