开云体育

I purchased the one piece worm from Losmandy hoping to get rid of some of my errors.
So far my 76 sec is worse than ever, and my PHD2 rms got worse.
I snugged the bolts for the worm block to worm housing first, then slightly snugged the bolts to the mount.
I then slightly loosened the worm block bolts to allow them to center with the worm, and then tightened them.
I've tried twice now and still get the same result
There's no noticeable backlash and it seems to run smooth, but obviously something is wrong.
The immediate thought is the worm blocks are "twisting" somehow and putting side load on the bearing.
Since I got almost the same results after R&R the assembly I wonder if the housing was drilled/bored off center.
Appreciate any help and input.

Thanks
Jim P

About mechanical indexing, are you referring to encoders?? If you just want to get the ring gear roughly in a predetermined position, cut the top off the cover plate, this exposes the ring gear, and mark it.? That way you can position the ring gear in a deterministic manner.?

Use of axial needle roller bearings for the RA and DEC axis shafts

1. ?They are not pre loaded.
2. Bearing load is uneven causing uneven wear on the axis.
3. required shaft clearance causes mis-alignment of the axis (axis 'sag') and non axial rotation
4. ?do not prevent wobble

Hi Henk
I'll try and address your questions

Don't the "touch points" as per Peter's drawing, depend on how the scope is balanced? If the scope is balanced towards the counterweight, these touch points would be just opposite.? And if the balance were perfect the touch points would be on the same side (low).Based on a deviation of 0.004" and a 16" distance between the bearings (just guessing), worst case, the angle between the shaft and the axis could be off by plus or minus (2×0.004/16)×(60×180/pi)=1.7'.? ?After a 180 degree rotation the polar alignment would be off by 3.6' if it were 0 at the CWD position.? So much for the arc second accuracy of polar alignment.? Frankly I doubt if it were that much.

The touch points don't change by balancing the scope. The total mass of the DEC axis, telescope and counterweight system is supported by the RA shaft and bearings. Even if there were no gap there would be uneven pressure on the needle bearings. The bottom half of the upper bearing and because it acts like a lever, the upper half of the lower bearing carry the load. The only way to equalize the load on the bearings is to pre-load the system with an opposite force.? The counterweights balance off the force on the worm gear and drive train.?

You are correct, the play in the system is far less than 0.004"? I don't have an easy way of measuring it, but i suspect it's around 0.001"

The number of 2' is normal for my AVX.? If I rotate it manually 15 seconds east then 15 seconds west while taking a 30 second image at 6400 ISO using a DSLR with the scope's RA center of rotation in the FOV, I see 2 star trail circles on the LCD with centers that are about 2' apart.? That's because the manual rotation positions the axis in different directions going one way vs the other because of friction.? I have not tried this with my G11S yet but that would be a simple test to measure the effect of the gaps between the shaft and the needle bearings.If the scope were perfectly balanced when tightening the clutches the touch points would be at the same side and the angle between the shaft and RA axis would be zero but the center would still be offset by 0.004" so the rong gear might still bind up unless using true SLWs or unless adding enough slack

The error from the required clearance of the RA shaft to be able to pass it through the needle bearings does not create the kind of error you describe. Peter has tuned his G11 to achieve sub arc-second RMS guiding accuracy. He posted it in one of the other threads. He was at the step limit of? 0.56 arc-seconds until he installed a higher ration gearbox which brought him down to the 0.14 arc-second guiding range.?

It's easy to lose perspective about the level of precision and accuracy being discussed.

?

