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On Wed, Jul 14, 2021 at 07:24 AM, Cyclone wrote:
Peter,
The backlash on my original Losmandy gearbox is 800 msec. With the gearbox installed on the motor and half the coupling on the output shaft, I took two pictures of the coupling on the output shaft, at each end of the backlash, measured the rotation angle difference in Photoshop and calculated the 800 msec at 0.5x guide rate. I did the same taking pictures of the other half of the coupling on the worm and calculated 6.5 sec. I don¡¯t have the spring loaded worm and the backlash is required due to tight spots around the Dec axis).
If I read your post correctly, it seems you have not identified the sources of backlash; you could try the method I described above.
Eric
Eric, So I'm focused on three things. The needle bearings. I've changed these out and added a third, but I will have more to say on that later. I have not touched my worm setup since last reading to get a comparitive measurement. Second, I will change the coupler to a rigid one and measure again, as the worn assembly can remain untouched. Third, I will follow some of the steps Allen has suggested.? I would die for 800ms! I should note that I set TVC to 0 based on something I read somewhere. Before that l had better backlash measurements. So forth, measure again with TVC set to 10. The problem is that so much of what we read here is antidotal. We need hard data that shows a progression of hopefully improving measurement to see what is significant. It's wonderful that we have to do all this work to get our mounts to perform! Pete
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Chip,
That's very interesting. I have posted here quite a bit about the caged bearings, but actually I might prefer the full compliment ones. They seem smoother. The thing is the shaft, at least mine, is 5 thou under size. The bearings are sloppy. The shaft touches the bears on just the top or bottom surface. That's a problems. ?Bearins are designed to be pressed in and the inner diameter reduces a little. These are only a very light press. Finger light. Makes them easily to change but they do no perform to spec. The 1.5" bore needs to be a few thou undersized to correctly compress the bearings I think.
Peter
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Peter,
800 msec is for the gearbox only, with the 6.5 sec worm backlash the total is 7.3 sec; ?completely unguidable.
Assuming the gearbox backlash is fairly constant with temperature, the TVC could be set to at least correct that backlash, but the worm is the major contributor to my backlash and it varies with temperature, so TVC is not an effective solution.
I don¡¯t see how the coupler would introduce significant backlash, relative to the other contributors.
Before you start replacing components, you may want to measure the backlash of each individual component, as I have done, hence my suggestion that you measure the backlash of your gearbox and worm separately.
Eric
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Bob H. here, though I have one of the newer G211g and have yet to try my hand at astrophotography and new to commenting in this forum ( so please forgive any ignorance on my part). Comments seem to be about a variation that is 2-3 digits beyond the decimal and and the mount was designed (previous comment) to be a visual tool, is this subject the equilivent to "beating a dead horse ", as i am having a hard time understanding this line of thought, and I am nowhere even close to being an expert on my mount, and no way is this comment of mine to be considered to a negative to the topic starter, just wondering. Respectfully submitted.? ?
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Hi Robert,
This endeavor (astronomy and astrophotography) runs the gamut...from novice to expert and from visual observer to dedicated astro photographer.
And the equipment also runs a gamut from lightweight 60 mm wide field scope to heavy 16 inch f/10 SCTs...or even larger scopes.??
Add to that heavy or light cameras, filter wheels, piggyback telescopes...?
So: we all have different equipment needs.? And all the mounts are not identical...there were changes along the decades these mounts have been on the market.? It can be like comparing car models of different years.??
However this forum is for those who are not getting satisfaction from their particular unit, and those folks are sharing their experience, reporting what they see or hear (sometimes a noise seems incorrect) and asking for suggestions to try.??
So if you are in the group of happy folks, just ignore these deeper dives.? Use your mount and enjoy it.? If you are thinking of diving deeper yourself, these discussions might offer some things to think about.
There are far more expensive and far more sophisticated mounts on the market.? These mounts are not expected to match that higher cost class, but sometimes owners can get to an amazing performance level by some personal efforts.? This forum is also for those who want to see how far their mount can be pushed toward the high end.
Stay well and enjoy your new mount!
Michael
On Thu, Jul 15, 2021, 10:28 AM Robert Hancey < rdhx@...> wrote: Bob H. here, though I have one of the newer G211g and have yet to try my hand at astrophotography and new to commenting in this forum ( so please forgive any ignorance on my part). Comments seem to be about a variation that is 2-3 digits beyond the decimal and and the mount was designed (previous comment) to be a visual tool, is this subject the equilivent to "beating a dead horse ", as i am having a hard time understanding this line of thought, and I am nowhere even close to being an expert on my mount, and no way is this comment of mine to be considered to a negative to the topic starter, just wondering. Respectfully submitted.? ?
