HEQ5 Pro elongated stars. backlash adjustment broken?

[teoria_del_big_bang]

I did the belt mod and replaced all bearings when I obtained my HEQ5 2nd hand.
I actually bought all bearings before even stripping it down and for me I think it was a waste of time replacing the bearings (but might not be in your case) as there relly was nothing wrong with the originals. 
The thing is none of these bearings are under any real load or work at speed and so even if they are not top quality bearings they are unlikely to wear over time.

The one caveat to this statement is unless somebody previously has stripped it down and put it back together wrongly.

If replacing the bearings anyway, to be sure all is in good, order then do not pack the bearings with grease, in the literal sense. They need grease but not packed to the gunnels with the stuff. Again they will not get hot and so the grease will run anywhere and packing just means the motors have to use more current to move them.

In my opinion the bearings that do need to be of good quality are the smaller ones on the worm gear shaft.
Also I found the biggest improvement to backlash was getting the worm gear shaft just right with no float, this is critical as there must be no float but if you overtighten the float adjusters then the whole thing is too tight and the stepper motors can stall and start missing steps.

The other critical step is the meshing of the gear and the worm. Again I found this the main cause of backlash and needs adjusting very (very) carefully which takes time. These mounts are not the pinnacle of engineering (although great mounts for the price, hence their popularity with astrophotographers) but they are not precisely machined and so you will not get this meshing perfect as there will be some runout of the gear to worm. This means in one position the mesh will be perfect, with no backlash at all, but then turn it 90 degrees and there will be a little backlash (or you will be able to waggle the gear a little without moving the worm) then at 180 degrees movement of the gear there will be a little more backlash. Once the gear moves a full turn it will be back to no backlash. So there is a periodic error every revolution of this gear. It maybe more complicated than this and there maybe some similar interaction every full revolution of the worm as well, but generally you will not eliminate backlash at all angular positions of this gear and worm. That is mainly just down to machining errors that you will not fix.
I suspect some mounts will be better than others and all will not be made the same just to a certain tolerance that will not be microns but 10's of microns.
So it takes some doing but basically get the meshing of these two gears too tight so they bind once per rev will stall the motors, so it is a matter of getting them tight enough so they do not bind and reduce any backlash at their loosest point (if that makes sense).
The final adjusts needs to be done when all back together and keep adjusting for minimal backlash until at the highest slew speed you find the motor just stalls when at high speed. Then back this off ever such a small amount and check you can slew both axes (with scope and counter weights) at full speed through full movement of Ra and DEC several times without stalling the motors.

Adjusting Worm Gear Engagement

All the other large bearings are easy to replace, as I say just do not pack the bearings totally with grease.
The ones to be careful with are the taper bearings. These need a minimal amount of pre-load. Pre load is applied when you tighten the slotted nuts as they compress the two halves together and these then wedge together.
The nut literally just needs to be hand tight and no more. Do not be tempted to start tightening with tools. If your grip on them is poor then maybe just use a screwdriver to turn with it in the slot of the nut but really just hand tight. It just needs to nip up so there is no movement in the axis up and down (in direction of arrows), that's all. Any tighter and again it means the motors need more current to turn and more likely to stall.
 

All pictures from Astro Babys' website that has a superb guide to all this.
Rebuilding-the-skywatcher-heq5orion-sirius-mount

One other note is make sure you bu Standard Clearance bearings and do not be tempted to buy C3 clearance bearings thinking they are better quality. C3 bearings are designed for high speed use where the bearing will get hot and the balls and races will expand so they are designed with extra clearance when cold to allow for the expansion and clearances will not be correct unless running at high speed. So as nothing is remotely high speed at all in this use you will actually have bearings that are less stiff than standard clearance bearings.

Steve

 

 

Edited August 22, 2021 by teoria_del_big_bang

[Tommohawk]

Well done for biting the bullet and stripping down, and lots of useful comments above to guide you.

One thing I would reiterate is the need to aim for smoothness in the drive and don't obsess over backlash especially in RA. RA can have a degree of backlash and it won't matter if you're always "east heavy". 

[Penguin]

Ok, so I've taken it apart, cleaned of the old black grease, replaced the bearings, regreased and re-assembled. No detectable backlash on Dec and that rotates smoothly, but I have what seems o me to be binding in RA even with significant backlash (horrible grinding noises at parts of the rotation and the the axis judders at those points). Do I re-do the RA axis? something else?

[Tommohawk]

Just now, Penguin said:

Ok, so I've taken it apart, cleaned of the old black grease, replaced the bearings, regreased and re-assembled. No detectable backlash on Dec and that rotates smoothly, but I have what seems o me to be binding in RA even with significant backlash (horrible grinding noises at parts of the rotation and the the axis judders at those points). Do I re-do the RA axis? something else?

