Using a hand crank for automation may sound like an oxymoron, but there are times when it makes sense. Sometimes an application requires the robustness, precision, and customization that is available from automation equipment, but it also requires manual control. The issue with this is that automation equipment is generally meant to be driven by a motor. Manufacturers of equipment such as linear actuators and gearheads make it very easy to mount any kind of servo motor you want, but if you ask them for a hand crank then you are usually met with confusion and resistance.
Figure 1: Hand crank mounting mimics a servo motor.
I recently ran into an application that required a large screw driven linear actuator. The payload was close to 1,000 lbs and the linear actuator could be tilted up to the point where it was almost vertical. The application also required very fine control over position. Normally I would specify a large servo motor with a fail-safe brake (to prevent the load from falling if power is lost). However, the customer wanted to manually control the system with a hand crank.
Figure 2: Components from left to right (gearhead, drive shaft, machined plate, bushing, lever brake, handwheel).
It would be fairly easy to put a hand crank onto the input shaft of the screw drive, but there are a few problems with that. Even with a fine lead ball screw, like 5mm per rev, it would be somewhat difficult for a person to lift the 1,000 lb payload. It would also be difficult to get very fine position adjustment. Last, the load would back drive when vertical, so we would also need to add a brake, which would be difficult to integrate onto the input of the screw drive.
Figure 3: Completed Assembly
The Hand Crank Solution
The solution to making it easier for the operator to lift the load, as well as adding fine position control, would be to add a gearhead to the system. However, the input on the gearhead is meant for a servo motor. I looked for a standard hand crank solution from different gearhead manufacturers but there weren’t really any options available on the market. So, I decided to design a hand crank system with a brake that mimicked the mounting dimensions of a common servo motor (See Figure 1). It uses a custom machined plate, bushing for radial load support, lever operated shaft brake, keyed drive shaft and a hand wheel (See Figure 2). The end products came out very nicely, turned smoothly, and looked great (See Figures 3 and the main image above).