MYDRAULIC CRANE, a type of crane in which the motive power is water tinder hydrau lic pressure. There are two general styles of hydraulic cranes: direct-acting and indirect acting. In the former class the lifting mechan ism is applied directly to the load; in the latter the power works upon a system of pulleys and chains, for the purpose of increasing the speed of operation, or of multiplying the working force. The mechanical action of the hydraulic crane is the same as in the hydraulic press (q.v.).
The direct-acting cranes are of two distinct designs; those in which the lifting mechanism is contained within the post of the crane, and those where the lifting cylinder with its piston and ram is suspended from the crane-head, as in the Armstrong crane. The indirect mechan isms are generally placed horizontally in a pit under the base of the crane, the operating chains passing up through the post and thus to the crane-head. Provision is generally made also for revolving the crane by the hydraulic power.
The hydraulic crane has a decided mechan ical advantage in conditions where a very great force is to be exerted through a comparatively short lift. In the Armstrong crane, however, lifts of 40 feets are attained with remarkable speed. It is economical as compared with other forms of cranes, because when the crane is idle no power is being used to keep up the momen tum of the driving machinery, as with steam and electric cranes. Moreover, the hydraulic crane has but very few moving parts, and there is therefore a minimum of wear and great dura bility. It is, besides, capable of instantaneous control, impossible in any form of geared cranes. In the direct-acting hydraulic cranes the effi ciency is close to 90 per cent.
In designing a hydraulic crane the formula commonly in use is that for each pound of pres sure per square inch on the lifting piston a °head° of 2.31 feet is required in the water supply — whence the required head of water will be the total lifting power desired divided by the area in square inches of the lifting piston, multiplied by 2.31. As a matter of prac
tice the pressure of the water used is not often due directly to a head of falling water, but to water from hydraulic accumulators which is under the pressure of weights and sustained by high-pressure pumps. The water pressures commonly in use vary from 400 pounds to 1,200 or even 1,500 pounds per square inch, that most frequently found being about 700 pounds. There are, however, a number of cranes known as low-pressure cranes in which the working pressure is 70 pounds per square inch, or even lower. These cranes are usually run by direct gravity water pressure from an elevated tank in a nearby tower, supplying a water main with hydrants, the crane itself being movable and arranged to be coupled to the near est hydrant.
Hydraulic cranes, especially if of the low pressure type, are susceptible to damage by frost, and in order to reduce the freezing point of the water with which they are operated to a temperature at least as low as the weather con ditions may produce, various substances have been added to the water. Glycerine has had the preference, but its cost has been almost prohibitive. Alcohol has been used, but the most economical substances have been cheap, non-corrosive salts. The cylinders, pipes and valves are covered with non-conducting coat ings, and the exhaust of the compressor-pumps has been utilized to impart some warmth to the water. In high-pressure cranes the danger of freezing is very much less. Consult Boettcher, A.. 'Cranes) (London 1908) ; Collyer, F., (Hydraulic, Steam and Hand Power Lifting and Pressing Machinery) (London 1892).