For edge profile or 'plate' cams such as the one shown, motion supplied by a cam rise is often refered to as being the 'positive' motion. Generally, positive motion is used to move away from any potentially interfering tool path such as nests on an indexed rotary dial or our in-line chassis. This precludes the chance of having a machine crash because of a broken spring.
The use of an air cylinder is often substituted for the spring to act as an 'air-spring'. Also, an air cylinder can be used as a 'lock-out' device. By actuating the air cylinder in the opposite direction, the cam lever is prevented from moving by holding the follower at the cam's major radius. This technique is often used, for example, at a rejection station where actuation is needed intermittantly.
Electric solenoids can also be used to lock-out motions. Although a spring is still required for the system, the solenoid is much faster than an air cylinder and therefore appropriate for high speed applications. Solenoids also require less maintenance.
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