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Re: G8 DEC axis backlash, fixing, questions

 

Henk, et al,

Just a quick overview...

The thread here has focussed attention on the newest version GM811 units with tucked motors and spring loaded worms.??

The major problem reported by many is a high hysteresis? meaning a long time lag between telescope movement and a worm direction reversal. This is making autoguiding difficult or ...PHD2 fails.?

Several possible causes must be ruled out one at a time.? Here is a checklist of tests and procedures:

1. Make the worm to ring gear mesh tight and look for a loose dovetail or slipping clutch disk.? ?How: On the DEC axis drive, back out the "back off" small bolt near the spring, until the worm will not turn.? Do not run the motor.? Grab the dovetail and try to wiggle it.? It should be completely immobile.? If it wiggles, suspect the dovetail bolts are either loose or bottomed out.? If bottomed out you must file the end down to shorten the bolt. Get the dovetail to be tightly held.

2. The GM8 clutch disks are small 3.0 inch OD and 1.25 inch ID.? Does the DEC axis clutch slip?? If so the disk may be oily.? Take the DEC axis apart and clean the plastic and mating metal surfaces. See attached PDF about alternative clutch pads.?

3. Now the worm still will not turn because we jammed it.? But the worm might still shift left to right.? Let's look at the chance the worm assembly can shift around.? How: when you grab the dovetail and try to rotate it, the worm is attached.? See if you notice the worm OPW assembly shift while you are wobbling the dovetail around in the DEC direction.? If the OPW can shift, then extra time will be needed for the autoguide system to respond.??

See if you can determine the cause of the shift.? You can first tighten the left pivot point block bolt.? Now wobble the DEC axis.? Do you see the worm move?? If the worm moves then the end block is not tight enough toward the left bearing block.? ?Perhaps the bolt holding the far block has loosened or shifted.? Fix this (or add a Belleville spring...some work here to remove and undersize the bearing, do the spring will work).??

4. If the worm won't move by wobbling the DEC dovetail, great.? Next step is to see how much the transfer gear on the gearbox needs to move before the DEC axis responds.? How: remove the DEC motor.? This allows the gearbox output shaft to freely turn.? Try to rotate the gearbox output gear back and forth.? How far can you rotate that before it feels the jammed worm?? If it rotates a lot, sustect a loose Oldham coupler metal part.? To get to that, bolt down both left and right bearing blocks.? Then remove the thick aluminum OPW cover.? Now you can see the Oldham coupler completely. Ensure both metal ends are tight.? (I often take out the tiny grub setscrews, and drill and tap to install 4-40 setscrews.? Those enable me to use a larger hex socket wrench.? If you need some if those set screws contact me.)

5.? Also check to see that the 2 transfer gears setscrews are firm.? (Sometimes these have tiny setscrews and can't be loosened or tightened!)..See note above about? replacing them with better 4-40 setscrews that can more easily be removed or tightened.

6.? Now that every item has been checked for hysteresis with the worm locked into the ring gear, there will be too much friction. You can't leave it like this in the evaluation condition with the worm jammed.? ?The next step is to drive the "back off" bolt in to relieve the heavy spring pressure forcing the worm into the ring gear.? ?Use just enough bolt inward movement so the worm will turn by hand using the gearbox transfer gear.? (The DEC motor is still off!).?

7. Now put the motor back on and run the Gemini DEC axis in both directions.? There should be no Lags or Stalled messages.? If you get a stalled message try reducing the digital setting for max Slew rate motor speed.? If that won't solve it you must back the worm off more with the back off bolt, but be careful...the more you back that off the more time there will be when it switches direction.?

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I'm done....I hope this step by step investigation will turn up good suspects and close the mystery case for you.

Best,
Michael?




On Tue, Apr 6, 2021, 4:13 PM Henk Aling <haling@...> wrote:
On Sun, Apr 4, 2021 at 10:21 PM, alan137 wrote:
I also tried a configuration with a very soft spring and no back-off bolt for a truly spring-loaded worm.? With this set up it is very important to have a hard stop that prevents the worm from lifting up and skipping over the ring gear.? I found that twisting the axis one direction lifts the worm up, and the other direction pulls it down into the gears, so there must be some kind of asymmetric shape to the ring gear tooth profile.? Anyway, I could sometimes also get a very low backlash, but this system was too unreliable in terms of always getting a low backlash.? I think the low spring force makes the worm block "wobbly" and it doesn't seem to sit on the ring gear consistently.? Too bad.? And if there is too much axis friction, a turning worm gear will try to "ride up" on the ring gear, whereas in a system where the worm is locked in place, it would be forced to push the ring gear along.?
Does this mean that the back-off screw is meant to keep the worm from riding up the ring gear because if it does, the assembly wobbles?? That would be disappointing.

From my own experiments (DIY v3 see the other thread), I have no trouble pushing the worm into the ring gear without the motor binding up.? However since both bottom screws are loose (by slightly loosened screws), the worm assembly can tilt and it does - mainly because I have steppers attached with a flex coupler.? Because the motor and worm are slightly misaligned the left block wobbles.? I will attempt to fix that by applying a Belleville washer at the bottom of the bottom screw of the near block so it pulls the near block down, hopefully tight enough to level it (and thereby the whole worm assembly) against the bottom. If that works I still have to worry about the block staying in place horizontally (the spring pushes it to the far side of the hole for the near bottom block) so in the worst case I have to build a stop so the block can't travel within its hole.

I have never seen the Losmandy spring loaded worms in detail other than the assembly/adjustment video.? If it has a wobble problem, would it help to pull the block down at the pivot by applying a (Belleville) spring underneath it?? An old design by Chip on CN had springs at the bottom IIRC.? In the worst case add a brace around the worm assembly that provides a pivot at the top to keep it from wobbling.? Pivots on both sides should greatly reduce the wobble.

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