Pressboard bend allowance supplies the straight legs and curved region of the finished barrier. Calculate developed length using the actual conditioning, compression and release route, then verify the formed geometry. [1]
Calculation basis. Project-specific values and gates in the illustrative screens below are assumptions; cited catalogue values retain their source test conditions.
Define the finished measurement state first: held in the forming tool, released on inspection supports or restrained in the transformer. These shapes need not share the same angle and radius. The blank calculation should target the state that governs fit.
A trial calculation with named assumptions
For a single bend, use straight lengths measured to the tangent points and an arc approximation for the developed bend. Let Rᵢ be inside radius, t the relevant formed thickness, θ the bend angle in radians and c the assumed distance of the neutral layer from the inside surface.
Trial bend allowance: BA = θ(Rᵢ + c). Trial blank length: L = L₁ + L₂ + BA.
The location c is a parameter to identify for the selected material and route, not a metal K-factor copied into a cellulose drawing. The simple model also assumes a reasonably consistent curved section. A wrinkled or strongly compressed bend may not satisfy that assumption.
In an illustrative calculation, Rᵢ = 12 mm, t = 3 mm and c = 1 mm for a 90-degree bend. BA = (π/2) × 13 = 20.420 mm. With straight tangent lengths of 60 and 40 mm, the trial blank is 120.420 mm. If the plausible trial range for c is 0.8–1.4 mm, the allowance ranges from 20.106 to 21.049 mm. That 0.942 mm spread shows why the assumed layer position must be checked.
Turn trial blanks into a process-specific calculation
- Identify the material grade, sheet direction and conditioning state used for the trial blanks.
- Measure the blank before forming using a defined datum and low, repeatable measuring force.
- After the qualified release sequence, measure leg lengths, inside profile, thickness and included angle.
- Compare several parts, then estimate the allowance needed for the target shape and its observed variation.
- Repeat the check when thickness, radius, material route or forming state changes materially.
Separate springback from blank-length error. A leg can be the correct developed length but occupy the wrong assembly envelope because the released angle is wrong. For an illustrative 40 mm straight leg, a 2-degree angular departure produces about 40 sin(2°) = 1.396 mm transverse displacement. Adding that amount to the blank would not correct the angle.
Keep the calibrated allowance and the released-angle correction as separate drawing or process entries. Also distinguish the inner radius imposed by the tool from the radius measured after release; fitting both with one unexplained coefficient hides the cause of a poor fit.
Verify the developed profile for folds, thinning, contact damage and the adjoining oil space. Combine reproducible dimensions with the dielectric evidence required for that geometry.
Allocate the blank-length tolerance to a calibrated neutral layer
Keep the original developed-length model and retain the conditioning boundary identified by pressboard mechanical evidence [2]. Assume a ±0.25 mm blank-length allocation, with other errors temporarily set to zero. Because ∂BA/∂c = θ = π/2, the neutral-layer offset uncertainty must be at most 0.25/(π/2) = 0.1592 mm.
The assumed 0.8–1.4 mm range around the trial c = 1.0 mm permits a +0.6283 mm allowance error, exceeding that allocation. Calibrate the layer position for the actual forming route before issuing a production blank length. Separately, the calculated 1.396 mm transverse movement from 2° springback exceeds an assumed 1.0 mm envelope allocation; correct the released angle rather than adding length to compensate.
References
- IEC, IEC 60641-2:2004, Pressboard and presspaper for electrical purposes — Part 2: Methods of tests.
- C. Oria, I. Carrascal, D. Ferreño, C. Méndez and A. Ortiz, 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.