Spiral anti-backlash couplers introducing backlash!

[MArruda]

Hello,

I've been working on my CNC router, built from a Fine Line Automation kit about a year and a half ago or so. Everything is going great, aside from some backlash issues with the Z axis.

My problem is odd- I hope others have expereineced it and can give some input. The backlash is being caused by the spiral cut motor to shaft coupler. With the Z axis carriage not attached, just the coupler shaft and upper bearing block, everyting turns fine. With Mach3 set to a .001 step per keystroke, everything moves fine with each step. When I attach the acme rod and Z axis carriage, there's .001 backlash- the motor turns with every keystroke, but the bottom of the motor coupler tied to everythin else stays stationary for one step- .001- of movement before it starts moving. With the router and dust collection shoe on the unit, there's a .002 takeup before it starts to move. You can visibly see the top of the spiral cut motor coupler turn and tighten the spiral like a spring before the rest of the assembly starts to move.

The problem with this, is that it's not repeatable backlash. The amount of "spring" being introduced by the motor coupler is variable when the motor is running, depending on if the Z axis is making a fast decel to reverse direction, etc. The problem is not exibited on the X and Y axes- I believe it's an effect of gravity and the weight of the motor and carriage on the coupler.

Any ideas? Would I lovejoy 3 piece type coupler be better for the Z axis?

Thank you,
Michael

[Al_The_Man]

If this is a helical coupler, is it aluminum instead of stainless?
Al.

[MArruda]

The couplers are these:

CNCRouterParts

I believe they're aluminum, actually, but could very well be wrong.

-Michael

[Al_The_Man]

They appear to be aluminum which is not the ideal type for stepper/servo drive, for applications like encoder they do well, but helical stainless would be a great improvement and takes motor flex much more without failure.
Al.

[MrWild]

i bought three like that from linear Motionbearings2006on eBay. Totally worthless. The ones off eBay had .800 (20mm) diameter bodies. Slinkies have better torque load stiffness. I've been looking off and on for affordable stainless ones with a 1' diameter. So far, no love.

[gio666]

All of the Spiral are not meant for torque ,and will get lots of backlash in the aluminium ones, you can turn in your hands and break in half ,best would be rigid coupling ,or this type is the best to use in aluminium and closed to rigid and still be a little flexible coupling you will find

http://x0cnc.com

[wantacnc]

Is your Acme shaft turned down and in a bearing block with angular contact bearings? Could you tighten the retainer nut to adjust the preload in the block? The helical coupler shouldnt have load on it only to transfer and couple the stepper to the rod, and to take up any alignment issue.

I could see it trying to tighten up if there is alot of drag or pressure in the z rails. Does your z slide smooth when moved by hand?

Im not a expert on this but just thinking and throwing it out there for discussion on your backlash
Joe

[RICHARD ZASTROW]

I usually use the stainless steel bellows type couplings. They transfer torque without backlash and still tolerate some minimal misalignment.

Dick Z

[leaveme]

@Michael:

I had similar problem in one of my machines. My couplers are own built helical couplers (L 30mm x D 25mm) from alu. I replaced them by steel but still some backlash is there. Then I discovered it is actually my ballscrew, which is flexing in the bearing. I locked the ballscrew at stepper end (bearing) and backlash is gone.

Just my two cents...

[MArruda]

Thanks for the replies everyone!

In the design, the assembly is motor->Helical Coupler->1/2" drill rod->Bearing block->ridgid coupler->acme screw->Z axis carriage. With the ridgid coupler unattached to the acme screw- thus taking the screw, carriage and router out of the assembly- there is no backlash. When the screw is reattached without a router on the carriage, there is .001" backlash. With the router, it brings it up to .002-.0025" backlash. It's definately the weight of the carriage and router that the coupler can't handle. You can visibly see the coupler "wind" up as the motor turns but the assembly doesn't.

Anyone know where to get bellows couplers in SS with 1/4" on one side and 1/2" on the other?

Thanks again,
Michael

[Connor9220]

Originally Posted by MArruda Thanks for the replies everyone! In the design, the assembly is motor->Helical Coupler->1/2" drill rod->Bearing block->ridgid coupler->acme screw->Z axis carriage. With the ridgid coupler unattached to the acme screw- thus taking the screw, carriage and router out of the assembly- there is no backlash. When the screw is reattached without a router on the carriage, there is .001" backlash. With the router, it brings it up to .002-.0025" backlash. It's definately the weight of the carriage and router that the coupler can't handle. You can visibly see the coupler "wind" up as the motor turns but the assembly doesn't. Anyone know where to get bellows couplers in SS with 1/4" on one side and 1/2" on the other? Thanks again, Michael

I would look at your bearing block... You shouldn't have any linear movement at all with the ACME screw when it's mounted in your bearing blocks. The coupler should only have to deal with rotational forces.. I had to add spacers in my bearing block to remove the play. Thus eliminating all the backlash. The helix design is to allow for a slight offset or miss-alignment of the motor and screw.

Thanks, Connor

[MArruda]

Thanks for the reply, but I don't think you're understanding the issue- when the bearing block is attached to the motor through the helical coupler, but not attached to the acme screw through the ridgid coupler, there IS NO backlash. The issue is DEFINATELY the helical coupler- you can see and feel it winding up to take up the rotational torque before it starts to move, when there is a weight- the carriage and motor- on it.

Thanks,
Michael