Pin Puller vs. Mechanical Actuator
As previously stated, a simple pin puller is a force-generating device. A common mistake for a designer implementing such a device for the first time is to confuse this device with a mechanical actuator, such as a linear screw or hydraulic piston, both of which are motion generating devices. What’s the difference? Let’s oversimplify and assume all conditions are ideal. A common linear screw or hydraulic piston will move at a constant rate regardless of the load (a “tractor”). This means the resistance presented to the actuator can be variable and ideally does not affect the overall behavior. However, a simple piston actuator is a force-output device (a “rocket”), and its motion is mainly governed by the familiar Force = Mass x Acceleration. It doesn’t take long to discover there is a big difference in the acceleration when the resistive forces vary from assembly to assembly, or even as the mass varies through iterations of design. When the mechanism is allowed to accelerate without restriction, the energy input into the driven system can be damagingly large (there is typically a shock load induced both at initiation and at end-of-travel). Compare the situation to pulling a plow with a tractor vs. a rocket. The tractor will push forward regardless of the resistance the plow encounters and overall the system’s behavior is easy to predict and model. The rocket can be made to work, but the resistance the dirt presents to the plow must be carefully balanced with the output of the rocket. Without that balance, the plow either never starts or it accelerates out of control, with the ideal behavior balanced precariously in between. As in the plow analogy, the resistances encountered in your mechanism aren’t always controllable.