# Helical Couplings



## grepper (Mar 7, 2016)

So, the motors I ordered have an ~8 mm shaft.  

http://us.stepperonline.com/3pcs-of...motor-35a-31nm439-ozin-24hs343504d-p-398.html

The CNCFusion site says the helicals they have will only work on 1/4" shaft - not exactly ~8 mm.  Is that going to be a problem?  If so, is it worth sending the motors back and getting different ones?

What about this?  Way less than $25 from Fusion.  
http://www.amazon.com/uxcell-5mmx8mm-Helical-Coupler-Coupling/dp/B00KHTVOEU
Shaft Usage : 5mm to 8mm / 0.2" to 0.31"(1/5" to 5/16");Length : 25mm / 0.98"

It seems to indicate motor shaft from 5 mm to 8 mm.   How does that work?  By carefully centering the shaft using the surrounding set screws?

Geeze.  I can't seem to do anything with endless questions arising!


----------



## omni_dilletante (Mar 7, 2016)

Misunderstood question.  Reply removed.


----------



## stupoty (Mar 7, 2016)

I hear the helical couplers tend to load up and cause backlash issues, apparently the flex disc type are better. 

Others with more experiance with them may have better info.

Stuart


----------



## TomS (Mar 7, 2016)

I have no experience using helical couplings so I can't comment on how well they will work with your setup.  For reference on my PM-932 conversion I use double disk diaphragm style couplings and I can't detect any backlash associated with the couplings.  Cost was about $21 each on eBay.

Tom S.


----------



## Jim_Hbar (Mar 7, 2016)

Mark:

You need to get couplings that have the correct bore(s) for the shafts being used - the one you linked to is 5mm on one end and 8mm on the other.
Couplings that are split through the hub and clamp onto the shaft are a better solution than those relying on setscrews.  Like the type shown with the integral clamps here.

Personally - I've never had great success with beam (helical) couplings, but that was in applications that usually ran 16 to 24 hours a day.  If you have three the same, I would recommend buying a spare.


----------



## dave2176 (Mar 8, 2016)

An Oldham coupling is very good in this usage. Available from McMaster and others. The helical are available in the size you need. Ebay is a good place for them.


----------



## grepper (Mar 8, 2016)

Thanks guys.  Until today I had not thought of how to connect the motor.  Really shows how unfamiliar I am with this stuff.  After a bunch of searching around I see that there are all sorts of different types; premachined  major & minor bores, some that can accept a range of motor shaft sizes, 5 mm to 8 mm for example, others that are called "zero backlash" but are split with different ID bore hubs available, etc.  Of course there are many to choose from.

I'll check out the ones you have pointed to.  Nothing like the voice of experience.

I don't know the length I need or the max OD that there is room for.  I've emailed Fusion (they don't have a listed phone), but have yet to hear from them.  Oddly, it appears that they only offer 1/4" motor shafts couplings.  Strange.

Really appreciate the input.  Thanks!


----------



## JimDawson (Mar 8, 2016)

If nobody mentioned it, you want the split ones, not the ones that have set screws that impinge on the shaft. 

The other option is to build your own zero lash couplings  I designed and built couple for a quick disconnect system for a stepper system on a special machine.


----------



## grepper (Mar 8, 2016)

Even though I hadn't given it any consideration, at first I thought there would be some sort of shaft, bored at each end and the motor would somehow just clamp to the screw.  I mean, how difficult is that?

I was staring at pages and pages of various couplings and it suddenly (I can be dimwitted sometimes) dawned on me the reason for all the fuss; that these things are acting like an auto u-joint.  Somewhere along the line I missed that step.  I'm guessing that once all this kit stuff is bolted together that things probably don't align perfectly, and if there was not some flex between the motor and the screw that something, probably the motor, would get trashed fairly quickly.  Duh.

Now it makes total sense why 


Jim_Hbar said:


> Personally - I've never had great success with beam (helical) couplings, but that was in applications that usually ran 16 to 24 hours a day. If you have three the same, I would recommend buying a spare.



The helical stuff appears to flex because of slits cut into the side of the coupling allowing the metal to bend.  That would seem to be doomed to failure from metal fatigue, or crud buildup in the cuts in the metal.

Obviously I don't know much about this, but I would think there would be a better solution.  Or maybe for this application it would be just fine. I'm clueless.   I'll check out the Oldham couplings that Dave suggested.  

Backlash in the whole drive assembly would seem to me to be really something to be avoided as much as is reasonable.


----------



## grepper (Mar 8, 2016)

JimDawson said:


> The other option is to build your own zero lash couplings



That will have to be my second project.  If you remember, my first project is to build a pump...


----------



## grepper (Mar 8, 2016)

Can anyone who has done this conversion tell me what length coupling I need or the max OD there is room for?  There seems to be different OD's and, of course, lengths available.


