Pulley system for Lifting/lowering a folding desk

Projects General Guidance
1

How do I hold the position of a pulley. I want to turn off the stepper motor and have less than 1/4 inch of slippage.

I want to design a folding desk, wall mounted desk. The idea is that when the desk is closed it hide computer monitor and keyboard.

My basic plan is to have two stepper motor and a pulley system to raise and lower the desk. I want each side of the desk to be designed to raise 50 pounds. I expect the total weight of the desk to be less than 50 pounds.

Total movement will be 30 inches.

2

You are better off using a conventional motor with a worm gearbox - that will hold its position with power off as you can’t drive a worm gear backwards .

(You will need a gearbox which ever motor choice you have anyway ).

3

Do you have any links to gear box and motor that would work.

What type of positional control would be possible with a normal motor. Can I reliable make it do one or two revolutions?

4

You might want to try an electric linear actuator. They "lock" in position when they are not moving. They're available in different lengths and capacities and usually run on 12V.

Here's an example:

5

I had looked into using a linear actuator, but I don't have the room for 2x the distance I want to move.

6

Hammy is right a stepper is completely the wrong choice. Have you looked into the actuators that raise a car tailgate? Anyway, if you want to use pulleys yes a dc motor and worm drive gearbox. Ans sensors taht detect fully up & fully down. Why not tell us what this is for?

7

ductcord:
How do I hold the position of a pulley. I want to turn off the stepper motor and have less than 1/4 of slippage.

I want to design a folding desk, wall mounted desk. The idea is that when the desk is closed it hide computer monitor and keyboard.

My basic plan is to have two stepper motor and a pulley system to raise and lower the desk. I want each side of the desk to be designed to raise 50 pounds. I expect the total weight of the desk to be less than 50 pounds.

Total movement will be 30 inches.

My advice is to get all the physical components made and operate manually as you want and then investigate how to automate the design.

Paul

8

By using some pulleys the travel of a linear actuator can be multiplied, but you will loose load rating if you use that method. Used on arresting gear for aircraft carriers I believe.

9

With projects such as this the mechanical aspects are the hardest - you need to be able to build or adapt something that is out there to do the job . There is little for an Arduino to do in a project like this , a couple of limit switches and relays would do it .

If you know the load and the time in which you want the desk to close , then you can work out the power needed . Double that to specify your motor/ gearbox setup . Some form of low powered winch might do it or a tailgate lift as suggested , electric drill driving a worm gearbox , maybe the motor for a car sunroof - you’ll need to be inventive !
You could also use some form of counter balance to lighten the load .

10

johnerrington:
Hammy is right a stepper is completely the wrong choice. Have you looked into the actuators that raise a car tailgate? Anyway, if you want to use pulleys yes a dc motor and worm drive gearbox. Ans sensors taht detect fully up & fully down. Why not tell us what this is for?

What details are missing for what this is for?

I will ask you the same question I asked Hammy. Do you have any links for dc motor and worm drive? And what would be the minimum "step size" or revolutions one could expect for a dc motor. The other problem with a dc motor is that the load is going to change as the desk closes. I would like to keep a constant speed.

I'm not sure what you mean for Tailgate lift. Are you talking about a gas strut? I consider it but don't have space.

I considered a counter weight, but rejected that idea because the load changes as the angle of the desk closes. So I would have to switch from raising the counter weight to lowering the desk. The other problem is I want the desk to be heavy so that it will be more stable.

11

saildude:
By using some pulleys the travel of a linear actuator can be multiplied, but you will loose load rating if you use that method. Used on arresting gear for aircraft carriers I believe.

I considered that option. I don't like the additional moving parts. One of my other design options is to use a lead screw. I'm concerned that the lead nut would spin or bind up as the angle changes.

12

Try googling for wheelchair motors; I'd guess they use a worm drive as you dont want em rolling away on a slope!

Here's an example that would drive a pulley system.

I will ask you the same question I asked Hammy. Do you have any links for dc motor and worm drive? And what would be the minimum "step size" or revolutions one could expect for a dc motor.

A DC motor doesnt DO steps. It has rpm and torque which you would need to consider what parameters you need.

The other problem with a dc motor is that the load is going to change as the desk closes. I would like to keep a constant speed.

