sizing 12 v motor to raise LCD TV

...with the correct gearing, just about ANY motor could be used, although your 10-second time-to-rise might not be met.

...it's motor torque and speed through gear-reduction (possibly a scissors-jack arrangement?) that you want.

...something to remember: DC-motors will yield more torque instantly, but AC-motors will be less costly.

There's a suitable circuit for reversing the motor on this thread.

For your limit switches, I'd suggest using a roller lever microswitch.

Too late tonight to start looking up motors & gearboxes (0:18am Monday my local time).

I bought an inexpensive motor operated car jack a while ago. It operates from 12 volts. The range of movement may not be enough for your purposes but you might be able to use a lever to extend the range. The jack has built-in limiting switches for up and down. Think it cost about 50 bucks Canadian.

Dual threaded rods with a single motor driving both rods simultaneously should do it. Adjusting the size of the motor pulley in relation to the pulleys on the threaded rods will allow poer change and timing.....use limit switches to control upper and lower positions.

I like this idea. I gather the shaft of the motor would have to be inline for a direct connect or parallel with a belt connecting to the threaded rod. I'm imagining a motor with a 1/2 in threaded shaft coupled to a 1/2 in threaded rod.

A 12v motor with a rheostat so I can slow the rise of the TV at appropriate moments.

I'm still having trouble knowing how to size the motor.

Also what would keep the weight of the TV and shelf from pushing itself back down from the forces of gravity and turning the motor in reverse after I shut it off? Would there be enough friction in the system to prevent this from happening?

Thanks for your input

Even a fancy lead-screw with ball races for moving the load (a ball screw) is very difficult to back-drive. A "normal" threaded rod with a nut connected to the load would be as hard to backdrive as trying to turn a machine screw by pushing a nut along it.

Unless you need to slow the motor for dramatic effect, I'd forget the rheostat - or any other speed control.

I like ball screws, however when I looked them up I got this reading:

If a Ball Screw is used to support a load against gravity, a rotational brake will be needed to prevent the load from rotating the screw and lowering the load.

But again how do I size the motor? I can guess and use a 110v 3/8 in drill to turn the threaded rod, but it looks bulky and awkward, particularly since I want to be able to sit back, press a button and up it comes. Maybe this is the way to go. 2 right angle drills on a rheostat or use variable speed drills and remove the variable speed switch to a remote location to power the drills from my sofa.

Since I am using ball screws I could probably use 3/8 in rod

"If a Ball Screw is used to support a load against gravity, a rotational brake will be needed to prevent the load from rotating the screw and lowering the load" - agreed. We make a 3-axis step motor driven sample changing m/c carrying samples up to about 2kg. Our earlier versions used a ballscrew, and would almost run down, backdriving the motor through a gearbox, under it's own weight with the motor power off.

Anyway, to your motor sizing. If you're going for a leadscrew, the torque (T) required to lift the mass (M) is given by:

T = P x M x g / 2 x pi x E

where P is the screw pitch, g is acceleration due to gravity (32 ft/sec/sec or 9.8 ms^-2), and E is the leadscrew efficiency.

For a ballscrew, E ≈ 0.85, and for an ACME tread, E ≈ 0.35.

(We'll neglect the torque needed to accelerate the system, which includes the inertia of all the rotating components plus the load, because at the speed we're talking about, it will be very small compared with T above).

Having got the torque, it's time to go to motor manufacturers catalogs & look for a motor/gearbox combo which will give than torque at the speed you want.

The linear speed will be P x (leadscrew rpm), but be careful of the units.

Most small motors will be running efficiently at something over 1000 rpm, so you'll need quite a lot of gearing in there. Remember to factor in the gearbox efficiency when sizing the motor.

Regarding braking, it's easier to use a less efficient leadscrew (increasing the motor power accordingly), and maybe short the motor leads when stationary (to give regenerative braking), than to muck about with electromagnetic brakes.

Good luck.

PS If you pm me your e-mail address, I can send you a .pdf which contains all the calcs for sizing motors, using a variety of drive methods.

Use normal threaded rod, no brakes required, keep lightly oiled. Use 4, one in each corner, all driven from the same motor via a toothed belt.