What is a regular hex nut?
The standard full-height nut. It is made to carry load: the nut's thread shares the clamp force with the bolt's thread, and its height gives the engaged length. Put simply, standard nuts hold parts together, and lock nuts resist loosening. A regular nut on its own does not resist vibration much. If the joint's preload falls during the vibration cycle, the nut can turn.
What is a jam nut?
A low-profile nut, typically half as tall as a standard nut. It is used in two ways. One is where a standard-height nut will not fit. The other is as a lock: tightened against another nut or a part, so the two push against each other. The effect comes from the forces made when two nuts are tightened against each other: a steady tension in the short stretch of thread between them. That steady tension holds the threads in contact even when the joint shakes.
The name follows the use, not the shape. Any nut used to jam against another, even a standard one, is a jam nut.
What is a lock nut?
A family, not one part. The common types are nylon-insert nuts, where a ring of nylon grips the thread; all-metal nuts, whose threads are slightly deformed; prevailing-torque nuts; and serrated-flange nuts. They resist loosening by friction or interference. Reusability varies, and nylon inserts wear faster.
The words get mixed up. "Lock nut" is used for the prevailing-torque types and also, at times, for a thin jam nut, and engineers do not always agree on the usage. When a drawing or a supplier says "lock nut", ask which kind is meant.
How do the three differ in what they carry?
| Regular hex nut | Jam nut | Lock nut (prevailing-torque) | |
|---|---|---|---|
| Height | Full | About half | Full or taller |
| Carries the working load? | Yes | Not in tension; it can share a push | Yes |
| How it resists loosening | Preload only | Jammed against another nut or a part | Friction or interference built into the nut |
| Reusable | Yes | Yes | Varies |
The difference in load follows from the thread geometry. The load a nut can take before its thread strips depends on how much thread is engaged, which scales with the nut's height. If a jam nut is half the height of a regular nut of the same material, it has about half the stripping strength. This is a proportion, not a rating: the real figure comes from the standard and the maker.
Which one should lock a rod end thread?
Usually a jam nut, and the reason is the load path. A rod end threaded into a tube, a clevis or a linkage end carries its working load through the thread engagement inside the mating part. Under a pull, the nut outside does not carry that load. It only has to hold the thread from turning, by pressing against the mating face and keeping the threads in firm contact. A thin nut can do this, and takes less room on the shank.
A push is different. The end face of the mating part presses on the nut, so the nut shares the load, and a load that reverses between pull and push makes the thread flanks change sides every cycle unless the nut's preload is larger than the swing. That is why a jam nut has to be tightened to a real preload, and why a thin nut is a poor choice at high loads.
A jam nut also lets you adjust. Run it back, turn the rod end to the length you want, and lock it again. A prevailing-torque nut drags the whole length of the thread each time, which makes fine adjustment slow.
There are limits. Jam nuts can be unreliable under significant loads. Two arrangements need to be told apart. In the usual rod end case, one jam nut is tightened against the end face of the mating part, which gives the nut something to push on. When there is no such face, two nuts are tightened against each other, and the torques matter: if the inner nut is tightened more than the outer one, the outer nut can yield, and if the outer nut is tightened more than the inner, the inner can loosen. A common guide (Lok-Mor's) gives the inner nut about a quarter to a half of the final torque, held with a wrench while the outer one is tightened. Practice differs on which of the two nuts goes on first; for the reasoning, and for the other ways to lock a thread, see K06-Q1-05.
How much torque does a jam nut need?
There is no general value, and here is why. The preload a nut makes from a given torque depends on the friction in the thread and under the nut. The usual relation is T = K × d × F, where T is the torque, d the thread diameter, F the preload and K a factor. K is not a constant. A commonly quoted spread for plain steel threads is about 0.15 to 0.25, and finish, plating and lubrication move it.
Take an M12 nut tightened to 24 N·m as an illustration. With K = 0.15, the preload is 24 ÷ (0.15 × 0.012) = 13.3 kN. With K = 0.25 it is 24 ÷ (0.25 × 0.012) = 8.0 kN. The same torque gives preloads about 1.7 times apart. A thin nut has half the stripping strength, so the safe torque for a jam nut is lower, and the correct figure is the one the nut's maker gives for its size, material and finish. A rule of thumb from a table, applied to a different finish, can strip the thread or leave the nut loose.
How should you fit it without damaging the rod end?
- Hold the rod end by its wrench flats, or by the shank. Never hold the eye or the ball. Twisting through the ball can load the race.
- With one jam nut, tighten it against the mating face while the rod end is held. With two nuts against each other, hold the inner nut while the outer is tightened, so the two lock against each other.
- Check the thread before locking. The nut should run freely by hand. If it binds, look for a damaged thread before reaching for a wrench.
- Stainless nut on a stainless thread can gall. Slow, lubricated assembly helps; see K06-Q1-09.
- Check the setting after the first period of running. A nut that has turned shows as a gap in the paint mark you made across the nut and the shank.
How does a jam nut cope with vibration and shock on a vehicle link?
The jam nut is the standard lock on adjustable vehicle links such as toe links, trailing arms, panhard bars and shift linkages. What differs is how hard it is worked.
The load swings both ways, and fast. Rough ground, landings and braking make the link push and pull many times a minute. That is the reversing case above: the jam nut has to hold a real preload, because a lightly tightened nut will shake loose. The preload spread in the example above (8.0 to 13.3 kN at the same torque) is why the torque must come from the nut maker for the exact size, material and finish, not from a table.
Dirt and corrosion change the friction. Mud, sand and rust in the thread alter K and make a nut hard to turn later. Clean the thread first, and do not force a rusted nut. Match the nut to the thread: a fine-pitch rod end and a coarse-pitch nut can start for a turn by hand and then bind, and forcing it on with a spanner strips the thread; the hand must match too (K01-Q1-01).
Mark it and look again. Put a paint stripe across the nut and the shank, and check it after the first period of running and after every hard event; a moved stripe means the nut has turned.
A jam nut on a steering or suspension link is a safety item. The tightening torque, the number of nuts and any positive locking (a rule requiring a lock wire or a split pin, for example) come from the vehicle's designer or the series rules; follow those. Jam nut types are on the jam nuts page, and the jam nut buyer's guide covers choosing one.