Gear mesh-Is this normal?

[Anthony RS]

Hello, 

 

Here are a couple of videos for the gears of my AVX.

 

In this first video, https://www.dropbox.com/s/btv9q9k8pkv3k1p/VID_20190319_155428.mp4?dl=0, do you think the gears are too close and tight together or is it fine?

 

In this second video, https://www.dropbox.com/s/qj3dygvrc0s17zc/VID_20190319_160042.mp4?dl=0,  is that kind of worm gear backlash normal or is it too much?

In this third video, https://www.dropbox.com/s/sd9dlx5d5qxa4ed/VID_20190319_165746.mp4?dl=0 , is that sound normal? It sounds like it's slowing down then speeding up?

Cheers!

 

Anthony

[Oddsocks]

Hello Anthony

Most engineers will tell you it is difficult to make an accurate assessment from video, you really need to be there with hands-on.

Looking and listening to the videos I would make a guess only.....

In the second video it looks as though you are not seeing worm-RA gear backlash but the spur gear on the end of the worm shaft is loose. If you look at the end of the shaft you will see a flat surface ground into the diameter of the worm-shaft, in the gear hub itself you will see one or more grub-screws, one of the grub screws should line up with the flat in the shaft and be tightened firmly, if this grub-screw is loose then the gear will rock slightly from side-to-side. I think this is what you can see in the video, not the worm shaft turning with backlash against the RA gear but the spur gear loose on the end of worm shaft.

In the first video the gear mesh looks ok, adjustment has to be made so that for a full rotation of the gear the mesh pressure is constant, or as near constant as it is possible given the manufacturing tolerances. This connects with the third video, it's not clear that the mount is speeding up or slowing down, only that the noise varies during one worm rotation and being a rattling noise this is unlikely to be from the worm-to-main-RA gear since it should be well greased and thus damped against noise, but is most likely coming from the spur gears or motor reduction gear. Because it appears to be varying with the worm rotation period it seems reasonable that it might be connected to the apparent looseness of the spur gear on the worm shaft and the variation is due to a slight bend in the worm shaft itself, or slight eccentricity in the bore of the spur gear, causing the mesh pressure to vary as the worm turns.

Only recommendation I could make is to check that the grub screw(s) in the spur gears of both the worm shaft and motor shaft are tight.

Most likely these are Allen headed grub screws so make sure the tool you use to tighten them is the right size and standard. Being an American mount from a Chinese factory the grub screws could be either metric or imperial, if you use the wrong size and standard of Allen key you may damage the heads and be unable to tighten or remove them.

Using the correct size and standard Allen key check the Allen screw(s) in both the motor shaft spur gear and worm shaft spur gear are tight, do not over-tighten, the grub screws just should be nipped up. If over-tightened they will strip the threads or fracture the heads. Apply plenty of grease to the spur gears then readjust the worm-RA gear mesh pressure and test again. Do not try to set too high a worm-RA gear mesh pressure as you will cause excessive wear and damage the worm and RA gear, the adjustment has to be just enough to remove backlash in the worm-RA gear mesh for as much of one full rotation of the worm as is possible. If too much force is applied when adjusting the worm-RA gear mesh pressure you may permanently bend the worm shaft, just a little pressure is needed, roughly equivalent to a bag of sugar (~1 kg) resting on the worm as it presses against the RA gear.

If you find the grub screws keep working loose after a period of time then you can use a very low strength screw locking compound to help keep them tight, but it has to be low strength or you may never be able to release them again for maintenance. We used to use a little nail varnish painted onto the threads of grub-screws before inserting into gear hubs, while still liquid, as it is easy to remove again later once set by soaking the end of the grub screw that protrudes from the gear with acetone, which softens the nail varnish and allows the screw to be released. If you use a commercial screw thread locking compound then it has to be very low strength such as Loctite-222, and this can be softened by applying a little heat with a hot-air gun or the tip of a soldering iron against the head of the grub screw.

Where are you geographically? are there no astronomy clubs near you? You can usually find at least one good mechanical engineer at most clubs that can help out.

William.

P.S. I'm travelling for the next three weeks so I most likely will not be able to visit SGL for a while. TBH with a mechanical problem such as this you would do better finding a local fellow astronomer through your nearest society to help out....

[Anthony RS]

15 minutes ago, Oddsocks said:

Hello Anthony

Most engineers will tell you it is difficult to make an accurate assessment from video, you really need to be there with hands-on.

