A load–deflection graph is the spring test in one picture. Deflection is normally plotted on the horizontal axis and force on the vertical axis. The shape tells you more than any isolated reading.
The axes and the datum
The horizontal axis should state exactly what its coordinate means. For STM-X automatic tests, test displacement is the nonnegative magnitude from the accepted preload datum. The vertical axis is the measured force magnitude in the selected unit.
An absolute machine position may be retained for traceability, but it should not be relabelled as spring deflection. That distinction prevents a fixture offset or change in machine origin from becoming a false change in the spring.
Five regions to inspect
- Approach and seating: the fixture contacts and stabilises the spring.
- Early travel: end conditions settle and the curve may not yet be linear.
- Working range: the intended measurement interval, often used for rate calculation.
- Target region: the maximum planned force or displacement.
- Return: the unloading path shows recovery and energy loss.
Slope is spring rate
Where the behaviour is approximately linear, the slope of the graph is the spring rate. A steeper line indicates more force is required for the same additional movement. Rate should be calculated between defined points or over a defined regression interval; choosing convenient points after seeing the graph weakens repeatability.
Loading and unloading
The loading curve records the outward test movement. The unloading curve records the return. They need not coincide. Friction, material damping, seating changes and fixture behaviour can produce a loop. The area between the two paths represents energy not returned during that cycle.
Patterns worth investigating
- A sudden step or drop that could indicate slip, coil movement or acquisition disturbance
- A changing slope that may be real progressive behaviour or an approaching limit
- A large return offset that suggests set, fixture movement or an incomplete return
- Unexpected differences between repetitions
- Clipping at the load or travel envelope
A graph is evidence for investigation, not a diagnosis by itself. Check the spring, fixture, calibration, recipe and raw trace together.
This guide provides general engineering information. Apply the governing drawing, licensed standard, risk assessment and approved procedure for the actual test.