belt grinder build -- a thought about speed control

[MattBower]

I know there are lots of very mechanically minded folks here, and I'd like to get your input on an idea I had.

I may soon be in a position to finally build a belt grinder, and I've been thinking about speed control. A VFD isn't in my budget -- at least not the sealed ones that are most suitable for this kind of project -- and I can't find a combination of pulleys that will give me the range of speeds I'd like, and that will also fit the shaft on my 2HP motor. I was pondering this problem last night, and I thought about those slip clutches you see on some mechanical power hammers, which allow you to vary the tension in the drive belt. That seemed like it had potential -- a simple pulley arrangement with a lose belt and a roller to tension it. But once I started thinking about power hammer clutches, it hit me: tire hammer! If you know how the tire hammer clutch works, I'm basically proposing to use a very similar arrangement to drive a belt grinder.

My idea is to mount a rubber surfaced drive wheel (basically a contact wheel, but with a keyed bore rather than bearings) directly on my motor shaft, then put the motor on a tilting mount just like on a tire hammer, so that by tilting the motor the drive roller can be brought into contact with a second drive wheel on a shaft. (The larger wheel would be on the motor, to step up the motor RPMs at the final drive wheel.) The drive wheel for the belt would be on the other end of that same shaft. The pivot arm of the motor mount would be moved up or down by turning a knob, rather than using a treadle as you would on a tire hammer, so you could set the speed and be free to move around the machine and use both hands.

If this'd work, it should give a nearly continuously variable range of speeds up to the maximum for that combination of motor and wheels.

What I'd like to hear from you all is what problems you see with this idea. One potential problem that has occurred to me is whether heat build-up in the drive roller(s) would be a problem. The surface areas we're looking at would be a tiny fraction of what you have on a tire hammer, and they wouldn't get the intermittent use that a hammer clutch does; they'd be running continuously for fairly long periods.

I also wonder if there'd be too much slippage in this arrangement. When I bear down on the belt, is the clutch going to slip?

What else? I'm kind of excited about this idea, but if it were really that great an idea it seems like someone would probably already be using it. So is there a fatal flaw that I'm missing?

[pkrankow]

Seems like a clutch is not going to provide what you want...but you can do a friction drive on a plate and get rather significant variation of output. I don't know how this would work on a long term wear standpoint though.

you could also do an idler using step pullies, seems that would give a lot of variation...

how much variation are you looking to gain? What size contact wheels, driving wheel(s) and intended belt speeds?

Phil

[CurlyGeorge]

Matt, the only concern that I would have would be the slippage and heating up the rubber drive wheel. The Idea sure sound feasible, though. Is there a way that you could make up a sort of test unit that you could put preasure on the drive wheel to see if it would drag down or slip, without making the full setup?

[MattBower]

Phil,

Specifically what problems do you see with the clutch idea?

To answer your questions, ideally I'd like to cover a range of around 500 to at least 5000 SFPM, which, with a 4" wheel driving the belt, means right around 500 to 5000(+) RPM on the driven shaft. (I'd like the lower range so I could use it for wood.) I spent a fair amount of time considering idler pulleys. (Or at least I think I did. If I'm misunderstanding what you're saying, let me know.) On a 3450 RPM motor, the hard part seems to be stepping down far enough. I could live with a two-step drive pulley on the motor driving a 4-5-6 pulley on the driven shaft, if the steps on the drive were far enough apart - like, say, a 2" step and a 6" step. That wouldn't get me down to 500, but it'd get me to around 1100. Not terrible. But they don't seem to make two-step pulleys that way; the steps seem to be pretty close together, which isn't that helpful. (I considered running two individual pulleys on the motor shaft, but the ones I could find were too wide to do that.) They make four- and five-step pulleys that'd do the job, but they'd hand way over the end of my shaft.

I don't have the drive wheels yet. I was figuring on a 4" wheel from Rob Frink to run the belt itself. I'll need that even if I end up going with a VFD (unsealed) or pulley system. As far as the other two drive wheels, it'd depend on what I could find that I thought might work. The diameters wouldn't be all that important if the clutch idea worked -- would they? Larger diameters would probably be better for durability. It sounds crazy, but I even looked at thisas a possiblity to mount on the motor. I know -- not balanced, probably not designed for 3400 RPM. I was just lookin' around.

Curly, I guess the reason I'm a little reluctant to experiment is that those extra drive wheels won't be cheap, so I'd kinda hate to put the money into 'em to find out it's a bust. Especially if finding out means something like delaminating the rubber on a wheel! If I do decide to go with this idea, it's going to be a sort of experiment no matter what. But before I make that decision I'd like to feel comfortable that there's a least a good chance it'll work out. That's why I'm asking you guys!

