4 Considerations When Choosing a Lead Screw

When engineers come to me looking to utilize a lead screw to actuate their application, I get them thinking about four main data points that can help determine the majority of questions I need to ask to help them utilize lead screws and polymer nuts properly. Those data points are payload, speed, duty cycle and stroke.

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In this blog post, I'll dive deeper into each of those data points and discuss additional factors that you should consider when choosing the right lead screw for your application. 

Payload & Speed

The first step is determining the quantified axial payload. This figure helps determine how much force is required to drive the application. 

Second, you want to take the application's speed into account. I usually ask an engineer a certain set of questions to help determine what the goal is for the application. It could be as simple as how many seconds do you want to complete this stroke in? Or, how quickly does it need to get from point A to point B?

The RPM (feed rate / lead of desired screw) is important but is typically determined after you have figured out the application&#;s linear feed rate, also known as the linear distance traveled per minute. Once you have that figure, you can determine the RPM of the application by taking a look at your desired lead screw and noting the distance traveled per revolution. 

Duty Cycle & Stroke

When working with a polymer lead screw nut, one of the more important factors that's going to determine the lifetime of the part is the application&#;s duty cycle. How often is it going to be running per minute, per hour, per day?

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This is extremely important for a polymer part because if the application has a higher load and a higher speed, the wear is going to be higher for the part. So, to properly spec a lead screw size, you need to take duty cycles into account to make sure that the PV values are in check.

Also, make sure you ask yourself how long is the application stroke and how long does the overall screw need to be?

Additional Considerations

Payload, speed, duty cycle and stroke are four extremely important data points; but that's not all that should be considered.

You should be asking yourself whether or not you need a lead screw that's going to be self-locking or if you want a part that is able to back drive. You should be thinking about the types of linear guides that you're using for an application. Are you going to be using a sliding guide? Or, are you going to be using a rolling guide, like a reciprocating ball bearing? Those types of questions are going to help determine whether or not you can use a smaller or a larger lead screw in your application. 

You also want to take into account how you're going to drive this application. Are you going to be using a DC motor or a stepper motor? Are you going to be hand cranking this application? These are some of the secondary factors that you want to consider when you're trying to narrow down what lead screw you use in your application. 

If you'd like advice on how to determine the best lead screw for your application, contact me here. You can also try out our lead screw product finder and lead screw configurator to calculate service life, build lead screws in 20 minutes or less, and more.   

I've experimented with heat forming uhmw around a threaded shaft. I made a two sided nut by simply clamping two softened pieces of uhmw around a threaded shaft. I used some wood to make 'clamping plates' that would allow the soft uhmw to press into the threads on the shaft. The wood pieces have a shallow groove cut into each one where the shaft will lie when it's pressed in a vice or a press. The end result is a two part nut that has 'wings' which can be used for mounting the nut, and also allow you to adjust the play by skimming material off the inside surfaces of these wings. In use, one side of this two part nut can be bolted solid to whatever it needs to propel, and the other wing can either be shimmed apart some to create some play, or clamped tight to eliminate play, or have some material skimmed off then bolted together to create a tight fitting nut. Or the second wing can also be used to hold the nut to it's home in the mechanism, and both wings can be shimmed or skimmed as needed to adjust the play. Whichever would work best in the room available for the nut.
I found the initial fit to be tight without any shimming or skimming, and if anything, there needed to be a thin shim between wings to create an easier turning shaft.
It's my feeling that a nut formed this way from uhmw is superior to one turned in the lathe, in the same way that a rolled thread in steel is superior to a turned thread. Smoother threads and more accurate.
During my experiment, I determined that an accurate two part press mold would be needed, as the uhmw faithfully takes on every little offset and wrinkle in the mold. You would be looking for accuracy without a next step machining operation, as this plastic is 'springy', and will take some skill to machine to an exact size. Best to mold it to finished size right away. Having said that, it wouldn't be hard to include markers in the mold to define where to drill mounting holes in the finished nut halves.
One thing about pressing the softened plastic around a threaded shaft this way is that the shaft tends to self-center. Of course your mold can always have alignment grooves to center the shaft automatically anyway. The two sides of the mold should have some means of aligning to each other as they are pressed around the shaft and softened uhmw pieces.
One drawback to a nut made this way is that the resulting threads in each nut half don't go full depth for a full half circle, but this could be offset by making the length of the nut longer.
Something which I thought of but didn't try is to coat the threaded shaft with a graphite lube or something, prior to the clamping. The idea was that some of that lube might end up locked into the plastic, giving a self-lubricating action.
Something else that helped propel me towards this experiment was the fact that the resulting nut made this way could be as long as you wanted it to be. The longer, the less stress per thread area, and the greater the accuracy in the leadscrew since more of it's threads are engaged in the nut at one time.

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