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Backlash and Axis-Play - Same Thing?

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#1 tenafterten

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Posted 10 September 2012 - 01:55 PM

I recently purchased a new EQ mount advertized as a precision AP mount.
In my old motor-driven EQ5 I would notice that stars would drift across the field of view for a few seconds before tracking would take effect. I interpreted this as backlash. It was not a big deal as I would compensate by placing an object off-center, after which it would be fairly well centered by the time the gears caught and tracking took effect. However, there was no 'play' (loose movement) in the RA axis when the clutch was tight.
In the new mount, there is very little drift (none that I can see at 50x) across the field of view after I use the controls to center an object. However, there is definite play in the RA axis. There is about 0.1 degree of free movement about the RA axis even when the clutch is very tight. This makes focussing at low magnification frustrating, and very difficult at higher magnifications. I understand that vibrations effect focussing, but this is entirely different. When my scope is well balanced I can nudge the mount to and from the two end-points of the rotation with the slightest pressure of one finger. I've rulled out the tripod as the culprit - When placing my finger on the RA axis joint on the mount I can feel about 0.5 mm of movement. The manual for the mount states that "some backlash is to be expected", but I suspect that this is not what it means.
Insights greatly appreciated.

#2 orlyandico

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Posted 10 September 2012 - 02:28 PM

which mount?

it should be possible to reduce the RA backlash by adjusting the worm mesh, but with the china mounts you cannot remove the backlash entirely.

#3 tenafterten

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Posted 10 September 2012 - 02:52 PM

Thanks for the repsonse. It's an iEQ30. Are you implying that RA axis play and blacklash are the same thing? They seem to be mutually exclusive on my mount, or maybe my understanding of the concept is off. There seems to be no or very little time lag between centering an object and when tracking starts, which is what I thought backlash meant. The problem is free movement in the RA axis when the clutch is locked. It's the opposite behavior of my EQ5. On that mount, there's lots of backlash, but the RA axis locks firmly with no play whatsoever - focussing is easy. Becuase of the play in the RA axis I could put the iEQ30 on a concrete pier and I would still have lots of movement when focussing.

#4 Cliff Hipsher

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Posted 10 September 2012 - 04:37 PM

You need to dig into the mount and physically inspect it to find out what is causing the "play."

#5 PatrickD

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Posted 10 September 2012 - 05:16 PM

Hello,

Long time lurker here, but I wanted to jump in as I've had a similar problem on my new iEQ30, but the relatively small play I experienced was on the DEC axis instead of RA.

I was apparently able to fix this play by tightening the tension of the drive belt which connects the motor to the worm screw. I say apparently because I was originally hunting for a way to fix what I deemed excessive DEC backlash instead of the play.

After inspecting the DEC assembly, It was obvious the drive belt looked too slack. What is nice on this mount is that you can adjust the tension of the belt by sliding the motor assembly in and out after unscrewing a couple of screws. I did just that and widened the separation just a little so the belt was nicely tight. Don't understand quite well how this could affect play, but after this tension adjustment I got no play at all anymore, while the backlash situation remained completely unchanged, which I guess lend credence that these 2 things not being related :)

I believe the RA assembly uses the same adjusting mechanism for belt tension, you might give it a try if nothing else works.

Patrick

#6 tenafterten

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Posted 10 September 2012 - 05:39 PM

Hello,

Long time lurker here, but I wanted to jump in as I've had a similar problem on my new iEQ30, but the relatively small play I experienced was on the DEC axis instead of RA.

I was apparently able to fix this play by tightening the tension of the drive belt which connects the motor to the worm screw. I say apparently because I was originally hunting for a way to fix what I deemed excessive DEC backlash instead of the play.

