Working with what I have here, there are not many options to correct the problem of the roundnut (inner pressure plate) loosening and turning on the axle. I came up with three to consider.
1. Assemble the parts as they are and continue to rely upon the 3 set-screws to keep the roundnut fixed and tight upon the axle.
2. Backing off the set-scews and tightening the roundnut enough to remove the lateral side play of the axle and hub in the side-arm and hope it remains stationary and fixed to the axle by thread tension alone.
3. Applying threadlocker compound - e.g., Loctite or Permatex - to the threading on the roundnut and axle.
The first option only continues the problem and risks further damage to the threads on the axle. The one thing I do NOT want to do is to re-tighten the 3 set-screws, should I find them loose again. Without knowing the exact, initial contact points on the axle where the set-screws were originally screwed into, I would only crush more of the relatively soft, aluminum axle threads and cause more damage. I need to remember that the axle is turned from the one piece, aluminum hub casting.
As I see it, there is simply no way these 3 set-screws can withstand the repeated forces placed against them and remain tight. The "fore and aft" pressure applied against them from loosening and tightening the clutch and then the constant torque force of the worm gear during slow-motion action is just too much for the tiny contact area of 3 set-screws. Add to this the tube's up and down (especially vertical) motion and the "to and fro" torque and friction this applies to the inside surface of the roundnut. As is, the repeated "to & fro" and "fore & aft" forces being directed at the roundnut are bound to loosen the set-screws and then the roundnut. This is the weak link in the chain of parts that make up the altitude control drive train.
The second option is being tried on both mounts (the 70 and 90) and so far, within one viewing session, the adjustment of the roundnut on the axle is lost and pauses begin to happen during directional changes in slow-motion control. In other words, with clutch engaged (tight) and when it should be fixed to the axle the roundnut is loosening and turning on the axle as the action from the worm and ring-gear are engaged. Even with the clutch fully disengaged (loosened), with the roundnut adjusted to remove all sideplay of the hub in the side-arm, there is still enough friction against the inside surface of the roundnut to create an unthreading action on the nut. This is particularly the case when the tube is being elevated in the vertical direction of free swing motion.
The third option may be the only one that has a good chance to work. Threadlocking compound could keep the nut tight on the axle, once properly adjusted. Loctite 242 Blue and Permatex Blue are both rated as medium strength and may be enough. Although, from my experience, these compounds are designed to set and harden best in an anaerobic situation. In order for optimum performance to be realized, once the Loctite is applied, the fastener's thread surfaces should be tightened enough to squeeze out the air (oxygen) between the threads. In this situation, though, the roundnut's threads are not able to be tightened enough to reach an optimum anaerobic condition. Anyway, I thought to give it a try.
Before Loctite application, I made certain the threads of both the roundnut and axle were thoroughly cleaned of any grease and completely dry. I shook the vial well, applied a drop to both thread surfaces, made the adjustment and waited a full 24 hours for it to set, dry and harden. It didn't. After finding that it again loosened, when removing the round-nut I found the Loctite applied to the threads the day before had not even begun to set and harden. It was still wet.
What I have been reading in some other mechanical theme forums is that for the threadlocker to work right, it must be fresh and that it has a shelf life of less than 2 years. Then again, others have written that Loctite they had for many years is still used with good results. At this time, both vials I have of the medium strength, blue Loctite and Permatex threadlocker are quite old and have been in the toolbox for 20 years, or more. I assumed this to be the reason the compound would not dry, set up and harden in the 24 hours as it should. I don't know why mine did not set up. Perhaps it was because the round-nut could not be tightened enough to create the anaerobic situation it requires for proper set up. So, another attempt will be made when what I have has been replaced with fresh Loctite. I also need a method to get the roundnut's threads tighter during the curing time.
Adjusting and Tightening the Round-Nut: To get around not being able to fully tighten the roundnut's threads on my first attempt, I may try adjusting the nut, while slipping a feeler gauge blade between the nut and the inner, *nylon washer on the side-arm (seen in photo above) and then tightening the nut just enough against the inserted blade to reach an anaerobic condition.
While adjusting and tightening the round-nut, for better control and leverage I use the makeshift pins fashioned from two finishing nails placed in the nut's two holes. A few attempts with different thicknesses of blades will need to be tried to get the best adjustment and I'll start with either .006" or .008" blades (0,15mm and 0,20mm). The adjustment aim is to have the nut tighten at the point where the set-screws line up with the pre-existing flats on the axle threads, or just off to the right of the flats. More on this below.
Once the correct adjustment is found and the nut well snugged up against the blade between it and the "nylon" washer, I'll then wait the full 24 hours to see if the threadlocker compound fully sets up and hardens. Afterwards, I can gently work the feeler gauge blade out from between the nut and nylon washer.
The plan is that If the medium strength blue Loctite 242 now fully sets and hardens, the round-nut should be tight enough on the axle threads to resist my being able to loosen it with thumb and fingers, using the pins in the round-nut's 2 holes for control and leverage, but without forcing it. With luck, there won't be any side shake of the hub in the side-arm and maybe there will even be a slight drag on the tube's vertical, free motion.
There is a temptation to use Loctite Red (271) High Strength here, but I am resisting this temptation. Should I want to change the roundnut adjustment, the Loctite Red 271 usually has to be heated (500°F) to melt the compound and allow for the nut to be loosened. Applying heat to the nut will most likely ruin the nylon friction washer directly behind and in contact with it.
Using Loctite 271 Red is my last resort. Should the Loctite 242 Blue not work out then I'll find some replacement, "nylon" washers somewhere and go with it.
Set-Screws (flat point vs. cone point): If able to reasonably secure the round-nut with Loctite, I am leaning towards replacing the original, "flat point" set-screws with M3 "cone point" set-screws. These are designed for more permanent and secure situations. Also, instead of crushing the thread, like a flat point, when tightened, the cone point is forced between the threads and not down on top of them.
If, as planned, the round-nut is sufficiently tight with using the 242 Loctite alone, I may want to include additional security with 3 "cone point" set-screws. Having adjusted the round-nut with the set-screw holes aligned just to the right of the pre-existing flats on the axle threads, when tightening the set-screws, the cone points will bore in between the threads just behind these 3 flats. The flats should provide a stop for the points, adding a little more security in helping to resist any unthreading force on the round-nut.
Were there more axle threads exposed on the outer side of the roundnut, I would like to try threading a low profile, spanner nut up against the roundnut and to act as a locknut. But, there are not enough exposed threads here and then a recess would need to be machined on the inside surface of the worm ring-gear to accomodate the profile of the locknut. Of course, this would then reduce the contact area of friction for the ring-gear on the roundnut. It's always something.
What I would really like to see happen is for Meade to replace this "round-nut on the axle as a pressure plate" function and to redesign it with a proper and traditional splined friction plate and matching, splined axle - as briefly described in the post above (#32). It would be great if a redesigned hub, axle and pressure plate were to be interchangeable with the existing mounts and then made available as an upgrade.
Of course, a wish like this may be about as fruitful as standing in Death Valley, with both hands out and waiting for my wishes to fill one hand, for the other to fill with rain and see which one fills first. Oh well, in a perfect world, the engineers would have the final say on the blueprints and not the accountants.
* I have been referring to the white, plastic washers as nylon. They look and feel like nylon, but may well be another plastic, e.g., ptfe, teflon, delrin, etc.
Getting back to the azimuth controls is next and I'll try to keep the following explanations more brief.
Edited by Veridian, 21 June 2021 - 05:03 PM.