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idler pulley with spring loading

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David Brown 911/07/2019 20:41:49
75 forum posts
4 photos

Does anyone know where I could buy an idler pulley with spring loading?

I have a mirror making machine, one of the belts is very slightly too long and is slipping. This only happened when I started polishing the mirror, there was no problem when grinding the mirror. Apparently there is a lot more torque when polishing.

Also, would I buy the same diameter idler pulley as the one I am replacing, and would it fit on the shaft the same? The diameter of the pulley I have at the moment is 8 cm.

I realise that I could buy a slightly shorter v belt, but I have been recommended to get an idler pulley as a belt can stretch with time. Also, getting belt that is exactly the right length is not that easy.

I also wonder if the fact that the belt is going from an 8 cm pulley to a 35 cm pulley could cause problems, is the difference in sizes to great.? I have a bit bigger pulley i could fit, but would need a new belt, as the one on the smaller pulley will not fit.


Pete Rimmer11/07/2019 20:51:26
721 forum posts
49 photos

Spring loading a belt tensioner isn't a great idea. When the load comes on the belt pushes the idler back and it slips anyway or they can start reciprocating. It's far better to make the pulley adjustable with the facility to be fixed in position.

A pic or a link to the machine would help a lot. It's hard to give advice without knowing what space constraints or mounting options you have.

old mart11/07/2019 21:10:52
1795 forum posts
138 photos

If the drive only rotates in one direction, then you could probably get away with a spring loaded tensioner on the slack side, otherwise a locking arrangement as Pete suggests would be best.

The idler wheel could be made from a rubber sealed ball race, as it can run against the flat outside of the belt.

 An 80mm pulley size would be fine for belts up to 13mm wide.

Edited By old mart on 11/07/2019 21:13:54

Michael Gilligan11/07/2019 21:15:28
15783 forum posts
690 photos

Converting to Poly-vee would almost certainly sort it


Clive Foster11/07/2019 21:39:40
2212 forum posts
73 photos

Belt tensioner pulley and mount off a car will work fine. Either cam belt or auxiliary drive will work just fine. Whatever you can find inexpensively. Either just use the spring loading as is or find one with the locking bolt built into the bracket rather than driving into the engine block. Most will be for serpentine drive belts so on the wide side but that probably won't be an issue.

In principle easily made from a sealed ball race, bracelet assembly and spring but these days its probably more trouble than its worth as serpentine belt tensioner assemblies are readily available inexpensively. Or ask you local garage guy if he has one in the bin as its quite common the change the tensioner at the same time as the belt.

Comments from Pete and old mart have some validity but I used spring loaded "slack belt" clutches on both my SouthBend 9" machines with no problems. Springs were ex BMW K or R series centre stand springs (don't remember which, whatever I had in the handy bits box) operating via an over centre lever so the clutch latched in or out. As I recall it the effective leverage ratio on the tensioner at pulley to belt contact was of the order of 1 to 1 and hand lever advantage about 10 to 1 giving a stiff but comfortable action. Lever probably about a foot long from the pivot so the spring abutment would have been inch and a bit I guess. Some pre-load. Design was pretty much replicated from the actual centre stand layout. I imagine the spring from almost any big motorcycle with dual centre stand springs would work. Not a Norton Commander one tho'. The design is a bit more sophisticated than usual with a swinging pivot arm involved and it will be way, way too strong.

My slack belt clutches were made as part of custom compact countershaft drives with an intermediate shaft. Single A section belt drive from motor intermediate shaft then two speed one up to the countershaft proper. Clutch on the two speed pulley set so belt could be shifted with clutch in the slack belt, disengaged, position. One tensioner pulley was made up from tube and two ball bearings. The other was a Lancia HPE timing belt pulley tensioner salvaged when the engine, a Fiat twin cam, went "pop". If I had to do one again right now I'd use the Rover V8 one I kept after doing the auxiliary belt on my P38.

Many self propelled mowers use similar, but smaller, slack belt clutch set-ups so one of those might do.  Hafta get new-new though and, probably surprisingly expensive.  Especially if looking for ac complete unit.



Edited By Clive Foster on 11/07/2019 21:42:25

Ian S C12/07/2019 12:11:03
7468 forum posts
230 photos

Depending on the load a skate board wheel might do the job. Mount it on a swinging arm that can be locked in place, think of the alternator in your car, or if you are like me(no car), a car.

Ian S C

Howard Lewis12/07/2019 15:33:36
3294 forum posts
2 photos

Based on what I've seen of them, the loading of a car engine polyvee Idler pulley would be a bit too much for such a small belt set up. An A section belt would barely fit around a 8cm pulley, and a Z would be only little better.

Also, the torque loading is going to be fairly constant, without the torsionals present in the drive to automotive valve gear, let alone an fuel injection pump as well.

The tensioner needs to maintain the tension that one would normally apply, allowing the normal amount of "spring" in the slack side of the belt. If all fails, make the ntensioner fixed, so that you adjust it to give the required tension.

Too little and the belt slips; too much and the belt and the drive and driven bearings will not blast long.


clogs12/07/2019 18:03:02
528 forum posts
12 photos

or u can find that springloaded idler pulley's eith a V or flat belt from a mower repairer's...

they are used all over the place on ride-on's.....

old mart12/07/2019 18:40:53
1795 forum posts
138 photos

The OP doesn't want the belt to slip, so slack belt clutches are irrelevant. Changing to a larger pulley would change the gearing and speed of machine depending on which end the drive comes from.

Edited By old mart on 12/07/2019 18:49:15

Ian S C13/07/2019 10:53:57
7468 forum posts
230 photos

Using a jockey wheel to tension the belt can also help to gain a greater wrap around, improving the drive efficiency.

Ian S C

Neil Wyatt13/07/2019 16:40:08
17917 forum posts
706 photos
77 articles

I think a car tensioner will be OTT too.

The skateboard wheel idea sounds good.

Now.. tell us about this mirror? blank size, focal length, how are you testing the figure?


vintage engineer13/07/2019 17:13:57
252 forum posts
1 photos

Most modern cars use spring loaded belt tensioners. Some cars use an adjustable pulley tensioner. You could make a tensioner with an arm and a weight on it so it gives constant pressure.

David Brown 913/07/2019 21:41:41
75 forum posts
4 photos

Thanks for all the advice. I have solved the problem by cutting in to the side of the table with a jigsaw and moving the shaft a few mm, this has now made the tension in the belt enough so it does not slip. The pillow block bearing at the bottom of the shaft is attached to a board with two nuts and bolts. I slowly tighten the nuts on the bolts, this moves the shaft and tightens up the belt.

Polishing now works fine,

It is a 20 inch f/4.1 mirror. I am testing it with Ronchi gratings and a bath interferometer, which I am slowly learning how to use, also the Foucault test.

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