Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
Here's a pic gearbox side.
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
On Mon, Jul 19, 2021 at 03:11 PM, <pcboreland@...> wrote:
The whole reason why I am testing with a rigid coupler is to ensure that the worm and motor shaft are axially aligned as much as possible. But, as you say, there is a potential problem here. If the worm and motor are not axially aligned by even a small amount something has to give, and it will be the motor. It will bob up and down relying on the spring action of the McLennan motor plate. This is a bad situation, so once the worm is set I loosen off the motor screws to make sure things are self aligned before retightening.?
I experimented with some success with rigid aluminium couplers on an original OPW design. I bored both ends and precision fitted for my DEC paying particular attention to concentricity and tolerances. So it wasn't exactly 4mm on the McLennan shaft and 1/4" on the worm shaft side. I also kept as much material and turned the OD of the coupler accordingly as to keep it as close to the worm bearing and the gearbox brass bush. I have 4x M3 SS grub screws at 90 degrees standard arrangement to secure both ends. When fitting it they're is enough play in the plate holding the gearbox on the side of the OPW and on the gearbox eyelets to secure everything aligned. so you first secure the worm then the worm shaft, the gearbox shaft, the gearbox and the gearbox plate last. Rotating by hand you can feel any hard spot. If there is one you release any pressure by losening the coupler grub screws on the same end then retighten. The McLennan has a few mm of longitudinal play so that takes care of any shaft runout. Took me a few iterations of turning alu stock to get it right as dimensions and concentricity are critical but I got one right in the end. I imaged one night with TVC set to 0 and an old version of ?PHD with DEC guiding set to auto to make sure I got N/S corrections imaging at the zenith scope pointing straight?up with camera weight hanging down. it did the job. Most importantly when the mount got very cold it was still fine. No change in behaviour.
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
I think if you can get to the bottom of your puzzle it will help others
know what issue to look for.??
***
Seriously??
Even an idiot like me knows the answer to this.. And even your text based
drawing shows this.. And you told me this.. in person.. three years ago.. ¡°You
can¡¯t adjust the worms without the clutches tightened¡±..
?
If
you adjust the worm to ring gear, and *then* tighten the clutches, you pull the
ring gear down.. However slight, this then moves it away from the ¡°perfect¡±
mesh you just set.. Tighten the clutches to how you would use them, then adjust
the worm..
?
Derek
?
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From: [email protected] [mailto:[email protected]] On Behalf Of Michael Herman
Sent: Saturday, July 17, 2021 9:09
PM
To: [email protected]
Subject: Re: [Losmandy_users_io]
Needle bearings and axial play, is this a significant performance problem and
how difficult is it to fix?
?
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?
Looking from the top down, the axes are composed of:
Upper part of axis ...dovetail if DEC say
Bottom of upper clutch surface
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)
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.??
?
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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Re: Price check for CG11 with Gemini 2
Thanks for the feedback and advice. I'm in no hurry - perk of being an astronomy professor is that I just get to have three setups for class instead of two until it sells. Shipping for both (this is my second G11, just the older one) was only about $200 - once from AZ to IL and the second from FL to IL. so slightly scary, but I hoped I mitigated that with the offer to meet in-person.?
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Re: Price check for CG11 with Gemini 2
Seems
good to me.. Either be patient (and edit the ad to keep it near the top), or
drop the price to a ¡°priced to move now¡± price, would be my suggestion.. .ie it
depends on how fast you ¡°need¡± to sell it..
?
There
might be some apprehension on if COVID is gonna shut things down and evaporate
peoples disposable cash.. Shipping costs might be scary, as I was able to drive
20 miles and do the transaction in person..
?
Derek
?
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From: [email protected] [mailto:[email protected]] On Behalf Of gsnarayan@...
Sent: Monday, July 19, 2021 7:47
AM
To: [email protected]
Subject: [Losmandy_users_io] Price
check for CG11 with Gemini 2
?
I'd advertised an older CG11 with the Gemini 2 upgrade on and despite lots of views, there haven't been
any bites. I was wondering if this is just because of the lack of the OPWBs and
brass worms, or my price was off for some other reason.
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
Peter,?
Glad you were able to make a solid connection between motor and worm. McMaster is a super supplier and you can find a whole lot of specialty bearings connectors, gears, etc. I have used them many times before. I looked at those flex couplers before but couldn't find the right size to fit into the standard arrangement with the tucked motors like I have. I was a bit concerned about the low torque ratings on this type of spiral cut aluminum, and also whether they hold up at temperatures below freezing.?
