4. Torque and fastening specifications

WorldSID is a precision anthropomorphic test device (ATD) whose measurement fidelity depends as much on mechanical integrity as on sensor calibration. Every bolted joint in the dummy — from the occipital condyle bracket to the lumbar spine interface and the pelvic-to-femur attachment — carries a specified torque value that must be reproduced during assembly, inspection, and post-test rebuild. Under-torque allows relative motion that introduces hysteresis and spurious acceleration offsets; over-torque crushes load-cell bodies, strips aluminum threads, and preloads strain-gauge bridges beyond their intended operating range. This section explains how to apply the correct torque values, select and apply thread-locking methods, and verify that each joint has actually reached its specified preload.

Where the torque values come from

Torque specifications for WorldSID are published in the manufacturer's assembly and maintenance manual and, for certified devices, in the calibration and verification documentation that accompanies the unit. A typical specification table lists the fastener size, thread pitch, material pairing, target torque, and the required thread-locking compound. The values below are representative of a mid-size WorldSID build and are given here as a teaching example — always use the values in the manual for the specific unit and revision you are working on.

Two properties of these values matter operationally. First, torque is a proxy for preload, not a direct measurement of it. Roughly 80–90 % of applied torque is consumed by friction under the head and in the threads, so the same nominal torque on a lubricated fastener produces a substantially higher clamp load than on a dry one. This is why the manual specifies thread condition (dry, oiled, or with a locking compound) alongside the torque value, and why mixing conditions across a joint is a common source of error. Second, the material pairing matters: stainless fasteners into aluminum inserts gall easily and require lower torque or anti-seize, whereas steel-on-steel joints tolerate the higher values shown above.

Applying torque correctly

Correct application is a sequence, not a single action. Work through the following for each joint:

1. Clean and inspect. Remove old locking compound residue with a nylon brush and solvent. Inspect threads for galling, cross-threading, or elongation; replace any fastener that shows damage. Verify the fastener length and grade match the specification.
2. Seat the joint. Bring the mating surfaces together by hand and run the fastener down until it is finger-tight. Confirm the joint is fully closed with no gap before applying torque — a partially seated joint will absorb torque without generating preload.
3. Apply the locking method (see next section) before final tightening, not after.
4. Tighten in the specified pattern. For multi-fastener joints such as the rib module bracket, tighten in a cross or star pattern in two stages — approximately 50 % of target, then 100 % — to distribute clamp load evenly and avoid distorting the bracket.
5. Use a calibrated torque wrench set to the target value, applied smoothly over two to three seconds. Do not use the wrench as a breaker bar or exceed the target to "make sure." A click-type wrench should be re-verified against its calibration certificate at the interval defined in your quality system.

Thread-locking methods

WorldSID joints experience vibration and repeated impact loading, so mechanical and chemical locking are both used. The choice depends on whether the joint is expected to be disassembled during routine service.

• Anaerobic threadlockers are the default for most instrumented joints. Medium-strength compounds (removable with hand tools) suit joints opened during annual calibration; low-strength compounds suit small fasteners such as the M4 rib screws where high breakaway torque would risk damaging the insert. Apply two to three drops to the fastener threads, avoiding the first two threads so compound does not migrate into the joint or onto a load cell. Allow the specified cure time — typically 24 hours at room temperature for full strength, though handling strength is reached in about 10–20 minutes.
• Mechanical locking — nylon-insert lock nuts, Nord-Lock washers, or safety wire — is used where chemical residue is unacceptable, for example on joints adjacent to open strain-gauge bridges, or where the joint is opened frequently. Safety wire requires drilled fastener heads and a consistent twist direction; it is a secondary retention feature and does not replace correct torque.
• Dry assembly is specified for certain high-load joints such as the hip fastener, where the design relies on metal-to-metal friction and the manual explicitly prohibits lubricants or compound. Follow the manual exactly here; adding compound to a dry-specified joint changes the torque-preload relationship and can reduce clamp load.

Verification techniques

Verification answers a simple question: did this joint actually reach its specified preload, and will it stay there? Three techniques are used in practice, in increasing order of confidence.

Mark-and-check (torque seal). After final tightening, apply a small bead of torque-seal lacquer across the fastener head and the adjacent bracket, or draw a witness line with a paint pen. During inspection, any misalignment of the mark indicates the fastener has moved. This is fast, non-invasive, and standard practice on flight and automotive hardware, but it detects only gross loosening — it cannot confirm that the original torque was correct.

Breakaway torque check. For a sample of joints, or after a test series, measure the torque required to loosen the fastener with a calibrated wrench. Breakaway torque on a correctly torqued, thread-locked joint typically reads at or slightly above the installation value. A reading well below specification indicates the joint was under-torqued or has relaxed; a reading far above indicates over-torque or compound migration. Record the value and the ambient temperature, since anaerobic breakaway strength is temperature-sensitive.

Preload verification by fastener elongation or ultrasonic measurement. For critical joints where torque alone is insufficient — typically the lumbar and hip interfaces — fastener stretch can be measured directly. Ultrasonic bolt meters compare time-of-flight before and after tightening to compute elongation, from which preload is derived using the fastener's known stiffness. This is the most accurate method and is used during certification builds, but it requires access to both ends of the fastener and a calibrated meter.

The diagram above ties the sequence and the verification methods together. Note that the three verification techniques are not interchangeable: the witness mark confirms that a joint has not moved since it was sealed, breakaway torque confirms that the installed preload was in the right range, and ultrasonic measurement confirms the actual clamp load. A complete quality record for a WorldSID rebuild typically includes the torque value applied, the locking method and cure time, the witness-mark status, and — for critical joints — the breakaway or ultrasonic result. Keeping that record per joint, rather than per dummy, is what makes a later discrepancy traceable to a specific fastener rather than to "the assembly."

Finally, treat the torque specification as a controlled document. When a load cell or bracket is replaced, confirm that the replacement part's fastener callout matches the manual revision in force; substitution of a different grade or coating changes the friction coefficient and therefore the preload at a given torque. If a substitution is unavoidable, document it, adjust the torque only with engineering approval, and verify the result by breakaway or ultrasonic measurement rather than assuming the original value still applies.