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That Pesky Back Lash & How To Fix It!
Almost every new LXD75 owner will at one time or another need to adjust the worm gear assemblies on their LXD75 German Equatorial Mount to reduce an increasing backlash in the operations of the mount as it breaks in. Some LXD75 GEMs come right out of the box needing the worm gears adjusted, while this can be frustrating to some, it's really not that big of a deal. Why do you think the mount was designed and built with a way to adjust these assemblies? Adjusting the worm gears due to usage and wear is something that is going to occur sooner or later with this type of GEM design, it's not that difficult. In this article I will explain how get to the adjustment screws on the LXD75; how to make the adjustments; and how to put everything back together without creating more problems.
How Do I Adjust The Worm Gear Assemblies?
While I do not take my OTA(s) off of the mount to make the simple adjustments to the worm gear assemblies, I suggest you do until you are experienced enough with the working of the mount to not do something you shouldn't and send the OTA(s) crashing into the side of the tripod or pier. You find in some of the pictures in this article my LXD75 GEM is still mounted on the pier with the OTA(s) still on the mount. I have enough experience with these types of mounts (the GEM designs) to know how to work on several aspects of it without endangering the OTA(s) by releasing the wrong levers or screws. As with everything listed and explained in this article, if you choose to do the maintenance procedures outlined in this article you do so at your own risk.
The first step in adjusting the worm gears it to bring the mount to a neutral position such as the polar home position and securely lock the Dec & R.A. axis clutches down.
Next you will need to remove the black plastic axis drive motor assemblies, this is accomplished by removing a single bolt in each axis drive gear assembly box, these units come off all in one piece as a unit. There should be no need to do anything else with these drive assemblies in this maintenance operation, although while you have them off it is a good time to check a couple of things, but we'll get to that another time.
Ok you should now be able to see the actual worm gear assemblies mounted to each axis, you will also notice at this point three Allen head screws in the front of each assembly, those are part of what we are after! Also look on the other side, the 'back side' of where these worm assemblies mount and you will see two more mounting screws for the worm assemblies. we will be using the front three screws to adjust the tension worm assemblies for sure, and may need to loosen the back two screws some too if things are really out of whack. Before we start on any of these adjustment screws though there is something we need to check first. We need to check the worm gear shafts themselves for 'end play'.
Take hold of the counter weight bar and gently work the axis in both directions, if everything is perfectly tight & there are no backlash or end play issues the axis will not move. You shouldn't be pushing or pulling so hard on the axis via the counter weight bar you can move it against the clutches, just enough pressure to determine if there is any looseness or play in the axis. If you cannot get your clutches to tighten down enough to resist some light pressure against the axis without slipping we are going to need to fix that now. This is a simple fix, there is a Phillips head screw in the top of each black clutch lever, remove it pull the clutch lever off & reposition it so you can get the clutch tight enough on the axis to resist the light pressure needed to check the worm gears. Put the Phillips head screw back in the clutch handle head and secure it, your done with that!
OK, now that the clutches are properly adjusted, should they have needed it lets check those axis for end play! Again on the R.A. take the counter weight bar and jiggle the axis. If you feel play -(And you should, because if after tightening the clutches you no longer have any play you're done - put it back together!)- OK, as you jiggle the axis you will notice some play in it, take your little finger and press it against the worm shaft where it comes through the worm collet. Now jiggle the axis again, if you feel motion or the worm shaft moving against your finger you have some end play in your worm shaft. We'll need to fix the end play first before we go after the back lash in the worm assembly/ ring gear engagements.
You'll need a 17mm wrench to loosen & take the lock nut off of the collet holding the worm shaft in place, remove the lock nut. Now take your fingers and turn the collet down as tight as you can with using just your fingers. Jiggle the axis again with your little finger against the end of the worm shaft in the collet. Is there any more precipitable end play there? If not then the collet is down tight enough to take your end play out and finger tightening should be enough to do it. You can overtighten the collet causing too much resistance and generating a motor fault error when you have everything back together and are running the electric drives. I suggest using a flat piece of steel like a piece of hack saw blade or similar to tighten the collet further if there is still noticeable end play in the shaft. trying to use a screw driver you can easily over tighten the collet and cause damage to the end of the collet. Once the end play has been eliminated from the Worm shaft you will need to put the collet lock nut back on the collet.