[MattBower]

And Phil, I didn't even know what a friction drive was until I looked it up just now. Sounds interesting, though it seems like it'd have its own set of problems.

[arftist]

Why do you think you need to go so slow to sand wood?

A clutch is not a good idea for a belt grinder, slippage is completely to be avoided.

Personaly, I don't see why you need variable speed at all, but I think this project is a bit over my head.

My first thought upon reading your OP was a snowmobile drive system, but I see you are not in an area likely to have junk snowmobiles laying arround.
Never the less, I suggest you research snowmobile drives. They are infinitely variable ratio and very positive.

Another posibility; if the step pulley's steps are too close together, you may have room to install two seperate pulleys on each shaft, giving you two very distinct ratios. You would either need two different belts, or if you could arange the ratios just so, the two different sysytems could share a belt, if there were just two pulleys sizes, one of each on each shaft.

A final option for your consideration, a variable ratio gear reducer. I have two, brand new such units I picked up at an auction in my stockroom. They both have a hand crank to vary output speed. New, it would be cheaper to buy a VFD, but finding one used, if you could, would give you lots of options.

[knots]

Phil,

Specifically what problems do you see with the clutch idea?

To answer your questions, ideally I'd like to cover a range of around 500 to at least 5000 SFPM, which, with a 4" wheel driving the belt, means right around 500 to 5000(+) RPM on the driven shaft. (I'd like the lower range so I could use it for wood.) I spent a fair amount of time considering idler pulleys. (Or at least I think I did. If I'm misunderstanding what you're saying, let me know.) On a 3450 RPM motor, the hard part seems to be stepping down far enough. I could live with a two-step drive pulley on the motor driving a 4-5-6 pulley on the driven shaft, if the steps on the drive were far enough apart - like, say, a 2" step and a 6" step. That wouldn't get me down to 500, but it'd get me to around 1100. Not terrible. But they don't seem to make two-step pulleys that way; the steps seem to be pretty close together, which isn't that helpful. (I considered running two individual pulleys on the motor shaft, but the ones I could find were too wide to do that.) They make four- and five-step pulleys that'd do the job, but they'd hand way over the end of my shaft.

I don't have the drive wheels yet. I was figuring on a 4" wheel from Rob Frink to run the belt itself. I'll need that even if I end up going with a VFD (unsealed) or pulley system. As far as the other two drive wheels, it'd depend on what I could find that I thought might work. The diameters wouldn't be all that important if the clutch idea worked -- would they? Larger diameters would probably be better for durability. It sounds crazy, but I even looked at thisas a possiblity to mount on the motor. I know -- not balanced, probably not designed for 3400 RPM. I was just lookin' around.

Curly, I guess the reason I'm a little reluctant to experiment is that those extra drive wheels won't be cheap, so I'd kinda hate to put the money into 'em to find out it's a bust. Especially if finding out means something like delaminating the rubber on a wheel! If I do decide to go with this idea, it's going to be a sort of experiment no matter what. But before I make that decision I'd like to feel comfortable that there's a least a good chance it'll work out. That's why I'm asking you guys!

The the moving parts of hammers with a tire drive are massive by comparison with the rigging for shaft driven sanding belt. Seems to me that high resistance to being driven is needed for the successful application of a slip clutch variable speed drive.

Some Clausing lathes have variable motor sheaves that are hydraulically actuated. My Rockwell wood lathe has a variable sheave speed control, lever actuated. Both of those work pretty well . Have a look.

[pkrankow]

I see heat and unsteady amounts of slipping. with a clutch

I would suggest a universal motor as a way of getting around the problem easily. Might be easier said than done on a budget, but they are out there, some pedestal grinders are based on them. A universal motor can be slowed with a pot (rheostat)


(excuse my rambling)
If we took 4 step pulleys, say
http://www.amazon.co...ref=pd_sim_ol_1
4, 3 1/2, 2 1/2, 2
and set them up with an idle shaft so

2 , 2 1/2 , 3 1/2 , 4 motor
4 , 3 1/2 , 2 1/2 , 2 idle input

.5 ,7 1.4 2

then

2 , 2 1/2 , 3 1/2 , 4 idle output
4 , 3 1/2 , 2 1/2 , 2 grinder input

.5 .7 1.4 2



your total outputs go to


from .25 up to 4

with a 3450 motor this gives you 862 rpm to 13800 (pretty stinking fast)

So not quite there. if the first pulley is instead a reduction of 4, so say a 1 inch on the motor to a 4 inch on the idle, then you only have 4 speeds then the range is

1 motor
4 idle input

2 , 2 1/2 , 3 1/2 , 4 idle output
4 , 3 1/2 , 2 1/2 , 2 grinder input

.5 .7 1.4 2

total ratio is
.125 .178 .35 .5

so with a 3450 rpm motor
431 614 1207 1725


How's this?