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On Thu, Jul 15, 2021 at 01:28 PM, Robert Hancey wrote:
Bob H. here, though I have one of the newer G211g and have yet to try my hand at astrophotography and new to commenting in this forum ( so please forgive any ignorance on my part). Comments seem to be about a variation that is 2-3 digits beyond the decimal and and the mount was designed (previous comment) to be a visual tool, is this subject the equilivent to "beating a dead horse ", as i am having a hard time understanding this line of thought, and I am nowhere even close to being an expert on my mount, and no way is this comment of mine to be considered to a negative to the topic starter, just wondering. Respectfully submitted.? ?
The G11 can be excellent for astrophotography when tuned and used properly.? This is not about 2-3 digits beyond the decimal of something, it is about real issues for minimizing tracking errors to a few pixels.? One of my favorite G11 AP galleries is here,?, to get an idea of what can be achieved.
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How do I do that?
As briefly described in my previous post, I measured the backlash directly by taking pictures of the coupling. To be more specific: -Remove the motor-gearbox assembly from the mount. Keeping half the coupling on the gearbox output shaft helps with the backlash measurement.?You should be able to rotate the output shaft with your fingers and feel the backlash. -Take a pictures at each end of the backlash. I installed my camera on a tripod so I can easily align the images. The camera was facing the end of the shaft (or coupling half). -In Photoshop, or other imaging software, measure the angle difference between the 2 images; this is the gearbox backlash. Convert the measured backlash to other units, for instance: Backlash angle * 239.34 /? 360 deg = backlash in seconds at sidereal rate -For the worm backlash, repeat the above taking pictures of the other coupling half on the worm which is still installed on the mount. -If you believe there is backlash in the coupling itself, you could assemble the coupling, fix one end, and measure the backlash at the other end using the same approach. Here is an example of the two pictures for the gearbox backlash, combined in a gif file: Eric
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Eric ("cyclone")...
Very nice work!? I love that 2 image timed video photo of the Oldham coupler end!
That you show is typical of the stock gearbox play.? That's part #1 of hysteresis and you can't get rid of that unless you change gearbox model (to a McLennan type say). But I think all gearboxes have some hysteresis in the gearing.? (There is a spec in the McLennan types...see attached document.)
On the Tucked motor system you'd find additional time lag #2 (seen by the worm movement delay from the motor) from the necessary gaps between the 2 transfer gears.? You must not allow those transfer gears to jam so there has to be a little gap.? You can adjust the gearbox mounting to optimize that.? (The gear driving the Oldham coupler is on a fixed shaft...)
On the SLW you'd get yet another time lag #3 if the OPW that's pivoting at the left pivot point can move left to right in it's "nipple" pivot hole.? That also has to have a gap.? That affects the DEC autoguiding but not normally the RA that's always forcing the RA toward the west as the sky rotates.??
( So I think that bolting down the SLW so it does not move (defeating the SLW action) is a good approach to try eliminating that time delay.? )
Another time delay #4 affects the DEC mostly too is any worm left to right slip along it's axis if the worm mounting blocks get loose.? I think these blocks must get pushed loose over mount temperature changes anyway, so I recommend putting in one R4 Belleville spring under the far worm bearing, and reducing that bearing OD and lubing it to slide, to get rid of that time delay.
{ The G11T has 3 transfer gears at the RA Titan worm drive.? I found that a 2 pulley belt drive eliminates that time delay in those gears (that thanks to Allan Ruckle who showed me his nice implementation).? }
Then there is the worm to ring gear gap.? That was always the obvious place where a gap must be optimized....going back decades.? For that, we used to (and I still do) use spark plug feeler gauges to adjust that.? That's hysteresis element #5.? Many owners get rid of that gap by using a weighted cord wrapped around the axis. [On the RA axis, hang the weight to the East side to minimize chatter of the ring gear when the worm rubs it perpendicularly].?
Recent emails suggest on some units there could be a gap between the 1.25 inch shaft and the needle bearing ID.? I don't find that only units but it's another thing to test for.? I guess that is potential hysteresis element #6.?
Lastly, check that your dovetail mounting at the top of the DEC axis bolts are tight and that dovetail is not able to wobble.?
If I left something out....I apologize.
Have fun all,
Michael
How do I do that?
