|
I was asked to start up a sperate thread on this subject, which I think is a good idea as this is a significant issue for some. After making numerous mods, I now realize I should have looked at this first. If buying a used mount I would recommend starting here with any tune-up. Whether or not wobble in Dec, RA or both axes is a significant problem is to be determined, but I suspect it is. Most certainly it must effect backlash and most likely the ability to carry heavier loads and maintain excellent guiding performance.? In doing some research I came across this posting from 2000.? /g/Losmandy_users/topic/g11_fix_source_for/21183709?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,1,40,21183709I just took apart my Dec axis and was shock by the degree of play. This is? also true of the Ra axis but not quite as bad. So what to do? In the note by Neal Barry from 2 decades ago be suggests adding a third needle bearing and shimming the shaft. There is room for a third bearing in both Dec and Ra.?? In researching bearings there seem to be two designs:? full complement drawn cup and caged drawn cup.? ?Torrington bearings used by Losmandy are the original full compliment design. I've opted to use the more expensive caged design. There advantage is the rollers stay axially aligned. This is what I ordered:? How hard is it to removed the old bearings?
Not hard at all. On my G11 there is a brass inset end on the housing. The bearing opposite the bass end I tapped out using a piece of 1/2" dowel catching the metal edge. It came out very easily. The brass end was more difficult because less of the edge shows, so a whittled down a piece of 1.5" dowel bought at Lowes. The overall diameter was reduced a bit, a small flat was added, and the business end I reduced the diameter further. I did this on a belt sander, but one could use an orbital sander, it will just take longer.? How hard is it to insert the bearings?Again this was much easier than I thought it would be. I was able to press in the bearing with my hand. The new ones might be more difficult. Nevertheless, they can be tapped back in using a block of wood. On the brass end the 1.5" dowel would be used to seat it in place.? How much play is there?In my case a lot. The shaft appears to be in spec. 1.25 - 0.05. I will not know if the bearings are the source of the problem until the new ones arrive. If the bearing is tight to the shaft job done. If not then I will shim. I bought this material just in case.??To test the effect of shimming I used tin foil and it worked great. What result am I hoping for?Significantly reduced backlash, better guiding especially in Dec. My current Dec backlash is around 2500ms. With all the mods I've made I have singerly failed to reduce it thus far. Keeping fingers crossed. Here is a quote from Neal Barry from the above thread.
"
The needle bearings on both the RA and DEC are set back about 2" from the
'business end' on each axis. As an experiment, I pressed an additional
1.250" ID needle bearing into the opening so that the shaft was supported
nearly to the end of the opening. Additionally, the shafts I have seen
measured 1.2470" dia instead of an optimal 1.250". I wrapped some .0015"
stainless steel shim stock around the undersized shafts as a temporary fix
to remove the bearing slop.
Well, it was like night and day. That simple change eliminated the
'shifting' on each axis, improved tracking and GOTO pointing accuracy
dramatically, there was no more slop in either axis, and best of all, the
backlash can be adjusted to virtually zero through a full 360 degrees of
rotation.
Anyone else have a similar experience?
Neal
|
This information is pretty interesting, Peter. If you have some pics of the mount with the bearings removed would love to see them. In fact, there should be a file section here for mount disassembly instructions and pictures if there is not one already. May I ask how old your mount is? Also wondering if Steve Losmandy has changed the bearing seats or manufacturing tolerances over the years. If he was responding to the forum 20 years ago, he may just adapted manufacturing techniques as a response. My shafts rock a bit more when the axis locks are loose, and less when they are tightened. After locking down, I assumed the remaining play would be from the worm rocking back and forth a bit inside the bearing blocks, but you may be indicating the needle bearings are playing a role too.
As a sidebar, I was checking online last night about possible techniques for worm polishing to improve surface profile. There seems to be two techniques available. One using the original machining equipment that made the gear, and replacing the cutting head with a polishing head which retraces and smooths down the surface profile. The second is placing the finished gear in a vibrating container of polishing media, such as ceramic shapes or even walnut shell fines. One company oft refereed to is REM:
It would be nice to see if they would do a single worm or a small batch of worms. Then we would have to check the guiding achieved by the unpolished vs. polished piece. A nice experiment for someone who has the time and inclination :).
Regards,
John
?
|
Yes, the shaft rattles, but that only has the saddle attached to it.? This does not affect the worm and ring gear engagement since that is set by a separate set of bearings that do not wriggle.? Thus it will not affect backlash.