1. Use of axial needle roller bearings for the RA and DEC axis shafts
   1. ?They are not pre loaded.
   2. Bearing load is uneven causing uneven wear on the axis.
   3. required shaft clearance causes mis-alignment of the axis (axis 'sag') and non axial rotation
   4. ?do not prevent wobble
2. Thrust bearings are not implemented properly
   1. Bearings are not pre-loaded.
   2. Bearings are loaded based on clutch operation not on proper setting for shaft rotation dynamics
   3. Bearing pressure not consistent. RA axis drag and dynamics not predictable or repeatable.
   4. Thrust bearings do not prevent non axial movement of the axis
   5. Bearing are exposed to the elements.
3. Clutch functionality
   1. Lack of mechanical indexing makes it impossible to recover from
      1. Releasing the clutch to perform required operations such as balancing disrupts the mechanical adjustment and calibration of the drive.?
      2. Clutch slip can occur and it is impossible to identify or quantify the loss of mechanical alignment calibration and adjustment
      3. It is impossible to accurately restore positional orientation after releasing the clutch.
   2. Operation is neither repeatable or reproducible
   3. Clutch pressure setting determines axis shaft thrust bearing load. Undesired cross effects
   4. Unloaded bearings can become misaligned.
   5. Thrust bearing load impacts drive motion dynamics and alignment.?
   6. No indicator or way to consistently set the clutch pressure
   7. Unwanted negative impact on drive performance.
   8. Clutch action is unpredictable
4. No indexing of mechanical components
   1. Impossible to maintain repeatability or reproduce performance.
   2. Mechanical alignment and adjustments are lost during the course of normal operations.
   3. impossible to reproduce mechanical alignment and adjustments.
   4. Inconsistent results even when following the same procedures.
5. Software based motion correction
   1. Software based motion correction gets out of sync with the mechanical position of the drive.
   2. Due to lack of indexing it is impossible to resync the software with the mechanical system.?
   3. Out of sync condition is only discovered by degradation of collected data and changes in guiding performance.
   4. Must eliminate other causes of poor data and guiding.
   5. Time consuming and data loss.
   6. Impossible to recover error correction data and apply it or re-apply it accurately.??
   7. There is no way to determine if the software motion control corrections are correct without fully testing and calibrating the drive.
   8. Fully testing the existing corrections takes as much or more time or more than starting over and collecting new data.?
   9. Stored PEC makes the guiding worse if it's out of sync by interfering with auto-guiding applications
6. Gear reduction ratios causes un-correctable PE errors
   1. Gear ratios are not even multiples so the periodic error is also not an even multiple. It is 'out of phase' so shifts over time
   2. Alternative gear box from McLennen has same 25:1 ratio but the intermediate ratios are even multiples and can be corrected with PEC.
7. Worm adjustment and mounting

This is a list of issues. I will try to bring forward ideas that are posted in other threads about solutions and link them in this thread.

Regards,

Michael

Don't the "touch points" as per Peter's drawing, depend on how the scope is balanced? If the scope is balanced towards the counterweight, these touch points would be just opposite.? And if the balance were perfect the touch points would be on the same side (low).Based on a deviation of 0.004" and a 16" distance between the bearings (just guessing), worst case, the angle between the shaft and the axis could be off by plus or minus (2×0.004/16)×(60×180/pi)=1.7'.? ?After a 180 degree rotation the polar alignment would be off by 3.6' if it were 0 at the CWD position.? So much for the arc second accuracy of polar alignment.? Frankly I doubt if it were that much.

The touch points don't change by balancing the scope. The total mass of the DEC axis, telescope and counterweight system is supported by the RA shaft and bearings. Even if there were no gap there would be uneven pressure on the needle bearings. The bottom half of the upper bearing and because it acts like a lever, the upper half of the lower bearing carry the load. The only way to equalize the load on the bearings is to pre-load the system with an opposite force.? The counterweights balance off the force on the worm gear and drive train.?

You are correct, the play in the system is far less than 0.004"? I don't have an easy way of measuring it, but i suspect it's around 0.001"

The number of 2' is normal for my AVX.? If I rotate it manually 15 seconds east then 15 seconds west while taking a 30 second image at 6400 ISO using a DSLR with the scope's RA center of rotation in the FOV, I see 2 star trail circles on the LCD with centers that are about 2' apart.? That's because the manual rotation positions the axis in different directions going one way vs the other because of friction.? I have not tried this with my G11S yet but that would be a simple test to measure the effect of the gaps between the shaft and the needle bearings.If the scope were perfectly balanced when tightening the clutches the touch points would be at the same side and the angle between the shaft and RA axis would be zero but the center would still be offset by 0.004" so the rong gear might still bind up unless using true SLWs or unless adding enough slack

The error from the required clearance of the RA shaft to be able to pass it through the needle bearings does not create the kind of error you describe. Peter has tuned his G11 to achieve sub arc-second RMS guiding accuracy. He posted it in one of the other threads. He was at the step limit of? 0.56 arc-seconds until he installed a higher ration gearbox which brought him down to the 0.14 arc-second guiding range.?

