Thread: Bugga@#$%%

Sorry Guys

Meant to add some pictures but forgot to upload them.

Here they are.

What sort of gearbox?

Sounds like too much sideways strain on the shaft.

Greg

Greg
Pretty much a straight knock-off of the gearing arrangement on your machine from memory.

Also remember copying from Daniel's machine - I think.

Using P.I.E.S. AT5 pulley and belts.

I am about to replace the motor with an old Keilng one from my first machine. I guess if the problem re-curs, I will have to have a hard look at the design of the gearbox - if there is too much side-pressure, I might be able to change the design of the belt-tensioner - what I'm using at the moment is pretty rudimentary.

Cheers

Noel

Hi Noel

First of all let me say up front I have no experience with rack and pinion, only ballscrews but my motor supplier always insisted there had to be a bearing to take the weight and whip of the screw, and to never be tempted to put a angular contact bearing at one end and then connect the screw/coupler directly to the motor at the other. I think the R&P setup would put a lot more load on the motor shaft than a screw would.
I know the last thing you want is more work but perhaps it could be redesigned so that the pinion is on a shaft supported and running on bearings to take all the side load with the motor coupled to the end required to only transmit torque.
Then again Greg and many others haven't had any trouble so maybe you were just unlucky?
Cheers
David

Hi Noel,

I had the same issue and ended up going through 3 steppers!

At first i thought it was due to me over tightening the belts, but it definitely wasn't that as through testing it happened whilst 'relatively' slack.

The only other thing I could put it down to was there may have been to much weight/resistance via the pulleys.

We ended up upgrading the steppers to 3/8" shafts and never saw the issue again.

Automatically went with this size shaft on my CH1 build.

Cheers
Glenn

Hi Noel

First of all let me say up front I have no experience with rack and pinion, only ballscrews but my motor supplier always insisted there had to be a bearing to take the weight and whip of the screw, and to never be tempted to put a angular contact bearing at one end and then connect the screw/coupler directly to the motor at the other. I think the R&P setup would put a lot more load on the motor shaft than a screw would.
I know the last thing you want is more work but perhaps it could be redesigned so that the pulley is on a shaft supported and running on bearings to take all the side load with the motor coupled to the end required to only transmit torque.
Then again Greg and many others haven't had any trouble so maybe you were just unlucky?
Cheers
David

Now that you mention it my steppers have 3/8" shafts too.

But for this shaft to break after only 100mm travel, sounds odd to me.

Greg

Couple of thoughts
Off centre pulley bore, pulley overhanging shaft, spring tension on pinion causing too much friction.
I wouldn't think the shaft would shear off by iteslf. The Kelling motors come out of China as well so perhaps as suggested you upgrade motors to 3/8 shaft.

Hi Guys
Thanks for all the input.
David - your suggestion about bearings etc on the pinion shaft is exactly what I have done. Everything is supported on bearings.

Rod - could be some misalignment as you suggest - have to have a much closed look at things - i am also a bit wary now of just how much pre-load/tension I have on the belt, but I would have thought that the belt would stretch before it caused the motor shaft to snap - but then again, maybe not.

Greg - do you have a belt-tensioner in your gearbox housings, or is it just a 'tight fit'design?
Also - what size motors are you running? The only motor with a 3/8 shaft that I could readily find is one from Keling, and that is a 570oz job.
This could be OK, but it does pull 5.0amps (??).

At the moment, I have swapped motors and am using one of the steppers from my old machine (now stripped and gutted). It will be interesting to see whether the problem repeats itself.

Any further suggestions greatly appreciated.

Cheers

Noel

Hi Noel,
I wasn't so much suggesting misallignment but more the forces that would cause the shaft to shear off. The most probable cause will be something binding and the motor screwing itself to death. I would be looking at binding (R&P allignment), too much tension on the pinion to rack, and lastly magnifying problems through the belt/pulley drive.

My money is on the belt being too tight.

Don't think the stepper has enough torque to twist the shaft off.

If the pulley bore was not concentric it could have a part of its revolution where the belt became tighter.

These belts have Kevlar cords in them and are amazingly strong and greatly resist stretching. Definitely not like a rubber band.

My belts are tensioned via slotted mounting holes for the stepper.

500 oz 3/8" shaft

Also don't have the belt as tight as I did at first. Concerned about overloading the stepper bearing.

Greg

Hi guys

Rod and Greg - your advice and inputs are always most welcome, and always extremely sage and informative. I think the best thing is that you both contribute so much, and are willing to offer help when required.

I think that Greg might be on the money this time - I checked things out - and - typical of me - if something is worth doing, then I tend to overdo it - and yes - the belt tension was very high (almost no flex). I have an idler wheel inside which bears down on the belt to maintain tension (didn't want slippage), and I have taken it out and turned it down to a smaller size. Result is less tension on the belt - hopefully this will fix things, but I am still thinking seriously about up-grading the steppers to 570oz with 3/8inch shafts.

Will wait and see what happens in the coming days.

I still have a bit to do to finish of the machine, so no immediate urgency.

I guess the acid test will be when I start to cut something substantial where the machine has to work for an ectended period. Should be interesting.

Thanks again guys for the positive input. A great help.

Cheers

Noel

I guess I got the terminology wrong with my suggestion when I said pinion. What I meant was for the pulley that is attached directly to the motor and relying on the strength of the motor shaft and bearings could instead be mounted on a shaft. This shaft would be supported at each end with bearings allowing you to apply as much tension as you feel comfortable, without any load being applied to the motor shaft or bearings. One end of the shaft would pass through the bearing allowing a coupler to connect the motor. This way the only load applied to the motor is torque.

Cheers
David

Noel,

Your other axis is exactly the same. Have you looked at this for potential wear?
By chance, did the motor stall just prior to the shaft shearing off? I was thinking this could have caused the rack to get misaligned with the pinion/shaft enough for it to shear.

Also, was this on the axis with the single drive or the two motor drive. Just wondering if there was any racking occurring causing the axis to twist or something? I mean you cut out a few jobs on this already, so it's strange.

Ps does Chris have any suitable steppers on his site?.

/ Michael