I briefly looked for a 1.25" ID 2" OD bearing (which would constrain the shaft relative to the black anodized body), but could not find one.? That's weird, since I know of several metric bearings (on bicycles) that are of even thinner profile.
In any case, you can't preload / adjust a needle bearing radially, so it hardly solves the problem.
Also, even though this joint CAN rattle, in practice it doesn't because the pre-loaded thrust needle bearing (at the clutch knob) constrain radial movements against small forces.
I don't care about pointing accuracy since I have never even bothered to align my telescope's dovetail with the optical axis.? I use plate solving anyway, so no big deal.
|
On Sun, Jul 11, 2021 at 10:58 PM, John Kmetz wrote:
This information is pretty interesting, Peter. If you have some pics of the mount with the bearings removed would love to see them. In fact, there should be a file section here for mount disassembly instructions and pictures if there is not one already. May I ask how old your mount is? Also wondering if Steve Losmandy has changed the bearing seats or manufacturing tolerances over the years. If he was responding to the forum 20 years ago, he may just adapted manufacturing techniques as a response. My shafts rock a bit more when the axis locks are loose, and less when they are tightened. After locking down, I assumed the remaining play would be from the worm rocking back and forth a bit inside the bearing blocks, but you may be indicating the needle bearings are playing a role too.
As a sidebar, I was checking online last night about possible techniques for worm polishing to improve surface profile. There seems to be two techniques available. One using the original machining equipment that made the gear, and replacing the cutting head with a polishing head which retraces and smooths down the surface profile. The second is placing the finished gear in a vibrating container of polishing media, such as ceramic shapes or even walnut shell fines. One company oft refereed to is REM:
It would be nice to see if they would do a single worm or a small batch of worms. Then we would have to check the guiding achieved by the unpolished vs. polished piece. A nice experiment for someone who has the time and inclination :).
Regards,
JohnJogn,
John,? I've spent countless hours trying to remove my Dec backlash. I have to assume there is the same problem in Ra, but masked by the fact that it is continuously moving, but would affect GOTOs. Here is my perception on the sources of backlash: 1. Worm-gear mesh. I resorted to applying a lot of pressure between worm and the gear. This is possible with the right grease. If too much force is used the Dec motor will likely stall as we all know, so it has to be backed off with the adjustment screw, thus introducing backlash so I plan to lap my worm. More on that later.? 2. Worm rocking back and forth between the bearing blocks. The worm ought not to move if the bearings are loaded with a bevel washer and screwed down. You can see any move easily enough with the cover plate removed if using the original worm block (not the OPW). 3. Gearbox. There is play in the bronze bushings of the output gear. It moves back and forth a little, but I do not think it plays a role since the worm blocks are locked into position. This is not however true is using the spring loaded OPW. I no longer spring load the Dec axis.? 4. Needle roller bearings. This is the possible elephant in room. Without being too negative hear, the Losmandy mount was originally designed for visual were a little play was OK if one could achieve 2-3 arc-sec of accuracy. Not true for astrophotography for long focal length setups. There was a design decision made to use inexpensive 3/4" long Torrington full compliment bearings (1" and 1.25 long bearings are available as are high precision bearings that have a larger OD). It's not clear what the tolerances are for these Torrington bearings as I could not find any values for the ID. This is not a good sign.? There is also the diameter of the shaft (1.25 +/-) to consider and whether there is wear on older mounts like mine. I believe Scot himself said that this was an issue (see conversation in the link I posted). If I'm not mistaken here recommended adding a third bearing to help with this problem (especially for Dec where the distance between the bearings is large), but indicated that in some cases it might be difficult to get the shaft through the bearings because each end is machined independently, so I assume there could be an offset. This is also perhaps why he went with 3/4" and not 1.25" long bearings. Clearly, a tight fit would make production more difficult so we have two interests working against each other each. Cost to manufacture (fitting and cost of bearings used) verses high precision. 5. The Oldham coupler. The only solution if this proves to be the problem will be to use a rigid one-piece coupler. Lapping One's Worm and Gear WheelThere is a excellent article on polishing the worm and gear wheel in combination here:??Sadly the images are gone. I plan to do this using higher rotational speeds and series of powders (Step 1. 1200 micro fine Aluminum Oxide, Step2. 3.5um Red Cerium Oxide - 3000 grit. Step 3. 2um White Cerium Oxide - 6000 grit.? Buffing One's Worm
I have quite the collection of worms so I will buff one to see what results I get. I intend to do this by holding one end with a bearing while spinning with a drill against the buffing wheel. I'm going to use a fine compound like jewelers rouge. Peter
|
On Mon, Jul 12, 2021 at 12:00 AM, alan137 wrote:
Yes, the shaft rattles, but that only has the saddle attached to it.? This does not affect the worm and ring gear engagement since that is set by a separate set of bearings that do not wriggle.? Thus it will not affect backlash.