It's easy to lose perspective about the level of precision and accuracy being discussed.

I keep pinching myself that I finally own this level of gear having rehabbed and hypertuned Celestron and Meade mounts.? Those designs have some good things about them. but this is a different beast.

I may be able to call in to get the date of manufacture of my G11 but it has a black ring gear and is a gem-1 with the last version of the hardware. Standard un-tucked motors.
I'll confirm the worm gear and bearing arrangement tomorrow. It does NOT have space for a wavy washer between the clutch plates.

Does that put it in the 2005 time frame.??

I suspect that you've got things too tight so there is binding going on. i suggest you loosen the connections at the coupler, the gear reduction mounts, housing and the bearing blocks so there is some play everywhere. You could do this work with the DEC axis pulled but then there is no pressure on the worm gear to keep it flat against it's thrust bearing. I suggest you leave the DEC axis installed but adjust the lattitude adjuster so that DEC axis is as horizontal as possible. Adjust the tripod legs to help that too. Run with the clutch relaxed so you can run the RA? motor without the dec axis moving. Put something in the way in case the DEC axis grabs and starts turning unexpectedly. Be safe.? Put an index mark on the worm gear and worm so you can visualize the rotation. You may notice that the worm gear may show some smoothing or wear on some section. Since this is a new worm, you may want to rotate the worm gear so the 'used' side is away from the worm. This will give your 2 fresh unworn surfaces to work with and avoids any uneven wear on the worm gear from the stainless steel worm.?
Shift the worm into position against the worm gear so that it's centered and fully meshed. Snug down the bearing blocks gently and then allow the worm to ease off being fully meshed hard against the worm gear so it's at about 75% snug down the housing being careful to not put any pressure or twist on the bearing blocks. The bearing blocks must be square to the worm on all three axis. Relax the clutch. You should be able to turn the worm with your fingers and feel easy smooth free movement. When adjusting things here you never want anything in the gear train to feel tight or snug. When you snug up the mounting screws and standoffs go slow. Sneak up on 'snug' and tighten things evenly checking each time for any change in the free movement of parts. It's easy to have a bearing block twist or tilt and cause the bearing to bind. Be gentle, go slow and work evenly. tighten the housing screws being careful to keep everything aligned. The bearing blocks should still be able to move around a bit so that nothing binds. Check as your go and relax bolts if needed to get everything in it's place and just snug. Always err on the side of too much play and too loose. We can correct that later.

The last two things to adjust are the transfer gears/gear reduction and the coupler.? I secure the transfer/reduction gear housing first being sure that the gears have a little float . Then run with the clutcand no tension on them and the shaft into the coupler goes in straight and is lined up with the worm so the coupler is all loose and straight.
I snug down and secure this gear train. The last thing is the coupler. the shafts should be fully in to the coupler and in line with each other.? On a shaft with a flat be sure that one screw is square to the flat and centered on it. The 2nd screw is then snugged down.? I found it's easy to have a set screw that's supposed to be on the flat only to find that the flat isn't square to the setscrew. the problem is with the relative size of the grub screw to the flat.?
Power up and run the axis motor watch the coupler to be sure that the shafts are aligned closely. You should see it run without bobbing up and down and the coupling should not be stressed or look like it's being pulled apart (tension).

A lot can be told by the sound of the gear system and motor.? Listen for any uneven sounds, ticking or other periodic changes when running at full slew speed.

If, before you started this process your made an index mark on the worm gear run the system with the clutch disengaged until the mark returns to it's starting point. If it all sounds smooth, reassemble and test.

Be sure you clear any stored PEC data. You must fully initialize the system check and set all parameters and then do another shutdown and and additonal cold start.

good luck

Michael

All great ideas.

