Motor cheap

[doveguy1]

http://www.surpluscenter.com/sort.asp?numrec=20&lastrec=220&sort=2&search=motor&UID=2008120220092829

[peter]

You like that one, hey?   

[grinderz]

Quick question, how much torque is necessary to run a BBQ drum? I notice some of these gear motors are rated in inch pounds.

[rasqual]

Quick question, how much torque is necessary to run a BBQ drum? I notice some of these gear motors are rated in inch pounds.

Where I confess my ignorance.   

Surpluscenter's a great source. The windshield wiper motor on the Open Sky has been turning 3 to 8 lb batches weekly for three market seasons. It overheats and STILL hasn't died.

We got the motor for the production model (yeah I know...) from there too.

[harryho]

Quick question, how much torque is necessary to run a BBQ drum? I notice some of these gear motors are rated in inch pounds.

Just a quick calculation:

A few assumptions:
1. RK Drum - 8" in diameter (or a radius of 4").
2. Max. 5# batch weight (factor in about 1# addition for uneven weight distribution for drum and spit)
3. Center of gravity for the bean mass when first loaded about 3" from spit.

5# x 3"= 15 in-lb to get the drum rotating. May be overkill, but still a good starting point.

[rasqual]

I think RK'ers would do well with this little guy.

The price is right, and pulleys are cheap. Actually cheaper from McMaster than from the hardware store, even with shipping.

[harryho]

If the desire is to plug them in, I would recommend this:

http://www.surpluscenter.com/item.asp?UID=2008120220092829&item=5-1417&catname=

It has enough torque to start a loaded RK drum rotating and is rated for continuous duty. It should last you quite a long time.


This one is pretty similar to what I used for my BBQ based roaster.
http://www.surpluscenter.com/item.asp?UID=2008120220092829&item=5-1682&catname=

This one will be a workhorse for years a thousands of pounds of coffee. It has an excellent torque rating, and rated for continuous duty, and even has a fan to keep the motor cool during operation. It may cost a few bucks more, but this will be a workhorse.

[milowebailey]

Quick question, how much torque is necessary to run a BBQ drum? I notice some of these gear motors are rated in inch pounds.

Just a quick calculation:

A few assumptions:
1. RK Drum - 8" in diameter (or a radius of 4").
2. Max. 5# batch weight (factor in about 1# addition for uneven weight distribution for drum and spit)
3. Center of gravity for the bean mass when first loaded about 3" from spit.

5# x 3"= 15 in-lb to get the drum rotating. May be overkill, but still a good starting point.

I like it when you use math....  


but where is the 1# uneven weight factor.  6# x 3 = 18 in - lb.  But you should also factor in the wight of the drum as it is part of the equation, so weight that and add that to the 6#.

Being an engineer I'd take that number and double it for severe overkill and a motor that will last and not overheat

[J.Jirehs Roaster]

Being an engineer I'd take that number and double it for severe overkill and a motor that will last and not overheat

A Caterpillar Tractor Co. Engineer would quadruple that size and pad it +15% for safety 

[harryho]

Quick question, how much torque is necessary to run a BBQ drum? I notice some of these gear motors are rated in inch pounds.

Just a quick calculation:

A few assumptions:
1. RK Drum - 8" in diameter (or a radius of 4").
2. Max. 5# batch weight (factor in about 1# addition for uneven weight distribution for drum and spit)
3. Center of gravity for the bean mass when first loaded about 3" from spit.

5# x 3"= 15 in-lb to get the drum rotating. May be overkill, but still a good starting point.

I like it when you use math....  


but where is the 1# uneven weight factor.  6# x 3 = 18 in - lb.  But you should also factor in the wight of the drum as it is part of the equation, so weight that and add that to the 6#.

Being an engineer I'd take that number and double it for severe overkill and a motor that will last and not overheat

Sorry if I was not too detailed.

1. A typical RK drum takes 4# of greens max. So the 5# I used includes the 1# uneven weight distribution.
2. The weight of the drum theoretically is negligible if the spit is located exactly dead center. Thus the 1# uneven weight distribution is a simple way of factoring in the added load of the drum's weight.

