Need a "constant torque" slip clutch

On Jun 15, 4:27pm, "Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote:


Ideally, aren't there constant-torque AC motors that can be stalled indefinitely?
So far your method is coming out a lot cheaper, I'm sure.
Dave
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yep... So called "impedance protected" motors. I've been looking for one that would produce about 20ft-lb torque at around 5 rpm.
No luck so far. But I'm thinking I can run a split-phase induction gear motor on reduced voltage and accomplish the same thing. Most split phase motors don't really care if they're running, so long as you don't overheat them. Reducing the input voltage would be one way to set the stall torque and keep one from burning up.
But that still only gives me constant torque, and what I really want is constant tension. so my dancer arm will need to control the motor voltage.
LLoyd
LLoyd
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On Fri, 18 Jun 2010 12:21:55 -0500, "Lloyd E. Sponenburgh"

Bodine calls them "torque motors," but they suffer the same size shortcoming as mag particle brakes. A 20 ft-lb torque motor would be enormous. When I was a kid I had a pair of torque motors I scavenged from a Univac 2 tape drive. They were about the size of a cast iron 1/3 HP induction motor and could be stalled by grabbing the shaft.
http://www.bodine-electric.com/Asp/ProductGroup.asp?Context=7
--
Ned Simmons

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But Ned, keep in mind that those motors were designed to produce that torque all the way up to several hundred RPMs.
Mine only needs 3+ RPM... so I multiply the motor torque with belting or sprocket drives.
LLoyd
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On Fri, 18 Jun 2010 12:21:55 -0500, Lloyd E. Sponenburgh wrote:

[snip re impedance protected vs split-phase induction gear motor]

Do you need the roving in constant tension when something is being done to it, or does it need to be spooled up at constant tension?
In the former case, it seems like you could have two drums set up with constant-torque motors or clutches. The roving would be stretched at constant tension between these fixed-diameter drums, wouldn't it? Might need idler rollers to pressed the roving against them.
--
jiw

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yep, that's one very proper way to do it, and if the real estate inside the (existing) machine would permit, I'd add a shaft with a "windlass drum" running at constant torque. No room, though.
LLoyd
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On Jun 18, 12:21pm, "Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote:

A DC motor has torque proportional to the Current. The dancer arm could control the Current command to the motor, but with a fair amount of low pass Filtering so that it does not hunt. A DC motor and drive may well be cheaper, simpler and easier than mechanical brakes etc.
The split phase motor may not enjoy the constant stalled condition. The DC motor shouldn't care.
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Primarily, it would not enjoy it because they're (gear motors) usually fan cooled, and the fan wouldn't be running during stall.
But I forgot something important, anyway. Split phase motors don't mind being stalled OR operating at reduced voltages, but they don't develop much starting torque below a certain saturation voltage. Only a "bang- bang" control would work with a typical split-phase type motor.
A DC motor would be perfect -- don't have one that fits my gear reducer, but I can get one. A 3-phase motor would be OK too, with the dancer driving the frequency pot of a VFC.
LLoyd
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What's that Lassie? You say that Lloyd E. Sponenburgh fell down the old rec.crafts.metalworking mine and will die if we don't mount a rescue by Fri, 18 Jun 2010 12:21:55 -0500:

Could you find one that would produce 10ft-lb at 10 rpm, and gear it down
Or one at 5ft.-lb at 20rpm? Or one at 2.5ft-lb at 40rpm? Etc.
--

Dan H.
northshore MA.
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snipped-for-privacy@privacy.net (dan) fired this volley in

Yep... probably. Already dealt with multiplying the torque via belting or chain drive in another post.
LLoyd
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On Fri, 18 Jun 2010 12:21:55 -0500, "Lloyd E. Sponenburgh"

The torque of a DC permanent magnet (DCPM) motor is proportional to current regardless of speed. The motor won't overheat as long as it's operated within its max current rating, even if stalled.
If you could rig up a 10:1 speed reducer with timing belts and pulleys (zero backlash) then your torque requirement would be 2 lbf*ft at about 50 RPM. A DC motor capable of delivering 2 lbf*ft at 2000 RPM (a more usful speed for motors in general) would be a 90 watt (0.121 HP) motor. I don't think that'd be hard to find on the surplus market. Or, you could go with a DCPM gear, e.g. power seat motors, power window motors, windshield wiper motors, etc. Some windshield wiper motors run at about 50 RPM. 20 ft-lbf at 50 RPM is about 23 watts, which is not quite 2 amps at 12 volts. Wiper motors are rated for more like 5 amps.
Not all DC motors in automobiles are PM motors, though. Some are series-wound. They'd work, but they're a bit trickier to control because their torque is not linearly proportional to current.
Another possibility might be an inexpensive cordless drill operated from a current regulator. Even a modest drill can deliver 200 in-lbf which is 22.5 ft-lbf, and it can do that at low to stall speeds all day.
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horse power = torque (ft lbs) * speed (rpm) / 5240 5 rpm at 20 ft lbs comes to .019 hp. www.mpja.com has all sorts of small motors; most are rated by output speed and voltage which gives no idea of power. But a few show amperage also. You might buy one or two of the motors that looked close and some resisters to place in series with them to convert to a "fixed" torque motor, ie fixed current rather than fixed voltage.
The motors probably wouldn't last long but the education would be quick and cheap.
Hul
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snipped-for-privacy@kbrx.com fired this volley in

ACtually... I have a couple of Ampex tape spool motors rated at 30VDC 6.5A.
It'll take some time in the machine shop to adapt them (they won't take the overhung weight necessary on the main belting run) not because of the fundamental difficulty of doing it, but because of real estate limitations in the machine.
LLoyd
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