Last updated: 31 August 2006
Fixing a Slipped Right Ascension Gear
One evening I had been observing Mercury low on the western horizon. I then did a GOTO to Jupiter, which was almost at the Zenith. The telescope slewed fine but Jupiter was not quite centered in the eyepiece (I had done just a rough alignment at the start of this observing session) so I began to slew the telescope using the Autostar arrow keys. I found that I could slew OK in Declination but the telescope would not move in Right Ascension even though I could hear the RA motor running. I checked the axis lock; it wasn't too tight and, afterall, the telescope had just finished doing a major slew in RA. I decided to start over so I redid the alignment from the start but when the telescope slewed to the alignment star only the Declination changed; there was no movement in Right Ascension. It was now obvious that something had failed in the RA movement mechanism. A few days later I was able to set aside enough time to "go inside" the mount head and see what I could learn about the failure and hopefully fix the problem. What follows is a report on my investigation and the (fortunately) simple fix. It took me about an hour of setup, locating the right-sized hex keys, disassembly, reassembly, taking photos at every step, and making notes for this report.
I removed the 8"SC OTA and the counterweight rod to make working with the GEM head easier and safer. The photo on the right shows the GEM head. The DEC gear box is on the top (with a cable coming out of it) and the RA gear box is lower down and includes the control panel with the power switch and jacks. You can see the gear inspection port cover on both boxes.
The two photos below show close-ups of just the DEC gear box. The first shows the closed gear inspection port and the second, the cover has been removed. With the cover removed you can see the two DEC axis gears. The silver gear is on the telescope side and the gold gear is on the drive motor. Note that these gears mesh correctly and that the end of the shafts are flush with the side of the gears.
Since I was not having any Declination drive problems, I moved on to the Right Ascension gear box. In the photos below you can see the gear box with its inspection port covered and opened. In the second photo notice that the silver (telescope side) gear is not flush with the end of the gear shaft and that it does not mesh with the gold (drive motor) gear's teeth.
The photo above definitely shows why the telescope would not slew in RA; the drive (gold) gear would just rotate without turning the telescope (silver) gear. With the RA inspection port cover off I was able to see this by pressing a slew arrow key on the Autostar. I then removed the power cord and cable connections from the control panel so that I could safely remove the RA gear box.
You might think that you would remove one or both screws on the RA gear box (circled on the photo below; there are two similar ones on the other side of the gear box) but you would be wrong. (I was!)
Instead you remove the gear box by turning a hidden hex bolt in the GEM head itself (circled in the photo below). You will need a 3/16" hex key for this hex bolt.
Hold the RA gear box with one hand as you loosen the hex bolt with your other hand. After several turns the gear box will come loose, exposing the RA gear (silver) on the telescope side (photo below).
I checked the condition of the RA gear box (next photo) and it looked good.
I removed the small setscrew in the silver gear using a 2/32" hex key and slid the gear along the shaft to expose the flat side of the shaft. The photo below shows the shaft. Note that the gear has been slid almost off the shaft to expose the flat portion.
I then pushed the gear back along the shaft to make it flush with the end of the shaft. I also made certain to keep the position of the setscrew fixed so that the setscrew would contact the flat side of the shaft (to prevent rotation of the gear on the shaft). Once the setscrew was tightened down the RA gear housing was held in place, making sure that the two gears properly meshed. Then the large hex bolt was retightened to secure the gear box in place. The photo below through the inspection port shows the final position.
My LXD55-8"SC slews in Right Ascension again!!!
Subject: RA not moving despite motor running Sent: Monday, April 21, 2003 22:54:38 From: "Michael F. Vasseur" (firstname.lastname@example.org) Hi Mike! I just read your latest post concerning your LXD55 mount, and noticed that you were having the same difficulties that I had, and that another friend of mine had. When the same happened to me, I took my mount to my machinist friend, and we promptly took the entire mount apart, as I wanted to get rid of the grease that seems made for California weather, but not for Ottawa weather. We noticed that the set screw holding the gear onto the worm gear shaft was loose, the very same set screw that I had tightened a few months back. The cause of this loosening? The hard metal of the set screw eats into the soft metal of the shaft, and even if you tighten it, it will come loose eventually. This problem does not seem to occur with the set screw holding the gear on the motor shaft, it being made of a much harder metal. My friend then proceeded to make a very small metal plate out of an old hacksaw blade and inserted it between the indentation on the shaft and the set screw. This has the effect of giving the set screw something hard to rest against (and prevent it from loosening again) while applying more even pressure on the worm gear shaft. I did this modification on both RA and Dec, along with re-greasing with lithium grease, almost a year ago, and I never had any problems with the mount since. Attached, you will find a picture of my ETX-125 on its LXD55 mount (Great combination!), and another picture of the piece of hacksaw blade in place between the set screw and the worm gear shaft (the flat, shiny piece of metal in the middle of the picture).
Cheers! Michael F. Vasseur Ottawa Valley Astronomy and Observers Group Observateurs Astronomes d'Ottawa-Gatineau OAOG
Subject: Re: Fixing a Slipped Right Ascension Gear Sent: Tuesday, August 22, 2006 06:11:46 From: Patrick Doyle (email@example.com) In your techtip on the above you state the following for removing the small set screw in the RA gear which had slipped: "I removed the small setscrew in the silver gear using a 2/32" hex key and slid the gear along the shaft to expose the flat side of the shaft." I interpreted 2/32" as 1/16" so I bought a 1/16" hex key. It was far too big. I took the gear wheel with the set screw to my local specialist tool supplier and tried several keys before finding one that fit. It was a tiny key, 0.050" (1/20"). Using that key I completed the repair and my RA drive is now working. I hope this is helpful. Best regards, Patrick Doyle
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