Alan, Interesting, this was not what Neal Barry said from his experience on this issue. It's no big deal to change out the bearings and/or shim the shaft, so we will see if you are correct on this. I will get a backlash reading with just he new bearings before lapping my worm. The other big issue for me in being able to carry a 70lb or bit higher payload and still have excellent guiding so I do not want any movement in any of the parts. "Well, it was like night and day. That simple change eliminated the 'shifting' on each axis, improved tracking and GOTO pointing accuracy dramatically, there was no more slop in either axis, and best of all, thebacklash can be adjusted to virtually zero through a full 360 degrees ofrotation." Plate solving rocks, Peter
|
Peter,
You made a list of gear elements that result in time-lag "hysteresis," but you left out one cause of DEC hysteresis on the newest SLW system's:
It's the loose bottom mounting bolts, holding the two worm blocks down.? These loose bolts have slight wiggle room at the left pivot hole near the gearbox, and substantial wiggle room near the far end of the worm near the spring.??
We who have older mounts (non SLW) don't have these mounting bolts loose, and don't have this potential hysteresis problem.??
Alan identified this problem from his new (March 2021) GM811.? (He has now replaced his DEC with a G11 DEC so he has now a G11... SLW on both axes.) ______
Solution to try:
After you have gotten your worm to ring gear spacing optimized (worm close to ring gear, and not stalling), cinch down both bottom bolts.? You now have disabled the SLW and eliminated the wiggle room around both mounting bolt nipples.?
Please compare your DEC hysteresis before and after locking down the SLW mounting bolts.??
All the best, Michael
[Edited Message Follows]
On Sun, Jul 11, 2021 at 10:58 PM, John Kmetz wrote:
This information is pretty interesting, Peter. If you have some pics of the mount with the bearings removed would love to see them. In fact, there should be a file section here for mount disassembly instructions and pictures if there is not one already. May I ask how old your mount is? Also wondering if Steve Losmandy has changed the bearing seats or manufacturing tolerances over the years. If he was responding to the forum 20 years ago, he may just adapted manufacturing techniques as a response. My shafts rock a bit more when the axis locks are loose, and less when they are tightened. After locking down, I assumed the remaining play would be from the worm rocking back and forth a bit inside the bearing blocks, but you may be indicating the needle bearings are playing a role too.
As a sidebar, I was checking online last night about possible techniques for worm polishing to improve surface profile. There seems to be two techniques available. One using the original machining equipment that made the gear, and replacing the cutting head with a polishing head which retraces and smooths down the surface profile. The second is placing the finished gear in a vibrating container of polishing media, such as ceramic shapes or even walnut shell fines. One company oft refereed to is REM:
It would be nice to see if they would do a single worm or a small batch of worms. Then we would have to check the guiding achieved by the unpolished vs. polished piece. A nice experiment for someone who has the time and inclination :).
Regards,
JohnJogn,
John,?
I've spent countless hours trying to remove my Dec backlash. I have to assume there is the same problem in Ra, but masked by the fact that it is continuously moving, but would affect GOTOs. Here is my perception on the sources of backlash:
1. Worm-gear mesh. I resorted to applying a lot of pressure between worm and the gear. This is possible with the right grease. If too much force is used the Dec motor will likely stall as we all know, so it has to be backed off with the adjustment screw, thus introducing backlash so I plan to lap my worm. More on that later.?
2. Worm rocking back and forth between the bearing blocks. The worm ought not to move if the bearings are loaded with a bevel washer and screwed down. You can see any move easily enough with the cover plate removed if using the original worm block (not the OPW).
3. Gearbox. There is play in the bronze bushings of the output gear. It moves back and forth a little, but I do not think it plays a role since the worm blocks are locked into position. This is not however true is using the spring loaded OPW. I no longer spring load the Dec axis.?