Phil

[Bryce Masuk]

I have a belt on mine and next time I wouldnt use one again

the bader grinders are direct drive this is much better
the belt will always slip some when you really lean into it

I would use one of those rubber and metal drive with the rubber star in them a vibration reducer and it blows out the star if its overloaded

with a 4 inch drive wheel with 1725 rpm motor

most people have belt grinders running too slow I followed the directions people had told me and it wasnt fast enough.

the only reason to have the grinder run slower is for belts above 22o grit

everything else will work better and last longer running as fast as possible especially the lower grit belts

you could easily setup a slow grinder beside that one with a gear reduction drive so it only goes a few hundred rpm for your high grit stuff
you dont even need a wheel for the front you use a wood triangle with a notch for the belt tracking and 2 wheels one for drive and the other for tracking/tension

the belt doesnt need to be very tight with this type of grinder you can take the belt along the blade really easily and because it has the sharpish corner of the wood you can easily radius the riccasso transition on a blade where you would normally have to take a file.

someone showed me this type of grinder years ago I thought it was pretty neat

[joshua.M]

what if, instead of putting somthing between the belt and motor, why not put somthing betweed the power scource and motor, if you wire in a dimmer switch like for a light would that work? i did it with an old electric blower that didnt have variable speeds
just a thaught, virtual slap me if im wrong
Josh

[Swamptroll]

A friend of mine built one using the motor and control from a treadmill. The motor was a 2 hp dc and it worked very well. The price was right to free

[MattBower]

OK folks, you've convinced me. It was a fun idea, but it's not suitable for this application. That's why I asked!

joshua, AFAIK dimmers/potentiometers (pots) will eventually fry most common types of AC motors. I think that's why Phil specified a universal motor with a pot. But I don't have a universal motor, and I'd like to use what I have. VFDs seem to be the accepted way to do what you're suggesting, but they tend to be expensive.

arftist, the only reason I'd like a low speed option is for wood. There are of course other ways to skin that cat, and using the same grinder for wood and metal might not be ideal in some ways.

arftist and Dan, the Reeves drives/variable diameter pulleys or whatever you call them are really neat. But yeah, you're right that they're more expensive than VFDs new, and finding them used at a good price seems like it could be a long process. I love the idea, though.

Bruyce, can you pics of your setup? I'd be very interested to see it.

Phil, it sounds like I did misunderstand what you were suggesting. I see now that you have an extra shaft and two more pulleys in the mix. Your first example would work. 500 RPMs was just a ballpark figure. 900ish is probably plenty low.

Larry suggested that for the cost of variable speed I might be able to build two fixed speed machines, low speed for wood and finishing, and higher speed for serious grinding, especially if I already have the motors -- and I do, sort of. (See below.) He might have a point. I'll think about that.

A couple related thoughts:

(1) How practical would it be to build an enclosure to protect an unsealed VFD, without frying the electronics? The unsealed ones are much cheaper than the ones with NEMA-4 enclosures. I mention this because if I start adding extra belts, pulleys, bearings and shafts, the price of mechanical speed control starts going up, and an unsealed VFD starts looking less and less expensive by comparison. And I have a massive old 3 phase motor. I notice that the VFDs I'm seeing can convert 1 phase to 3 phase. So with a VFD maybe I could get some use out of that motor?

(2) I have a variety of motors that I've picked up cheap or free over the last five years or so: 1/3 HP/1750 RPM, 3/4 HP/1750 RPM, 2 HP/3450 RPM, 1-phase, and that massive 3 HP, 3-phase beast. (I'm not sure of the RPMs on that one, off the top of my head.) I was planning to use one (now maybe two?) of them for this project. The main problem with all these motors is that they have ODP enclosures. What are your thoughts on how I can adequately protect them from dust and grit?

[MattBower]

A friend of mine built one using the motor and control from a treadmill. The motor was a 2 hp dc and it worked very well. The price was right to free

I've thought about that. I just haven't run across a treadmill at the right price.

[arftist]

A friend of mine built one using the motor and control from a treadmill. The motor was a 2 hp dc and it worked very well. The price was right to free

This is a really good idea. Those things show up at the metal pile at the dump all the time. DC was the original variable speed electric motor before VFDs became fashionable.

