Torsion spring vs Torsion bar 2

[ATLENG]

I have been researching torsion springs and torsion bars for a small spring loaded folding table (imagine a retractable changing table (10"x15") where the spring works to return the table in the stowed position. The table should hold about 4-6lbs (without retracting) and when these items are removed then it should return the table to the upright position. (see attached)

Torsion spring or Torsion bar?

 

[israelkk]

The calculated torque with reference to the hinge axis is at least ~73 lbf-in. It is not clear how much residual torque the spring should apply in the stowed position. Torsion bar may not fit because it has to be very long to be able to rotate ~45 degrees without yielding under the calculated torque.

If the space is limited a coiled torsion spring will be easier. Remember that in torsion bar the spring is loaded in shear therefore you use only 60% of the material potential. In torsion spring the material is loaded in bending/tension thereby utilizing the material full potential.

 

[ATLENG]

Ok, which leads into my second question, I have the spring that I figure will work however given the limited space in that area, i am having an issue with the torque transfer mechanism, (in other words how does the torsion spring transfer its work to the linkage arm? I have seen plugs and keys used in the pass or is this an issue for the spring manufacturer?

 

[israelkk]

Spring attachment or the shape of the ends is the designer responsibility it is always a good practice to consult the spring manufacturer if he can actually manufacture what you design.

When you select a spring you should consider the required life cycle from the stowed position to the loaded position.

 

[israelkk]

Looking again at the drawing I notices that the space for the spring is 10" long. Therefore, I believe you can use a torsion bar if it carefully designed.

 

[ATLENG]

What would be the challenges with locating the spring in the hinge part of the table locally. Keeping the support arms, and doing away with the torsion bar contraption below.

Basically a spring hinge loaded to the open position where the table is located. It seems to simplify things but I am unsure if the spring can accomplish the requirements in that location. Where M= (3lbs table) * 15" = 45lb in

I dont know how to factor in the slider friction, which will make it harder for the spring to return the table to the stowed position. What do you think?

 

[desertfox]

Hi ATLENG

I think your mechanism will jam if you try and place a torsion spring at the table hinge, ie there will not be
enough force to overcome friction during the rotational movement.
I did a very quick analysis which might help.

regards

desertfox

 

[desertfox]

hi ATLENG

Just an idea instead of a torsion spring why not use a tension spring mounted above the table and attached to the wall.

regards

desertfox

 

[ATLENG]

The table will be used often and quickly during flights and from what i understand from your suggestion of the tension spring mounted idea, the springs would run along side the table and attach above the table, this could cause interference with the overall operation, as well as limit the area from which the table can be used, from the front only, the sides need to be free and clear once the table is down.

 

[desertfox]

Hi ATLENG

Wasn't aware of those limitations, the operation would I think be okay ie it would work, however your other restrictions are correct.
You need to analyse the table at different positions to obtain your friction loadings and spring loading in a similar fashion to the calcs I posted

 

[TheTick]

Torsion springs exert torque (force * distance). Your spec should reflect that. If you want to hold 4 pounds, you need to specify the position (distance from axis) at which 4 pounds will be located. Think of what happens when the 4 pounds in 1 inch from the axis.

To conserve space, you may want to hold the spring in place with bushings to keep radial position. The twist of a torsion spring around an undersized pin greatly increases the amount of volume required for the spring to move freely. An alternative to using bushings is to use cone-shaped ends to get the spring ID to match the pin ID.

Two positions are enough to get your spring curve (i.e. torque at bottom and torque or preload at top). Also remember that the torque required to hold position is reduced as table goes up, since CG is moving closer to the axis.

I've designed torsion spring assemblies for car consoles. Usually I plot the spring torsion and moment of lid weight vs. angular position (angle from horizontal). Also plot ±15% tolereance for spring performance.

 

[ATLENG]

Thx Tick, Now that we know what type of spring we need, We are discussing the options of transfering the springs torque to the lever/leg arm. The challenge remains that the pin/bar has to remain fixed on both ends while the transfer assy rotates around it, the left end is easy to fix but the end leading into the lever arm is the challenge due to space.