Page 599 - The Mechatronics Handbook

0066_Frame_C20 Page 69 Wednesday, January 9, 2002 5:49 PM

FIGURE 20.95 Symbols of hydraulic rotary motors.

FIGURE 20.96 Hydraulic rotary actuator (Parker Hannifin).

Linear Actuators
Linear hydraulic motors constitute the most commonly used type of actuator. They provide a rectilinear
movement realized by the stroke of a rod connected to a piston sliding inside the cylinder. A distinction
is made between single acting and double acting cylinders. The former only permit a single work stroke
and therefore the pressure of the fluid is exerted on the surface of the piston in one single direction; the
retract stroke is made by means of the force applied externally to the cylinder rod, or with the aid of a
helical spring incorporated with the actuator inside a chamber. The latter permit both strokes, so that
the fluid acts alternately on both faces of the piston, generating both the advance and retract strokes.
Double acting cylinders may have a single rod or a double through rod. These are composed of a tube closed
at the ends by two heads, and a mobile piston inside the barrel bearing one or two rods connected
externally to the load to move. As it is fitted with sealing gaskets, the piston divides the cylinder into two
chambers. By sending the oil under pressure into one of the chambers through special pipes in the heads,
a pressure difference is generated between the two surfaces of the piston and a thrust transmitted to the
outside by the rod. Figure 20.97 shows the constructional solution of a hydraulic double acting cylinder
with a single rod. Single rod actuators are also known as asymmetrical cylinders because the working
area on the rod side is smaller than the area of the piston, as it is reduced by the section of the rod itself.

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