Likelihood is that the main gear is just somewhat eccentric. Did you put on the belt conversion? Either way you can test the worm / big gear mesh best by removing the internediate gears or belts if you've converted, and then just turn the worm / gear to  test the backlash  and binding. Loosen until there is no binding. then whatever backlash  you have youre probably stuck with - BUT as Ive said before this wont be an issue if you always set up East heavy.

You can put your scope and weights on and make sure the East side is slightly heavier and then turn the worm manually as described above - you will always be working against the weight and consequently will not feel any backlash. You just need to make sure you always set up like that. If the backlash is massive your pointing may suffer though especially if crossing the meridian. Its more of a problem if you have DEC backlash because you have to drive both ways. (Unless you deliberately misalign PA slightly so DEC always drifts same way, but that's a skill I havent mastered)

[Penguin]

12 hours ago, Tommohawk said:

Likelihood is that the main gear is just somewhat eccentric. Did you put on the belt conversion? Either way you can test the worm / big gear mesh best by removing the internediate gears or belts if you've converted, and then just turn the worm / gear to  test the backlash  and binding. Loosen until there is no binding. then whatever backlash  you have youre probably stuck with - BUT as Ive said before this wont be an issue if you always set up East heavy.

You can put your scope and weights on and make sure the East side is slightly heavier and then turn the worm manually as described above - you will always be working against the weight and consequently will not feel any backlash. You just need to make sure you always set up like that. If the backlash is massive your pointing may suffer though especially if crossing the meridian. Its more of a problem if you have DEC backlash because you have to drive both ways. (Unless you deliberately misalign PA slightly so DEC always drifts same way, but that's a skill I havent mastered)

Ok, so if it is that the RA main gear (do you mean the worm gear?) is slightly eccentric, what can I do there? Reseat it? It seems to be binding at the same point every time (at least when going clockwise), with the scope down so not an area it will be in very often. I haven't done the belt mod and I'm not guiding so I appreciate backlash shouldn't be a massive issue for me.

Thinking about it, I am noticing, as @teoria_del_big_bang said, I can feel more backlash in some positions than in others. I'll see where the least backlash is and whether that correlates with the binding.

Dec seems to be fine, nice and smooth, no obvious backlash and no binding anywhere in the rotation.

[teoria_del_big_bang]

3 minutes ago, Penguin said:

Ok, so if it is that the RA main gear (do you mean the worm gear?) is slightly eccentric, what can I do there? Reseat it? It seems to be binding at the same point every time (at least when going clockwise), with the scope down so not an area it will be in very often. I haven't done the belt mod and I'm not guiding so I appreciate backlash shouldn't be a massive issue for me.

Thinking about it, I am noticing, as @teoria_del_big_bang said, I can feel more backlash in some positions than in others. I'll see where the least backlash is and whether that correlates with the binding.

Dec seems to be fine, nice and smooth, no obvious backlash and no binding anywhere in the rotation.

Without accurate re-machining and sleeving the gears, or getting new gears made that are more accurate, there is nothing easily you can do. I don't think.
It is really just down to the tolerances they allow during manufacture that they allow some eccentricity errors. In true engineering these would be limited to a few micron so negligible, but for relatively cheap goods like these (I know £1000+ is not cheap really but if you look at the cost of precise mounts, relatively it is).
Because of the tolerances some gears will be made with little eccentricity and some may be near the maximum allowed (depending on how good the quality control is some may be over) so I think it is generally pot luck whether you get good ones or bad ones.

You could buy a spare gear, if you can get them, and hope it is better than the one you have but again it is pot luck.
But I would just get it the best you can so that where it is binding it is free to rotate and then a bit of backlash 180 degrees away, but at least then the backlash is the best you can get. I think all of us have to live with these errors in these types of mounts and let guiding correct for it and PEC if you need it.

HERE is a good article on Periodic Errors.

Steve

[Penguin]

Thanks so much for all this help. I did move it to where it was binding and loosened it off and now it runs smoothly all round in both directions. I think I understand what to do now and I will tighten it up to minimise the backlash at its tightest point. You are probably right that it has always been like this but I have not really noticed before now.

[Tommohawk]

3 hours ago, Penguin said:

Ok, so if it is that the RA main gear (do you mean the worm gear?) is slightly eccentric, what can I do there? Reseat it? It seems to be binding at the same point every time (at least when going clockwise), with the scope down so not an area it will be in very often. I haven't done the belt mod and I'm not guiding so I appreciate backlash shouldn't be a massive issue for me.

If the worm is eccentric or if the main gear with which  the worm engages is eccentric - any/both of these will cause variation in the speed of the drive (periodic error) and binding in some spots if too tightly engaged and backlash where it is loose.

Couple of things. There are periodic error routines you can run which try and anticipate the changes in drive speed. You can read up on this BUT if you are guiding this wont be an issue. My mount has heaps of backlash in some areas but I routinely guide at sub arc-second. Not perfect but pretty good for the class of mount.

