The screw closest to the optical tube is a lock screw. It is designed to set the draw tube at that point and it disconnects the adjustment, which just spins without effect. I have had several Bintel 1:1 and 10:1 focussers and have always been able to find a spot on the tension adjustment (the screw furtherest from the optical tube) to have the crayford mechanism work effectively, even with heavy eyepieces. Yes, you might have to adjust it as the temperature changes and as you change eyepeice weight. Maybe I have been lucky - I've never had to service one. See a few previous threads and links:-
http://www.iceinspace.com.au/forum/s...ad.php?t=36706 http://www.iceinspace.com.au/forum/s...ad.php?t=51192