Pulley sizes - how to measure & advice

Hi All,

I have a pully on an old electric 1/2HP motor, 1/2" bore. It is approximately 3 3/4 diameter, and I want a driven pulley (again 1/2" bore) slightly smaller for an increase in speed. Where am I measuring? Outside diameter? Depth of the valley?

Just so I get a slightly smaller pulley!

I would need a V belt for this also, but I'm not sure of the size yet.

Also, where do I get these in the UK - or if anyone has one spare I'll buy it off you !

Thanks in advance,

Steve

Hi Stevo, first of all is your pulley "A"section or "Z"section, formerly known as FHP? If it's A then the measurement across the top of the vee will be 13.29mm. If it's a Z section this will be 10.18mm. Once you know the belt section you can work out the pitch circle diameter of the pulley. The pitch circle of a Z section pulley sits 2.75mm below the pulley outer diameter. On an A it sits 3.75mm below O/D. You will need the PCD of both the driver and driven pulleys and the centre distance to work out your belt length. Another figure comes into play depending on the relative sizes of the pulleys and this is called the Arc Correction Factor but in some cases especially with drives which are provided with belt tension adjustment this figure can sometimes be circumvented by other methods. You should find that bearing stockists also sell vee belts, wether endless or sectional. They certainly do where I live. I got a couple of drive belts last week cos I've made a clutch and fitted it to myML7. The pair of them, one for the countershaft to mandrel and the other for the motor to countershaft cost six pounds. Hope this helps

Rick

I'm always amazed at the standard of reply on this forum...

Thank you Rick, It is indeed an 'A' section, 101mm outside diameter, so PCD = 101mm - 3.75mm = 97.25mm (3.8" or approximately a 4" pulley!).

I have no centre distance yet as I have not redisigned the machine, which is a Flee sander around 14 - 1/3 full size. The motor mount will be made adjustable to tension it correctly.

Once again thank you.

Steve

Not quite Stevo, if it's A section 101 O/D then you must subtract the 3.75mm from both sides otherwise you're only subtracting it from the radius. So 101mm minus 7.5mm gives you a PCD of 92.5mm or make it 92 for ease of belt calculation. Obviously do the same for the other pulley and you wont go wrong. To get your speed ratio divide the pulley PCD's and if you have 50mm driver and 150 mm driven you have a speed reduction of 3 : 1. All the best

Rick

Even I got that wrong! Your pulley is 101mm OD so work on a 93.5mm PCD. It's my toolpost grinder pulley thats 92.5 PCD. Too many things in me 'ead at once. Sorry about that.

Rick

Hi Rick and Steve, Rick when I was in maintenace a few years ago we changed many V pulleys of all sections and your depth of the PCD is more than we used; i. e. Z/SPZ was only 2mm A/SPA was onl 2.75mm therefore Steve's 101 A secton pulley will be 95.5mm PCD. Upon looking up in my Fenner Drive design and mantenace manual I see this is correct, However in the pulley listing they round the figures down to the whole number i. e. a 101mm O/D pulley has a PCD of 95mm.

Here is a **LINK** to thier design manual.

Regards Nick.

P. S. scroll down to page 64 for A/SPA 101mm O/D pulley and to page 69 for pulley groove dimentions.

Edited By Nicholas Farr on 24/03/2014 01:04:16

This gets better and better

I'm sure that slight change in PCD from your above posts won't change a lot really... Its just for a sanding drum driven from a 1500rpm motor - 1500 is fine for a 4" drum but I'm using a 3", hence my speed increase!!

Thanks to all

Steve

Before you worry too much about the odd 1mm on pcd - if your "1500rpm" motor is an induction type it won't be doing 1500rpm - more like 1425rpm. For this application I wouldn't worry too much about getting the pcd of the pulley exact.

Keith

For a job like this just measuring the O/D of the pulleys will be near enough, assuming the same groove in both of course.

Hello Nick Farr, I admit I'm a bit behind the times. I'm using my old Fenner Design manual, number 110 of 1979. Very last century.! I think the changes came about when the overall belt depth was reduced slightly probably to get more belts out of the same volume of raw material. As a point of academic interest Tom Walshaw,( Tubal Cain) also qoutes these figures in Model Engineers Handbook of 1980, but as you say your given figures will provide a satisfactory value. We all live and learn.

Rick