Price vs accuracy
In our last posting, we talked about the positional accuracy of a point of the moveable plate or work surface of our ball screw table with the terms roll, pitch, and yaw. The application that you’re trying to solve should be with the most cost-effective positioning solution. Using the application’s specified positional requirements should define these parameters. You do due diligence in your search for a product that meets your requirements and you find that ball screws systems are significantly more accurate than what you need and they are pricey too. Well, don’t fret. There are ways to make these rotary-to-linear converters less expensive.
Using a PTFE coated ACME screw and polymer nut
“I’ve got one word for you…plastics.” (Sound familiar? It’s from the movie “The Graduate” ©™®*)
Hmmm, should this have to have a footnote or something?
Ok, plastic is good, but Teflon©™®*, also called PTFE in its non-branded form, is better. Replace the ball nut with a Teflon nut. Replace the ball bearings that support the ball screw with bronze or Teflon bushings. Replace the linear bearings that support the work plate with PTFE-on-PTFE surfaces. And replace the ball screw itself with an acme screw.
Yes, the Road Runner©™®* and Wile E Coyote©™®* have come up with a viable ACME product, the ACME screw. (Hmmm, should I have to footnote Loony Tunes ©™®* too?) Go ahead and Google “ACME screw vs. ball screw” and see what you find. I’m not kidding. I’ll wait.
OK, so you didn’t find any reference to the Road Runner©™®* or Wile E Coyote©™®*. That part I was kidding about, but you should have found that another name for an ACME screw is a lead screw. An additional benefit for using this screw thread form, besides cost, is that its operation is quieter. If the pitch is high enough, an acme screw won’t back drive. This is an important feature if the application requires moving the load vertically. You typically don’t want to have the load drop if there’s a power failure or the motor’s coupling breaks. One of the reasons a high pitch lead screw won’t back drive is because its efficiency is in the order of 20% – 30%.
Weighing the application: Ball or ACME?
Advantages of an ACME screw are:
- ·They cost less.
- Self-locking and doesn’t require a braking system
- It’s a viable solution for vertical applications
- Reasonably quiet operation
Disadvantages of an ACME screw are:
- Less efficient
- It may require more torque to move the load, thus the motor and drive may be larger and more expensive
- Higher nut friction (low efficiency) creates a higher nut temperature.
- They’re not well suited for high speed and/or high duty cycles.
Advantages of a ball screw are:
- Highly efficient (90% or greater)
- It may require less torque, thus the motor and drive may be smaller and less expensive.
- Well suited for high speed and/or high duty cycles.
Disadvantages of a ball screw are:
- More expensive
- Requires a braking system for vertical applications.
Now this one really needs to be footnoted: Google: ”Helix Nook The Engineer’s Guide: Lead Screws vs. Ball Screws©™®*
More next time.
* I figured I’d cover my bases by using ©™®. One of them has to be correct. Right?