Self-locking screw threads: specifying prevailing torque in the RFQ

Free-running versus prevailing-torque: what the buyer is actually specifying

A free-running thread is specified to assemble with no intended resistance beyond the normal friction of fit. Its acceptance is a fit and gauge question: does the thread conform to the stated system, diameter, pitch and tolerance class, and does the mating part assemble within the agreed fit?

A prevailing-torque, or self-locking, thread is specified to provide a defined resistance to rotation in the installed condition. That resistance is usually produced by an integral feature such as a deformed thread segment, or by an applied locking element such as a nylon-type patch or an adhesive system. The resistance is a functional property of the assembled joint, not something the nominal thread designation describes.

The same M6 designation can therefore sit behind two completely different requirements. If the drawing only states the thread system and class, the supplier has no way to know whether locking behavior is expected at all. State which of the two expectations the joint needs, and if it is the self-locking type, write the requirement as a separate line item with its own check. The pre-applied locking patch article explains what the locking element itself needs; this article is about the functional condition the finished thread must meet.

What to state on the drawing and in the RFQ

A supplier can quote and inspect consistently only when the locking requirement is described as inputs, not as an adjective. The following belong on the drawing or in the RFQ document:

- Locking mechanism, if one is mandated (integral deformed thread, applied patch, adhesive). If the mechanism is open to the supplier's proposal, say that instead, and state the functional requirement the proposal must meet.
- Locking zone location and axial extent relative to the usable thread and the intended mating engagement. This is the geometry the mating part must actually reach.
- Mating thread: material, class, engagement length and the condition of the mating part.
- Assembly tightening method, since the locking torque interacts with how the joint is driven.
- Required number of assembly and reuse cycles, stated explicitly rather than assumed.
- The final supplied condition in which the behavior is judged, including any coating or plating on the screw.
- The functional test method and acceptance conditions for prevailing torque.

Each of these is a field to be filled, not a marketing phrase. Exact torque values, reuse cycle limits and pass/fail thresholds must be agreed for the actual part. They cannot be inherited from a generic figure or from a different screw. Treating a general self-locking claim as if it were a measured value for your part is one of the most common causes of later disagreement.

Prevailing torque as a functional check, not a gauge result

Two checks answer two different questions, and a self-locking screw usually needs both.

A thread gauge verifies that the thread form and fit conform to the specified thread system. It answers: is the thread the right size and shape? It does not measure locking torque. A go/no-go gauge does not tell you whether an applied patch is doing its job, whether a deformed thread segment resists rotation as intended, or whether the resistance survives a coating.

A functional torque check measures resistance to rotation under defined conditions. It answers: does the thread lock, and within what range? The method must state the engagement, the driving speed or condition, the number of cycles, and the seating or starting condition, because each of these changes the measured resistance.

Both checks can apply to the same part, and both should be recorded. A successful gauge result must not be treated as proof of locking behavior, and a functional torque result must not be treated as proof of thread-form acceptance. When a supplier reports "torque passed," confirm which test and which condition it refers to. Records should separate the base thread acceptance from the locking-function acceptance so that a repeat order preserves both.

Mating thread, coating and the final supplied condition

Several conditions change the measured locking behavior, so they have to be pinned down before the check is defined.

The mating thread matters: its material, class, engagement length and condition affect both the resistance felt and the reliability of the lock. State them. A locking feature checked against one mating condition is not automatically valid against another.

Coating changes the result. A coating or plating on the screw or the mating part can alter the effective fit and the friction at the interface, so the acceptance condition must be referenced to the final supplied condition. If the part is checked before coating, that has to be stated and justified, not left implicit. This is the same principle as coating allowance for threads, applied to the functional check.

The locking zone must actually overlap the intended mating engagement in the installed condition. A well-formed locking feature positioned where the mating thread never reaches does nothing. That is a drawing-position question tied to usable thread, and it should be checked on the installed geometry rather than assumed from the manufacturing route.

Reuse and reapplication limits are a drawing and RFQ decision. An applied locking element may lose effectiveness after repeated assembly, and reapplication is a separate operation with its own handling and records. State how many assembly cycles the part must sustain, whether reapplication is permitted, and if so, under what conditions. Do not leave this to be discovered in service.

Failure modes this prevents

Writing the requirement this way closes specific gaps:

- A part that gauges acceptably but does not lock, because thread-form acceptance was mistaken for functional acceptance.
- A locking torque outside the intended range after coating, because acceptance was defined on the pre-coated condition.
- A locking zone that misses the mating engagement, because the zone was not tied to the installed geometry.
- An acceptance dispute on first delivery, because the functional test method, cycle count and final condition were never agreed.
- Premature loss of locking in service, because reuse and reapplication limits were never stated.

Practical specification pattern (illustrative)

The following is an illustrative specification pattern, not a measured result and not a set of acceptance values.

Suppose the part is a shoulder screw with a short working thread engagement. The drawing would:

- State the thread system, diameter, pitch and class for the working thread, so the gauge requirement is unambiguous.
- Name the locking mechanism, or state that a mechanism is proposed by the supplier against the functional requirement.
- Define the locking zone by its position and axial extent relative to the usable thread, and annotate the expected mating engagement so the overlap is visible.
- State the final supplied condition (for example, the specified coating) as the condition in which locking is judged.
- State the required assembly and reuse cycle count.
- Specify a functional prevailing-torque check with its method, and list thread gauging as a separate acceptance item on the same part.

The exact torque window, cycle count and test conditions are left as fields to confirm with the supplier against the approved drawing. They are deliberately not fixed here, because they depend on the geometry, material, coating and joint, and must be agreed rather than assumed.

Next steps

Put the locking requirement, the reuse expectation and the functional check on the drawing, reference the process route and final condition that the part will be supplied in, and ask the supplier to confirm the proposed mechanism and exactly how it will be verified. When you send the RFQ, include the mating thread and engagement so the locking zone can be checked against real geometry.

Start with the RFQ preparation checklist, then submit your drawing for review. For the locking element's material and placement, see pre-applied locking patch requirements. For thread-form acceptance, see thread gauges and custom screw acceptance. For coating effects on fit, see coating allowance for threads and shoulders.