A couple of questions on Losmandy Drives

[[1ponders]]

I seem to be having a bit of a Losmandy Hoodoo atm when it comes to drives and backlash. What would be causing rubberbanding in the RA drive, particularly RA-, and a unusual backlash in Dec.

The rubber banding is quite pronounced through a C14 viewed via a ToUcam. Approx half the FOV. Could this be caused by RA worm too tight. I'd only just loosened the worm by 2 toi shortly before, but I may not have loosened enough.

In the backlash issue the Dec starts almost immediately when a key is pressed. But when the opposite key is pressed, the star starts to move immediately in the opposite direction, but then stops for a number of sec and then starts again. I have this problem on both my own mount and the Mapleton Observatory mount. I recently serviced my mount and reset the worm mesh, the Observatory one was done professionally. Both have the same issue.

On mine if I tighten the worm mesh up by more than 1 toi, the Gemini sends a Dec Motor Lag message, meaning the worm is too tight. So I can't go any tighter. Loosening seems to make the matter worse by lengthening the backlash period. It is currently about 12 sec at 0.5x guiding. The attached image shows the Dec through a cycle in each direction. The dump is from K3 using a ToUcam 840 in an ED80. The arrows indicate the start of a direction reverse. I find that PHD is having trouble accurately calibrating because of the issue.

[doug_parkes]

I once owned a G8 and a G11 Losmandy mount. After a while, both mounts exhibited the same fault—lost motion on both shafts. If the worm/worm-wheel engagement is correct and you still have some lost motion then your mount probably has this fault too.

I had lots of backlash problems which turned out to be caused by the set screws that hold the end-cap of both the RA & Dec axis onto the shaft, progressively coming loose.

To check if you have this fault, tighten the clutch knobs and hold the OTA and attempt to rock it back and forth in RA & Dec travel. If you discover any excess movement then the set-screw retaining the end-cap onto the particular (RA or Dec) shaft is loose. My C11 had more than 1° of movement. (One year, at the SPSP, I did a little survey and found four other Losmandy mounts with the same fault).

I fixed the problem by machining a V-shaped groove into the shaft (it is normally a flat ground onto the shaft but the tip of the set-screw 'rocks' on the flat) and replacing the original set-screw with two shorter ones. By tightening down the first and then tightening down the second onto the first (providing a lock-nut effect) the lost motion disappeared since the end caps were again firmly locked to the shafts. I used Lock-Tite to assist in locking the set-screws. The main thing is to be aware that the set-screws can loosen and check them frequently.

The Losmandy mount performs very well and provided they are well maintained, they will give very good service.

[[1ponders]]

Wow, thanks heaps for that info Doug. My G8 and both the 2 G11's exhibit this rocking motion when I rotate the end caps. I wasn't aware of the set screws so they haven't been checked at all. That's fantastic. I'll check it out.

[Exfso]

Doug, I am interested in this one as well. I must admit I am not sure which set screws you are referring to. I have just re greased my mount and cant recall any set screws. Any chance of some sort of drawing or image showing the ones you are referring to. Thanks in advance

[[1ponders]]

Good move Peter. I hadn't seen any set screws before, but I hadn't looked for them. Knowing where they are in advance could be of some assistance. :doh: :lol:

[multiweb]

Watching this thread with interest too. As Peter pointed out I have no idea either where those set screws are... and I've put my mount apart too many times to recall as well. If it's to be loktite'd may be the attachement of the DEC and RA cones onto the axis bearings cylindrical housing? :question: That's all I can think of.

[doug_parkes]

hi guys, I have edited my original reply by adding a jpg image which hopefully fully illustrates what I said as text in my original post. If you still have any probs, send me an email.

Doug http://www.iceinspace.com.au/forum/../vbiis/images/smilies/happy19.gif

home-made 381 mm Dob, Stellar Cat, xy secondary mount.

C9.25 Schmidt Cass

various Ethos & Nagler eyepieces

member SASi

[Exfso]

Thanks Doug, very much appreciated

[multiweb]

Sorry guys, I must be thick but I still don't get it. Here's a couple of shots from mine DEC and RA. Looks like the gear is threaded straight onto the shaft. :question:

[doug_parkes]

hi Marc,

My G11 was a much older model than yours appears to be so the design may have changed. I bought my G11 in the U.S. in the late 1980s/early 1990s and transported it home myself.

Even if the shafts are threaded into the endcaps, there still must be a way of locking those end caps to the shafts.

When you tighten the hand-clutch knurled knob really tight, and then attempt to move the end caps back and forth, is there any movement between the endcap and the shaft?

If there is you will still have to tighten the allen-headed setscrews. They were about 6 or 8 mm ones.

The setscrews may not immediately be apparent. I have a vague memory of the setting circles covering up the threaded holes where the setcrews go. I seem to recall having to remove (or simply move) one or both setting circles. I think that there was a small screw holding the setting circle to the periphery of the endcap.

Doug