4. Needle roller bearings. This is the possible elephant in room. Without being too negative hear, the Losmandy mount was originally designed for visual were a little play was OK if one could achieve 2-3 arc-sec of accuracy. Not true for astrophotography for long focal length setups. There was a design decision made to use inexpensive 3/4" long Torrington full compliment bearings (1" and 1.25 long bearings are available as are high precision bearings that have a larger OD). It's not clear what the tolerances are for these Torrington bearings as I could not find any values for the ID. This is not a good sign.? There is also the diameter of the shaft (1.25 +/-) to consider and whether there is wear on older mounts like mine. I believe Scot himself said that this was an issue (see conversation in the link I posted). If I'm not mistaken here recommended adding a third bearing to help with this problem (especially for Dec where the distance between the bearings is large), but indicated that in some cases it might be difficult to get the shaft through the bearings because each end is machined independently, so I assume there could be an offset. This is also perhaps why he went with 3/4" and not 1.25" long bearings. Clearly, a tight fit would make production more difficult so we have two interests working against each other each. Cost to manufacture (fitting and cost of bearings used) verses high precision.
5. The Oldham coupler. The only solution if this proves to be the problem will be to use a rigid one-piece coupler.
Lapping One's Worm and Gear Wheel
There is a excellent article on polishing the worm and gear wheel in combination here:??Sadly the images are gone. I plan to do this using higher rotational speeds and series of powders (Step 1. 1200 micro fine Aluminum Oxide, Step2. 3.5um Red Cerium Oxide - 3000 grit. Step 3. 2um White Cerium Oxide - 6000 grit.?
Buffing One's Worm
I have quite the collection of worms so I will buff one to see what results I get. I intend to do this by holding one end with a bearing while spinning with a drill against the buffing wheel. I'm going to use a fine compound like jewelers rouge.
Peter
|
I had the same problem with undersized RA shaft on my old G11. It really needed plating and re-grinding to the correct size, but an MI-250 came along and I took the easy option and sold the mount.
The problem manifested itself during imaging as unpredictable poor tracking as the shaft "climbed up" over a needle and then dropped off the other side.
|
Michael,
I actually have an older mount. There is no wiggle and the left hand bearing block mount hole. I did the spring loaded worm mods myself. For me the jury is out as to whether a SLW is better or worse for guiding. I think it might be worse. It might be better to lap in the worm so no high spots and lock the sucker down. The nice thing about using higher ratio gear boxes is you have more torque. With the 25:1 ratio box I could stall out very easily. Not so the 80:1 and 125:1 boxes. Never stalled in fact. A lot of worm pressure and eventually it will wear in. I'm going to help it along a bit by lapping the worm to the gear wheel. The Hysteresis problem you mentioned on the new mounts maybe what is ailing someone else on this forum with high Dec backlash. My advice for what it is worth is to lock down the Dec axis. I'm not sure what spring loading buys you in tracking performance.?
The interesting thing to me is allan137 says a sloppy needle bearing and shaft assembly should have no effect on backlash. He has got me thinking, but I'm now committed to replacing them since I have removed them all. I have not touched my worm at all since my last backlash test, so will run the numbers again once re-assembled.?
If this is not the problem than the only left is the coupler. I found this great <$10 rigid coupler solution that will actually fit inside the worm housing. First the 1/4" x 1/4"? coupling:
3/4" long for Dec
1" long for Ra
Now you need a bushing. For the Losmandy motors you will need 3/16" ID and 1/4' OD. You will have to look this up (tubing bought from Lowes will suffice), but for the replacement gearboxes with a 4mm shaft this is what you need??.
Peter
|
Mark,
So you took the easy why out hey! :) The problem you describe may be attributable to the design of the needle bearings. The stock bearings are full complement Torrington. They do not necessarily stay parallel to each other, and perhaps this created the climbing effect.? I'm going over to caged needle bearings which keeps everything parallel.??
Peter
|
Good investigations by all you great folks.
Yes, your remark about the needles able to shift to be angled to the 1.25 inch shaft....another fellow reported this had occurred in his (GM8?) mount year's back.? I did not know they made needle bearings with the needles locked in the axial direction.? Thanks for describing the options for possibly better bearings.? That sounds like a great idea.? I must look at the needle bearings I bought long ago but never used...?
My problem is this: no problem.? I have no motivation to change anything on my 5 mounts as they are all working fine.? Now if I could just get home to do some imaging!?
Stay well and thank you for your deep investigation into the needle bearing design.??
