Why do different companies get different load test results when testing the same batch of springs. Having been asked this question we decided to look into it a bit deeper and here’s what we found…
After carrying out a standard load test on a compression spring we found the results recorded were different from those of the spring manufacturer (using the same test equipment), the end customer had also recorded a different set of values. Although all three sets of results were within the load at length tolerances for the spring design, the variation had, unsurprisingly, raised concerns on the part of the end customer.
Many spring manufacturers will appreciate there is a natural non-repeatability in springs such that, every time they are tested, the result can be slightly different, but many companies struggle to understand why this happens. There is also the matter of tester to tester variation; all of this raises the question as to what would constitute a natural variation, and what would become a significant difference – in other words, which set of results truly reflect the performance of the spring.
The variation in spring load test results is made up of four main factors: Length Measurement Effects, Load Cell Effects, Machine Calibration and Spring Effects. Let’s explore these a little further…
Length Measurement Effects
This is the error produced by the length measuring system and the load frame deflection. All measuring systems have an error, and different test machine manufacturers may use length measuring systems with different degrees of accuracy. For example, the length encoders used by us for its range of test machines have a resolution of 0.005 mm and are accurate to ±0.003 mm, but that accuracy degrades when they are bolted into a testing frame due to imprecise deflection compensation and nonlinear deflections such as those from bearings. Hence a real-life accuracy figure on our machines is ±0.01 mm. On a high rate spring, 0.005 mm resolution can produce significant load variation.
The machine datum or zero setting can also introduce errors if the platens are not parallel or if they are worn/damaged. A high rate spring placed offset from the machine center can cause the frame/load cell to deflect differently and hence introduce a length error.
Load Cell Effects
The type of load cell used in spring testers is important because many types of cell are badly affected by non- axial forces and moments. Irrespective of how careful the load cell selection has been, all cells will exhibit some effect to a greater or lesser extent. It is therefore good practice to locate springs centrally on a test platen. It is very difficult to quantify load cell effects, but they can be very significant and cause several percent differences in load measurements.
For spring testing, the machine load cell should be accurate to ± 0.5% of reading or better. This means that, if the machine used by one company was reading high and that of another company was reading low, a difference of 1% of load could be seen.
Human error could be a factor here whereby length systems are left un-adjusted or calibration not completed, in these cases we have seen errors of greater than 0.1 mm.
End squareness has a considerable effect on the spring load as turning it upside down will usually give a different test result. This means that, unless the spring design is such that it will naturally only be tested one way up (e.g. for conical springs, beehive springs, springs that have a number of closed coils only at one end, etc.), it is advisable to test both ways up. Springs should also be placed in the same orientation on the test platen (i.e. with the end tip at the same position) in order to minimize possible variations.
Springs that are laterally stiff produce high side forces that will impact on the load cell and frame (as mentioned above). Repeatability and reproducibility tests (R&R) assume that the test pieces are absolutely stable, and all variation is due to the machine/operator, but spring effects can make R&R tests appear very poor even though the machine is good.
Springs themselves are not always stable items (design dependent) and are subject to recovery when left unloaded or relaxation when stored compressed. Hence loads may appear to increase or decrease simply because of the delay between testing at each company.
… There would be no way to quantify what would be a natural non-repeatability in spring load as it dependent on the design in question. The variation would only constitute a significant difference when that variation falls outside of the load tolerance for the spring.
We hope you found this article interesting, if you have any questions relating to the material please get in touch.