[ButtWhiskers]

I'm pretty sure that you get away with a lot less than you guys are calculating due to the rotation angle, which multiplies against the force vector and the radius to come up with the necessary torque.  The weight of the top half of the drum adds to the momentum, as well, so once the thing is moving (even a little) you need even less torque than you did to get it moving in the first place.  If you were turning something on a horizontal plane (like a carousel), you could calculate it with the simple cross product (as Harry showed).  But since you are turning on a vertical plane (the spit is horizontal), I'm pretty sure that you get to toss sinθ into the equation, which buys you some extra power.   To put it another way, all of the weight is not directly opposed to the direction that the motor is turning the rotisserie, which is sitting at equilibrium with half of the weight directed toward moving the drum each way (clockwise vs counterclockwise).  I think that it really only takes about 5 or 6 inch-pounds of torque to turn a loaded RK drum, provided your rotisserie spit turns smoothly, and the beans can roll around inside mostly unfettered.  Just make sure that the motor is rated for continuous, rather than intermittent, duty.

[grinderz]

OK! Now this is a useful conversation!

For the sake of discussion, what would you all consider the minimum/optimum/maximum RPM's for, say, a six or eight inch diameter drum? How fast can it go before you've got a Green Bean Gravitron and what's the minimum speed needed? The rotisserie I saw on a display BBQ at a Home Depot seemed V E R Y slow. Like more than a 10 count per revolution.

[ButtWhiskers]

OK! Now this is a useful conversation!

For the sake of discussion, what would you all consider the minimum/optimum/maximum RPM's for, say, a six or eight inch diameter drum? How fast can it go before you've got a Green Bean Gravitron and what's the minimum speed needed? The rotisserie I saw on a display BBQ at a Home Depot seemed V E R Y slow. Like more than a 10 count per revolution.

I seem to remember that Ron Kyle recommended something in the 48 to 60 RPM range at one point for people that were procuring their own motor.  The higher grade motor that he sold was a 57 RPM model (I think that it had 10 inch-pounds torque, vs 7 inch pounds and 50 RPM on his 'home' model motor).  I always felt that was a little bit fast, so I run at 40 RPM.

I have seen people roast successfully at 15 RPM.  I also saw a guy use a drill motor that had to have been going over 120 RPM, which resulted in unevenness - I'm pretty sure that was from the centrifugal force.  Both of those speeds were with an RK knockoff, though, so the genuine article could be different due to its superior agitating fins.

[rasqual]

The Open Sky is about twice the RK's diameter, and 60 RPM is perfect.

That RPM is good for a couple reasons. First, the more energetic the agitation, the better the heat transfer (in a perf drum) and the lower your environment temperature, if you wish. You can add more beans and/or ramp faster, if you have the heat for it. Second, there's a sweet spot where the tumbling beans no longer resist rotation, but are nearly neutral in their effect because you're pitching the beans over to the downward-moving side of the drum, and even adding force to any vanes descending on that side.

Too fast, and the beans just hug the inner side of the drum. Too slow, and you're using mostly conduction (less efficient by far in a light perf drum).

The ideal, IMO, is to be sure the beans go airborne and land on the far side of the drum. If they only roll back down on the same side they're ascending, that's a terrible use of the fine airflows a perforated drum allows, and a worse reliance on predominantly conduction for heat transfer on a surface not ideal for that. It just doesn't make sense.

There's the story of the trucker who pulled over before he got to the weigh station and whacked the heck out of the side of his truck just before he pulled onto the scales. Turns out his load of pigeons put him over weight, so if he could just keep 'em in the air until they read his weight . . .     

Ya wanna keep these pigeons in the air, not crowding each other in a slow-rolling pile.

[Ascholten]

I would also think that a slower turn rate would bog down the motor more as well.  Id say 50 to 60 rpm is good.  Fast nuff to keep everything moving, which is what you want but not fast enough to start making things fly out or hang on the walls.  I would also think that the slightly faster speed will help ensure the beans are actually tumbling more and agitating and not just sliding along until the vane tips them slightly to slide down the other side.

Aaron