[nick mulder]

Hi,

I have a 16mm motor shaft about 35mm long that I need to couple to a another shaft 18mm in diameter but it has been milled flat on both sides to make the shape you see in the pictures. It is 8mm wide, 6mm deep down the shaft and there is an area of about 1mm of true round shaft that could be used if need be, otherwise all there is the flat um, 'thing' to grip to.

The motor is a 1.8Kw 5000rpm BLDC that will couple to this ER32 live tool for a DIY metal milling CNC machine, no numbers but a fair amount of torque involved - its will be servo driven and will be expected to be able to reverse with no backlash for work like rigid tapping and so on...

How would you design and make a coupling here ?

At present I have no lathe, but I do have a mill with an ER40 26~25mm collet in which I can make a kind of home-brew lathe if need be (and some steel rod of the same dimension, but maybe this is too small) - also a boring head will arrive in the mail shortly... A slitting saw of unknown width and radius, but I'm pretty sure sub 1mm and sub 100mm is available on loan also.

Maybe this is a job for a real machinist ? in any case, how would you design it ?

What are the considerations/implications involved ?

Many motor couplings have rubber inserts or helical spring sections to account for the slop and concentricity issues in installation. What I was going to build was very rigid, but the flip side of that is that I need the alignment to be spot on...

Any help appreciated!

Kind Regards,
Nick Mulder

[nick mulder]

well - this is really annoying

uploading images isn't working ...

Is my verbal description enough ?

#1 Round shaft - simply 16mm diameter 35mm long

#2 Round shaft 18mm diameter, 7.5mm long - with two equal flats on it cut 5mm in each side, 6mm down the shaft - resulting in an 8mm 'bar' to couple to - not enough roundness, except that 1mm or so to deal with ...

[nick mulder]

yup... just stalls on 'uploading files'

they are all well within the range of acceptable size and res

[judleroy]

If what I'm picturing is correct then I would say drill and tap your coupling 180 deg apart for set screws so you can push your flat to the center of the hole. Hope that makes since. Good luck.
Judleroy

[nick mulder]

Hmmm, yes - that could work nicely ! 4 set screws - 2 per side ? and would you put in bit of something for the set screws to work on so the shaft just 'felt' a nice flat face (steel, alu?)

[judleroy]

I would say 4 on 90 deg from each other. 2 to center in the hole and 2 to grip the shaft. If you can grind some small flats like you mentiond I think that might help. That sure is a small shaft to have to grip. Have fun.
Judleroy

[nick mulder]

I'm thinking of using an existing coupling I have here - its a 16mm one that I can bore out to the 17.8mm shaft (I got out the micrometers)

Pretty much like this but with a relatively bigger hole in it already:



So with its clamping force I already have two of the set screws sorted - they'll bear on the still round ends of the flat shaft and the tapped set screws will be 90deg to them to bear on the flats as you've suggested (with a bit of filed 'just so' steel to act as a middle man on both sides)- just drill and tap right on through pretty much huh.

I thought maybe 2 set screws per side still 90deg angle to the flat faces but offset about centre line so they bear further out on the flats than centre - more lever arm that way... I'd just to do two on the relevant ends, but since its going to be spinning both ways four would be required...

Its nice (and fiddly) in that along with fastening the whole situation it'll also allow a small amount of centering adjustment to get rid of any wiggle waggles ...

[RICHARD ZASTROW]

I like to use something similar, the stainless steel bellows type. They do not have the compliance or "give" of the plastic or rubber center member. They still allow for minor misalignment.

For your short shaft though, you will still need some modification.

Dick Z

[mactec54]

nick mulder

Putting set screws/grub screws, will not work for what you want to do, This has been done this way for a special coupling, you need to mill a matching slot in the coupling, for it to work as it was intended

[Al_The_Man]

I would also mill a slot in the hub of a coupler, the shaft appears that it may be tapped?
If so a socket heat screw or similar could be used to retain the hub, if not tapped, drill out deeper and tap.
Misumi offer a few nice alternatives in their coupling section that go down to small hub centre hole dia.
Such as their CPO Oldham or sleeved jaw spider CPF type.
Al.

[nick mulder]

nick mulder

Putting set screws/grub screws, will not work for what you want to do, This has been done this way for a special coupling, you need to mill a matching slot in the coupling, for it to work as it was intended

Hi,

I fear milling a matching slot is beyond my current machining ability here - if one face of the slot is too 'inwards' then the whole lot will wiggle around, once I reach the exact point, which is to say, cut past the exact point I'll have a slot that gives backlash. I have to do this twice, and once I get to test it, if it's wiggling then the other side is too far by definition...

Why wont the 4 set screws bearing against some small filed flat bar work ? - I thought that once they were adjusted correctly the whole lot could be set in place with something, at the very least thread lock the set screws, but no reason the whole lot couldn't be set in something more substantial, it might not look the same to the eye, but it is effectively a slotted coupling

yes/ no ?

I would also mill a slot in the hub of a coupler, the shaft appears that it may be tapped?
If so a socket heat screw or similar could be used to retain the hub, if not tapped, drill out deeper and tap.
Misumi offer a few nice alternatives in their coupling section that go down to small hub centre hole dia.
Such as their CPO Oldham or sleeved jaw spider CPF type.
Al.

Shaft isn't tapped - there is something in there that looks intentional I fear I could be drilling into something special! its a live tool, any ideas ?

But as it's going to be spinning both ways, wont the screw just unscrew itself ? Especially if there is backlash ...


P.S. the rubber style couplings are called 'zero backlash' - but then that rubber... must be some backlash, however small right ?

[nick mulder]

A friend who supplied the live tool drew this after I mentioned the set screws scenario:



only two set screws but a much more substantial insert with a bit of flex in it for the set screws to bite on and clamp the shaft

[mactec54]

judleroy

That will work just fine, if you make it that way,That's also is the easiest way to do it, Make the bushing a tight/press fit in the coupling

[nick mulder]

judleroy

That will work just fine, if you make it that way,That's also is the easiest way to do it, Make the bushing a tight/press fit in the coupling

I figure not larger than the shaft itself though - or the coupling cant bear against the outer edges of the still round shaft flat

Maybe I can flat the insert at those points a little...

17.80000000000001mm for the coupling bore hole - 17.7999999999999mm for the insert - 17.8mm shaft


If I did it really nicely and nailed the slot in the insert I wouldn't even need the set screws - but I wont nail it, trust me :drowning: