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Help with DEC backlash on GPD2

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#1 roryt

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Posted 26 February 2011 - 10:01 AM

Hello,

After my older kid hit by accident the counterweight bar :foreheadslap: , I had to take the declination axis out and check what was wrong and fix it. Meanwhile I hypertuned DEC a bit (clean, smooth, lube etc).

But still I can not trim the backlash correctly. I think I had it in one night but then the next, pointing at zenith, PHD could not calibrate for DEC and I had enormous backlash (5-6 secs and more, depending the position, this DEC never had more than 1sec before the accident :bawling:). I have followed whatever I found at Net, like for example astronomyboy's pages for CG5 that is a clone. I thought I got it right but apparently I'm doing something wrong or I'm missing something.

Still, based on this photo http://www.astronomy...es/dec_worm.jpg , what is the correct procedure for the screws ? I put the worm assembly back at place, as parallel with the worm gear as I could, and then I tighten the top screws (the ones at the right at the photo). Then I tried minimizing the backlash based on the three screws ("Worm Gear Mesh" chapter at AB's pages). First I was trying to trim it out without a load, but then I loaded everything on and tried again. I reached a point that I thought everything was correct, but if I put the motor back there is still backlash. It is more apparent at specific OTA positions and looking with the CCD.
Maybe the order I tighten the screws is not correct? Or, is there a way to trim the backlash out with the motor on and looking through the ccd, with the OTA pointing to (eg) a far away daylight target ?

Thanks in advance :help:

#2 EFT

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Posted 27 February 2011 - 12:43 PM

Make sure there is no endplay in the worm since that can look like backlash between the gears as well. If there is, then you will need to remove the worm block and tighten that up first.

The adjustment is pretty much like you described. The top bolts need to be as snug as possible while still allowing the worm assembly to move. The adjustment on this mount is completely by feel so tighten the two outside adjustment screws until the worm gear starts to get too tight to turn easily with your finger on the spur gear. If you cannot turn the worm using one finger on the spur gear then it is probably too tight. Check the slop (back and forth movement) in the axis. When you get it to a point where the slop is minimal and the worm turns without too much force, then tighten the top two bolts and check things again. You may need to go through this a few times before you find just the right spot for everything. You will also need to check the entire rotation of the ring gear in order to make sure that you don't have any high spots. Once you have done this then install the motor and try slewing. Of course you might have to back up a couple of steps at this point as well. Overall, you are on the right track. Just have patience and you will find the sweet spot in the adjustment.

#3 roryt

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Posted 27 February 2011 - 01:23 PM

Thank you Ed,

Actually yesterday I took it apart and started from scratch. I had to add some Teflon tape around the threads of the counterweight bar (apparently the hit loosen the threads and it was "jumping" around inside the axis), and carefully re-apply lithium to all bearing surfaces (it was not symmetrical applied).

I also added lubricant to the external surface of the worm gear, but I'm not sure if this was correct, it is a bit stiff and balancing is not easy :question:

Found carefully the sweet spot for the ring nut that holds the Dec shaft in its housing so to rotate without play.

Fortunately there is no end play in the worm. It is only matter for the sweet spot with the screws. Actually I followed by instinct a procedure more or less similar of what you described, and -I think- I got it correct , albeit a bit stiffer than my previous attempt. It is with minimum backlash and OTA's direction does not seem to affect it, but I can not go beyond 20X (which I guess should not be an issue, I never used anything more than 25X with my controller anyway).

So I say I'm back in business :smirk: (let see when the clouds will allow me to do a real test :( )

Again thank you

#4 roryt

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Posted 27 February 2011 - 01:40 PM

Forgot to say, I took apart both MT1 motors and cleaned the old lube from the internal X120 reducers.
Man, it was black and stiff and looked like glue (I guess due to many years of use ? ).

Does it really need lubricant here ? Just cleaning it with gasoline seems a lot better than applying lithium to the spur gears.

Lets hope that this will improve tracking also (usually I have -guided- tracking 0.5 to 0.9 arcs RMS, with 1.5 - 2 arcs spikes).

Apparently some of the backlash comes from the motor/reducer but I could not find a way to remove it.






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