As briefly described in my previous post, I measured the backlash directly by taking pictures of the coupling. To be more specific:
-Remove the motor-gearbox assembly from the mount. Keeping half the coupling on the gearbox output shaft helps with the backlash measurement.?You should be able to rotate the output shaft with your fingers and feel the backlash.
-Take a pictures at each end of the backlash. I installed my camera on a tripod so I can easily align the images. The camera was facing the end of the shaft (or coupling half).
-In Photoshop, or other imaging software, measure the angle difference between the 2 images; this is the gearbox backlash. Convert the measured backlash to other units, for instance:
Backlash angle * 239.34 /? 360 deg = backlash in seconds at sidereal rate
-For the worm backlash, repeat the above taking pictures of the other coupling half on the worm which is still installed on the mount.
-If you believe there is backlash in the coupling itself, you could assemble the coupling, fix one end, and measure the backlash at the other end using the same approach.
Here is an example of the two pictures for the gearbox backlash, combined in a gif file:
Eric
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On Fri, Jul 16, 2021 at 02:17 PM, Michael Herman wrote:
That you show is typical of the stock gearbox play.? That's part #1 of hysteresis and you can't get rid of that unless you change gearbox model (to a McLennan type say). But I think all gearboxes have some hysteresis in the gearing.? (There is a spec in the McLennan types...see attached document.)
?
(...)?
?
If I left something out....I apologize.
?
For the DIY minded I would mention that OnStep requires no gearbox - the stepper is coupled directly to the worm axis.? Mode-switching stepper drivers slew the mount in slewing mode and track in micro-stepping mode.? I don't know if OnStep performs as well as Gemini but I have not seen or heard evidence of the contrary.
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Michael,
I replaced the gearbox on the RA axis when I first got the mount in 2013 with a McLennan gearbox, but if I remember correctly it has more backlash then the Losmandy one, so I still use the original gearbox on the Dec axis. The bulk of the backlash is due to the worm mesh anyways (no spring load on my mount), so the gearbox is not the main contributor on my mount.
Eric
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Hi Eric,
I liked your visual approach.
Alan has told us of his innovative approach too using a laser pointer attached to an axis.? You can then readily see how long it takes to take up the time lag from hysteresis.? Just watch the spot on the wall and see how long it takes to get that to move.??
As for me, I was just trying to list all the things you have to check on as aount owner.? Like a pilot's checklist before you take off.? ?
Anyone selling or making mounts needs to have a similar quality checklist.??
My gearboxes are all the older type and all are riveted. They have some time lag.? I didn't see any lag in the McLennan ones I have but I expect all gearboxes to have some slack.? Earlier this year I went to the Losmandy company and bought two new gearboxes.? I think those are riveted too...I wasn't expecting anything different.? I'll ask if I can exchange them for the Allen bolt assembled ones you have described.? I have not seen one of those yet.? Maybe the Allen bolted ones also have a shorter lag, ...who knows.? Any product that has no spec on it will have a wide variation.??
Very best, Michael
Michael,
I replaced the gearbox on the RA axis when I first got the mount in 2013 with a McLennan gearbox, but if I remember correctly it has more backlash then the Losmandy one, so I still use the original gearbox on the Dec axis. The bulk of the backlash is due to the worm mesh anyways (no spring load on my mount), so the gearbox is not the main contributor on my mount.
Eric
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Michael,
I don¡¯t know what those Allen bolt assembled gearboxes are; you must have heard about them from someone else.
Eric
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Just as a clarification, the laser pointer trick only works when clamped to the worm or anything before it in the drivetrain.? If you clamp it to the actual axis saddle, that moves too slow to detect.? Or you have to shoot the laser beam really far away.
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It was JKmetz who recently reported this in this thread:
" And the newer?gearboxes?you can dissemble to clean and lube, or check from wear. My originals were riveted and could only be drilled out. Now they have Allen head screws which you can easily take out. If you stick your finger into the?gearbox?hole and hold down of the first gear that contacts the pinion gear, then rock the output shaft/spur gear, you can see how much play there is.
"
Stay well, Michael
Michael,
I don¡¯t know what those Allen bolt assembled gearboxes are; you must have heard about them from someone else.
Eric
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Alan,
I could only think of how to attach a laser to an axis.? I cannot think of how you'd do it on a worm.? So ...
Can you say something about what size laser you attached, and how you attached this laser to a worm??
?Do you have a photo of this?? ?Seems like you have a magic trick to teach at least to me!
Thanks again, Michael
Just as a clarification, the laser pointer trick only works when clamped to the worm or anything before it in the drivetrain.? If you clamp it to the actual axis saddle, that moves too slow to detect.? Or you have to shoot the laser beam really far away.
