Small electric motor for raising/lowering jack + bonus tinkering forums

July 7, 2016 2:41 PM

I'm using a motorcycle jack as the base for an adjustable desk I'm building to rest on my desk at work. I'd like to rig an electric motor to raise/lower the ~75lbs of the desk and it's contents. What are my options for small electric motors up to the task? Bonus - is there a good forum for general tinkering questions?
posted by pilibeen to Grab Bag (15 answers total)

Best answer: It won't take much torque to lift 75# with a motorcycle jack. Typical electric motors, however, turn too fast to direct-drive the jack, so you'll probably need a gear reduction of some kind. I'd start making some guesses about how fast (in RPM's) you'd like to drive the jack. Count how many turns it takes to move the jack through its full range of motion. Make a guess as to how quickly you'd like it to move through that range. That tells you an approximate number of RPM's you are looking for. It won't be enough to lead you directly to the correct motor, but it will dramatically narrow the field.
posted by jon1270 at 3:16 PM on July 7, 2016

Not sure if you can find one as cheap as an electric motor, but a linear actuator is basically a motorized jack. They are commonly used in electrically-operated recliners, and I've seen some DIY projects to use them for standing desks.

Or, how abou connecting a Rotisserie Motor to your motorcycle jack?
posted by reeddavid at 3:59 PM on July 7, 2016

Response by poster: I estimated I need a motor in the range of 400-500 rpm. How would I go about estimating the necessary torque? Thanks for the help.
posted by pilibeen at 6:47 PM on July 7, 2016

How about an electric drill? Pretty cheap, speed control, probably run slow enough to drive the jack nicely.
posted by straw at 6:57 PM on July 7, 2016

Response by poster: Using an electric drill was my first thought and a backup if I can't find a cheap dedicated motor solution. Wiring up something that's mounted to the desk w/ a switch sounds like more fun though.
posted by pilibeen at 7:16 PM on July 7, 2016

How about a car window motor? They're cheap and already geared down to the right speed.
posted by monotreme at 8:24 PM on July 7, 2016

Best answer: I would look at the problem in terms of power required. Let's say you want to lift the desk in 2s. The power required is the change in potential energy over time: mgh / delta t (34kg*9.8m/s2*0.5m/2s) = 83W. So I'd round that up a bit and select a motor around 0.125KW / 1/6 HP. Bear in mind that small motors are not typically all that efficient and the power consumption will be more than 125W.
posted by ssg at 8:25 PM on July 7, 2016 [1 favorite]

Best answer: The reason to look at it in terms of power is that for torque you need to know the properties of the jack in question and that gets into the properties of the hydraulics and that is going to be complicated.
posted by ssg at 8:40 PM on July 7, 2016

Best answer: Surplus Center has all sorts of gear motors ,you want a DC one so you can reverse the direction.
Gear Motors
posted by boilermonster at 11:27 PM on July 7, 2016

Also look at seat motors (used for moving your car seat back and forth). They are low-ish rpm with a strong worm gear drive. Generally take a square shaft, so not too hard to interface with the mechanics.
posted by ryanrs at 11:29 PM on July 7, 2016

Response by poster: Thanks for all the good advice.

Is there any reason I couldn't load up the jack w/ weight and put a torque wrench on it to estimate the necessary motor torque?
posted by pilibeen at 11:48 AM on July 8, 2016

Yes, that will work. I'd move it through the full range of motion you need, because the torque required may not be constant.
posted by ssg at 11:58 AM on July 8, 2016

And don't forget to adjust for the pulley ratio (assuming you are coupling the motor with pulleys and a belt).
posted by ssg at 11:59 AM on July 8, 2016

Response by poster: Looking at motors on Surplus Center - this looks to be a pretty close match.

What is the purpose of a hollow thru-shaft? Will that work for my application?
posted by pilibeen at 12:22 PM on July 8, 2016

Best answer: I guess it means it can drive a square rod socketed into it from the left or the right. The latter bit is so they can use the same motor and part number for the left and right seats which have mechanisms and motor mount points with mirror symmetry.
posted by sebastienbailard at 11:35 PM on July 8, 2016