Measuring size for vee belts

What is the best way to measure what size V-belts I need for the machine that I'm renovating? I don't have the original belts, unfortunately. Presumably the width of the belt needs to be slightly narrower than the width of the pulley at the top of the vee? And presumably the belt should ride part way up the vee in the pulley, not at the bottom of the vee? So I'm guessing that I measure the width of the pulleys at the top of the vee, and find the next down vee belt size (aiming for 1 or 2 mm less than the width of the pulley). Then I try to measure the length of the belt by wrapping a rope (of approximately the same width as the belt) around the pulleys and measure its length. Presumably I do all this with the machine adjusted to the middle of the range of adjustment for belt tension.

Will that work?

Any other suggestions that you can make to help me get this right? And to avoid buying too many belts that turn out to be the wrong size

Thanks - Rowan

Measure width for section, calculate length and visit my local ABC - very easy if only two pulleys. They are very accommodating re returns.

Look at the online bearing and belt suppliers. Many have a calculator facility which will give the length required, given the pulley diameters and centre distances, and they will have dimensions to identify the section that you need.

I would suggest that you size the belt section to be close to the width of the pulley V groove, if you go narrower the belt will run too deep in the groove and may bottom out. Some of the modern metric sized belts can be a little fatter than the old imperial versions, but I would not go smaller than the groove width.

Measuring the belt length can be a bit tricky, in the past I have run a dress makers fabric tape measure over the top of the pulleys to get the outside length of the belt. I have then used the "Simply Bearings' website to find out what belt I need. They give both inside and outside dimensions for V Belts, which makes life easier. You can get the belt from wherever is most convenient for you once you know the belt designation.

Measuring at mid-adjustment should be alright, but it might pay you to measure the lengths at both extremes to see what difference it makes, just to make sure you can get the belt onto the pulleys at the minimum centre distance.

Neil

The belt's outer width is more or less the same width as the top of the groove; but more importantly, you are right that it does not contact the groove floor.

Measuring with rope won't give a very accurate answer because the lines of contact will be wrong, but as Redsetter says, there are dedicated calculators or the formula for the purpose.

V ee-Belts are designated by their circumferences and standard widths each designated by a code-letter, and the pulley groove reflects that, so you need know:

- the width of the top of the groove, to establish the belt type;

- the outside diameters of the pulleys (some, by no means all, are marked);

- the distance between centres.

Obviously too the machine needs a good range of adjustment from slack enough to slip the belt off completely or to change ratio on a stepped-pulley (move the smaller end first); to correct tension for running.

A tip: having bought and fitted the correct belt: cut the label from its box and keep it in a safe place so you know the designation (width-code and circumference) for future replacements, by which time the markings on the belt itself may have become illegible.

Similarly with the packaging-labels for other standard parts that wear more rapidly than the machine as whole.

Sorry if I have missed it in the previous posts but if you say which belts for which machine, it might be that someone on here has the information you want ?

It's a Qualters and Smith QSH6 power hacksaw, and the belts I need are the motor-to-mid-shaft belt (which may need to be non standard because I have changed the motor) and the mid-shaft-to-main-shaft belt.

Thanks - Rowan

Most common belt width for that sort of old machinery was the standard A section belts, so that might be your starting point. Plenty of online sources telling you A section dimensions etc. Myford had to be different and use a smaller Z section belt from motor to countershaft though. But rarely seen on other common workshop machinery of the era.

Hi Rowan, these scans may be of help to you.

Regards Nick.

From the information kindly given by Nick above, you could draw the belt sections full size on a cereal box, cut them out and offer up to the pulleys to see which fits best.

Kempes Engineers Year Book quotes, for pulley pitch diameters, groove dimensions as:

Z section 10 mm for pulley pitch diameters < = 80 mm ( 10.2 > 80 mm )

A section : 13.1mm <= 118 mm ( 13.3 > 118 mm )

B section 16.6 mm <= 190 mm ( 16..9 mm >190 mm )

These should help you find the belt section, and the previous post should allow you to calculate the belt length, usually quoted as INSIDE length. So a belt may be marked as "A 280"

The O D will be larger than the pitch diameter by :

Z 5 mm, A 6.6 mm, B 8.4 mm.

The groove angle can vary between 34 and 38 degrees, so is 38 nominal.

HTH

Howard

I measured the pulleys and the centre distances and the groove widths, used the calculator on simplybearings.co.uk and ordered the belts from them. The motor to inter shaft belt was A40.5 (groove 12mm wide), and the inter shaft to main shaft was B54 (groove 17mm wide). They should arrive soon and we will see whether they fit!

Thanks for all the advice on this forum.

Rowan