After inspecting the DEC assembly, It was obvious the drive belt looked too slack. What is nice on this mount is that you can adjust the tension of the belt by sliding the motor assembly in and out after unscrewing a couple of screws. I did just that and widened the separation just a little so the belt was nicely tight. Don't understand quite well how this could affect play, but after this tension adjustment I got no play at all anymore, while the backlash situation remained completely unchanged, which I guess lend credence that these 2 things not being related :)

I believe the RA assembly uses the same adjusting mechanism for belt tension, you might give it a try if nothing else works.

Patrick

Thanks very much Patrick, particularly as it is your first post. The belts are easily accessible, so I'll check them out.
A

#7 orlyandico

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Posted 10 September 2012 - 08:42 PM

i thought the iEQ30/45 had spring-loaded worms. These would eliminate any play..

backlash and play can be the same thing.

as you say, delay when reversing direction is caused by backlash - which can be in the worm mesh, or in the motor gearbox. whereas play in the axis when locked is due to too-loose worm mesh (the worm is not firmly seated against the gear so the teeth wiggle back and forth a bit).

it is the 2nd case which a spring-loaded worm is supposed to solve...

#8 PatrickD

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Posted 11 September 2012 - 12:47 AM

Worm mesh too loose would make sense for that play, I agree.

In my case, I focused on the belt tensioning because it looked really abnormally loose, but I recall also tightening all screws before putting back the cover, so I may have inadvertently solved my play by tightening something related to the worm screw...
I do recall in particular now a small metal tab with 2 screws flat against the worm housing and pushing against a small spring which then went down a small hole at the worm level... Looked weird and the screws already tight, but who knows maybe the system is very sensitive.

Maybe tenafterten can look if the RA assembly has also some kind of similar spring system which may need adjustment. Can't say from experience, I didn't have a look at the RA box as that axis is working very smoothly on my mount.

Good luck!

Patrick

#9 tenafterten

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Posted 11 September 2012 - 01:24 AM

iOptron sent me some instructions for things to check, but the play is still there. The worm spring seemed fine, belt was tight, no play in the worm shaft that I could feel. The one thing they said I would not be able to check for is "gaps between the worm bearings and worm shaft, or a misalignment of the entire RA drive unit. In this case, it needs a full disassembly of the RA drive unit."
If you saw the amount of movement when the counterweight shaft is moved back and forth - hardly noticable to the eye - you wouldn't think it would be an issue, yet when focussing, the play equates to free movement of almost 20% the diameter of the full moon.
Back to the shop it goes I'm afraid.

#10 orlyandico

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Posted 11 September 2012 - 01:58 AM

20% of the diameter of the full moon is 6 arc-minutes.

I'm not sure but that sounds almost within spec. :tonofbricks:

On my CGEM even after a lot of messing around, there is still about 1-2mm of play at the end of the counterweight shaft.

#11 tenafterten

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Posted 11 September 2012 - 02:08 PM

Seeing as my old EQ5 had zero play in the RA axis when locked (but plenty of backlash), I can't see this as being normal for something considered to be an upgrade to that mount. 6 arc-min backlash might be within spec - I don't know if it is or isn't - but I've come to think that play and backlash are not the same thing and that 6 arc-min of loose play is not within spec.

#12 EFT

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Posted 11 September 2012 - 09:06 PM

Calling anything in this category a "precision AP mount" is kind of wishful thinking. However, there are a lot of different posibilities for things that can cause axis play. Backlash is on thing but is is not the only thing. Backlash is the play that is inherent between gears. Gear teeth cannot fit perfectly without getting stuck. The higher the precision of the gears, the less space that is necessary and the less backlash that results. For the most part, backlash cannot be completely eliminated in mounts like these because the gears are not very high precision. However, because of the low precision, the gears can sometimes be jammed together giving the appearance of a tight fit. In reality it is just close fit and will usually result in jamming due to excessive friction between the gears and high spots in the gears. The only choice is to get things as close as possible without binding. If the worms are spring mounted, this can help to eliminate backlash, but spring loading comes with its own challenges, especially in lower precision gear systems, as seen with the LX80.

