through the use of a crank-slide oscillator (slotted link motion) as shown in Fig. 7.7. The sliding
block in the slotted link is fitted to an eccentric disc which is driven by an electric motor through
a gearbox and clutch. The ram is connected to the slotted link. The ram speed is dependent
upon the rotational speed of the eccentric disc and the length of the stroke which can be
adjusted by varying the amount of eccentricity. Because of the kinematic principles applicable
to a crank-slide oscillator, the velocity conditions illustrated in Fig. 7.8 (left) apply. The unequal
velocities obtained are used to advantage by applying the slower velocity to the forward stroke
and the faster velocity to the return stroke ( generally known from applied mechanics ).
The main advantage of a hydraulic drive is (as may be seen in Fig. 7.8, right) the near-
constant cutting speed during the whole working stroke. Since the working stroke demands a
large power for a relatively low ram speed and the return stroke requires little power for a high
ram speed, the piston has differing surface areas on each side to give these conditions.