Pulley and belt for Samyang 135 lens and SW motor ?

[kirkster501]

Hi, I want to get motorfocus working on my Samyang 135mm lens (what a superb lens).  At F2, it is a swine to focus and the focus movements are minute and wobble the image when touching the lens.

This is the motor I have in mind, only £45: https://www.firstlightoptics.com/skywatcher-focusers/skywatcher-auto-focuser.html

I'd need a pulley wheel to go over the shaft of the SW motor and a belt to go around the focus ring of the lens and the pulley wheel that is long enough.

Any ideas please what wheel and belt I'd need? 

Many thanks!

[johninderby]

Pulleys and belts intended for 3D printers will be ideal. Search for GT2 closed loop belts and GT2 drive pulleys. Amazon is a good source for the drive pulleys although usually sold. In small quanities not individually.

Buy drive pulley with a 5mm bore and drill out to 6mm. The shaft on the Skywatcher Autofocus is about 5.95mm. Pulleys are available in 1/4 inch bore but that’s a bit too sloppy a fit. About a 40 tooth pulley should do. Belts come mainly in 6mm and 10mm widths. I prefer the 10mm but not essential.

This belt length calculater is invaluable in working out what belt length to buy.

https://www.bearingboys.co.uk/Timing-Belt-Finder--Calculator-24-a

You can buy lengths of the belts and make up your own drive belt. Youtube has vids on how to do this but takes a bit of practice to get a strong joint.

You'll have to work out a way to mount the motor of course but each installation will be different.

 

Edited August 6, 2019 by johninderby

[kirkster501]

Many thanks John.  I think I understand this now and how it works.