Torsion spring size

[Phizinza]

I have a 50mm square tube arm approximately 600mm long coming off another piece of square tube (sitting horizontal) pivoted with a 14mm pin. At the end of the arm there will be about 25kg. The arm will sit vertical which is 90 degrees, and it will swing down to about 190-200 degrees. What I need is a torsion spring that will take 15 to 20kg of the weight so it is easy enough to lift back into place (vertical). What size spring would do this?
Thank you.

 

[desertfox]

hi Phizinza

Suggest you look at this site:-

http://www.roymech.co.uk/Useful_Tables/Springs/Springs_Torsion.html

failing that why not give a spring manufacturer a call.

desertfox

 

[PaulPepka]

I have a spread sheet I use for things like this (ramp lift assistance spring) I would be glad to try it for you. Let’s see if I have the correct assumptions.
The 15-20 kg you want to counteract is mostly weight at the end of the 600mm long arm, or is it more spread evenly along the 600mm?
Is it okay if the spring has almost no force once the arm is up or near up, or do you need the spring to hold the arm vertical?
You want the spring to fit over the 14mm pin.
Your arm will rotate about 95 degrees down (just making sure I have the same ordinates as you).
Correct any of the above.
Other questions:
If this is a low quantity item, you might be better off money wise to use a simple fix. I agree a spring is a clean fix, but it can be a bit expensive. Stock (on the shelf) springs often don’t come in the size – torque you likely need.
For a general solution I can tell you that...
Things don’t balance out perfectly. Torsion springs will provide linear assistance, but your weight will be non linear. The torque of your weight adds slowly, but increases fast, but a spring just builds at a constant rate. Often I find the best solution if the torques balance when the ‘arm’ is about 30 degrees from horizontal.
Some more basic alternates would be to extend you rotation arm in the opposite direction and add a counter weight. As in add 60mm in the opposite direction and add weight to it until your ideal is found. This won’t work if it’s hinged at the ‘floor’, nowhere for the weight to drop.
Another alternative is to put a pulley off to the side from your hinge point, you could run a cable from your rotation arm, around the pulley then to a spring anchored to a point (on your horizontal arm?). Then you can experiment with an off the shelf compression or extension spring to provide lift, changing your attachment point on your rotation arm and your anchor point and springs to get what you need.
For any of these options, I can still give you a torque estimate, based on the questions I asked above.
Paul

 

[Phizinza]

Sorry for the very late reply, I appear to have skimmed over my email notification.

You are right about the weight on the arm that it is mostly at the end of the arm.
Once up there doesn't have to be much force on the arm, only enough to stop it moving (rattling).
Yes the spring needs to fit over the 14mm pin.
Indeed about 95 degrees is right.

Thank you for your other suggestions. The placement of the arm doesn't allow for a counter weight and a pulley system wouldn't suit. I do realise getting a spring may most likely be getting one made. But it would be the most elegant solution.
I understand what you are describing about the non-linear weight. Being it only has to assist the weight of lifting and not counteract it altogether I think it will still work.

Thank you for your help!

 

[PaulPepka]

In my off time, the best design I have so far is this: It meats your target but is a ery large spring. Let me know what you think. I work in inches and pounds but if we are close to what you want, I can provide a graph/chart in metric.

The mounting position can be hard to work out in words.
What I have so far would be a spring that starts free position with 16 and 1/8 coils.
10.3mm wire diameter of Oil tempered wire.
62MM Outside Dia.
Length needed over pin would be about 185mm

You would install it by bending it almost 1/8 of a turn. Torsion springs are supposed to be wound as if adding more coils not taking out coils. So this means your 1/8 turn now brings the spring to almost 16 and 1/4 of a turn. The fixed end of the spring you’d be 'pointing' in the same direction as your arm would fall. This spring arm does not need to be very long.
The installed torque would but enough torque to hold your bar vertical.
Lifting pull forces below are at the far end of the 600mm bar.
It would take about 5kg force to start pulling it down. The weight of your bar would start adding up. After about 17 degrees it nearly would balance. At 20 degrees from level it has the maximum force you need to lift it (or hold from dropping) of 8kg.

All the way down it would have about 3kg of force to start the lift?


If the size of this spring is a problem, I am looking at an off the shelf torsion spring, but its way under sized, and would require a 'bank' of 5? or 6? springs mounted to the pin with some way of reaching the several springs the catch the arms and provide the torque needed. This can go may ways in design and springs usage. If you wanted this instead do you have an idea how you would want to mount them?

Let me know how I can help Paul

 

[Phizinza]

That was very well explained and very helpful. I am a little taken back at how large the spring needs to be though. Perhaps the torsion spring is not the solution as I only have 50mm of room on the pin.