----------



## TomS (Mar 8, 2016)

grepper said:


> Can anyone who has done this conversion tell me what length coupling I need or the max OD there is room for?  There seems to be different OD's and, of course, lengths available.



Install your motors and make sure you have about 1/8" between the shaft ends.  Measure the length from the ball screw shoulder to the face of your motor.  This is the max possible coupling length.  Realistically the coupling length will be somewhat shorter based on what is commercially available with the bore sizes you need.  The max OD is determined by the clearance hole in your motor mount and any other obstructions on your machine.  Now search for couplings that meet your criteria with the thought that the more shaft engagement the better.  As someone else mentioned the clamp style couplings are better but it doesn't hurt to drill and tap for a set screw also.  Hope this is clear and I didn't confuse you.

Tom S.


----------



## grepper (Mar 8, 2016)

Thanks, got it.  Very helpful, guys.  Guess I'll have to wait to order couplings until after assembly.  I was surprised to see even Amazon stocks a pretty good selection.  Free delivery is good.  Would have been nice if the Fusion folks had a wider selection of couplings so I could have just placed one order.  Oh well...


----------



## TomS (Mar 8, 2016)

grepper said:


> Thanks, got it.  Very helpful, guys.  Guess I'll have to wait to order couplings until after assembly.  I was surprised to see even Amazon stocks a pretty good selection.  Free delivery is good.  Would have been nice if the Fusion folks had a wider selection of couplings so I could have just placed one order.  Oh well...



You might also try eBay.  Shipping time usually is a couple of weeks for items of this size and often free shipping is available.

Tom S.


----------



## Jim_Hbar (Mar 8, 2016)

With couplings, you pick your poison.  They all have their pro's and con's - and if there was a product/design that was clear winner, the rest would have largely disappeared.
Beam, Oldham, Curved Jaw, and other designs are all used successfully in this hobby CNC market.
Here's a link to just one manufacture's treatise on the topic.

If you are purchasing a kit, then you need to have a dialogue with the kit supplier, rather than second guessing their design.
If you don't agree with their design, design and make a "kit" yourself.
The axial length of a coupling changes with the design of the coupling, and changing the type and size of the coupling will change the geometry of the mount - and not all couplings require an 1/8" gap between the shafts - many require a significantly larger gap!

Have you thought about the possibility of boring the couplings to the size you need??


----------



## grepper (Mar 8, 2016)

CNCFusion does not seem to be that conversational.  I emailed them yesterday asking what model # coupling they were using and what current lead time looks like but have yet to get a response.  They don't list a phone number.

I did think about boring out the coupling, but I'd rather just purchase the size I need if possible.  For all I know the inside bore of the coupling has nice machining to enhance it's grip on the shaft.  I haven't seen one so I don't know.

I'm just speculating since I have not seen the Fusion kit installed, but I would think that the distance between the motor shaft and the screw would be more or less fixed, determined by the length of the screw, motor shaft, and the thickness of the motor mount.  I would guess that the screw position is pretty much fixed, and the only easy way to increase the motor shaft to screw distance would be to cut the motor shaft, or make a new mount, or add spacers, etc., or decrease the spacing by machining the mount.  Hmm..., come to think about it, I hadn't thought that various motors may come with different length shafts.

As long as I meet the spacing requirements of the coupling, it fits the shafts and doesn't bind against anything I would think it would work.  Aside from getting a zero backlash coupling, simply connecting the motor to the screw can't be that big of a deal, can it?  (, said Mark, all the while realizing he might be laboring under a delusion).


----------



## grepper (Mar 8, 2016)

Jim_Hbar said:


> Here's a link to just one manufacture's treatise on the topic.



Thanks for the link Jim, very informative.  In fact I was looking for a summary of the basic different types.

My, my.  Look at all the wonderful choices... High angular misalignment capability, Low reactionary loading; low inertia, Fail-safe design , Four different types of urethane elastomers available (what more could I ask for?), Highest torsional stiffness!  Oh boy!  That sounds good.  I want that!


----------



## JimDawson (Mar 8, 2016)

grepper said:


> I did think about boring out the coupling, but I'd rather just purchase the size I need if possible. For all I know the inside bore of the coupling has nice machining to enhance it's grip on the shaft. I haven't seen one so I don't know.



A reamed or bored hole would be fine



grepper said:


> Hmm..., come to think about it, I hadn't thought that various motors may come with different length shafts.



The NEMA spec sets the shaft length (and the diameter), but some of the NEMA 23 motors have 5/16 shafts and others have 8mm.  The actual NEMA spec is 5/16 for a NEMA 23.  The lengths have all be pretty much the same.  I have seen a number of stepper motors from Asia that have metric shafts.


----------



## grepper (Mar 8, 2016)

I'm using Nema 24's.  Maybe that explains the 8 mm shaft.

http://us.stepperonline.com/3pcs-of...motor-35a-31nm439-ozin-24hs343504d-p-398.html


----------

