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Offline acdcTopic starter

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ac motor
« on: April 04, 2013, 03:14:48 PM »
Hi,
Trying to replicate an automatic 12v car jack (youtube) but clueless what 110 ac motor to pick.  Looking for high torque, reversible, slow rpm (like an auto car seat moving vertically), load 100lbs.  I have a hand crank car jack and thinking of motorizing it to lift vertically a box that's about 100lbs.  Lift height approximately 6inches.  Preferably a very quite ac motor & avoiding an ac-dc adapter if I opt for a dc one.  Unit will be housed in a cabinet.  Eventually will be needing a timer for the gizmo.  Appreciate if you guys can give me some lead.  Thanks.

Offline newInRobotics

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Re: ac motor
« Reply #1 on: April 08, 2013, 03:07:59 AM »
Any particular reason why You want to go for AC motor? DC motors are easier to control, also, by going DC you isolate AC input from DC output making it safer to work with.
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Offline jwatte

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Re: ac motor
« Reply #2 on: April 08, 2013, 10:52:59 AM »
I would go for a suitable industrial AC/DC power supply, and a suitable 12V or 24V motor with very high gearing. Pololu used to have some 1000:1 gearmotors, but I can't find them now. Others will have them, I'm sure.
Here's an example of a 24V power supply that you can integrate into the cabinet: http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_123450_-1
Also, what is the lifting mechanism? Are you still using a car jack, with a worm gear/screw?

Offline acdcTopic starter

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Re: ac motor
« Reply #3 on: April 11, 2013, 04:29:35 PM »
Hi guys,

Appreciate the comments/questions/and link.  Been out awhile due to workload, apologies for the late response.

(NewinRobotics)  Opting for an ac motor cause just to avoid an adapter.  Wouldn't mind going dc if it offers more advantage. 

(jwatte)  Will be using the worm screw still.  Hand cranked the jack with a 100lb load one night, and that took some effort.  The jack did maintain its lift position without bowing or collapsing though which is good.  Need to do a longer test to confirm that that will be the case if I leave the load in the up position.

Can you recommend a quite dc motor?  I'll be housing all these in a buffet cabinet and would like to make it as "stealthy" as possible. 



 


Offline jwatte

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Re: ac motor
« Reply #4 on: April 11, 2013, 04:43:48 PM »
Quote
Can you recommend a quite dc motor?

Quiet makes it significantly more expensive, unfortunately.

Mostly, it's the gearhead that makes noise in my experience, so I would look at a strain wave ("harmonic drive") gearing if you need it to be super smooth.

Offline acdcTopic starter

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Re: ac motor
« Reply #5 on: April 11, 2013, 10:13:57 PM »
Your right, lots of $$$ for a harmonic drive.  Too much for my pocket.  For starters, what dc motor spec would you recommend?  Something with a good torque to turn the jack, reversible, and something that can be connected to a controller.

Offline waltr

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Re: ac motor
« Reply #6 on: April 12, 2013, 04:58:50 PM »
good torque??
What torque numbers will turn the jack?

There may be some gear motors used in automobiles that may work and not cost too much. Especially if obtained from a junk yard.

Offline jwatte

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Re: ac motor
« Reply #7 on: April 12, 2013, 09:12:26 PM »
You don't need a lot of torque in the motor itself, necessarily, if you can get a gearbox geared high enough and are OK with waiting longer. The main thing to do is to match the strength of the motor with the friction and losses of the gearbox so you don't get an entirely unreasonable combination. (I think this is why most gearheads are in the 30:1 - 300:1 range, and 1000;1 is kind-of hard to find.)

Without knowing the equivalent gearing of your jack / worm screw, there's no way anyone can calculate whatever motor you need.

Do this:
1) Put your expected load on the jack.
2) Get a foot-long handle for the crank on the jack.
3) Get a spring scales (the kind used to weigh caught fishes at sea, say)
4) Attach the spring scale to the end of the handle.
5) Pull on the scale until the handle just barely moves.

Now, multiply the "weight" you see on the scales, by the length of the handle from center of rotation to spring scales attachment point.
The product you get is the torque you need to move the jack.
Now, get a motor whose stall torque is at least 2x that amount, for intermittent duty, or 5x that amount, for more continuous duty.
« Last Edit: April 12, 2013, 09:15:01 PM by jwatte »

 


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