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Highlands Astronomical Society

EQ3 Drive Mod - A “Direct Dec Drive” for the EQ3

A simple solution to reduce large backlash in some of the declination drives of the EQ3 type mount, by Maarten de Vries

When I bought dual axis drives for my EQ3/2 mount I hadn’t expected to get so much backlash in the declination drive. The cause of this appears to be the fact that the little shaft which is connected to the declination worm gear has a bit of lateral movement in it. Because the declination drive is connected to this via a gear wheel, when changing direction on the drive the shaft first moves across its free play before the gear starts to turn. The result, in my particular instance, was that I got a backlash of a good 10 arcseconds or more (depending on the direction the scope was pointing at the time).

I found that a bit over the top, so I decided to try and fix it. Because I could not find a way to fix the free play on the shaft, I decided to take the entire gear wheels out of the equation by finding a way to fit the declination drive straight onto the shaft. This turned out to be easier than I thought.

I used a bit of aluminium box section (50mm x 25mm). I cut it to 160mm in length and fitted this to the bracket on the mount that normally holds the drive. The drive was then fitted to the other end. With a bit of 3mm thick steel strip I managed to fit the drive nearly in line with the shaft. In order to do this I needed to drill a hole on the outsideof the box section in order to be able to tighten the screw that holds the drive in place. I also need to take a small section out of the top inside of the box section in order to make place for the connection socket of the drive.

To make sure that any misalignments wouldn't cause any stress on the gears in the drive, I ordered a flexible in-line coupler (the same idea as the one that goes onto the RA drive) from RS components (you can do a search on part number 814-590 to get the correct page. Check the diameter of the shafts before ordering). The holes in the coupler were slighly too small, so I needed to drill them to make them fit snugly around the drive shaft and the worm gear shaft.

In the photograph you can see the final assembly. I used the gear with the built-in clutch to lengthen the worm gear shaft, because you will find that you can’t get close enough to the drive in order to fit it directly. Please also note that the drive ratio is no longer correct. However, since this is the declination axis, you will find that it is of no consequence at all.

I finally used some black Hammerite spray paint to neaten the whole bracket up a bit. The results are excellent. The backlash is now quite acceptable, a few arcseconds at most, and similar to the RA axis.  

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