Question (from a layman) about torque

I recently used a surplus windshield wiper motor in a drum coffee
roaster application. The drum has an 11 inch diameter, and the motor
can reliably do about 5 pounds of coffee beans. It's coupled with a
pulley that's almost 1:1 just now.
The drum has agitation vanes inside which are optimized to keep the
beans tossing about, but the simple picture is that they're being
lifted on a 5" radius (on average).
The drum's size easily allows for a 10 - 12 pound batch, but the motor
I've been using can't handle that at all.
What this layman needs is a bit of guidance on how to understand what
kind of torque specifications I should look for in a gearmotor that
will do the trick.
I'd be glad of any tips!
- S
Reply to
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1. windshield washer motor may not like running continuously - you want a continuous duty motor 2. if I understand it right, you want to lift 12 pounds by 10 inches - to be conservative, assume all the weight is at the extreme end of your 5 inch radius, so you have 12X5 inch pounds of torque (e.g. 60 inch pounds or 5 foot pounds) of torque needed when the weight is exactly horizontal. I'd probably add another 100% to that and look for 10 foot pounds of torque.
Reply to
William Noble
Has it gotta be a wiper or 12vdc motor? Why not press into service a washing machine motor or the likes ?
Reply to
Rheilly Phoull
You might find a copy of the Bosch electric motor (aftermarket program) catalogue useful. I have a printed copy, it may be available for download or you may have to request one from a Bosch rep. Basically lists all sorts of motors which I think derive from automotive applications but which have a wider use. It give sizes, duty, voltage and power consumption figures, torque curves. It may be helpful as you might find the same or equivalent motor and may be able to find a more powerful alternative.
Regarding William Noble's comment about duty cycle, those listed which appear to be windscreen types are listed S1 duty (continuous) which makes sense as it rains a lot in some parts of the world.
It might be worthwhile looking at a motor from a truck as they have larger wipers but are likely to be 24V, at least they are typically in the UK. Also the wiper motors are often only intended to run in one direction and don't run as well in reverse so that may be worth checking in your app.
Reply to
David Billington
The power of google, searching for "bosch electric motor aftermarket" got
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as the first hit and you can search through their online literature about the various motors.
Hope that helps.
David Bill>
Reply to
David Billington
I think you guys missed the point, he has a wiper motor, and wants to know how to select a commercial gearmotor replacement that will be suitable.
Reply to
If you have 12 pounds of coffee beans at 6" radius (1/2 foot for easy math) it would take 6 foot pounds of torque. I don't know what your ideal RPM is but I'll attempt to calculate approximate HP at 30 RPM. 30 RPM is 1/2 rev per second, at 5 inch radius I'll estimate 1-1/2 feet per second 6 pounds of torque at 1.5 fps = 9 ft/lbs/second. IIRC 1 HP = 550 ft/lbs/second. So you need 0.0164 HP, or 1/61 HP for 30 rpm, scale for the actual rpm you intend to use. Since gearboxes, bearings, etc are not 100% efficient, upsize a bit from this. I would look for something perhaps 1/40 HP or larger. Since 746 Watts = 1 HP, you need a little over 12 Watts of motor power @100% efficiency, a 1/40 HP motor = 18.65Watts (not actual electricity usage, output HP, some motors are rated in HP, some in Watts).
Gearing - What you have / what you want. If you find a 1750 RPM motor and want 30 RPM, 1750/30 = 58.33, you would look for somewhere around a 60:1 gearbox.
To just answer your torque question, you need around 6 ft /lbs, or 72 in/lbs, or 1152 in/oz of torque at the drum.
Hope this helps RogerN
Reply to
Adding to what Roger said about gearing down to reduce RPM: when you gear down to reduce speed, you multiply the torque that the motor can deliver by the same ratio as you geard down the rpm. So, for example, say you gear down to reduce the speed to 1 tenth. That makes your motor effectively 10 times stronger. A 100 RPM motor capable of delivering 1 foot pound of torque geared down to 10 RPM will deliver 10 foot pounds of torque through the gears. There will be some loss due to friction, so the actual delivered torque will be something a bit under 10 foot pounds.
