Making pulleys (sheaves)

[AdeV]

One of the many projects I'm currently mulling over in my head (whilst I've no access to my workshop...), is a generator for the race car transporter/motorhome.

The engine I want to use for the genset works best at 2200rpm. The alternator needs to spin at 3000rpm. Seems simple enough, right? Ratio is 1-4/11ths (naturally), so pick a pulley size for the alternator such that there's one 1 4/11ths ish bigger available for the engine.

Except.... is it REALLY that simple? I'm aware that Pitch Diameter is the all-important measure. I'm assuming I'll have some engine speed adjustment, so I can wriggle out any minor discrepancy, but can I just match the ODs of sheaves and expect them to be "more or less" right?

OK, second example, assume I slow the engine to 2000rpm, making a nice easy 2:3 ratio. Can I put a 9" pulley on the alternator, a 6" pulley on the engine, and expect it to work at about the right speed?

PS: I ask here because my google-fu is sorely lacking today, for some reason.

[AdeV]

PS: The reason for the title of the topic...? Well, if it IS just a case of matching up ODs, then I'll have a crack at casting my own of the exact size(s) I need...

[mexican jon]

Provided the engine is of a suitable size then yes  I have a Honda generator that is belt driven so it's the same principal 

[John Rudd]

The smallest pulley is determined by the minimum wrap of the belt you use as a general rule......

And dont forget, you may need to govern the engine to maintain speed as you load/unload the generator.....

[AdeV]

Thanks guys, what I'm after is: Is using the ratio of the OD of the pulleys generally regarded as "close enough" when sizing up?

e.g. if my power source ran at 3000rpm with a 5" pulley, and my output ran at 1500rpm, is it close enough to just ask for a 10" pulley?

Or is it more scientific than that & actually I neeed a 10.25" pulley due to the difference in pitch diameter?

I'm probably not explaining this very well, hence the google problems...

[John Rudd]

If your driver is running at 2000rpm and the driven needs 3000, the ratio is 1:1.5
So the bigger 5" pulley is on the engine and the smaller 3.33" on the gen.

Speed adjustment on the engine will accomodate any size irregularity in the pulleys.....( governer?)

[Jo]

Thanks guys, what I'm after is: Is using the ratio of the OD of the pulleys generally regarded as "close enough" when sizing up?

Yes. (Assuming they are both for the same size belt)

Jo

[AdeV]

Perfect, thanks 

Jo - yes, they'll be for the same size belt (probably "A" section twin groove).

So.. next question... as 10" pulleys cost ££££ (or $$$$, or even ¥¥¥¥), and are mostly cast iron & therefore _heavy,  I'd thought I might cast my own out of aluminium. This appears to be quite an uncommon thing to do, at least for a home workshop, as one doesn't see many references to home-made pulleys. I did find one useful page here, but there's very little information given about machining the inside of the pulley grooves.

If anyone has any tips/tricks, please do share them, I'm all ears

[mattinker]

Perfect, thanks 

Jo - yes, they'll be for the same size belt (probably "A" section twin groove).

So.. next question... as 10" pulleys cost ££££ (or $$$$, or even ¥¥¥¥), and are mostly cast iron & therefore _heavy,  I'd thought I might cast my own out of aluminium. This appears to be quite an uncommon thing to do, at least for a home workshop, as one doesn't see many references to home-made pulleys. I did find one useful page here, but there's very little information given about machining the inside of the pulley grooves.

If anyone has any tips/tricks, please do share them, I'm all ears

I've cast quit a few pulleys, I would not cast them as in the example you've shown. It is far simpler to turn the grooves afterwards. The pulley pattern in the photo below is one that I made early on, originally I split it, not a good idea, it needs taller molding boxes and is much more difficult to get out of the sand that way! simple flat forms are much easier to make and use!

All the best, Matthew

[mattinker]

A passing thought, you probably know this, but Al has a shrinkage factor of 1.8%

Not a lot, but...............

Cheers, Matthew

[wgw]

I have made V pulleys from sheet, turn half a groove on each sheet. Easier than parting tool and form tool.

[philf]

Ade,

I've got a surplus of pulleys. I have 3 off 241mm OD A Section pulleys and a number of smaller A section pulleys 4, 5, 6, 7".

