a winkler-technology Insights Article
Its tiny, its easy to design and implement and yet it has caused so many failures and costs: the set screw. And therefore I’m dedicating an article to this seemingly mundane, little component.
No offense: if you never heard of a set screw and don’t know what it is, then this article is not for you. But if you are a design engineer or if you are a manager of engineering, the following 2..3 minutes might be well spent 🙂
Most of us operate a mechanical device with an integrated set screw every day and it works just fine: the lever of the door handle at your home or at your office. Typically a little set screw is used to keep the handle form slipping off the axle.
If you ever mounted a door handle you might have noticed that the axle inside is square and the handle has a correspondent square-cut fit for the axle. This ensures a form-fit or positive locking for the torque transfer. Good solution!
Unfortunately a square axle and especially a square-cut are not how a typical axle or your typical tube, wheel, hole etc. looks like. Normally they are round. Because that’s what you get when machining an axle with a lathe or when creating a hole with a drill.
Most every machine, most every mechanical device contains one or several round axles with matching hubs – also round – but also the requirement to transfer a torque between hub and axle.
Let’s be honest: for every designer, for every engineer the quick fix solution is tempting: use some kind of pinion or stud going vertically through hub and axle. It can be used to transfer the torque and it holds the two in place relatively to each other. Isn’t this a simple, perfect solution?
And what easy way to achieve this using a set screw: It is that “kind of pinion or stud going vertically through hub and axle” and because it is a screw it can also be easily assembled or disassembled.
Failure alert !
Some designs I’ve seen rely on the point of the set screw to dip into the axle and to transfer the torque. Even without heavy start-stop operation this ‘solution’ will fail quickly.
But also if there is an actual external thread in the axle: a screw thread is not made for transverse forces. Over time the set screw will increase the gap to the external thread, especially in start-stop or reversing operations modes. Until the point of mechanical failure.
Yes there are connections with screws to transfer torque. Your typical flange is mounted with screws. But here the screws are used to tighten the two pieces of the flange together and to create a pressure between the surfaces. The torque is transferred via the pressed surfaces and not via the screws.
I learned the following the hard way and hope this little article makes your learning experience more pleasant:
- never use a set screw to transfer torque
- a set screw might be used to prevent a hub from sliding along the axle _if_ the dynamic forces are low or infrequent.
Please let me know about your experience with set screws and which solutions you prefer for a hub and axle torque design.