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.

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