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I used one of those cheap $3 mini laser pointers at the hardware store, designed to make a spot for harassing / entertaining your pets.? It's about 3/8" diameter and 2 inches long.
I used a binder clip on one of those transfer gears and just put the laser on the binder clip.
For the worm, you'd clamp onto the bare exposed portion (obviously not the helical part), or if there's not enough room, maybe use something narrower like clothespin, etc.
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I think I may have discovered something interesting regarding why it is so difficult to both reduce and get a consistent backlash value for Dec.
First, while out in the field disassembling my mount for the umpteen time, I had a very close call. Focused on the work at hand, I did not notice a thunderstorm moving in over head. Once I became aware of the suituation, I closed the observatory roof and started heading back to the house, when I felt a strange sensation, followed by a crackly noise, and I immediately hit the group. Almost instantaneously I heard and felt the crack of lightning. Several minutes later fire engines arrive at my neighbors house less than 800 feet from where I was standing. I never thought of this as a dangerous hobby?
After changing out the needle bearings, and adding a third, I noticed how stiff the worm gear was to turn after I had tightened the clutch. When I later measured my backlash I had actually gone from 2500ms to close to 4000ms, even though a visual inspection of the mesh seemed good. Actually, I set the mesh with the clutch undone, using finger feel to sense if the alignment was good (no high-low spots) and little to no blacklash. The loading placed on thrust bearings seem to be were the problem resides. I ordered new clutch plates and just received new thrust bearings from McMaster- Carr. I'm concerned by the stiffness and I think this means considerable pressure has to build up before there is any rotational movement in the worm. This pressure (force) has the created in the gearbox and coupler. This is both bad for the gearbox and means a large backlash.?
I will post my findings with the new parts next week.
Peter
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Peter,
Glad to read your last posting, not your "hit by lightning" obituary!? ?What a story!
You may need a lightning rod near your observatory.??
On to the question: why does your clutch force affect your worm to ring gear spacing?? Ideally, it should have no effect, but you are not the first to report that it does have an effect. I always wondered what's causing this effect, as it is not observed on my systems.
So let's think it through.? Something on your system is getting distorted by the tightening of your clutch knob. The distortion is changing the worm to ring gear mesh.? What piece is distorting causing the worm to tighten or loosen on the ring gear teeth???
In worm drive systems, one critical setting is the center of the worm gear axis (center of the worm long axis) to the center of the ring gear teeth (center plane of the ring gear.)
Like this:.? --- ((((o ---- with the (((( being the ring gear and the o being the worm seen end on.? The center of these is supposed to be equal height on the center level I indicated with? dashes? ----
Looking from the top down, the axes are composed of:
Upper part of axis ...dovetail if DEC say Bottom of upper clutch surface Clutch disk (soft) Ring gear ((((( upper surface? ? ?is the top of the lower clutch surface Top race of the thrust bearing (thicker plate) Rotating center of the thrust bearing Bottom race of the thrust bearing (thin plate) Thick mounting plate, holding the worm gear bearing blocks. Mount axis body (thick aluminum cylinder) Bottom thrust bearing Bottom clutch knob
From that construction it seems impossible to shift the ring gear to worm gear height just from axial compression.?
But the ring gear is a metal disk and is being pulled from the center.? ?Can that ring gear disk curve down in the center from the high force, and curve up at the outer edge?? That must mean the upper flat surface is distorting also.? Seems unlikely....but all those arts are aluminum.? If the worm gear gets tight all the way around 360 degrees rotation this may be what's going on.
Or else something else must be shifting.?? ?I think you found you have a slight gap between the 1.25 inch shaft and the needle bearings.? If that were present, consider another possible movement:? We now know there can be a gap between the 1.25 inch steel shaft and the tubular needle bearings inside the mount axis body.? It is therefore possible that the axial force causes the ring gear to shift or? "tilt" in the axis body.? The ring gear could tilt toward or away from the worm.? That is, the axial force causes the 1.25 inch shaft to go slightly non perpendicular.? As you rotate the axis around 360 degrees you would find one side would go "tight" and the area 180 degrees would go loose.? That would be the test for that effect.
If the latter is the problem, I'd ask Losmandy to replace the 1.25 inch shaft with a larger OD shaft.? That should eliminate the gap and the potential for tilt during clutch force changes.??
It is also a benefit to reduce clutch force as much as possible yet maintain the grip.? I recommend you try my high friction clutch disk to reduce clutch pressure, and reduce distortion of the mount materials.??
