Torsion spring vs Torsion bar
Torsion spring vs Torsion bar
(OP)
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?
Torsion spring or Torsion bar?





RE: Torsion spring vs Torsion bar
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.
RE: Torsion spring vs Torsion bar
RE: Torsion spring vs Torsion bar
When you select a spring you should consider the required life cycle from the stowed position to the loaded position.
RE: Torsion spring vs Torsion bar
RE: Torsion spring vs Torsion bar
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?
RE: Torsion spring vs Torsion bar
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
RE: Torsion spring vs Torsion bar
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
RE: Torsion spring vs Torsion bar
RE: Torsion spring vs Torsion bar
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
RE: Torsion spring vs Torsion bar
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.
RE: Torsion spring vs Torsion bar