Finding Solutions – U Spring

The problem

Our regular supplier who we had been using for some years was no longer supplying a U-Spring that we used in a testing instrument build. A new supplier was required.

Process

We provided specification drawings to potential suppliers, received quotes, and placed orders.

Upon receipt of the new parts these were tested as part of our goods-in process. The U-springs were found to be deflecting significantly more than the old ones.

This was a problem as the U-Springs form the basis of the Mooney rubber testing process. When the calibration force is applied and a test is run, the instrument mechanism should deflect the U-spring by 0.10” our new springs were giving us readings of around 0.15”.

There were several things that could cause a difference in deflection:

Material – the old supplier may not have made a change to the drawing years before we inherited the job and never fed this back. Without any Part Material Inspection testing on site, we could not eliminate this, but the similar density and appearance made it unlikely.
Forming – The manufacture of the spring could put residual stresses into the surfaces that resist deformation. The old springs looked slightly overbent and the edges were also scalloped which would affect the 2nd moment of area (moment of inertia).

We decided the best action would be to see whether we could match the measured deflection of either spring in our finite element analysis software and manufacture the part ourselves.

The spring was modelled to drawing and then constraints and forces were applied.

The simulated spring matched the new spring.

Once a size of spring had been ascertained in a basic model, with slightly unrealistic constraints, it was modelled again in a more representative simulation. The portion of the instrument where it sits was taken from CAD. This allowed us to check the effect of adjusting the position of the u-spring in the throat (hence adjusting the moment on the beam end of the spring).

This gave us confidence that were we to design a spring of suitable deflection in the FEA software, we could replicate this result in real life.

Solution

It was decided to keep the thickness of the material the same, as this suited supply and fit to instrument, but to increase the width of the flange, and thus the moment of inertia and resistance to deflection iteratively until the correct result was achieved.

A prototype part was created and allocated a number. A Works Order was raised with Operations to review the product after every stage of manufacture to test deflection pre and post heat treatment (hardening). Hardness does not affect deflection (technically it does, but in a highly localised way), but we were worried about the part’s changing shape or springing back during stress relieving. To this effect we tack welded some laser cut braces into place.

Prior to this however we performed a deflection test.

The results of the deflection test correlated very closely with the Finite Element Analysis (within a few percent).

We performed the same test after stress relieving and found the same results. The part was then tested on the instrument and achieved the desired results.

Outcome

The U-spring part could now be produced in-house ensuring guaranteed quality, continuity of supply and suitability to instrument build.

This part also proved our ability to solve problems quickly using theory and the FEA software available to us and to put the theory into practice in a quick and efficient manner.

Finding Solutions – U Spring

The problem

Process

Solution

Outcome

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