Ok so I briefly talked about torsion springs and with compressions springs they can be one of the most vital components in your pocketqubes and cubesats. Torsion springs are useful for solar panels, hatches, any other mechanisms that have rotational movements.
Material.
Similar to compression springs torsions come in a variety of materials and coatings. Since we are talking about cubesats and pocketqubes it is best to go with stainless steel since it has good corrosion resistance.
Mandrel size
Some suppliers will indicate the mandrel size the torsion spring is designed to fit around. To prevent jamming it is best to pick the right spring size for the mandrel.
Angular travel
Torsion springs can come in a wide range of sizes and opening angles in which you need to keep in mind when designing your mechanisms.
One of my recommendations when using torsion springs is to use one’s that have an opening angle that is larger than the maximum angle of your mechanism. For example, if you want to open a hatch at 90⁰, select a spring that opens at 180⁰ and if you want to have a solar panel open 180⁰ have torsion springs open at 270⁰. Then incorporate a hard stop in your hinges so that the mechanism only opens at the desired angle. This is to ensure your mechanism is always preloaded and to prevent it from bouncing back. This is because as the torsion spring loses torque as it’s travels.
Orientation
Torsion springs come in two orientations, left and right so if you are using two springs, make sure you have one on each side.
Calculating force
Calculating the force from a torsion spring is more complicated than a compression spring as you need to carry out multiple steps.
To calculate the force of a spring you first need to calculate the bending stress which can be calculated from Equation 1, Equation 2, Equation 3 and Equation 4.
Equation 1 Bending stress
Equation 2 Stress correction factor
Equation 3 Spring index
Equation 4 Average diameter
Once you have calculated the bending stress you can calculate the linear force acting onto the spring by using Equation 5.
Equation 5 Linear force on spring
You can use the results of the linear force calculations to calculate the torque of spring from it’s displacement. This can be done in two ways one by using t=Fr where you use the linear force and length of the spring or total deflection and spring rate of the spring.
Spring calculator
To make things simpler a torsion spring calculator (Figure 1) has been added to the mechanical calculations DLC which can be downloaded in this link.
Figure 1 Torsion spring calculator
Just like the compression spring calculator all you have to do is fill out the yellow cells and the calculator will provide you with the bending stress and forces based on displacement and the design of the spring.
If you need assistance in your pocketqube or cubesat mission please don’t hesitate to contact me at Andrew.dunn@wyrmengineering.com