Other sources of axis play include improper assembly and poor design. If the worm is not tightly held in the housing, it can slide back and forth or otherwise move around causing axis play that can mimic backlash. This is not too common on new mounts because the tendancy for most complanies is to over tighten the worm to avoid this problem (a cause others). Depending on what type of support the axis shaft has in the housing, it may also cause axis play, but this does not usually look like backlash in that it doesn't just cause play in two opposite directions. The best example is a mount that uses two taper bearings to hold and center the axis shaft in the housing (like a CGE). If the axis nuts are not tight enough, then the axis will exhibit play. If they are too tight, then the axis will be stiff.

Finally, poor design and construction can result in axis play that is either loose and shifting or tight and wobbling. For example, if the axis shaft is held in with two bearings and they are not straight and parallel, then the axis may wobble. If the axis head is not straight and perpendicular to the axis shaft, then it will wobble, but not necessarily show any play.

When it comes down to it, the first and easiest things to check are the worm spacing and the worm play. These are the most common and generally correctable (or at least reduceable) causes of axis play. The other possibilities are obviously more difficult to deal with.

In mounts at this level (i.e., cost), you can only expect so much and precision is not necessarily one of the things you can expect. However, improvement is frequently possible.

#13 Eddgie

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Posted 12 September 2012 - 07:42 AM

There are two kinds of play and my quesiton would be which kind do you have?

Worm play can be identified by simply by locking the RA clutch and rocking the counterweight shaft east and west.

If the counterweight shaft moves in this direction even a tiny tiny bit, this indicates that the worm gear is not fully meshed with the worm wheel or the worm gear itself has end=float. There should perhaps be a tiny bit of movment east/west, but excessive would be something like 3/16th of an inch. This is not unheard of in these mounts.

The other kind of end play is when you can pull on the endo of the counterweight shaft and rock the counterweight shaft towards the north and south. This kind of play would denote some kind of assembly error or improper tolerance in the RA axis such as excessive end float in the RA shaft, or a bearing moving in its recess.

I do not own the mount in question, but again, it would be useful to know exactly what kind of play you are experienceing. If it is worm gear mesh, that is pretty easy to adjust on most mounts.

There should be no movement at all when you do the north/south movement of the counterweight shaft, but east/west may be OK unless it is excessive. The purpose of training the mount is to let the software know how much slack the gears have so that the computer can automatically drive the motor a little extra as required after a slew to take out the slack. I hate the slop in a lot of these mounts but the designers must have figured that rather than having assembly people take the time to get the gear mesh well adjusted on every mount was just to long a process, and it would be better to allow loose assembly tolerances and deal with it with software.

This is to bad though because a worm gear that is not fully engaging the worm wheel can cause pre-mature wear on the teeth of the softer gear (usualy the worm wheel). All of the pressure is put on the very tip of the gear faces. A worm gear can have pretty enormous pound per square inch loading and if the gear faces are not positioned properly with respect to one another.

But it would perhaps take a couple of decades for the average person to see a problem.

But if your problem is North/south play, something may be wrong in the RA housing itself, such as too much end play or bearing issues.

#14 tenafterten

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Posted 12 September 2012 - 10:27 PM

Thanks for the responses. Lateral movement of the worm gear sounds highly plausible. iOptron sent me instructions for an inspection and possible fixes, but I could not delve to deeply into them becuase one of the screws I was supposed to remove for the inspection was factory overtightened and the alen key spun freely within it. Suffice to say I could not complete the inspection. The mount was sent back. In the meantime I re-evaluated my desire for a goto and decided to redirect the cash into a new SV90T, which will sit on my trusty M2 alt-az. My 5" will remain on the EQ5 for now, which has proven a sturdy mount for it. Maybe the future holds a non-goto G8 for me at some point - I prefer well built simple things of good repute as apposed to hastily built complicated things of little repute.






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