Screw Design Basics

Actuator design is a series of tradeoffs to reach an optimized solution.  The problem can be very complex because there are no rules for the design process.  The connection between the mechanism and the source that powers the load seems arbitrary, but the impact is significant.  Since all of the major components have to be considered in terms of  cost, the best solution for a given requirement can appear to be somewhat obscure.

The most common and inexpensive linear actuator technology is the screw.  Its been around for thousands of years and can be very versatile.  The screw is mostly known as a fastener, but in the actuator world, it can be engineered to achieve accuracies in the ten-thousandths of an inch and forces in the tons.  So the first set of tradeoffs to consider are the accuracy needed and the best style of screw to achieve it.  There are a lot of great technical resources available from several vendors like Thomson and others that will provide extensive details on screws.

We’re not going to get into details in this post, its too big a topic to cover here.  What I hope we can accomplish is to come up with some good ideas for creating a strategy for your project.

One key parameter is finding the right screw pitch.  Often, the screw pitch is driven by simple issues.  A 5 to 1 screw pitch connected to a 200 step per revolution stepping motor results in a easy position control of 0.001″ per step.  So if you need a bit more accuracy, you can increase the pitch to a 10 to 1 and you can have accuracy of 0.0005″.  By using microstepping controllers you can drive the accuracy to almost any level.  There are limits to the measurable repeatability, but you get the idea.

Increasing the leadscrew pitch works against the speed or throughput of the mechanism.  So this is a major balancing act.  If you have a high throughput requirement, the pitch has to be limited to what the speed requirement dictates.  But another element of the screw pitch is the mechanical advantage that allows increase torque capacity in the actuator.  This allows screw actuators to operate in very high load applications like hydraulics.

Sometimes the screw ratio is used primarily for mechanical leverage.  Higher ratios mean either more torque from the same source, or a smaller input torque requirement for the same load.  The mechanical advantage often leads to a very economical solution.

The other parameter that speed works against is life expectancy.  The faster the actuator has to go, or the more cycles per hour, the lower the total number of hours the actuator will survive.  In the industrial world, with 8000 production hours per year, engineering a screw actuator for years of operation is a big issue.  Huge effort has been invested in understanding lubrication, surface coatings and bearing materials so that high life expectancy is achievable.  In the industrial environment, a 20 million cycle life is not unusual.

When there are many variables in an application, the goal is to manipulate the constraints to get the best overall results.  Fortunately, that’s relatively easy.  For a change.