"Over time the condition you report of a 'tight worm' at 180 degrees from CWD is expected "

I assume you are making this statement because you expect wear to occur.? Lets assume wear is negligible for a moment. The fact that the Dec shaft can not sit perfectly centered on it's axis because of the needle bearing slop due exacerbated by the weight of the scope. When the clutch is tightened and you rotate the full 180 degrees it is unable to adjust it's position, hence pressure is placed on the ring gear that forces it into the worm at the 180 degree position. Of coarse as one rotates from CDW position this force steadily increases with the degree of rotation. If there was no movement between the shaft and the needle bearings then this problem would not exist.?

Actually, my thinking is that most of the slop in the RA shaft is taken up when the thrust bearings are loaded as the clutch is tightened, at least in theory.? I agree that the wear should be occurring on the brass worm, not the gear. However I do think that some smoothing and wear of the gear does occur. The 'tight worm' occurs when when during maintenance the worm mesh is readjusted on the smoothed side of the worm gear with very tight tolerance to minimize backlash. In your case, enough variance to eliminate the gap for lubricant as your pictures clearly document and even cause a motor stall.? In my case, the original owner had a steel worm, and only much later upgraded to precision brass and a OPW assembly. I expect more variation between the 'worn in' and 'unused' sides of the gear.? I agree that the problem wouldn't exist if there were no movement between the shaft and needle bearings and also a press fit worm gear on precision bearings rather than a bushing. Solutions to those problems are really fundamental changes in the design.?

The indexing issue I raise is a matter of repeatability.? We have a software correction system that we can't keep in sync with the hardware and a hardware system that we can easily throw out of sync with itself.??

I've been following several threads and the work being done to reduce guiding error and corrections.? The results have been amazing.??
Having put in that much effort and money it's important to understand what will cause performance improvements to be lost. Any hope of using stored PEC? depends on indexing
to be maintained.? My suggestions were to establish a consistent 'zero starting point' reference orientation.? Establish a 'Home' starting position for the worm gear and for the orientation of the DEC housing to the worm gear by indexing the across the clutch. If you are going to disturb the worm mesh or motor drive train, index the components so their relative positions can be restored.
It's the only way to create reproducible and repeatable results.?

I see enough design issues that I am going to start a separate topic for them.? Many of the suggestions, ideas and solutions go beyond process, procedure or part replacement with matching higher spec equivalents. We should recognize the difference for the sake of the larger user community who should not equate suggestions to use specific skills and techniques to optimize results as product shortcomings. Yes, in some cases shortcomings are avoided. This is true for any complex high precision equipment.? Technique and mindful process only becomes more important as the precision goes up.

I can't think of a recent better example than Peter' improvement on guiding and tracking accuracy.? A casual reader of the topic might think at the top
"that the G11 has poor performance and isn't very precise". However pretty quickly if you're paying attention you see that Peter is already getting repeatable arc-sec accuracy at the start of this effort. MOST people don't have the kind of gear or sufficient skill to achieve where Peter starts.? He reduces his RMS guide error down to 0.56 arc-sec and then down to 0.14 arc-sec, the limit being his motors smallest single step. Perhaps it's even a little better and he should slow the guide rate? I think at this level further effort has diminishing returns. Clearly his success is due to his skill and knowledge which he generously shares.

I learn a lot reading here.?
Especially - Don't set my goals too low

I purchased the one piece worm from Losmandy hoping to get rid of some of my errors.
So far my 76 sec is worse than ever, and my PHD2 rms got worse.
I snugged the bolts for the worm block to worm housing first, then slightly snugged the bolts to the mount.
I then slightly loosened the worm block bolts to allow them to center with the worm, and then tightened them.
I've tried twice now and still get the same result
There's no noticeable backlash and it seems to run smooth, but obviously something is wrong.
The immediate thought is the worm blocks are "twisting" somehow and putting side load on the bearing.
Since I got almost the same results after R&R the assembly I wonder if the housing was drilled/bored off center.
Appreciate any help and input.

Thanks
Jim P