I royally screwed up my first big project involving ball screws.
The project goal was to observe the effects of dipping test specimens into two separate temperature controlled oil baths repeatedly over a period of months. One chamber was kept essentially boiling and the other was maintained at cryogenic temperatures the whole time. The setup required a 2 axis linear gantry that moved like an overhead crane to automatically transport the specimens from one bath to another.
One last detail: The test specimens were irreplaceable.
The problems were relentless right from the beginning. Accurately measuring the bath temperatures and keeping them in the proper range was a difficult task all on its own, but I’ll leave that for a future post. The big problem with the gantry system was that the ball screw was not designed rigidly enough. The screw diameter I selected should have been larger considering its long unsupported length and the high speeds I needed to operate at. On top of that, the screw was supported by track bearings that were too small and too close together to adequately prevent vibrations. So this was a noisy machine, but I was pushed for time so I decided to let it go. It might have turned out OK if it didn’t have to last for months of endless use. It worked for a while at least.
…Until it didn’t.
Eventually the excessive the vibrations freed one axis from its support and the machine lost track of where it was. Since I didn’t use any physical limit switches to verify its position, the machine didn’t know it was poised directly above a steel plate when it decided to dip the specimens…
When I came to work the next morning it wasn’t a pretty picture. Because this project has to remain confidential I can’t share pictures of the actual carnage. What I can do is communicate the feeling with these pictures instead:
My managers didn’t have to tell me I messed up, I knew and I felt terrible. I never want to be in a position like that again so for future projects I created a worksheet to calculate the ball screw critical speed and buckling load before committing to a design. And of course I invite you to use it for your own linear positioning and CNC projects!
This calculator can also be used for lead screws, so I’ll leave you with this quick reference table for comparing the merits of ball screws and lead screws.
CLICK HERE FOR THE RESOURCE DISCUSSED IN THIS ARTICLE.
Wow how agonizing. It’s amazing how critical a screw can be. It seems this info is priceless for someone in your shoes. Interesting stuff and written in a way that can be understood. Good job!
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