Spacer force and dimensions must be compared at defined material and loading states. Pressboard compression data show nonlinear response and loading-history effects. [1] Separate drying-related dimensional change from mechanical seating before using either to predict axial preload; the clamp and restraint system determine the force response.
Record dimensions in defined wet and dry states under a stated measuring load. Identify material direction, spacer position and whether the measurement represents individual parts or a complete stack. An unloaded coupon height cannot be inserted directly into a loaded winding model.
Follow the chain from material to force
| Step | Evidence | Distinction |
|---|---|---|
| Material dimension | Moisture, direction, temperature and load | Hygroscopic change versus mechanical compression |
| Stack height | Actual spacer/contact sequence | Accumulated shrinkage versus seating |
| Clamping travel | Spring, tie-rod or rigid restraint response | Available compensation movement |
| Preload after process | Qualified force or displacement evidence | Calculated expectation versus verified state |
A linear stiffness estimate is only a screen
For a linear restraint with effective stiffness k, shortening Δh changes preload by approximately kΔh in the specified force-releasing arrangement. Use k in N/m and Δh in metres. Bolt/member compliance and the actual load path determine the effective stiffness; contact opening and nonlinear spacers need a fuller model. [2]
A spring arrangement may maintain force over some travel, while a rigidly fixed geometry can lose compression after shrinkage. Neither behavior should be assumed from the word “clamped.”
Retain the measured loaded-state height and force together, so that a dimension change cannot be interpreted using an incompatible restraint or free-coupon state.
Translate spacer settlement into restraint force
Assumptions: effective linear restraint stiffness is 25 kN/mm, the relevant loaded stack shortens 0.6 mm, initial force is 60 kN and the design requires at least 50 kN after processing. Force loss is 25 × 0.6 = 15 kN, leaving 45 kN, deficient by 5 kN. Hold clamp release and verify the actual nonlinear force/travel state against the design; unloaded spacer shrinkage cannot establish this result.
References
- C. Oria et al., Experimental dataset on the compressive mechanical properties of high-density pressboard used in power transformers spacers, Data in Brief 50, 109471, 2023. DOI: 10.1016/j.dib.2023.109471.
- R. T. Barrett, Fastener Design Manual, NASA Reference Publication 1228, 1990.