Drive belt tension on milling machine

[Paul RadfordParticipant @paulradford79614]

Please can somebody advise me what sort of tension should be on the drive belts on my Tom Senior M1 milling machine? Many thanks!

[Chris CrewParticipant @chriscrew66644]

Just pulling them taut with a light leverage is all you need do, after all this is what happens with an eccentric tensioning lever on belt drives, e.g. the back-gear drive belt on your TS. I have recently replaced all the original belts on my Tom Senior after 40 plus years, things just don’t seem to last these days! No doubt someone will come up with a mathematical formula as regards tensioning but as I can’t work to such NPL standards my motto as always is if it fits right, works right and looks right then it is right!

[Mark RandParticipant @markrand96270]

To add to that, just make them reasonably stiff when you try to deflect them with a firm forefinger. If they squeal and get hot when running, they’re too loose.

[Paul RadfordParticipant @paulradford79614]

Great thank you Chris! Whilst you’re here- don’t suppose you know where I can get another of these spring clips from do you? 

[Chris CrewParticipant @chriscrew66644]

Sadly, I don’t think TS spares are available any longer but as long as the spring clip holds the back-gear drive pin in or out engagement it doesn’t need all that much tension, although I would agree it’s not a very good design in the first place. Denford’s used to supply spares just after TS went out of business but I think that even they are now long gone or absorbed into Boxford who now, as I understand it, only produce laser CNC machines and software.

[Nigel Graham 2Participant @nigelgraham2]

On any conventional Vee-belt you should be able to push the middle of it in with light finger-pressure. On some machines the tensioner has a spring designed to give the correct tension, but if not about half-inch depression on about a foot of belt seems about right. They should never be bar tight.

The clip is most likely a “special”, made for the machine, but from its appearance it does not seem something too hard to make from spring wire or strip if need be.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, the link below should give the correct tension, and how and why to achieve it.

Proper Belt Tensioning

Regards Nick.

[Paul RadfordParticipant @paulradford79614]

Many thanks for all your replies! Much appreciated as still fairly new to this! 😊

[Howard LewisParticipant @howardlewis46836]

Could you make the spring clip from piano wire of the correct gauge?

Howard

[Martin ConnellyParticipant @martinconnelly55370]

I think I would add that usually too much belt tension is a bad thing. I spotted a neighbour tightening the alternator drive belt on his car by levering the alternator away from the engine with a crowbar. I told him that was probably excessive force and there should be some play on the longest leg of the belt. He thought he knew better so carried on. A month or two later he told me he had to replace his alternator because one of the bearings had disintegrated. For a milling machine replacing the bearings damaged by over tightening is probably far worse than any outcome from slightly loose belts.

Martin C

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