Contact pressure across a bolted joint
Torque is an input, not a result. Two joints tightened to the same torque can carry very different pressure distributions, and the difference surfaces later as leakage, fretting or a fatigue crack. A pressure map makes the distribution visible while the joint is still open.
Torque does not describe clamp load
A bolt tightened to a stated torque produces a clamp force that scatters by tens of percent, because most of the torque goes into friction under the head and in the threads. Even with perfect preload, a group of bolts interacts elastically: tightening the last one relaxes the first. Add face flatness, coating thickness and gasket compression and the interface pressure becomes something nobody designed directly.
The consequence is not a lower total clamp load. It is an uneven one: a ring that is tight on one side and barely closed on the other. That is where leakage starts, where fretting begins, and where a fatigue crack eventually initiates — usually far from the bolt that was checked.
What is measured
Contact pressure across the whole interface face, at the preload the joint will actually see — not at a hand-tight condition extrapolated upwards.
What is compared
Tightening sequences against each other. The same joint, two sequences, two maps: the difference is the elastic interaction, made visible.
What is decided
Sequence, washer or shim placement, face flatness tolerance, gasket choice — decisions that otherwise rest on a torque table and experience.
How the interface is measured
A PD-FS sheet is placed between the mating faces, the joint is closed and tightened in the production sequence, and the pressure field is captured. Each sheet carries 4096 points on a 64 × 64 grid with ±5% FS accuracy and a response under 1 ms; collectors sample at 20 Hz over USB, and multiple collectors merge into a single map in one software session.
For a round flange the sheet is a full-area map; for a large diameter the coverage is assembled from several sheets, with seam positions agreed in the layout drawing so that no seam sits where the answer matters. A ring or annulus pattern is a custom geometry — the array is not restricted to the standard rectangular sheet.
The film is not invisible in a bolted joint. 0.3 mm of compliant material in the load path redistributes pressure locally and slightly reduces clamp for a given torque. Two honest ways to deal with it: design the stack-up to include the film where it stays in service, or use the film to characterise the interface and re-check without it. A reading taken with film and applied to a joint without it is an approximation, and should be described as one.
Where this method stops being valid
The published range is 400–6000 kPa (4–60 bar), configurable. That fits composite flanges, gaskets, clamped polymer and castings, and moderate-preload joints — the majority of what this method is asked to check.
It does not fit a high-utilisation steel joint. An M24 or larger bolt at high preload concentrates far more than 6 MPa under the head and washer. Reaching that range is a different sensor specification, and saying so before a quotation is cheaper for both sides than discovering it during a site visit.
What we have published, and what we have not
No bolted-joint record has been published on this site. The closest published work is a press platen uniformity study — the same question on a different interface: is the load actually where the design says it is, over the whole face rather than at the points someone chose to check.
If a bolted interface is your case, the honest starting point is a paid pilot on your own flange: one face, one tightening sequence, a map before and after a change. It is a small enough scope to be decided on data, and it produces a baseline that later campaigns can be compared against.
The nearest published record
Not the same interface, but the same question: is the load where the design says it is, over the whole face?
Press platen uniformity before a curing change
40,960 points across ten sheets measured platen flatness under load, so the process change was decided on data rather than a sample.
Where this setup stops working
Every one of these has cost somebody a re-test. They are listed here rather than discovered on site.
Film thickness is part of the joint
0.3 mm in a clamped interface changes the local pressure field and the achieved clamp for a given torque. Account for it in the stack-up, or re-check without it — but do not ignore it.
Pressure is not preload
The map shows interface contact pressure. Bolt preload is inferred from contact area and pressure, not measured directly; a load cell or ultrasonic bolt meter answers a different question and is often used alongside.
One shot per assembly
The map describes the state it was taken in. It is not a permanent instrumented joint unless the film is designed to stay in service, which is a different (and usually custom) build.
20 Hz is not an impact wrench
Tightening transients are faster than the sampling. What is captured is the held state after each step, which is the state that matters for the joint — but not the dynamic event that produced it.
Range ceiling
Above roughly 6 MPa the standard film saturates. High-strength steel joints need a different specification, and a different conversation.
The parts this measurement is built from
Film, collectors and software are quoted as one chain — the sheet count follows the surface.
4096-point sensing film
Sensing principle: Piezoresistive · Thickness: 0.3 mm
Compact 4096-point collector
Model: PXS064mini · Dimensions: 90 × 40 mm
Large-format collector
Model: PXS128 · Dimensions: 180 × 180 mm
One session, all sheets
Platform: Windows desktop application · Input: Multiple collectors via docking hub
Frequently asked questions
Yes, at pressures inside the published range. The film is 0.3 mm with a 1.5 mm sealed border; it is a compliant layer, so the local pressure field around bolts redistributes slightly compared with a metal-to-metal contact. That is a known bias, and it is repeatable.
No. It measures interface contact pressure. Preload is inferred from the pressure over the contact area; direct preload measurement needs a load cell, a strain-gauged bolt or an ultrasonic bolt meter. The map tells you how that preload is distributed, which is usually the part nobody knows.
Measure the same joint under two sequences and compare. That is a one-day experiment with a clear answer in the maps, and it is more convincing to a design review than a simulation of the same question.
Yes. The standard sheet is rectangular, but the array geometry is a custom item — a ring or annulus pattern for a round flange is a normal request. Lead time is longer than a standard sheet, so it belongs early in the project, not at the end.
Start from the expected mean pressure at the interface, not the bolt stress, and leave headroom for the peak under the washer. If the estimated peak is above 6 MPa, say so at the enquiry — the standard film is not the right answer and pretending otherwise wastes a site visit.
Different surface, same method
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Send us the surface and the decision it has to support
Area, peak pressure, the feature you are looking for, and what happens if you miss it. That is enough for a first assessment.