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Bearing Location - Pulley or mounting block

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Peter Cook 611/07/2021 11:33:46
307 forum posts
88 photos

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.

jackshaft pulley.jpg

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.

bernard towers11/07/2021 11:45:00
689 forum posts
141 photos

As a Peatol user I am interested to know the use of this. Personally I would have the pulley fixed to the shaft and the bearings fitted to the block with retaining caps/plates/screwed rings. If the block is bored with a hole clearance size for the shaft and a stepped bore for the bearing minus 5 thou screwed cover plates would do the job and make for ease of maintenance.

Peter Cook 611/07/2021 12:45:05
307 forum posts
88 photos

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.

Martin Connelly11/07/2021 14:35:17
2179 forum posts
227 photos

The step in the shaft and nut on the end will work in a bearing mounted in the block as well. If the block is wide enough you could put a couple of bearings in it for better rigidity in the setup.

Martin C

Peter Cook 611/07/2021 15:15:59
307 forum posts
88 photos

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.

old mart11/07/2021 17:41:49
3891 forum posts
268 photos

You are lucky to have a choice, both ways would be usable, but I would tend to lean in towards having the bearing/s in the pulley for slightly better rigidity.

Nigel Graham 211/07/2021 18:06:54
2273 forum posts
33 photos

Also, if you over-hang the pulley as in the drawing, minimise the overhang by reducing that gap between jounal / axle-block and pulley face as far as possible.

duncan webster11/07/2021 20:33:39
4108 forum posts
66 photos

Whichever way you do it, the bearings should be a light push fit in the bit that goes round, ie on the shaft if the bearings are in the block, or in the pulleys if t'other way.

A purist would say that the bearings will last longer if in the block , but either will see me out, so I wouldn't get worked up about that.

John Reese17/07/2021 23:24:31
1038 forum posts

Have you considered a water pump bearing? **LINK**

not done it yet18/07/2021 09:51:08
6882 forum posts
20 photos

Operating with the load axial to the bearing is always the better option. Outboard bearings will always be pulling off-centre - imagine a huge overhang. The usual way, if large over-hang is unavoidable, is bearing support on both sides of the pulley. Moving shafts are far better employed in transmitting the power to the other end of the shaft.

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