I did buy a few flex couplers like these months ago:??which are similar to the Oldham couplers Losmandy uses, but haven't had the chance to test them out yet. They are supposed to be very low backlash and you can buy separate halves for the different shaft diameters.? The one draw back is that the center disc readily pulls away from the shaft caps unless the shafts are rigidly held in place, nothing to hold the assembly together other than locked position.?
Replacing the Oldham coupler must result in lowering the overall backlash of the gearing, and lessen the total guiding error as well all seek :).
JK
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Re: At some point, I may begin commenting on the G11 DEC axis...
On Sun, Jul 18, 2021 at 08:58 AM, <pcboreland@...> wrote:
You can set the step size on he brain/algorithm form. Default is 500ms.
I set the step size smaller, but it uses 500ms anyway. For TVC, I used 10, which is what I derived from the G8 axis.? I didn't change it when I upgraded to the G11, but from the backlash test, at large excursions there is almost no hysteresis lag, so this value is about right.
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
What I meant was that our methods determined a TVC value for the gear backlash only.? This part is generally fixed and doesn't depend of sky location, gravity, temperature, etc.? I don't have an opinion on using extra TVC to take out worm backlash too.? I encourage you to keep increasing the value until "something bad happens".
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
On Mon, Jul 19, 2021 at 02:19 PM, alan137 wrote:
Both our methods (laser or allen wrench)? use TVC to remove all the gear backlash BEFORE it gets to the worm.? There is additional backlash between the worm and ring gear (including ring gear eccentricity), and this is the part that varies due to all those factors mentioned.
?
Allen, I have to quibble with you here. TVC does not just remove the gear backlash (I assume you mean gearbox), it removes all the backlash including the worm "mesh" backlash. Did I understand correctly what you said? For others reading this thread, ring gear eccentricity is a valid point. I hand rotated my ring gear 360 degrees by hand winding the worm and found one small spot where I felt I had a different level of rotational resistance. I made sure this area was not within the operating area of the Dec axis. Previous to my open worm setup, I could? feel high/low pressure situation for each worm rotation. This I took as periodic-error/alignment problem. When I strapped the 1/4" aluminum bar between the bearing blocks it went away. Now this is the old worm design and the OPW does not suffer from this problem as far as I can tell. Also, if you have the old worm you may have the bearing blocks that are silver underneath. These are a different specification the the new blocks. Specifically, the round insert portion on the left hand block is under 3/8" and wobbles in the hole. This is obviously going to lead to big problems. Further, the new block are a little larger and do not fir into the old cover plate, hence why I had no choice in going to an open worm design, which prove fortuitous as I learned a lot from having access to the entire assembly while in operation. Peter
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
>>
So the long and short of it is my TVC value is equivalent to 20 in regular parlance.?
yes, that sounds about right :)
Thanks guys! I suppose I could have looked this up but I was lazy.? This makes a lot of sense and 8x guiding rate for the adjustment is not quite instantaneous, but probably good enough. I hope folks don't too envious of me having ?5x more integers to work with, a tick in favor of smaller step sizes. ? So the long and short of it is my TVC value is equivalent to 20 in regular parlance.?
Peter
-- Brian?
Brian Valente portfolio
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
On Sun, Jul 18, 2021 at 11:04 PM, John Kmetz wrote:
Also I am not sure how you are replacing the Oldham coupler with a rigid Allen screw attached one, as shown. Unless the motor shaft and worm are really tightly axially centered and parallel, the set screws would probably loosen over time, or start grinding a depression into the shafts. Or the motor shaft or worm shaft would move around instead. This connection must need a Oldham coupler, flex shaft or universal joint of some type to allow for the differences.?