Reply to
On Mon, 13 Nov 2006 19:22:44 GMT, ehsjr proclaimed to the world:
This reminded me of my past in the US Navy. I took care of the main engine controls on the last conventional powered aircraft carrier built. It has four main engines consisting of huge GE steam turbines. The reduction gears tower above you. All that turbine torque reduced down to less than 100 rpm to a 30 inch drive shafts. There is no clutch or reverse. A separate turbine drives the whole thing in reverse. Anyway, the weight of the drive shafts and span between bearing blocks made it necessary to slowly turn the shafts while in port to keep the shafts from sagging. We "jacked" the turbine, reduction gears and main shaft with a fractional HP electric motor. Because we could connect the jacking motor to a huge ring gear on one of the turbine's larger rotors, there is little gearing loss.
One night while looking down into the dry dock, I had a vision of the worlds largest barbecue pit, with a tied and spittled Godzilla slowly turning over the fire. In this application the jacking gear would have to be sped up or the big lizard would get burnt on one side. I don't remember the exact turn rate anymore. I knew then. It's somewhere around 2 RPD.
Reply to
Paul M
On 11/12/06 10:27 PM, in article, "" wrote:
Read a high school level physics book on simple machines.
Bill -- Fermez le Bush
Reply to
Salmon Egg
This won't solve the problem, but one place to start would be to measure the torque you have. Since torque is measured in foot-lbs, you just need to measure the force exerted by a lever-arm one foot long. Make up a bar with a distance of one foot from the motor shaft to a large hole at the other end. Then connect a spring scale (such as a fish scale) to the large hole and secure the scale and the motor to a firm bench or something. Turn on the motor and read the force on the scale. This is the stall torque of the motor. If I remember correctly, a rough estimate for optimum power is between 65% and 80% of the stall torque. The exact value for the optimum power is measurable from the motor torque curves, but these may be hard to find for a surplus motor. Hope this helps you get a handle on what you have now. ww88 wrote:
Reply to
How about roasting a bunch of lawyers instead of Godzilla? ;-)
Reply to
Michael A. Terrell
I'm not sure that the stated reason for "jacking" is correct. Many power plants use gas turbines to run peaking generators for short-term loads When those are idle, they are kept moving slowly by "turning gear" (different industry, different name). The purpose is not to prevent the steel shafts of alternator and turbine from taking a set, but to keep oil from being squeezed out of the lower parts of the bearings by steady unidirectional pressure. Some modern installations use pumps to force oil flow even when the shafts are stationary. This has the advantage that recovery from power failure is simpler and safer.
Reply to
Jerry Avins
On Tue, 14 Nov 2006 10:21:49 -0500, Jerry Avins proclaimed to the world:
From my experience, I suspect that rotor warping is as much a reason for turning those gas turbines. I've do some work with gas turbines in both the power generation and marine industries. BTW "turning gear" is also used in the marine field, more so in the merchant marines than Military. I know the Brits have some odd names for things too.
My stated reason for jacking the turbine and power transmission system in a marine propulsion system is only one of several. There may be more but here are the ones I know of.
1. Drive shaft sag 2. Lubrication 3. Warping of the turbine due to temperature variation.
Lubrication was never a big issue that I knew of other than making sure that the pumps were on and you had pressure at the bearings before you started jacking. There were several conditions we keep the engine in which had to do with the amount of time it took to turn screws under power. The most "shut down" state was with no oil to the bearing, no jacking. With a completely cold main engine turbine it took around a day to warm it up to the point where the throttle valve was cracked open. I remember a few times where we pushed the edge of safely when we had to get underway unexpectedly. Shaft sag posed the possibility of the longest delay. We kept logs of when and how long we jacked and had a setup to measure the shaft sag. There are times when it is unavoidable and the shafts had to stay in the same position. They do not sag all at once of course. I saw a graph of shaft sag once and remember that sag is a curve with sag decreasing with time. After several weeks the shafts are as bent as they are going to get. It takes several weeks of straightening to get underway. This is unacceptable for a combat ship, so they jack the shaft as a rule, only letting it set when necessary, such as when in overhaul.
It might be a good idea for me to add that the jacking gear was sometimes used intermittently.Some ships had two speeds. We might want to keep the shaft stationary most of the time, so we would jack the shafts 180 degrees ever few days. The longest shaft is over 200 ft. I was told that this shaft has 2 1/2 twists in it when under full load. I find this difficult to believe, but the guy who told me was pretty credible, as he was one of the naval engineers working on refurbishing the bearing blocks for the shaft. It gives you a better idea of the flexibility of these shafts.
