An interesting aspect of actuator design popped up in a couple of recent applications. The efficiency value of the actuator isn’t often the top parameter on the list of things to check when making a selection. But it should be. Efficiencies vary wildly, even from the same vendor, and will have signficant impact on the resulting machine design.
Why? First because losses in the sytem result in friction and heat. Every time energy is converted from one form to another, there are losses. In electromechanical systems these losses are most often heat. If the application has high cycling rates, the heat created by the losses need to be considered.
Acme screws are a great solution that is inexpensive and widely available. The design parameters for Acme screws are pitch, diameter, material, surface finish and nut. The most significant issue is the pitch. At low pitch ratios, the efficiency can be 60% or higher. At high pitch ratios, like 10:1 lead, the efficiency of some Acme screws can be very low, 35-40%. When thermoplastic materials are used for the drive element, the efficiency becomes a critical issue. The efficiency translates as heat, with high cycling the heat will cause deflection in the material that can make for added difficulty in the actuator.
A recent project I worked on had a screw with 35% efficiency, which means that 2/3 of the power into the mechanism was being dissipated as heat. Not a good situation when the load is moving constantly or at high speed. As the parts heat up, the polymer nut finally heated up to the point where it melted. By changing the drive train to a planetary gear reducer and a screw with low pitch, the screw becomes a low speed actuator with double the efficiency, cutting the load requirement and heating in half.
In ball screw actuators, the circulating ball bearings and lubricant reduce the friction dramatically regardless of pitch. Some ball screw actuators are 99% efficient, so there are no significant issues to consider in the losses. The increased efficiency comes at a substantial price, but it also comes with high precision. So for high precision actuators, the ball screw is a natural choice.
Rolling bearing actuators are another solution to linear motion, which also have incredibly high efficiency. There are many vendors using rolling bearings of various configurations. With low coefficients of friction of 0.2%, rolling bearings are an incredibly efficient solution at a very low cost. Most often the rolling bearing systems are driven by belt and pulley systems which are also very efficient.
Again, frictional considerations become very significant in subtle ways. Early evaluation can insure higher performance and lower cost in many designs.