Very best, Michael
Mark,
So you took the easy why out hey! :) The problem you describe may be attributable to the design of the needle bearings. The stock bearings are full complement Torrington. They do not necessarily stay parallel to each other, and perhaps this created the climbing effect.? I'm going over to caged needle bearings which keeps everything parallel.??
Peter
|
Peter,
These are some interesting mods you are undertaking. I'm sure you will let us know if they are successful. With regard to the needle bearings, I imagine there needs to be some level of play with regard to needle movement. If the needles were perfectly rigid within the cage, both bearings on the shaft would need to be more perfectly parallel as seated inside the mount or there would be more significant wear on the shaft. The needles in both bearings and the main shaft would all need to be well aligned axially or you could be riding high on one end of a needle, and low on the other. So less play in needle bearing means more precise machining of the bearing seats in the housing. But if you can gain less shaft wiggle with the new bearings mentioned that would be great if the seat tolerances allow.
And yes I have done the Bellville washer mods on both RA and Dec. I tried a number of ways to tighten down the blocks as per Michael's instructions. I found if I used a C-clamp on the blocks before tightening down, I could flex down the Bellville washers to gain that axial tension. But if I clamped too hard, the worm was too difficult to turn and the motor stalled. Even after the mod, I still had some wiggle if I rocked the counterweight shaft back and forth, but yes much less than beforehand.
My feeling is still the worm machining and then polishing is a fine art, yet to be perfected here. Since mount performance relies heavily on worm profile, if there was a simple solution HGM would already be doing that. If you have the skills to gain a better outcome, please share when you have it down. Just adjusting the blocks the first time took 5 or 6 nights out for me. Make an adjustment, assemble rig, guide at night, check results, then tear all down again if not right.?
Best of success,
John
|
For those interested in cage guided needle bearings, how they work and their relative advantage over the Torrington full compliment needle bearings used by Losmandy, I found this manufacturer article very informative (nice graphics):?Their design intrinsically keeps the rollers parallel with the shaft.?
|
Have u seen my long post where I talk about characterizing the DEC backlash with PHD2 guiding assistant?
Too tight worm mesh will cause backlash as well, and I suspect you have a too tight worm mesh because you keep talking about having to "power thru" motor stalls.
The "correct" worm mesh is a really light touch with low clutch pressure and shouldn't take more than about 300mA to slew.
|
Alan,
First, I never power through motor stalls. In fact,I do not even get them. ?Second, no, can you post a link please. I like the idea a checking the current and will no that.
When you are over 2500ms of measured backlash it has to come from some place. ?Others here has even larger values on a brand new mount.?I've already gone over this. 1. Gearbox, 2. Coupler, 3. Worm mesh, and 4. needle bearings (which I know you do not agree play a role). I will be checking my backlash again tonight after some mods made and will report back.
I'm going to nail this sucker,
Peter
|
/g/Losmandy_users/message/71205You can use the laser pointer trick to tell whether the backlash is in the gearing, or the worm to ring gear junction.? Try to have the worm adjusted to its regular operating condition, and clip the laser pointer to... either the worm or whatever before it, depending on whether you can access it.? It will easily detect motion even at "guide speed". On my G8 DEC axis, I had approx 2s worth of backlash in the gears before the worm would even turn.? You are "allowed" to use the TVC value to compensate for this :)
|
Alen,
first what a great great piece you wrote on backlash. From the graphs you posted I might be too tight? Second, you mentioned going to I think guiding rate and counting. You say you counted 2a before any movement in the worm. When I do this test it moves immediately and a see no visible backlash, yet I still measure 2.5s in guiding assistant. What might be going on here?I looks like a possible clear night so I'm anxious to get this sorted.
Peter?
|
Where did u clip the laser, and was the worm "loaded normally"?
We'll have to see the PHD backlash graphs to figure out what is going on.
Dirty little secret : Start the guiding assistant, then click stop right away, then hold down the Ctrl key and left click on the "cancel" button.? It will start the backlash test right away and you don't have to wait two minutes.
|
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
|
My 2005 vintage G11 has caged needle bearings supporting the RA and DEC axles. I have rebuilt several G11 mounts for friends and club members and some may have been newer than mine but almost all had caged bearings on the axle shafts. I wonder if there was some substitution in production over the years. Scott often slipstreams updated parts and newer designs into production without noting or documenting the changes.
?
--
Chip Louie Chief Daydreamer Imagination Hardware?
? ?Astropheric Weather Forecast - South Pasadena, CA?
|
|
|
Michael Herman
Chip Louie