In bolted joint engineering, the double-nut locking method is one of the oldest and most widely used techniques for preventing loosening. Yet, a seemingly simple question has long puzzled countless engineers and assembly technicians: should the jam nut be placed at the bottom or the top?

The answer to this question directly determines whether the double-nut assembly effectively secures the joint or serves no real purpose at all.

Basic Principle of the Double-Nut Anti-Loosening Method

The dual-nut locking method—also known as the jam-nut or lock-nut method—relies on a straightforward principle: when two nuts are tightened against each other on a bolt, a continuous axial force is generated between them. For either nut to rotate, the friction between the engaged threads must be overcome; this friction is the fundamental mechanism that prevents loosening.

This configuration typically employs two nuts of different thicknesses: a jam nut and a full-height nut (a standard or thick nut). The jam nut is usually about half the thickness of a standard nut; its primary function is not to bear the load, but rather to prevent the other nut from loosening through a "wedging" action.

Correct Assembly Sequence: Jam Nut on the Bottom, Standard Nut on the Top

Industry-Standard Consensus

Both ASTM fastener installation guidelines and standard heavy-machinery construction practices in the U.S. specify the same correct sequence for doublenut locking: install the jam nut first, against the workpiece (on the bottom); then install the standard-height nut on top of it (on the upper side).

Failure Risk When the Sequence Is Reversed

If the standard nut is installed on the bottom and the jam nut on the top, three critical problems arise – and these are the root causes of most field failures of double‑nut assemblies:

Greatly diminished locking effectiveness: When the jam nut is finally tightened on top, the threads of both nuts engage the same flank of the bolt thread, failing to generate opposing wedging forces. The thread clearance is not fully eliminated.

Thread overload and stripping risk: The jam nut has fewer engaged thread turns. When it is used as the final locking element and subjected to full torque, its threads are prone to plastic deformation or shear failure.

Preload transfer failure: The jam nut lacks sufficient rigidity; it deforms elastically during tightening and cannot stably transmit the intended preload. The resulting clamp load in the joint is far below the design value, compromising the reliability of the connection.

Comparison of the Two Methods

Correct Installation Steps

Based on the above analysis, the correct installation procedure for double‑nut locking is as follows:

Step 1: Run the jam nut onto the bolt first, positioning it against the workpiece (on the bottom side).

Step 2: Tighten the jam nut to 25%–50% of the final total torque.

Step 3: Using a wrench to hold the jam nut stationary so it does not rotate with the upper nut, use a second wrench to tighten the upper standard‑thickness nut to the specified final torque.

Critical cautions:

1. If you do not hold the jam nut while tightening the upper nut, both nuts will rotate together. No opposing axial force is generated, and no locking effect is achieved.

2. If the jam nut is not tightened at all before installing the upper nut, even if the upper nut is torqued down hard, both nuts will back off together – there will be no locking whatsoever.

3. Do not over‑torque the jam nut. Excessive preload on the jam nut can actually counteract the wedging effect.