Coupling stepper to ball screw

[garagefela]

Hi everyone,
I have a query regarding the coupling of a stepper motor to a ball screw.
Can you use a solid coupler when the ball screw is supported with back to back angular contact bearings at the motor end?

Or do you still need some sort of flex coupling with some give in it?

Obviously the solid coupler is easy to make and costs nothing more than a bit of material and some time, but real flex couplers can be a bit dear.

Cheers all, M

[dertsap]

i use spider couplers , they are cheap

[garagefela]

Hi, Thanks for that.
I suppose I was sort of asking in the original question, do you have to use a non solid coupler? I probably was not specific enough in the first place

Asked another way, please give me a reason if/why I should NOT use a solid coupler? (In terms of zero axial movement)

The amount of shaft growing in length from the motor to the angular contact bearing would be very minimal in my mind and not worth worrying about.

There might be something I have not considered, so thanks.

Cheers all, M

[dertsap]

no reason why you can t use a solid coupler other than the fact that if everything is not abasolutely dead nuts you will have bouncing or binding happening , if you work in a shop then there is no reason you can t make everything to a perfect fit

[GAWnCA]

dertsap,

Do you have a good source to buy them from? USA perferably. Thanks

Originally Posted by dertsap i use spider couplers , they are cheap

[HuFlungDung]

You might be able to use a solid coupling if you don't over-constrain the motor. A lot of motors have a C-face with a boss to fit into a precisely made opening in a support bracket. However, this might be worse than a very generous sized opening if the bracket is sloppily attached off of the true ballscrew centerline.

What you want to do is make a solid coupling with an extremely good fit on both shafts. This way, you do not introduce angularity between the two shafts. Both diameters at each end of the coupling must be perfectly aligned, and this means bored in one setup in the lathe.

Then, install the coupling on the ballscrew and the motor. Heat expansion of the coupling may help get a close fit to actually go together without excessive force (like hammering the hell out of something).

Leave the mounting bolts in the flange of the motor slightly loose. Run the motor. See if it vibrates during operation. This can be checked with a dial indicator on the motor housing. If there is any oscillation of the motor, remember when you tighten the bolts up, that the bearing in the motor is going to have to absorb the force locked in by the misalignment. It may fail prematurely if the runout is excessive.

It would actually be better to remove the front bearing from the motor housing and let the ballscrew bearings guide that end of the rotor than to have 3 bearings running close to one another in poor alignment.

[dertsap]

Originally Posted by GAWnCA Do you have a good source to buy them from? USA perferably. Thanks

any industrial supply store should have them

[fpworks]

Hu hit the nail on the head with shaft misalignment. You're going to cause premature bearing failure on your stepper.

I think you "might" be able to get away with it if you were to surface grind all the mating surfaces, from ballscrew support bearing mount to stepper motor mount. At that point, you might as well use flexible couplers.

I've used Ruland before, and they will make it to your size if you have a unique application.

www.ruland.com

[GAWnCA]

Thanks all for the information!

[NC Cams]

Here's why solid couplers are tough to implement:

What is solid???

A noted VMC maker tried a "solid coupler" (metal hard rubber metal) and found that it "chuckled" when it turned. We ran into similar problem with a slotted pin joint when we tried to affect a "flex joint" between two non-planer drive/driven shafts.

So they went to a flex joint that had essentially a coil spring between two couplings tied to the shafts. The spring wasn't rigid engough and it would torsionally oscillate at certain start stop sequences. WE lived with the chuckling in our case. The machine tool guys went thru huge internal grief as it turned into a corporate pizzing contest.

Until/unless you can quantify the torsional rigidity (as in ft-lb per deg of flex) to superb levels, you can get into some real messy drive harmnic situations unless you're careful.

This is a reason why cogged belt drives are SO forgiving - their torsional rigidity is pretty darn high and they do have charm with respect to packaging and torque multipication.