Keep on questioning things Jerry. It gives me the opportunity to babble more. :-)
Reply to
Paul M
["Followup-To:" header set to rec.crafts.metalworking.]
Once you load the drum up, you'll have more coffee in toward the center of the drum so your average moment will decrease. Also in the light load, all your beans are carried up beyond horizontal on the vanes before they spill back to the bottom while the larger load has much more of a rolling effect. You still have the same amount of beans riding the vanes up to horizontal, but you also have a mass that's just rolling around the bottom... it's only rising from something like 180° to 230°. Obviously, this is all highly dependent on rpm, and how close that comes to the sticking speed for that radius. Even if I knew all the variables, I wouldn't know the math of it.
In this sort of application, I don't think that doubling the load necessarily requires doubling the torque though.
My Millenium uses a 3/4 hp motor to rotate 15kg of beans in 15" drum at 57 rpm, and it's not even a strain. I have to guess that it does require more than 1/2 hp though, or they would have cheaped out on the lighter motor. (The blower motor is 1/2 hp, for instance)
Purely back of the napkin then, and ignoring other variables I know about and others I can't imagine, your drum is .733 the radius of mine x .333 the load of mine, so should require roughly 1/4 the torque.
I'm going to guess that 1/6 hp ought to be more than enough @ 60 rpm or 1/12 hp @ 30 rpm...
Reply to
Steve Ackman
Paul M wrote
interesting information and
I stand corrected.
I have a collimator that fastens into the bore of my rifle and provides a target for the scope. I can remove the scope from the barrel and the barrel from the receiver, then align the scope to the collimator's projected image at reassembly and it is dead on *provided the barrel is right-side up and level when the adjustment is made.* The weight of the barrel and collimator flexes the barrel enough to throw the sight line off otherwise. My barrel is about two feet long and 3/4" in diameter, with a 15/64" (.22 cal.) bore, yet it flexes enough to throw the sights way off is not used gingerly.
I drove a '50 jaguar XK-120 that had such a hard-grabbing clutch that nobody I knew, not even the dealer's mechanic, could slip the clutch a bit without chattering vigorously. (That's why it sold cheap.) The drive shaft was about 1.25" at the clutch spline, tapering uniformly down to 3/4" at the differential. My way to start the caw was sliding my foot sideways off the pedal so the clutch didn't have time to chatter, and rely on the drive shaft's windup to absorb the shock. The shaft probably wound up two turns before the car moved an inch, then unwound, returning the stored energy to forward momentum. Properly playing the accelerator avoided oscillation. That car started like a goosed antelope!
Reply to
Jerry Avins
On Tue, 14 Nov 2006 14:52:32 -0500, Jerry Avins proclaimed to the world:
I hope the car had head rests. I would imagine that having your head against the rest made the start a bit better. Forget the coffee.
BTW a fine clutch and gearbox is a real joy. Coupled to a beefy engine it really is a treat. I really should have become a rally racer.
Reply to
Paul M
My son has a one year old Mazda 6. I have a 15 yr old Jeep. I can beat him off the line for the first 50 feet or so. He then smokes me. Torque is what makes the Jeep jump out ahead.
Reply to
On Wed, 15 Nov 2006 14:16:25 GMT, Al proclaimed to the world:
In younger years I had a Suzuki 380 motorcycle. It was a hot 2 stroke back in the 70's. I used to street drag race with it against all the Hondas around then. The Honda 360 and 550 were really popular four stroke bikes back then and had more torque than most. The two strokes were laughed at then but they had a lot of top end. For me to beat a 550 Honda, I had to due a burnout start, hell to get off the line, I had to do this. On the start the Honda would get maybe twenty feet ahead while I was leaning over the handle bars, leaving a cloud of smoke with the tire. Once I got moving a bit, I would lean back and transfer weight to the tire. The bike would squat, the front wheel would come up and the tire would stick. It was like being shot from a cannon. At around 150 ft off the line, I would shoot by the Honda like it was standing still.
Now I have a old Honda CX500 touring bike with lots of torque. Torque is better than speed for day after day pleasure in driving.
Reply to
Paul M
The Jag belonged to a friend. It had the same weight, engine displacement, and cylinder count (6) and as my father's '50 Dodge sedan. Not much similarity beyond that.
Reply to
Jerry Avins

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