What details are missing for what this is for?

How much do you hope to spend?

Not sure why constant speed should be important. You can balance the load with a properly engineered take up spool.

I'm not sure what you mean for Tailgate lift. Are you talking about a gas strut? I consider it but don't have space.

No I meant a POWERED tailgate mechanism.

13

No links for suitable motors and gearboxes , you need to sort out the mechanical design as your requirements are very specific and without designing the system in detail or doing some experimental work , who can say .

14

Cool now that I know what to look for there are stepper motor with brakes attached

15

ductcord:
Ok, thanks for trolling my threaded with your half hearted suggestions to use magic pixie dust.

You might want to check your smirk at the door.
and

"Hey, you accidentally answered the actual question."

no, you did not ask a question.

go back and read your first post.

please define "1/4 of slippage"

and "Total movement will be 30 inches"

You have left out a huge amount of information and have not offered a drawing of your design.
I would use sliding rails with fixed length bars as the main support to hold the desk at level.
and only use the motor to open and close. eliminate the nonsense of trying to hold a motor or drive in a position. yes, nonsense. unless you expect it to have multiple positions, which you did not mention.

You are asking for selection of a device without any real information.

If your desk is a hinged table top that rotates from the wall to create a work platform, total weight of the platform and keyboard being 30 pounds. add to that your leaning, pressing, writing, coffee cup and what-not.

The hinge carries the bulk of the weight. the mechanical bits you want to add to open and close should be engineered. not cobbled together, but calculations and clever design to allow for additional forces not expected. (I may have the only overweight cat that ever jumped on a desk....)

The linear actuator located 6 inches from the hinge would be out of the way when extended and while the circumference of the arc of travel of 90 degrees is 30 inches, and your 19 inch table (doing the math) you only need a short stroke.

If you have cables that are 30 inches long , fixed at the end of the desk and you rotate 90 degrees, your 21.2 inch desk (again, doing the math) would open and close with different holding forces. or, did you mean that the platform would extend 30 inches ?

As was mentioned, you really have not offered much in the way of details.
As for your question. if you wanted to know how to hold a thing in position, then all the rest was not needed, a simple "how can I lock a thing in position with a 30 pound load ?"
would have sufficed.

sketches, crayon on paper and your cell phone would be more than enough.

We are here with the best intentions and try to do higher engineering without the whole project, details, scope, etc
I have not offered solutions on this thread as there was just so little information.

maybe you did not have enough coffee, or I had too much.....

16

no, you did not ask a question.

Very first sentence of very first post. "How do I hold the position of a pulley". Sorry the units got lost in the next sentence, but that has been fixed.

As was mentioned, you really have not offered much in the way of details.
As for your question. if you wanted to know how to hold a thing in position, then all the rest was not needed, a simple "how can I lock a thing in position with a 30 pound load ?"
would have sufficed.

I'm sure that if I had left out the details people would be asking for what application.

17

I’m out , what sort of help does this guy expect !

18

@ductcord

Inflammatory posts DELETED

Could you also take a few moments to Learn How To Use The Forum.
It will help you get the best out of the forum in the future.
Other general help and troubleshooting advice can be found here.
It will help you get the best out of the forum in the future.

19

ductcord:
Very first sentence of very first post. "How do I hold the position of a pulley". Sorry the units got lost in the next sentence, but that has been fixed.

I'm sure that if I had left out the details people would be asking for what application.

Most of the time, we ask the questions that help define the problem.
as you know we are all human and we all talk in different ways.
Not trying to beat a dead horse, but if you said you wanted to lift a 30 pound weight and then hold it in a position,...
most of us would say, use a worm gear. I think that was mentioned early on.
simple question, simple answer.
you might ask to think out of the box.... what else can you use ? If there is one thing I am certain, we are a VERY creative lot !
Yes, you can get brakes for motors, but they are more rare to find as they are more of a compromise for other problems.
maybe questions of speed ? worm gears are slow.
linear actuators are slow because of their gearing.
steppers loose position when they are shut off, and get HOT when powered. often we use an end switch to figure it out on power-up.
But, it sounds like we ruffled a few feathers, but you did get what you needed. and in the end, that is what we are trying to make happen.