Anyway, I feel that somehow your undersized 1.25 inch shaft is the root cause of your worm to ring gear difficulties.? I think that can be addressed by the factory.? I know you are trying to solve this by needle bearings, but I think the alternative answer could be the shaft replacement.??
I think if you can get to the bottom of your puzzle it will help others know what issue to look for.??
All the best, Michael
I think I may have discovered something interesting regarding why it is so difficult to both reduce and get a consistent backlash value for Dec.
First, while out in the field disassembling my mount for the umpteen time, I had a very close call. Focused on the work at hand, I did not notice a thunderstorm moving in over head. Once I became aware of the suituation, I closed the observatory roof and started heading back to the house, when I felt a strange sensation, followed by a crackly noise, and I immediately hit the group. Almost instantaneously I heard and felt the crack of lightning. Several minutes later fire engines arrive at my neighbors house less than 800 feet from where I was standing. I never thought of this as a dangerous hobby?
After changing out the needle bearings, and adding a third, I noticed how stiff the worm gear was to turn after I had tightened the clutch. When I later measured my backlash I had actually gone from 2500ms to close to 4000ms, even though a visual inspection of the mesh seemed good. Actually, I set the mesh with the clutch undone, using finger feel to sense if the alignment was good (no high-low spots) and little to no blacklash. The loading placed on thrust bearings seem to be were the problem resides. I ordered new clutch plates and just received new thrust bearings from McMaster- Carr. I'm concerned by the stiffness and I think this means considerable pressure has to build up before there is any rotational movement in the worm. This pressure (force) has the created in the gearbox and coupler. This is both bad for the gearbox and means a large backlash.?
I will post my findings with the new parts next week.
Peter
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On Sat, Jul 17, 2021 at 04:36 PM, <pcboreland@...> wrote:
I think I may have discovered something interesting regarding why it is so difficult to both reduce and get a consistent backlash value for Dec.
After changing out the needle bearings, and adding a third, I noticed how stiff the worm gear was to turn after I had tightened the clutch. When I later measured my backlash I had actually gone from 2500ms to close to 4000ms, even though a visual inspection of the mesh seemed good. Actually, I set the mesh with the clutch undone, using finger feel to sense if the alignment was good (no high-low spots) and little to no blacklash. The loading placed on thrust bearings seem to be were the problem resides. I ordered new clutch plates and just received new thrust bearings from McMaster- Carr. I'm concerned by the stiffness and I think this means considerable pressure has to build up before there is any rotational movement in the worm. This pressure (force) has the created in the gearbox and coupler. This is both bad for the gearbox and means a large backlash.?
Peter
This is totally expected. Remember I said there were three concentric parts of the DEC axis: a) the saddle and 1.25" shaft b) the assembly that holds the ring gear (which I have never taken apart, so I don't know much about it) c) the black outer anodized body Part b) rides on it's own set of bearings that you don't see.? When everything is put together, tightening the clutch knob pulls a) downwards against c).? The clutch pad itself pushes b) downwards relative to c).? Well, c) holds the worm and b) holds the ring gear, so you'd expect the ring gear to move down a LITTLE bit, depending on what the bearing preload characteristics are.? With that said what's important for low backlash is that the axis still turns freely when the clutch is engaged.? To test the b) bearings, remove the worm (or push it out of the way), loosen the clutch knob all the way, push down hard on the saddle dovetail with your palm, and see if the axis still rotates freely.? All bearings should rotate with minimal friction, and even the innermost needle bearings because they aren't loaded by the weight of the scope and counterweights. Next, with the worm still out of the way,? Pull UP hard on the dovetail and try to rotate it.? Now you are testing the needle thrust bearing that is part of the clutch knob assembly.? BTW, did you rearrange the spacers so that there is a thick spacer between the wavy washer and the thrust bearing?? Because that would be pretty dumb to have the wavy washer pushing against the bearing surface.? (There should be a set of rollers and two thin plates for the bearing assembly, and a correct orientation) Next, start to tighten the clutch knob and see if the friction increases.? On my G8 DEC axis, it increased a little bit, but on the G11 it is still fairly free to turn even with moderate clutch pressure. Finally, with the worm still out of the way, load up the counterweights and a sacrificial scope ;) to see if the DEC spins freely with the clutch tightened somewhat.? NOW you are testing out the inner needle bearings with a realistic radial load. With all that said, adjust the worm pressure / spacing with the clutch engaged the "usual" amount.? Also, yes, too tight worm mesh can increase backlash.? Also, too much axis friction can increase backlash.
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Michael Herman