Putting the collet locking nut back on the collet and turning down tight enough to lock the collet in place is tricky. there are a couple of ways to ensure the collet is locked enough to hold it position. One is to use some Blue Loc-Tite on the collet when adjusting it and then snugging the lock nut down on the collet once the Loc-Tite has had sufficient time to set. As you tighten the lock nut down on the collet the nut has a tendency to grab the collet and turn it too, causing it to become over torqued letting the Blue Loc-Tite set can reduce this possibility. Another way to secure the collet with the lock nut is to use a #9 plumbers rubber O-ring over the collet before putting the lock nut on. This serves as a quasi lock washer that will not bite into the worm assembly or if shifted against the collet not damage or cause it to turn with the lock nut. Before proceeding to the worm assembly adjustments we need to check to be sure the collet is not over torqued.
What you need to do is check the ease with which the axis will turn. Take the drive gear on the end of the worm shaft and turn it lightly with your finger tips. Move the axis 180 degrees in both directions this way, it should move through the entire range either direction without becoming so tight you have to apply a lot of finger pressure to turn it. It should take some light pressure but nothing that will hurt your fingers or cause the spur gear to bite into your finger tips too much. You nay encounter some spots a little more resistant than others, but nothing that is to tight. If everything is OK we can proceed, if the axis is turning to tightly with too much resistance you'll need to back the worm collet off slightly, it is too tight. If everything checks out OK we're ready to move on to the Worm assembly adjustment to reduce the back lash if there is any.
OK, We have now eliminated the clutches being loose & the end play of the worm shaft causing a back lash like problem. It is finally time to address the adjustment of the worm assemblies. First now that the clutches & worm shafts are fixed jiggle the axis again. Is there play on the axis, if it is tight and there is no movement in the axis when you jiggle it, your done there was no serious back lash to deal with, put it back together. If however, (and there probably is) you feel looseness or slop in the axis still you have some back lash from worm to drum/ring gear mesh looseness.
OK, now it's time to do some adjustments! Go to the three Allen head screws on the front of the worm assembly and turn the middle one back (loosen) about 1/16 - 1/8 of a turn. Then turn the 2 outside screws down about that 1/16 - 1/8 turn each. Jiggle the axis to check for back lash, continue this process until you have eliminated the back lash from your giggling of the axis. You may find your adjustments do not seem to be making any difference at all, you'll need to back those two rear screws off about 1/8 - 1/4 turn do the front three adjustments & tighten down the two screws in back. This usually is not the case or just backing off & re-tightening the two back screws for the first adjustment is all you need to do before proceeding with more adjustments of the front three screws. OK now that it seems you have taken the back lash out of the axis with the adjustments once again you will need to rotate the Axis 180 degrees in both directions. Use the same procedure with your finger tips as you did for the end play adjustments. If the axis becomes too tight at any point through the rotations you will have to back the worm assembly off a 1/32 - 1/16 of a turn on the adjustment screws until you can do the finger tip rotation through the rotations of the axis without excessive finger pressure. If the Axis has a spot in it that is too tight or the axis as a whole is too tight it will cause a "Motor Fault" error when you operate the mount with the electronic drives.
Ok time to put it back together! Once back together put the OTA(s) & counter weights back on the mount and run each axis out at full slew as far as you can without mount to OTA or drive gear assembly box collisions at full slew speeds. If this generates a Motor Fault error something was allowed to go back together too tight. Disassemble and fix the problem, reassemble reset the handset and do it again. Don't worry about loosing your PEC training, or drive training when you took everything apart and adjusted it you negated any training. You will need to do new ones, the positions and such of the gears has been changed.
Unfortunately you will probably not be able to adjust all of the back lash out of the gears, the machining required for that kind of precision only comes in the mounts costing thousands of dollars. You should however be able to adjust the back lash down to an acceptable level for the mount, just don't expect to get gold from lead. It's not going to happen, but properly adjusted and maintained the mount should give you an above the mark performance in it's price class!
The Mad One
39 47' 06" North / 85 46' 10" West