[Dave Hammer]

If you have a 3 HP 3-phase motor, you can use that for your grinder by adding about $100. Buy a static converter (Anderson Converters is a source) to run it. You will get about 2 HP out of the motor with the static converter, which should be plenty for a belt grinder. I use this type of setup on a 3 x 132 belt grinder.

[MattBower]

If you have a 3 HP 3-phase motor, you can use that for your grinder by adding about $100. Buy a static converter (Anderson Converters is a source) to run it. You will get about 2 HP out of the motor with the static converter, which should be plenty for a belt grinder. I use this type of setup on a 3 x 132 belt grinder.

Thanks, Dave. That would be a good option, but if I'm going to cut down to 2 HP I might as use the 2 HP motor I already have.

I was considering Larry's idea of building two grinders, but after pricing out some different options I think I may have found a cheaper way to get acceptable results. I have a 3/4 HP, 1725, 120V motor, and a 2 HP, 3450 RPM, single-phase 240. I'm thinking of building a single grinder with a drive shaft driven by a step pulley, then mounting both motors on tilt mounts with the shafts roughly equidistant from the driven pulley on the grinder shaft. With the right pulleys I should to be able to get speeds from around 850 to nearly 8000 SFPM, with a 4" drive wheel, using the same drive belt. The lower speeds would be at lower power, but that's fine because they'd be used for wood, deburring, finishing, etc. The higher speeds would come from the 2 HP motor for serious stock removal. I'd just have to make sure the belt was connected to the correct motor, and flip the proper switch. A little crude, yes, but it'd do all I need at a lot less cost than a VFD, or building two separate grinders. This way I'd only need to buy one set of wheels, which is nice because they're a big part of the expense.

[ironstein]

When i built my grinder i found the vfd's too expensive. Instead i bought a dc motor which was a bit cheaper than an ac motor, and a dc motor controller which was ALOT cheaper than a vfd. Works great.

[MattBower]

When i built my grinder i found the vfd's too expensive. Instead i bought a dc motor which was a bit cheaper than an ac motor, and a dc motor controller which was ALOT cheaper than a vfd. Works great.

Good idea. For economy, I'd like to use the motors I have.

[peacock]

I think your idea will work. Some riding lawn mowers use rubber friction drive wheel against a flat drive disk on the motor. The friction wheel is moved from near the center to the outer edge which gives you an infinite amout of speeds. My old snapper mowed for years on this setup several hours at a time. The parts are at every lawn mower shop. Or pickup an old rider and rob the parts. I do think could work.

P. S. If you move the friction wheel to the other side of center of the disk you can also change direction.

[clinton]

I do not understand why you would need a speed control on a belt sander but you can get a router speed controller from harbor freight tools for around 20 bucks http://www.harborfreight.com/catalogsearch/result?q=router+speed+control

[fciron]

Clinton, he's not really looking for variable speed, just two different speeds. That actually does make sense whereas continuously variable speed seems like overkill and trying to feather the clutch while sanding could be a disaster waiting to happen. I like the two motor solution.

Your router controller states in the description

# Works with any universal AC/DC brush-type motor, 15 amps and under

I don't think that 220 volt 2HP motor Matt is planning on using qualifies on either count.

As Matt pointed out earlier, unless it says 'universal' or 'ac/dc' somewhere on the nameplate, hooking a motor up to a dimmer will let the smoke out somewhere. This is a safety issue! With an AC motor speed is a function of frequency, not voltage, that's what Variable Frequency Drives do.

Lewis

[MattBower]

I do not understand why you would need a speed control on a belt sander but you can get a router speed controller from harbor freight tools for around 20 bucks http://www.harborfreight.com/catalogsearch/result?q=router+speed+control

Because it's a good idea to run slower belt speeds on wood to prevent burning, and with fine grit belts on metal to keep the heat down. But more belt speed means faster stock removal for hogging, and some of the fancier belts don't work as well or last as long at slow speeds. So it's nice to have a least a couple speeds available.

I have one of those HF router speed controls. I used it to dial back a nice portable blower for forges, smelting, and that sort of thing, before I knew any better. Bad idea. Didn't seem to agree with the blower at all. I didn't let the smoke out entirely, but it got very hot and developed a whine that wouldn't go away. Bummer. Should've used a damper.

Continuously variable speed would be neat, but the price is just too high for this project. I can always add a VFD later if the opportunity arises.

[BIGGUNDOCTOR]

Plastic is another one that needs slow speeds to keep from melting. I can also see slower speeds when all you want to do is grain a piece. I have a 20" disc sander, and I would like to slow it down at times.