Ballscrew coupler question

I need to replace the coupler on my Fadal, it uses a keyed lovejoy coupler now and the new ones I see available are non-keyed.

Does anyone have experience with this type of coupler for a ballscrew: MTR-0154A

Are they secure enough?

I'm worried that without a keyway it will rotate on the motor or ballscrew shaft.

That does not look like a zero backlash coupling. The cheapest supplier of zero backlash couplings that I know of is Misumi USA.

Couplings - Disc, Stepped, Clamping, For Servo Motor

I prefer the ones that clamp to the shafts and don't rely on a key.

GAM and R+W both make good elastomer couplings (as well as Lovejoy, of course). You can generally order them with or without a key.

An elastomer coupling with the proper spider is zero-backlash, but it's torsional stiffness generally isn't as good as a bellows or disc coupling.

The key thing you're going to want to make sure when you replace your coupling is that you match the durometer of the old spider. They are color coded, but I think the color-durometer correlation might vary by manufacturer.

CNCPros carries the bellows style too, in the size I need.

My concern is over the lack of a keyway in the coupling, if the clamping of the shaft is sufficient for the torque transmission necessary.

Usually for a servo axis, the clamping hub carries the torque, as a keyed connection usually has backlash. A keyed and clamped connection does fail safe though, so if it's a vertical axis, I'd want the key just in case the clamping hub slips.

If you're trying to get your machine up and running fast, McMaster sells elastomeric couplings, all metric and sizes over 1/2" have keyways.

If you have more time and want what is probably a better product (I don't know what brand McM sells), I'd call R+W.

Dont put a cheap coupler on your motor. Those couplers with the soft plastic are slightly forced together and the cuts are for misalignment. Your motor will live longer with a good coupler

Those soft plastic spiders can be pretty hard. I have a R+W coupler between my spindle and 5hp motor and have had no issues in 2 years, but it does have a different clamp design. There is nothing cheap about it, very well designed and made. Don't worry about the keyway missing, if you need it you have other problems. Just make sure you torque the screw down to spec.

R+W has some good info on these and several clamp designs.

I'd want a key, but also want the coupling to be a clamp style. If you have a light mishap with the machine, you don't want to be wondering if the coupling slipped a degree or two. The momentum of the moving components during a crash will most likely move the joint.

Keyed fits have clearance around the key, so any amount of actual looseness of the shaft fit will cause the connection to fret and act loose. It is likely worthwhile to fuss with fitting a key to the actual width dimensions of the key in the shaft and the key in the hub. Off the shelf keystock has a lot of tolerance in width.

That appears to be a zero backlash lovejoy style. (The spider is red)
I have never had a clamp on style slip but we do clean it and the shaft with acetone before mounting.
Key type couplings offset the center slightly when the set screw are tightened. This leads to small errors, usually not a problem unless arc-seconds are a concern.
Bellows, disc, spring, oldham, or the above are never zero backlash under load and reversal. They all have published wind up ratings and curves.
Fussy people will check these numbers against their systems torque and allowable error. Particularly during rapid reversal in the early section of the acceleration curve or ramp.
No matter how stiff everything bends. Screws wind up also when the nut is out there a ways.

They also have differing cyclical errors from shaft misalignment. They lead and lag through one revolution even with no load.
Close to zero would be a better name.
Zero tolerance keys and key slots don't exist except maybe press fits.

Used inside it's load rating and on a shaft within print you won't spin a clamp on style.
Crash a decent sized motor at 3000 IPM into the end stops and somethings gonna give before that servo shuts down.
This coupling won't break, many of the other styles will. If you are lucky it will spin on the shaft. Unlucky... well that's me in spades.
They do seem gut feel not so good and I wondered the same as you and felt "I just would really like a key here" as it just seems sensible.
They do work very well.
Bob

Whether or not a lovejoy-style coupling has backlash is a function of the geometry and fit of the hubs and spider. If there are gaps, you will get backlash. Good modern couplings shouldn't have any backlash (at least by the definition the coupling guys use). I'm purely talking about backlash here, where you turn the input and get no motion on the output. Torsional stiffness (windup) is another matter. The color of the spider is coded for its hardness. A harder spider will have a higher torsional stiffness and torque capacity but less capacity for radial and angular misalignment.

Bob, I think we're talking about two different types of keyed couplings. In the type you are talking about, the key carries the load under normal circumstances and is held in by a radial set screw. The type I was talking about is almost identical to a clamping hub, but also has a keyway. Under normal operation, the torque (in at least one, probably both direction) is transmitted by friction from the clamping hub. In the event of an overload, the hub will slip enough to bring the key into contact. This can be good or bad depending on the application. In ordinary, low-ish speed operation, there won't be a difference between a clamping hub with or without a key. You need to decide what you want to happen in the event of either a) an overload or b) the clamping hub coming loose. Choose key or no key based on which failure mode is better for your application.

If you do decide to switch away from a Lovejoy-style coupling to a bellows coupling, be very careful installing it. If you ding up the stainless bellows, you'll get a fatigue failure, and bellows couplings definitely don't fail safe.

If you're coupling has a clamping hub, regardless of whether it has a key or not, you can find the torque spec for the clamping bolt in the manufacturer's documentation. Like Bob, I've never seen a clamp on style slip when properly torqued up.

You will have the best luck replacing a missing key with a new key and running the same type of coupling unless Brian Denny tells you otherwise. Brian was installing these machines in that area way back when. He is the President of fadalcnc.com and will still answer an email.

I have this style of coupling on my lathe and never had any problem with it. One ofy costumer also use this in the paper printing industrie and they cant afford it to slip or have anu backlash and they never had i think this jas been proven in the past.

I ordered that keyless zero-backlash lovejoy style coupler. Worse comes to worse, I'll just shim the gap and broach a keyway that the coupler has a keyway and clamp.

I'm just gonna run it as-is and see how it goes, my biggest concern is that the keyway creates an interrupted area of the shaft, so the shaft isn't completely smooth.

I could make up a dummy key for the motor to make it a smooth shaft, the ballscrew has a woodruff key, so that doesn't concern me as much, as there is a mostly round shaft there.

Whatever the outcome, I have plenty of stock on hand to make rigid couplers or modify the bought coupler. The Lovejoy coupler just didn't have the tolerance I wanted.

I bought mine from ITSCNC.
Backlash is backlash.. and you have settings for that. It cannot be entirely eliminated, however, it is easily compensated.
Mark

My Toyoda HMC's don't use keys, they use taper locks. Very secure but not a cheap coupling. The older cinci's used a piece of steel tubing ith a collar to drive a pin into a v groove on the end of the shaft. Not elegant but it worked, until it fatigued and broke in half. I'd be looking for taper lock if changing styles.