Also you will notice if you set up your scope and weights as I described, ie East heavy, when you turn the gear manually (with the intemediate gears/belts removed)  in the normal direction of RA drive it will be much soother than if you rotate in the other direction. It's just an oddity of how the gears mesh even if perfectly adjusted and lubes. Dont worry if it seems jerky when rotating in the opposite direction to normal - thats of no consequence. It should be smooth though when going in the normal direction. Remember when guiding the correction pulses dont reverse the direction - it just  doesnt drive forward quite so fast.

Edited August 28, 2021 by Tommohawk

[Penguin]

Hopefully this video will upload...

Dec is lovely, smooth and free and I've got rid of most of the backlash (I'm not guiding so hopefully it shouldn't matter)

RA was a bit of a mare: I'd stopped referring to videos for each step so of course I missed a bit: the top bearing! Took it apart again and got the bearing in after a bit of persuasion. Again, I've got rid of most of the backlash but as you can see from the video, it's still very stiff, though I think it is better than it was before I started this whole adventure.

Any further ideas on the RA or is this acceptable for this kind of mount?

[Tommohawk]

To me that looks like the lower tapered thrust bearing may be too tight - assuming its clean and nicely greased which I'm sure it is now!

Edit - but just to add that mine is much like that too, and it doesn't seem to cause an issue.

Edited August 31, 2021 by Tommohawk

[City9Town0]

I agree with Tom... I'd loosen the taper bearing just a touch and give it another swing. Too free and it'll be sloppy and you'll feel it wobble as the scope goes over the top of the axis.

[Tommohawk]

Just one other quick though on this. In use, be careful not to overtighten the clutches - this "ovals" the housing and can markedly increase friction.

[Penguin]

Loosened it a touch and it seemed slightly better but not to anything like the Dec. I might have a bit more of a fiddle.

[Penguin]

Had a bit more of a fiddle. Much happier now. I think it's ready to do back on the pier. Massive thanks for all you help!

[Penguin]

Well, after trying it on a few targets of the last few months, I have come to the conclusion that it's now worse! I'll confess this may have been caused when, one night, I forgot to park before disconnecting the laptop and then also neglected to turn the power off before closing up the observatory and only realised a couple of days later that it had continued tracking, fouled a shelf and been stuck there all that time. Could that have forced things out of alignment or worse?

Both axis are still free moving with the clutches disengaged so I have improved that aspect, but once the clutches are engaged, there is massive backlash in both. There is a few mm free movement of the ends of the scope in Dec and a similar amount at the end of the counterweight bar in RA. It's obvious to the eye, even from a distance. The slightest further tightening up via the grub screws results in binding.

I have not done guiding before and had bought a guidescope and camera to try it out but PHD2 fails to even calibrate and I am suspecting the backlash is the cause of that. Trying to image targets unguided now leads to me having to throw away around 75% of my subs. Looking back at the history of this thread, you guys have said not to obsess about backlash so maybe it's something else that is wrong and I am barking up the wrong tree.

I'm not sure what to do now. I could look for another 2nd-hand HEQ5-Pro or even an EQ6 but wasn't really expecting to be spending close to 4 figures right now and then what am I going to do with my exiting one? I'd have to inform any potential buyer of my troubles with it. I'm tempted again to send it to DarkFrame for a hypertune. I know people have concerns about their service but I'm hoping that was a temporary blip that they have since sorted.

Edited March 20, 2022 by Penguin

[michael8554]

Examine the belts for damage, if you fitted them.

There are strategies for guiding with backlash.

RA can be slightly unbalanced east heavy to take up the backlash.

A PA Error will cause Dec to drift either north or south, gentle guiding in the opposite direction can keep Dec at one end of the backlash.

If you get PHD2 to Calibrate, following the instructions to clear backlash first, you can run the PHD2 Guide Assistant for a worm period.

That will show how the RA worm drive is behaving.

Michael

[Tommohawk]

18 hours ago, Penguin said:

I have not done guiding before and had bought a guidescope and camera to try it out but PHD2 fails to even calibrate and I am suspecting the backlash is the cause of that.

If PHD fails to calibrate, at least you have something definite to work with! If you are perfectly balanced but have lots of backlash, PHD calibration may fail. But if you deliberately imbalance - fairly markedly for the purpose of experimentation - backlash in either DEC or RA will simply not be an issue. 

Have you looked at some of you basic settings? ASCOM pulseguide settings are important. I believe the default for RA and DEC rate is 0.1 - but most folk advise 0.5-0.9. Mine is set at 0.8 though TBH I cant recall how I arrived at that!. If it's too low you may not get enough movement to calibrate. Minimum pulsewidth should be 20-30 msecs I believe (mine is 20) and DEC backlash on mine is 0 ms. TBH I'm not sure what that setting does, but there is a useful guide on how PHD deals with DEC backlash and other importatn PHD settings here. 

BUT, as I say, initially just keep it simple - ensure there is imbalance and try and get PHD to calibrate. It all seems a monster faff initially, but once you get it sorted you'll be fine! Right now there's lots of clear skies and a big moon - perfect for faffing!