Slight Slop in Dec Axis of Mach2 Mount


mjb87@...
 

My Mach2 has a bit of slop in the Dec axis. With clutches tight I can move the axis a very small amount. The RA axis, in contrast, is rock solid.

Another individual posted a similar problem regarding slop in the RA axis of his Mach 2: https://ap-gto.groups.io/g/main/message/81162

Do I need to follow the same process, in this case on my Dec axis?

Marty


Roland Christen
 

Won't affect you imaging at all but if you want, go ahead and adjust.

Roland

-----Original Message-----
From: mjb87 via groups.io <mjb87@...>
To: main@ap-gto.groups.io
Sent: Mon, Oct 4, 2021 3:01 pm
Subject: [ap-gto] Slight Slop in Dec Axis of Mach2 Mount

My Mach2 has a bit of slop in the Dec axis. With clutches tight I can move the axis a very small amount. The RA axis, in contrast, is rock solid.

Another individual posted a similar problem regarding slop in the RA axis of his Mach 2: https://ap-gto.groups.io/g/main/message/81162

Do I need to follow the same process, in this case on my Dec axis?

Marty

--
Roland Christen
Astro-Physics


mjb87@...
 

Thanks, Roland.

I certainly did not notice any impact on imaging -- getting excellent unguided results. I'll leave it alone for the time being.

Marty


Roland Christen
 

Slight mesh issue can be caused by a couple of things. In the Dec axis we set the spring tension to be very minimal to avoid stiction (static friction). Stiction is the main cause of poor guiding in Dec because it results in retrograde motion which is common in almost every mount that has spring loaded worm mesh. If the mesh is not 100% in a non-encoder mount it causes backlash delay when reversing, but encoder mounts don't have backlash even if the worm mesh is not 100%.

However, the problem of stiction is much worse for both encoder and non-encoder mounts. Retrograde motion means that when reversing, if the star error is slightly above the zero line it will first go further above the line before finally heading towards zero point. If it would stay there, that would be fine, but it usually overshoots at that point (release of static friction) and the whole cycle repeats in the opposite direction. It results in what control theory calls limit cycling and is a type of oscillation instability that is contained within boundaries.

Therefore to prevent stiction, we purposely set the Dec spring pressure low so as not to get into that possibility ever. We let the encoder eliminate the backlash delay if there is slight de-mesh between worm and worm wheel teeth. The mounts go out with the Dec axis tested for proper mesh, but possibly during shipping and later use with heavy loads the adjustment of the spring pressure is not enough for full mesh. The spring pressure can be increased, and I found that after a number of weeks of steady use, the problem of stiction is reduced considerably just from the worm and worm wheel polishing themselves thru use.

If you want to do a preliminary adjustment I would recommend checking the pivot bolt first before adjusting the spring pressure:

Your pivot bolt might be too tight which is preventing the worm from pivoting fully into mesh. It's an easy adjustment.
Remove the outer motor box cover and set it aside with its 4 screws.
Remove the 4 screws that hold the inner cover and put those screws aside in another place, but do not mix them with the outer screws!
Now slide the inner cover back and out of the way to expose the worm assembly. You don't need to remove it entirely.

First make sure that the 3 Allen screws that hold the motor assembly to the RA axis are fully tight. If they are, then proceed with the next step:

Referring to the image below, loosen the locking screw #1 (5/64 Allen Key). Then with a 1/4" Allen key loosen the pivot bolt #2 about 1/2 turn ccw while gently pushing on the scope. Gently! not with guerilla force please. It should stiffen up as the worm mates with the worm wheel teeth. Once it is in full mesh, tighten the pivot bolt a small amount and again feel the axis to make sure that it is meshed. You can then push down on the top back of the motor to bring the worm slightly out of mesh, then let go to allow the springs to pop the worm teeth back into mesh. The motor assembly should be able to rotate slightly in and out of mesh when you apply force to the back of the motor.

If everything feels right, re-tighten the locking screw. Then replace the inner motor cover with the shorter 4 screws (very important that you do not use the outer cover screws!!) Finish by replacing the outer cover.

Roland Christen
Astro-Physics Inc.







-----Original Message-----
From: mjb87 via groups.io <mjb87@...>
To: main@ap-gto.groups.io
Sent: Mon, Oct 4, 2021 3:52 pm
Subject: Re: [ap-gto] Slight Slop in Dec Axis of Mach2 Mount

Thanks, Roland.

I certainly did not notice any impact on imaging -- getting excellent unguided results. I'll leave it alone for the time being.

Marty

--
Roland Christen
Astro-Physics


mjb87@...
 

Thank you for the very detailed explanation. It is both very helpful -- and also enlightening as to the details you folks at AP apply in your engineering.


Roland Christen
 

I do have one request, the heading of Slop in Dec axis is a huge turnoff for me. For 2 reasons.

First, it suggests sloppiness, both in design and execution. Our design engineer is 6ft 3" and built like Arnold and he would take unkindly to the design being sloppy ;^)). Both Dave and I assemble the mounts and we are very careful in making sure that everything works exactly as intended. In fact, the tolerances are so tight that we have to shrink the bearings in our deep freeze in order to set them in their housing.

But the second reason is a bit different. back when my hobby was off-road endurance racing, we had one event each summer in south-western Wisconsin that was about 65 miles in length. The course went thru many farmer's back woods in the hills of the Driftless area (Driftless = untouched by the ice sheets that scoured the rest of the Midwest). This is rugged country and ideal for all kinds of off-road events.

Normally the farmers are all eager to lend the enduro clubs their wood sections for these races, but one of the most desirable sections was owned by a farmer who made one stipulation: the race had to go thru his pig wallow before heading up into his hills. He would set up a comfy chair on the hill overlooking the wallow, beer in hand and watch the racers run thru his 1/2 acre of foul smelling muck. Some of the novice racers would inevitably slide out and do a head-first dive into the mud. I always managed to make it thru without sliding out, but the stench stayed with me the rest of the race. Some of us would load the bikes on the trailers and or pickup trucks and head for the nearest do-it-yourself carwash. Of course we would also spray ourselves down too. So, every time I see the word slop, it brings back those memories and the foul stench thereupon.

Rolando

-----Original Message-----
From: mjb87 via groups.io <mjb87@...>
To: main@ap-gto.groups.io
Sent: Mon, Oct 4, 2021 7:14 pm
Subject: Re: [ap-gto] Slight Slop in Dec Axis of Mach2 Mount

Thank you for the very detailed explanation. It is both very helpful -- and also enlightening as to the details you folks at AP apply in your engineering.

--
Roland Christen
Astro-Physics