Looking and listening to the videos I would make a guess only.....

In the second video it looks as though you are not seeing worm-RA gear backlash but the spur gear on the end of the worm shaft is loose. If you look at the end of the shaft you will see a flat surface ground into the diameter of the worm-shaft, in the gear hub itself you will see one or more grub-screws, one of the grub screws should line up with the flat in the shaft and be tightened firmly, if this grub-screw is loose then the gear will rock slightly from side-to-side. I think this is what you can see in the video, not the worm shaft turning with backlash against the RA gear but the spur gear loose on the end of worm shaft.

In the first video the gear mesh looks ok, adjustment has to be made so that for a full rotation of the gear the mesh pressure is constant, or as near constant as it is possible given the manufacturing tolerances. This connects with the third video, it's not clear that the mount is speeding up or slowing down, only that the noise varies during one worm rotation and being a rattling noise this is unlikely to be from the worm-to-main-RA gear since it should be well greased and thus damped against noise, but is most likely coming from the spur gears or motor reduction gear. Because it appears to be varying with the worm rotation period it seems reasonable that it might be connected to the apparent looseness of the spur gear on the worm shaft and the variation is due to a slight bend in the worm shaft itself, or slight eccentricity in the bore of the spur gear, causing the mesh pressure to vary as the worm turns.

Only recommendation I could make is to check that the grub screw(s) in the spur gears of both the worm shaft and motor shaft are tight.

Most likely these are Allen headed grub screws so make sure the tool you use to tighten them is the right size and standard. Being an American mount from a Chinese factory the grub screws could be either metric or imperial, if you use the wrong size and standard of Allen key you may damage the heads and be unable to tighten or remove them.

Using the correct size and standard Allen key check the Allen screw(s) in both the motor shaft spur gear and worm shaft spur gear are tight, do not over-tighten, the grub screws just should be nipped up. If over-tightened they will strip the threads or fracture the heads. Apply plenty of grease to the spur gears then readjust the worm-RA gear mesh pressure and test again. Do not try to set too high a worm-RA gear mesh pressure as you will cause excessive wear and damage the worm and RA gear, the adjustment has to be just enough to remove backlash in the worm-RA gear mesh for as much of one full rotation of the worm as is possible. If too much force is applied when adjusting the worm-RA gear mesh pressure you may permanently bend the worm shaft, just a little pressure is needed, roughly equivalent to a bag of sugar (~1 kg) resting on the worm as it presses against the RA gear.

If you find the grub screws keep working loose after a period of time then you can use a very low strength screw locking compound to help keep them tight, but it has to be low strength or you may never be able to release them again for maintenance. We used to use a little nail varnish painted onto the threads of grub-screws before inserting into gear hubs, while still liquid, as it is easy to remove again later once set by soaking the end of the grub screw that protrudes from the gear with acetone, which softens the nail varnish and allows the screw to be released. If you use a commercial screw thread locking compound then it has to be very low strength such as Loctite-222, and this can be softened by applying a little heat with a hot-air gun or the tip of a soldering iron against the head of the grub screw.

Where are you geographically? are there no astronomy clubs near you? You can usually find at least one good mechanical engineer at most clubs that can help out.

William.

P.S. I'm travelling for the next three weeks so I most likely will not be able to visit SGL for a while. TBH with a mechanical problem such as this you would do better finding a local fellow astronomer through your nearest society to help out....

Thank you William for your great explanation. Unfortunately, I live in Beirut, Lebanon so I have no one to go to. I've hypertuned the mount myself and so I'm checking if everything is as it should. I think the grub screws are tight enough but I'll double check. As for the worm-Ra gear mesh, this is where it's getting tricky with me. I seem to have a lot of backlash in the RA axis which shows as slop and free play. No matter how much I reduce the spacing (till the motor binds) the slop is always there, I've also tightened the worm's locking ring but the free play is still there. I was wondering if you know what could be the cause of that. According to Ed from DSP, free play is unavoidable since before hypertuning, grease was masking this slop, but after cleaning and re-greasing, there's no way I could fully get rid of this slop. I'm worried now that I might have the worm-gear mesh a bit too tight to avoid even more free play but I don't know if loosening it a bit more would cause my tracking to get worse. I've got good tracking results during my last test but I don't want to cause any damage to the worm and gears ?

