By Tom Little, Sales and Engineering, on May 10th, 2021.
Through the years I’ve worked with many different manufacturing companies in Automotive and Office Furniture. When working with the companies, there was a torsion spring design that was consistently discussed.
Torsion springs can be used in a multitude of ways, but the design that was most discussed among the Automotive and Furniture industry was a torsion spring design that magically floats in the air and never contacts the pivot pin. The reason they do this is twofold and I fully understand why they do it. Here are the reasons why:
- Convenience: It’s convenient where the centerline of the pin matches to the centerline of the coil spring.
- Position Awareness: If the designer puts the spring in a better position, how do they know that the position is correct?
I’ve spent a lot of time working with designers that didn’t understand this concept, so I will attempt to explain it and the risk involved if you let the magical torsion spring float in free air without contact with the pivot pin.
Typically a torsion spring must have three accurate points of contact. (see below) There are also pinless designs that must contact 3 points to work correctly, but the design can be debated. I was a skeptic for many years until it was explained to me on a deeper level. I’ll do another blog post about that another day…
Above, is a picture from a CAD model from a customer that I received from their designer. I explained the pin shift concept and understandably, it took him a while to get the concept.
The torsion spring will always try to move back to its original free position with the force of the spring moving in the blue arrow direction (below). To position the pin in the correct direction, Apex typically does a bi-sect of the installed angle and moves the pin against the ID of the spring.
To minimize the pin shift in this scenario, you need to do two things:
- Increase the pin O.D.
- Reduce the I.D. of the spring.
This has some major pitfalls, since there is an indexing problem (manufacturing), and stress calculations that are affected by the amount of wire in the spring. Space is also an issue to be aware of.
Here is the spring in the correct position. (Left) The other major pitfall to the designers is if you don’t do the pin shift, the installed angle (in this instance) was 18° different! Doing 30G crash analysis based on the inaccurate data will put a door handle and the latch system in serious jeopardy (Low Torque Value) of having a system failure and may cause some serious problems from a crash of a vehicle.
The ‘magical’ torsion spring concept can be difficult. It takes a team of expert engineers to ensure the success of the concept. Apex Spring and Stamping is equipped with CAD System software that can accurately position the spring. Details as meticulous as this should not be left for questioning.
Connect with one of our experienced engineers to see how Apex can help. How Can We Help?