Monday 15th July 2019: Motor focuser Pt.3. Belt drive.

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Monday: Another grey day.

The 14T "timing" pulley and matching GT2 belt duly arrived in the post. I found it easier to drive the direct drive focuser knob rather than the slow motion knob. The direct drive wheel is 44mm in diameter [with 48T] while the 14T is only 8mm. Meaning a gear reduction of 5.5:1. I had to bore the 14T pulley out to 6mm in the lathe because 5mm was the nearest standard size. I feared the tiny pulley would fall to pieces but it survived. My 6mm drills seem considerably oversized despite my infinite care. Beware that some pulleys have a 1/4" bore which is considerably larger at 6.35mm than the "Skywatcher" motor shaft.

When fitting the new pulley system I quickly discovered that the FT didn't like the thickness of my motor mounting plates. Because I had fitted the plates over the rack housing boss they overlapped the conical section. Which caused stretching and locked up the rack pinion assembly! So I put the plates on the disk sander and quickly removed  a couple of millimeters off the thickness. Problem solved. Though I could have safely left them at 9mm thick rather than 8mm. I may end up making new motor mounting plates anyway. To tuck the motor closer in to the rack assembly.

The pitch of the black FT 3.5 focuser knobs is 44mn Ø x Pi/48T = 2.9mm. I'm not sure whether it makes any sense to try and match this with GT3 pulley & belt components. Would it really gain anything to be much closer in pitch? The smaller pitch of GT2 just rides over the FT teeth as it it were a smooth pulley. While wrapped over half a pulley circumference the error would be even smaller with GT3 @ 0.5mm per tooth. So would almost behave like a toothed pulley. While avoiding any chance of slippage.

The little, "Skywatcher" clone, focuser motor had no problem rotating the rack pinion but I haven't tried lifting any loads on the focuser itself yet. The rack will have to go back on before I can try lifting the rather "lumpy" Lacerta 2" Herschel solar prism against gravity. If the motor lacks enough torque I can easily move the motor over to align the belt with the slow motion knob. Though that would provide an incredibly slow drive reduction at ~39:1.

Having now experienced the new belt drive under power I am delighted to report that it works fine in lifting the Lacerta prism vertically. Plus a 4" x 2" extension and an eyepiece. Turning any of the focuser knobs by hand allows the belt to slip remarkably smoothly but with a desirable level of friction. Providing safe, manual override if so desired.

Motor driven, end-to-end of the complete focus range takes almost exactly five minutes either up or down. [Or in-or-out, if you prefer] It would have to be roughly focused first to avoid a long wait. When the drawtube reached the end of its travel in both directions the motor simply stalled.

Fitting a 30 tooth pulley would double the speed of movement of the focuser. With the FT slow motion knob rocking back and forth slightly. I am rather pleased with the results. Fearing the little motor wouldn't have the guts to lift such a heavy load I had chosen the smallest possible motor pulley. It seems I could have been far less conservative. Particularly as I will soon have easy access to higher voltages than the 9V battery.

Click on any image for an enlargement.

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