John, The whole reason why I am testing with a rigid coupler is to ensure that the worm and motor shaft are axially aligned as much as possible. But, as you say, there is a potential problem here. If the worm and motor are not axially aligned by even a small amount something has to give, and it will be the motor. It will bob up and down relying on the spring action of the McLennan motor plate. This is a bad situation, so once the worm is set I loosen off the motor screws to make sure things are self aligned before retightening. In the case of Ra, which is spring loaded, the left side worm block acts as a pivot point, so there is the possibility of things not staying precisely aligned. Thus far, I've not seed any movement in the motor once set, but I have seen a very significant reduction in the native 240s worm error. At this point I have no PEC, but my guided 240s error is under 0.2 arc-sec. There is a coupler I could use that allows some flex. Part number:? It's quite expensive so one could do it a different way:??plus??In this arrangement one needs to drill and tap though the bushing and get slightly longer set screws. The coupler may be a tight fit because its 5/8" diameter. If I had not spent so much money on couplers already I would likely just go straight for? With that said I'm unlikely to rework the Ra at this point as things are really working well.? Peter
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
On Mon, Jul 19, 2021 at 05:29 PM, <pcboreland@...> wrote:
I hope folks don't too envious of me having ?5x more integers to work with, a tick in favor of smaller step sizes. ??
You only have up to TVC value of 255 you can set. You may just run out of integers ;)
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
Thanks guys! I suppose I could have looked this up but I was lazy. ?This makes a lot of sense and 8x guiding rate for the adjustment is not quite instantaneous, but probably good enough. I hope folks don't too envious of me having ?5x more integers to work with, a tick in favor of smaller step sizes. ? So the long and short of it is my TVC value is equivalent to 20 in regular parlance.?
Peter
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
Paul,
Thank you for confirming the TVC speed (8x). I could not find it anywhere except an old post which stated 20x which did not match my measurements.
Eric
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
On Mon, Jul 19, 2021 at 11:20 AM, <pcboreland@...> wrote:
I cannot quite get my head around why the pointing position should affect backlash, unless something is shifting, which I already went into previously with regard slop in the needle bearings. We have here a gimble with a balanced load for Dec, the worm/ring gear should not care what position it's in? Perhaps one here could explain this?
Both our methods (laser or allen wrench)? use TVC to remove all the gear backlash BEFORE it gets to the worm.? There is additional backlash between the worm and ring gear (including ring gear eccentricity), and this is the part that varies due to all those factors mentioned.
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
On Mon, Jul 19, 2021 at 04:05 PM, Cyclone wrote:
According to the Gemini serial commands, the TVC value is measured in encoder steps, programmable from 0 to 255. If your step size is 5 times smaller, your TVC value will be 5 times larger for the same absolute backlash.
Exactly. TVC value is the number of encoder steps to take at 8x the guiding speed. The smaller the encoder step, the larger the TVC value required to compensate for the same amount of backlash. Regards, ? ? ?-Paul
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
Peter,
According to the Gemini serial commands, the TVC value is measured in encoder steps, programmable from 0 to 255. If your step size is 5 times smaller, your TVC value will be 5 times larger for the same absolute backlash.
Eric
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Best result with new mount so far
Hello all
After getting my new G811G last month, I had some trouble with the random slews and trying to configure PHD2 to get better results with guiding, but things are working pretty well now. This graph is from last night's session, and show the HFR for each shot (the green line with white dots on top) and the average?RMS for guiding on the yellow lines. Of course there are the occasional wind gusts throwing things off but on average the results?are very good.?
This has been a constant view in my sessions after i fixed the issues above and balanced my mount well.
Given that my imaging setup nas 1.43 arcsec/px, i can say the results are pretty good! Here is my last image, the Cygnus Wall in NGC 7000:
Hope you guys enjoy!
Guilherme
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Re: Price check for CG11 with Gemini 2
I'm in the market for a second mount. I posted an add yesterday in Astromart. But I"m really only wanting the head, no tripod, controller, etc. Please feel free to PM me.?
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Re: Needle bearings and axial play, is this a significant performance problem and how difficult is it to fix?
Brian,
Tomorrow is a clear night and I'll run some PHD2 backlash tests to confirm my bench results one way or the other. The value of 100 I determined to cancel out the backlash seems to be at odds with your experience ?and others. Allan137 I know has looked in this issue in great detail. It may be my value is different because I have 32000 steps per worm cycle corresponding to 0.11 arc-sec per step, not 0.56 arc-sec per step in the case of you and Alan. Since I do not know the calculations behind TVC, and can only assume this to be significant. One thing is clear, I have far greater resolution in setting my TVC value.
I cannot quite get my head around why the pointing position should affect backlash, unless something is shifting, which I already went into previously with regard slop in the needle bearings. We have here a gimble with a balanced load for Dec, the worm/ring gear should not care what position it's in? Perhaps one here could explain this?
Peter
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Paul Kanevsky