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Belt tension

Toothed belt tension

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Speedy Builder515/02/2021 12:36:16
2590 forum posts
207 photos

I am adding a stepper motor drive to the Warco Economy mill x axis. All going well however, how tight should the toothed belt be ?

I have 5mm pitch by 10mm wide pulleys, but when I put a 5mm pitch x 9mm wide belt onto the pulleys, the belt doesn't sit well and skips a tooth each rev on the 20 toothed pulley. The belt is tight and I am concerned that if I pull it tight enough to stretch the belt to "fit" that it will put too much load onto the bearings of the NEMA 23 motor. The fit is even worse if the belt is really slack.


noel shelley15/02/2021 12:45:52
1278 forum posts
21 photos

Are you sure you have the right pitch belt ? Cold it be 3/16" not 5mm ? The width might then be 3/8" Noel.

bernard towers15/02/2021 13:33:52
568 forum posts
109 photos

Also do the profiles match, I believe there are at least two profile shapes!!

SillyOldDuffer15/02/2021 14:00:35
8469 forum posts
1885 photos

I suspect something other than tension, unless the belt is already too tight,

Plain Flat and V belts rely on friction for grip, and there is value in tensioning them for best effect. But toothed teeth are indexed by the pulley and don't rely on friction at all. I think toothed belts only need to be just tight enough to stay in place, which doesn't require much tension at all.

What could possibly go wrong? I can't think of much other than Neil and Bernard's suggestions, i.e the belt doesn't match the pulleys. Like gear wheels and threads, it's important they are designed to work together. Best I can do is suggest checking the pulley alignment. Belts don't like running at anything other than 90° between pulleys and this one might be riding up due to a fairly small misalignment.


Clive Foster15/02/2021 14:50:38
3103 forum posts
107 photos

Time to hunt up the data from the belt and pulley makers.

As Dave says the tension on toothed (timing) belts isn't great. Maybe a pound or two force to deflect its sideways at the mid point between pulleys. 5mm - 3/8 is small so tension at te low end methinks.

Metric / imperial sizes are very close in many cases so its easy to get mixed up. As there are two common metric tooth shapes and one different one for ordinary timing belts its important to buy by type not specification.

If you start looking beyond the types generally available off the shelf from bearing and transmission suppliers there are more variations. Mix and match between suppliers probably unwise if you don't already have expertise.


Chris Evans 615/02/2021 15:33:51
2050 forum posts

I have seen a "Gates" catalogue giving information on useage. Maybe worth a look online.

DC31k15/02/2021 15:38:12
654 forum posts
2 photos
Posted by Clive Foster on 15/02/2021 14:50:38:

As Dave says the tension on toothed (timing) belts isn't great. Maybe a pound or two force to deflect its sideways at the mid point between pulleys. 5mm - 3/8 is small so tension at the low end methinks.

A good reference is here:

Inter alia, it shows three common belts at 5mm pitch and one at 0.200". Belt and pulley profile must match.

Section 10 of the document discusses the correct tension and it should be noted that for a positioning application (perhaps where a stepper motor is used), the tension is greater than just a drive application.

Dave Halford15/02/2021 15:56:55
2004 forum posts
23 photos


Your belt should match pulley perfectly all the way around if it starts to drift out of sync then it's wrong and more tension won't help. It's the same if the tooth shapes are different.

Your OP says to me the tooth pitches are not the same.

Speedy Builder515/02/2021 16:54:35
2590 forum posts
207 photos

A whole new world of confusion. Round teeth, double teeth simple teeth.

Speedy Builder515/02/2021 17:37:40
2590 forum posts
207 photos

I contacted the supplier - My belt is a 'double tooth synchronous' and the pulleys are simple tooth. Different belt type required.

Thanks for the information folks.


Pete Johnson 128/11/2021 10:33:17
1 forum posts

I recently treated my Boxford BUD lathe to a electronic lead screw conversion following the excellent Clough42's video's on Youtube.

The only problem I had was that I made the belt slightly to tight between the stepper motor and leadscrew and when starting up from cold, the stepper locked out probably from being out of step with the encoder and showing an error on it's driver board, I built in a little slack to the belt and it has never happened since.

My set up is a 5mm pitch belt,15mm wide using 2X 40 tooth pulley's so I think you may have a miss match of belt and pulley.


Tim Stevens28/11/2021 14:27:30
1584 forum posts

To try an answer to the original Q: How tight?

It depends on what can happen if there is slackness - how much can you turn one pulley to and fro without moving the other. If the belt drives a cutter, then the slack should be eliminated, and it should in other cases never be slack enough to risk skipping a tooth or sliding over the pulley flange. This latter is more likely (so tension is more critical) if the alignment is iffy, or if it goes iffy under load. This depends on the design details.

Otherwise it is a good idea to try the belt and pulley temperature now and then, and be aware of rubbery smells, which would point to overtight conditions (or overload, mislignment, etc).

Toothed belts are generally more forgiving than roller chain drive, or direct gear drive, but no so capable of really heavy loads.

But as you have discovered, the belt must fit the pulleys.

Regards - Tim

John Haine28/11/2021 15:51:45
4622 forum posts
273 photos

Another issue is how accurate you want the drive to be. If you just want the table to move, not so much or a problem, but if the belt is too slack then it will add some backlash as either one side or the other will have to be tensioned to transmit the drive. I tend to have my belts all pretty tight, have had no wear, overheating or locking problems so far.

Howard Lewis29/11/2021 16:48:21
6005 forum posts
14 photos

Polyvee belts used to be tensioned by extending the distance between two marks by a specified amount.

Possibly toothed belts could be tensioned in the same way, but tension gauges are available..

I have a gauge for engine timing belts, which presses on the back of the belt between two pins, to indicate the tension in the belt. In this way the tension is indicated as being too little,,within the correct range, or overtensioned.

The object has to be to tension the belt so that it does not jump the teeth, to ensure that the shafts are synchronised, but not overload the bearings of the shafts carrying the toothed pulleys..


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