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piston rod[′pis·tən ‚räd]
a round rod that connects a piston to a slide block in reciprocating machinery—for example, pumps, compressors, steam engines, and some internal-combustion engines—or in the hydraulic or pneumatic mechanism of a reciprocating drive. A piston rod is used when the compartments formed on both sides of a piston are employed. When only one of the compartments formed in a cylinder of a reciprocating machine is used, a piston rod is usually not provided and a connecting rod is connected directly to the piston.
A unilateral piston rod is located on one side of a piston and passes through one head cover. A bilateral piston rod passes through two head covers; such a piston rod is used to provide better alignment of a heavy piston in steam-driven machinery or to connect a second cylinder in a compressor.
In some reciprocating hydraulic drives, a cylinder is attached to the moving part, or table, of a machine and a piston rod is attached to the bed. Hydraulic fluid is supplied through the piston rod to the compartment formed in the cylinder.
The diameter of a piston rod is usually determined by the rod’s strength, stiffness, and stability. However, in reciprocating hydraulic drives, it is sometimes determined by the required velocity ratio of the to-and-fro motion at a constant pump capacity.
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