right electric motor for this...?

good evening, and please let me know if i should post this to a different more appropriate group.
i'm looking for help with electric motors.
the goal is to raise a pole very very slowly, about 2-3' and then lower it back down again and then back up again. every cycle should be something like 1-2 hours.
i was thinking i can hang the pole from a rope, run the rope to a pully on the ceiling and then over to another pully on the wall and down, where it'll be attached to one end of a big gear. a motor can slowly turn the gear and as the gear moves around it'll pull the pole up and down.
the problem with this design is of coure the big gear.
can you help me with a better design? perhaps, are there any electric motors which run very slowly one way for a set amount of time or distance and then automatically switch back the other way and then back and forth? if such a motor exists it could be used to simply wind the rope.
thanks for any help you can offer. max
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Search 'stepper motor.'.. on google. That would drive an eccentric cam mounted near the secured base of the pole... round and round.. in one direction... fit so as to raise and lower the pole. You can buy those with gear reduction also to get a range of speeds (cheaper than variable speed drives)...
How heavy is the pole?
drive would have to be fit so that wind and forces from the pole would not put inappropriate mechanical loads on the motor shaft. A loose roller arrangement would work for that.
If you want it to last be sure to get motor rated for the load, a larger shaft and with ball bearings, my guess in the 1/2" shaft size or larger due to the weight of the pole. If its more than a few lbs it the motor shaft should be extended with a flex drive coupling and suppored at both ends independent of the motor
Phil Scott

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There are gear motors (mostly DC) which are suited to this and they come in various sizes. All that is needed is a limit switch, top and bottom to reverse motion
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Don Kelly @shawcross.ca
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Phil Scott wrote:

You will need some sort of mechanical advantage, such as a reduction gear or block and tackle. How much load on the pole, and how much side force, etc. The right device for a display model of a dam spillway will not be the right device for a damper on a blast furnace.
--Dale
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the pole will weigh about 30 lb. and there shouldn't be any side force,
although the pole needs to hang from the ceiling.
can a limit switch be put on any motor? how is it applied? can it be "programed" or set to wind a certain amount and then reverse and then back again?
if so, perhaps a spool on the shaft of the motor to wind the rope would work best.
i take it there are no slow motors, hence the need for the reduction gear. what is a block and tackle?
also, dale, what would be the right device or a damper on a blast furnace? in a separate project i've been thinking about, i need to make a device that will open and close a duct based on temperature - this sounds like a similar thing.
max
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snipped-for-privacy@gmail.com wrote:

    Well, the dampers I've seen on blast furnaces (only once, many years ago on a tour) looked like they weighed tons, and they got moved by huge hydraulic rams. For a 30 pound load, I find a low RPM motor such as

http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?xi=xi&ItemId11756316&ccitemor
http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?xi=xi&ItemId11576821&ccitem
Attach that to a worm gear which then drives a rack and pinion to raise and lower the thing. There ought to be a controller in the grainger catalog to match the motor.
--Dale
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wrote:

not at 2 rpm per hour. maybe 2 rpm per minute at the slowest. to get 2 rpm per hour you will need a stepper motor. imo thats without blowing a lot of time on research... the OP needs to learn how to use google.
.
on the duct issue the OP mentioned, any air conditioning supply and Grainer has motorized duct dampers in the 12" diameter to 4" diameter range for under 50 dollars usually, 24 vac. You need a 40va rated transformer to go with it.
Phil Scott

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Phil Scott wrote:

Huh? RP*M*! M as in minutes

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You gave no information as to the weight of the pole. However at the speed indicated, the HP may well be quite low.
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Phil Scott wrote:

Now I see where you got confused on rotation. He wants the raise/lower *cycle* to take something like 1-2 hours, not the motor to rotate 1-2 times per hour. The pole is to go up 2-3 feet, then back down the same distance. In rough numbers, he wants to move the pole at a rate of 1/2 inch per minute. An 8 RPM gearmotor (# TM01MTR4540) from http://www.herbach.com/ for $23.95 driving a 16 TPI linear actuator will do it, but the actuator would need a 30" + travel. Expensive to buy, but he can make one, cheap, if he's clever. Still, the actuator needs over 30" of horizontal or vertical space, so even if he can do it and it's cheap, the space it takes or the appearance may make it a no-go.

Do you have a url for such a motor? What kind of money for it and a controller? Perhaps it could direct drive a 5" drum. That seems more attractive than a 30" + linear actuator.
Ed

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Who says that the motor needs to rotate continuously? How about using a garden variety gearmotor and pulsing it once per minute, for example (or more frequently for a smoother motion)? With a high mechanical advantage, it is unlikely that the load will move when the motor is not energized, especially if you use a worm gear.
Ben Miller
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Benjamin D. Miller, PE
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Hi Ben... I think the problem with the garden variety gear motors is that most are driven by 2. 4. 6 or 8 pole AC motors the 4 pole running at 1750 RPM... the 8 pole at half that speed.
pulsing power to those driving a gear motor you would get gradual but imprecise control... thats no doubt what brought the stepper motor into existence... these are designed to do exactly what you are suggesting.. with no gear box in most cases.
also with the pulsed approach to a standardly wound motor you would be getting locked rotor amps through the contactor at each pulse... and of course the wasted residual flux. thats probably why its not done. Maybe with a shaded pole motor the approach you suggest would be more viable...but that takes us back to various forms of stepper motors.. built specifically to do that.
Phil Scott

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----------------------------

And what sort of inrush currents would you get from a stepper of the same power rating? Similar transient electrodynamics exist. Note also that many gear motors are series DC motors. Possibly a hydraulic system with a small continuously operating pump would be as effective.
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I think some of your comments assumed I was talking about "pulsing" an induction motor as you would a stepper. I was not. I was referring to just running the motor for a short time once per minute (or 30 seconds, or whatever works). A gear reducer could easily give you 1 or 2 rpm if necessary. Feedback via an interrupter or other pulse generator could tell you how many shaft revolutions it makes, if he needs that precision. Over a two hour period, this would have a similar effect as slow continuous operation. It may or may not be an acceptable motion. Only the OP can tell us that.
Ben Miller
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He wants two revolutions per **HOUR...and gradual...so he can waive a pole up and down. ..slowly, twice over the period of an hour.
that gear ratio would be 100,000 to 1... you wont find it on the market.
Feedback via an interrupter or other pulse generator could tell

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Here is his original specification:
"the goal is to raise a pole very very slowly, about 2-3' and then lower it back down again and then back up again. every cycle should be something like 1-2 hours."
He is specifying a stroke. There is no mention of "revolutions", as this depends on how you mechanically link the pole to the motor. Add a 24"-36" leadscrew of appropriate pitch to a 1 RPM gearmotor, and you can easily get that stroke rate! Use a VFD if necessary to slow down the motor. I am just pointing out that there are options besides a stepper, and they could be less expensive when you consider the cost of a stepper motor and the drive electronics. A stepper is of advantage if you need to precisely position or make small changes. He doesn't.
Ben Miller
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B. MILLER ENGINEERING
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amazing.... truly. This has to be our final conversation Ben.... I will leave you with the notion of the crankshart, rod, wrist pin and piston. An example of a rotary / 'stroke' motion interface.
and please note once again its 2 or 3 revolutions per HOUR

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