The big pulleys have bearings in a flanged boss which is bolted to the pulley.

When you get round to picking up the machine vice (which I just had to move to get at the pulleys) you can take your pick of the pulleys!

Cheers.

Phil.

[John Stevenson]

Dumbell weights make good pulley blanks

[Pete W.]

Hi there, Ade,

Rule of thumb: to get the effective diameter of the vee pulley, subtract the thickness of the vee belt from the pulley outside diameter.  That's a more accurate figure to use in predicting the effective ratio of a pair of vee pulleys.  This assumes the neutral level of the belt wrapping round the pulley is halfway down - in practice, I think it's a bit shallower than that.

I did write 'rule of thumb' - you can get more accurate data from the vee-belt manufacturers' literature.  In the 'old days', the name to look for was Fenner (not to be confused with the famous Fenner on Cape Cod!!).

I just got off my chair and dug out 'Belt Drives in the Small Workshop' by "Duplex", published by Percival Marshall (price three & sixpence net!).  That title might have been carried forward to the 'Workshop Practice Series'.  The best I could find in its pages said the pitch diameter is the OD less the belt thickness as in my opening sentence. 

[mc]

The easy option to calculate pulleys/belts which will eliminate any guesswork, is to download Gates Design Flex software, put in the figures, select the belt type(s) you'd like to use, and it will come up with various combinations.

Download page is http://www.gates.com/catalogs-and-resources/resources/repository/engineering-business-applications/design-flex
There is a mobile version, but I've never used it, so I have no idea if it's as in depth as the download version.

[AdeV]

Many thanks everyone!

 

Matt - Yes, I know aluminium (like all materials except H₂O) shrinks when it freezes... I didn't know the percentage though, thanks! Not as bad as Machinable Wax then, that's about 5% IIRC. Maybe more. SERIOUS shrinkage...

wgw - interesting idea; isn't it a bit tricky getting the two halves properly concentric though? Mind you, I like your idea because it would allow you to make a variable pitch pulley with ease... that is where the groove width can be adjusted (usually by screwing one half into or away from the other half), thus fine-tuning the ratios... definitely one to consider! Thanks!

Phil, as soon as I'm off the slow boat from China, I'll drop you a line! Don't know when that will be yet, probably another month or two. Not sure where I'm going to put any of this stuff... the workshop looks like it's been hit by 1/2 dozen cyclones, a hurricane and a fairly severe earthquake at the moment... it's going to take a solid week of work just to get to some of the machines!

John - I've heard about dumbell weights (well, strictily speaking, weightlifting weights), wasn't the consensus these days that the metal was so poor it was positively dangerous to machine? I'm thinking of cast iron weights, I don't suppose there'd be any point making aluminium ones...

Pete - someone's selling one on Amazon. £14.50!! Now that's inflation for you.... I'll keep a lookout for a more reasonably priced example I think.

MC - I'm going to give that a go right now, thanks for the link!

[AdeV]

The easy option to calculate pulleys/belts which will eliminate any guesswork, is to download Gates Design Flex software, put in the figures, select the belt type(s) you'd like to use, and it will come up with various combinations.

Download page is http://www.gates.com/catalogs-and-resources/resources/repository/engineering-business-applications/design-flex
There is a mobile version, but I've never used it, so I have no idea if it's as in depth as the download version.

Heh, many thanks for that link, it made the process almost boring! It found a perfect match anyway, 180mm and 132mm pulleys are a dead nuts fit for my requirements. They're not even particularly stupidly expensive... although I reckon I'll still have a go at casting them myself.

Thanks again for the link!

[mc]

Gates DF Pro is a revelation the first time you use it. You used to have to jump through various hoops to be able to download it, but now it's a straight download.

It's brilliant for seeing what belt/pulley combinations will/won't work for my latest daft ideas. Stick in the power required, ratio or speeds, shaft sizes, centre distance, and you get a list of possible solutions. Then for each solution you get the full design details, with part numbers, safety margins, and any other notes. Gone are the days of guessing what belt size is needed, and I particularly like the fact it tells you how much adjustment you need to be able to get belts on/off.