Break-Off Groove: Two Hexagons, One Function
Working nut: lower hex carries thread and seats against the panel. Above it is a taller installation hex for a standard socket, and between them is a circumferential groove whose depth and corner radii set shear torque. The groove depth controls break torque, while the radii prevent the fracture from commencing elsewhere.
After separating, the upper hex leaves a chamfered, drive-less cone. Because the fracture plane sits above the last engaged thread of any properly designed mating bolt, the thread below the groove is unaffected.
Manufacturing struggles with batch consistency. On our lathes, the groove and both hexes are cut together, maintaining drive section-nut body concentricity and narrowing break torque across batches. Before manufacturing, torque-to-shear testing verifies sample batches for thread pitch diameter within +/-0.1 mm.
Installation hex adds driving depth. This height allows the socket to seat entirely before the wrench touches the panel, which is important on recessed bosses where a shallow hex would bottom out the driver before the nut seats.
Shear Nuts Compared with Other One-Way Fixings
|
Fixing |
After installation |
How it comes off |
Best suited to |
|
Shear nut |
Smooth cone, no drive faces, full female thread preserved |
Destructive only - split, drill, or grip and tear |
Enclosures fastened from outside, rarely opened |
|
One-way machine screw |
Special drive recess stays in place |
Only with the matching odd-profile driver |
Joints where a male thread is preferred |
|
Flush bulbed insert, female thread |
Reversible with the installation tool |
Service panels that must reopen |
|
|
Left-hand thread |
Looks ordinary |
Turns the opposite way with a normal spanner |
Low-cost deterrent on rotating fittings |
The distinction that matters most is against the shear bolt, its male-threaded cousin: the bolt breaks its head and leaves a threaded stud, while the nut breaks its drive hex and leaves a threaded socket. Choose by which side of the joint your mating part occupies, not by security level - both are one-way.
Installation: Tighten Until the Hex Lets Go
1. Hand-start the nut on the bolt and bring the lower hex square against the panel.
2. Fit a standard socket over the upper installation hex.
3. Tighten at a steady rate until the nut shears at its preset torque with a clean snap. Avoid repeated stop-and-start tightening, which can deform the groove before the break.
4. Check that the remaining cone sits flush and the mating bolt is torqued home. The fracture plane sits above the thread, so nothing needs re-tapping.
5. Log the batch and torque setting if the line runs audit trails.
Which side faces out? The installation hex always faces the tool-access side - the outside of the enclosure - because that is the only side from which it can be driven and sheared.
If a sheared fixing must come off for repair, the route is destructive by design: split the remaining cone with a cold chisel, or drill it and extract the pieces. Budget one replacement nut for every planned opening and keep the break-torque record with the panel's service file, so the refit matches the original specification.
FAQ
Q: Can shear nuts be removed after installation?
A: Only destructively-chiseling, drilling, or grasping and tearing the cone. The design preserves authorised access but leaves evidence and costs hardware.
Q: What determines break torque?
A: Material, groove depth, corner radii. Since shear nuts are tuned per drawing, two pieces with similar threads can break at different grooves.
Q: Are shear nuts reusable?
A: No. No tool can drive or back off a nut without the drive hex. The same socket, torque, and snap procedure installs a replacement, minimising downtime even though the hardware is single-use.
Q: The drive hex faces which panel side?
A: Tool-access side. The fastening is one-way since the cone cannot be driven from either direction after shearing.
Q: Do you offer small prototypes?
A: Yes. The shear nut you qualify is the one you scale since small batches and prototypes use the same groove-tuning and gauge workflow as production.
Q: What thread sizes do you make?
A: Integrated M4–M12 metric and #8–5", with configurable pitches for instruments. Beyond M12, the groove section fights the clamp load the joint needs, hence bigger security joints use two fasteners.
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