Gears/Pulleys/Torque/RPM

[RobustZA]

Kindly help.

I am looking to build a linear actuator to make a lifting device for a television. The weight of the television and the stand are about 30kgs.

I have a motor with the following details:-
12V DC
120W
8mm D-Shaft
RPM No load 3200rpm
RPM with load 2500rpm

What gearing system would I need. I am looking to use a ball screw with a 10mm diameter and a pitch of 5mm.

 

[RobustZA]

As far as gear ratios go ... if I attach a gear to the drive shaft of the motor and a gear to the bottom of the ball screw I think I would need a gear ratio of ±1:2 to get the rprm right, but would this give me sufficient torque? Would the ball screw add to the torque?

 

[CWatters]

How fast do you want it to move?

 

[CWatters]

The force required to lift the TV slowly is 30kg * 9.81m/s/s = 300 Newton's approx.

For safety I would de-rate the motor to 60w.

Power = force * velocity
so..
Velocity = power/force
= 60/300
= 0.2 meter per second

To go faster needs more than 60W.

If the pitch is 5mm then 200mm/S is equivalent to 200/5 = 40 revs per second.

The loaded speed of the motor is 2500 rpm whic is about 40 revs per second.

So 1:1 gearing or direct connection might work.

Is 200mm/second too fast?

If you gear the motor down 2:1 it will go up at about 100mm per second and consume about 30W.

Peak/starting load currents might be higher.

 

[CWatters]

PS I would test it with a dummy weight before trying it with the TV. It might go down much faster than you expect.

You might consider adding a sensor to detect and control the speed with feed back loop.

 

[RobustZA]

Thanks, I was thinking that 100mm/s was a good speed to work on so a 2:1 gearing would be fine. Also I was planning on including a limiting switch ±150mm before reaching the top to slow down the speed and then a final limiting switch to end the run.

How would one de-rate a motor?

 

[CWatters]

De-rate just means not operating it at full power. Thats achieved by gearing it down as you propose. Eg When you gear it down the TV moves slower so it requires less power to move against gravity.

There is quite a lot of uncertainty in the calculations because friction in the ball screw drive, bearings and gears is unknown. We have also ignored the extra power needed to accelerate the TV from stationary. I think it should be fine but I would measure the current to check how much power it actually draws. Most motors can cope with short overload so I wouldn't worry too much if it approaches or briefly exceeds 120W at start up.

 

[RobustZA]

Great ... thanks for your help. This confirms all my thoughts on the matter. I was also thinking of including a potentiometer in the circuitry to fine tune the speed, but to let the gearing do most of the work.

 

[CWatters]

I think it would be worth designing the circuit so you can change the up and down speeds independently. That might be as simple as putting diodes in series with the resistors. The resistors may need to be high power versions.

Sorry if you know all this stuff already.

 

[jrmichler]

Ball screws have very little friction. When you turn the power off, it will fall. Better to use an Acme thread screw so that it self locks when the power is off. McMaster-Carr (www.mcmaster.com) has a huge selection of Acme threaded rod at reasonable prices.

 

[RobustZA]

I think it would be worth designing the circuit so you can change the up and down speeds independently. That might be as simple as putting diodes in series with the resistors. The resistors may need to be high power versions.

Sorry if you know all this stuff already.

Thanks again. I have a mate who is an electronics boff who will help me with the circuitry thanks for the advice as I didn't think of that at all.

 

[RobustZA]

Ball screws have very little friction. When you turn the power off, it will fall. Better to use an Acme thread screw so that it self locks when the power is off. McMaster-Carr (www.mcmaster.com) has a huge selection of Acme threaded rod at reasonable prices.

Another thing that I didn't consider. Acme threaded rod was my first thought when I started this project but when I stumbled across the ball screws I thought the low resistance factor would work in my favour. I didn't consider that the weight of the TV assembly would push the motor backwards when not running. Thanks for that.