 

[Oddsocks]

1 hour ago, Anthony RS said:

I seem to have a lot of backlash in the RA axis which shows as slop and free play. No matter how much I reduce the spacing (till the motor binds) the slop is always there, I've also tightened the worm's locking ring but the free play is still there.

If the motor is binding and stalling when you increase the worm-RA gear mesh pressure then backlash can not be the cause of the slop. By jamming the worm and RA gear into mesh with enough force to stall the motor then backlash between worm and RA gear that can be felt cannot exist because the mesh pressure forces the worm and gear tightly against each other. 

Backlash may be present during motorised rotation of the worm under such extreme conditions but only if the worm or gear are badly worn, or poorly manufactured such that the points of the gear teeth are riding along the valley floor of the worm thread, or vice-versa, but you would not be able to feel this, only measure it with mechanical and optical testing. Normally, in a well made gear and worm the point of the gear tooth does not touch the valley floor of the worm thread, rather the mesh is along the walls of the threads leaving the gear points clear of the valley floor.

Difficult to explain in words I'm afraid.

I have never seen an AVX mount so the advice I can give is only generic but from your description I doubt the backlash you are seeing is anything to do with the principle RA worm-gear mesh, with the possible exception being if the worm and gear are not aligned with each other, i.e, the RA gear is sitting further up, or lower, down the RA shaft with respect to the centre of the worm long axis, which is possible if you took the gear off the shaft at some stage and did not put it back in exactly the same location, or removed or added spacers to the RA shaft bearings etc, even so, if the mesh is tightened to the point that the motor stalls then backlash between the principle RA gear and the RA worm than can be felt should be impossible.

If you increase the worm-RA gear pressure to the point that the motor stalls then you run the risk of damaging the worm by bending the worm shaft or gouging the RA gear/ Worm, the mesh pressure should be no more than about 1kg maximum, as described in the last post.

William.

[Stargazer33]

I can't really tell from the video, but there doesn't appear to be a lot of grease on those gear wheels. Especially for somewhere like Lebanon.

[Anthony RS]

1 hour ago, Oddsocks said:

If the motor is binding and stalling when you increase the worm-RA gear mesh pressure then backlash can not be the cause of the slop. By jamming the worm and RA gear into mesh with enough force to stall the motor then backlash between worm and RA gear that can be felt cannot exist because the mesh pressure forces the worm and gear tightly against each other. 

Backlash may be present during motorised rotation of the worm under such extreme conditions but only if the worm or gear are badly worn, or poorly manufactured such that the points of the gear teeth are riding along the valley floor of the worm thread, or vice-versa, but you would not be able to feel this, only measure it with mechanical and optical testing. Normally, in a well made gear and worm the point of the gear tooth does not touch the valley floor of the worm thread, rather the mesh is along the walls of the threads leaving the gear points clear of the valley floor.

Difficult to explain in words I'm afraid.

I have never seen an AVX mount so the advice I can give is only generic but from your description I doubt the backlash you are seeing is anything to do with the principle RA worm-gear mesh, with the possible exception being if the worm and gear are not aligned with each other, i.e, the RA gear is sitting further up, or lower, down the RA shaft with respect to the centre of the worm long axis, which is possible if you took the gear off the shaft at some stage and did not put it back in exactly the same location, or removed or added spacers to the RA shaft bearings etc, even so, if the mesh is tightened to the point that the motor stalls then backlash between the principle RA gear and the RA worm than can be felt should be impossible.

If you increase the worm-RA gear pressure to the point that the motor stalls then you run the risk of damaging the worm by bending the worm shaft or gouging the RA gear/ Worm, the mesh pressure should be no more than about 1kg maximum, as described in the last post.

William.

That makes a lot of sense actually. I'm not sure if the worm and gear are perfectly aligned although I did use the same spacers that were before. So I guess there's nothing I could do except wait till I can upgrade to a better mount. 

[Anthony RS]

33 minutes ago, Stargazer33 said:

I can't really tell from the video, but there doesn't appear to be a lot of grease on those gear wheels. Especially for somewhere like Lebanon.

I did use a very small amount of grease after I cleaned since that's what others usually do when hypertuning their mounts. Wouldn't using more grease make the mount sticky and sluggish again?

[Stargazer33]

Depends on the grease. I think a lot of members here who do their own servicing/upgrades/regreasing, use a white lithium based grease, or PTFE (Teflon) grease. You need enough that it isn't going to dry out in hot weather or under prolonged use. The last thing you want is metal grinding against metal. Take it from someone suffering with arthritis in their knuckles!