I am making a small "backshaft" for my Taig lathe, and am wondering where is the best place to put the bearings. In the pulley with a fixed shaft, or in the bearing block with the pulley fixed to the shaft.

The large pulley is 80mm, the small one 20mm, the shaft is 8mm (or 10mm) steel, and will use a couple of 8 x16 x 5 bearings. Block & pulley will be aluminium. The Pulley profile is for a 3mm Gates belt ( as used on the lathe).

If I mount the pulley firmly on the shaft, I could use the shaft as a mandrel for turning the pulley and ensure concentricity – but I then need to work out how to stop lateral movement of the shaft in the bearing block.

Putting the bearings in the pulley would make it a bit harder to ensure concentricity, but makes lateral positioning easier – small step on the shaft, and a nut or similar on the small pulley side.

Could I ask the the wisdom of the forum for thoughts, ideas or gotcha's before I make yet more scrap.

Thanks Bernard for the ideas more to mull over.

If you are interested in what I am trying to do I have added an image in my "Taig lathe" album called "backshaft overview.jpg" which shows the general idea. It is based on the DC motor I have already mounted on the lathe. The backshaft is mounted on top of the headstock using the T slot. The smallest motor pulley drives the big pulley, and the small pulley drives the next to largest spindle pulley – hence the alignment. The bearing block will be made to slide (slot & tenon) on the part fastened to the T-slot for tension & belt change adjustment.

As designed it gives me about a 10:1 reduction (vs the 3.5:1 available on the standard pulley set). I can't see any reason why the design wouldn't work with a motor mounted behind the lathe if you adjusted the lengths of the belts.

Hope this is of interest.

Thanks Martin,

The block will be 20mm wide and I will be using a couple of bearings in either case. A concern with the step and nut approach on the bearing block is where the axial squeeze forces applied by the nut would be carried. To avoid them being transferred to the sides of the ball races, I would need a spacer between the two inner races and the bearing block would need sufficient room for that to rotate. That also implies that all the radial loads caused by belt tension get transferred to the block via the ball races – so two 5mm supports at opposite ends of a 20mm block.

For bearings in the pulley the two bearing centres could be clamped together ( with a very small spacer to allow rotational clearance) giving in effect a (small and cheap) double row bearing. The radial loads get transferred through the whole length of the 20mm shaft set in the block.

I think I am convincing myself to go the pulley route. As usual dialogue here clarifies thinking in a way I can't do on my own.