An excellent brake for very large cranes is Matthew's hydraulic brake, in which water is passed from end to end of cylinders fitted with reciprocating pistons, cooling jackets being provided.
It has been shown that this behaviour of dielectrics can be imitated by a mechanical model consisting of a series of perforated pistons placed in a tube of oil with spiral springs between each piston.
I, the lift cylinder is in hydraulic connexion with a pair of short cylinders placed one above the other, the pistons working in them being connected together by a common rod.
With this arrangement the lift - ram and the two balance pistons are always in equilibrium, or, in other words, the ever-changing displacement of the lift-ram is automatically in balance.
To work the lift, pressure-water is admitted to the annular space C above the lower of the two balance pistons (the space B above the upper one is always in communication with the pressure-water), and the combined pressure on the two pistons is sufficient to lift the cage, ram and load.
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