MkIII drive

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I had been mourning the loss of my synchronous drive to the MkIII for some time. The tiny wires on the original 50 year-old motor had snapped off flush with the motor housing.

The MKIII perfectly suits my little 90mm F:11 Vixen refractor. This is an ideal telescope for quick looks and solar observation. The latter using a full aperture Baader Solarfilm filter.

Luckily my modest, lifetime's collection of synchronous gearbox motors included a dual voltage one with a 1/5th rpm final drive. All I needed was a 2:1 final gear reduction from the output shaft to the worm. The 144T wormwheel requires a 1/10th rpm drive to match the rotation of the sky. Or, more accurately, to counteract the rotation of the earth relative to the sky.

1/10th rpm = 1 revolution in ten minutes x 144 (worm reduction ratio) = 1 revolution in 1440 minutes. The ideal speed for the polar axis is is one revolution in 1436 minutes. The sidereal (star) rate is one revolution in 23:56:04.

Four minutes too slow, in 24 hours, is no problem, at all, for visual use or taking snaps at the eyepiece.

After much searching, and failing, I finally found some suitable gears online at Technobot of the UK. I ordered their largest plastic gear and one of half the number of teeth. Their Module 1 60T and 30T. By sheer coincidence these proved to be the perfect size for the worm housing. Only later did I mount the motor on the reverse side of the alloy plate.

The gears were not expensive and their service was fast, affordable and efficient. So much so that I sent an email of thanks for their excellent service. Praise where it is due when so many other online companies are a miserable waste of time.

My initial experiments use a scrap of thin, sheet alloy, simply cut out with tin snips. Some long M3 screws, with lock nuts, support the motor. Once I am satisfied that it works well I shall cut a thicker piece of alloy and make some proper pillars for the motor. An image of the first trial is shown. The MKIII is looking rather scruffy after several years outdoors under a small tarpaulin. At least the replacement stainless steel shafts haven't rusted like the originals!

An interesting discovery was that simply reversing the worm housing allowed the motor to drive the wormwheel the correct way. I hadn't even considered the direction of rotation. Initially the polar shaft of the mounting turned the wrong way. After reversing the worm housing and motor it rotates as it should. I bought some smaller gears at the same time and may see if they can be fitted into a more compact arrangement. By the way, you can have any colour of gears you like as long as they are red.

I decided to turn a new and larger, brass, clutch/adaptor. To go between the motor-gearbox output shaft and the worm shaft. The small diameter of the original adaptor made very short threads for the gearbox shaft clamping screws. The threads easily stripped with the slightest, enthusiastic tightening. The new motor(gearbox) shaft was also larger than the original.

I used a hole punch to make a small disk of PTFE/Teflon to go in the screw hole in the worm shaft end. This disk rubs against the worm shaft. Acting as a slipping clutch. Which allows the slow motion hand wheel to be turned independently without damaging the motor and gearbox.The hand wheel only turns the worm. With the PTFE disk slipping smoothly on the worm shaft. Yet providing enough grip for the motor to drive the worm. With its matching wormwheel attached to the polar axis. 

Click on any image for an enlargement

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