A jam nut is a low profile type of

nut Nut often refers to: * Nut (fruit), fruit composed of a hard shell and a seed, or a collective noun for dry and edible fruits or seeds * Nut (hardware), fastener used with a bolt Nut or Nuts may also refer to: Arts, entertainment, and media Co ...

, typically half as tall as a standard nut. It is commonly used as a type of

locknut A locknut, also known as a lock nut, locking nut, self-locking nut, prevailing torque nut,. stiff nut or elastic stop nut, is a nut that resists loosening under vibrations and torque. Prevailing torque nuts have some portion of the nut that def ...

, where it is "jammed" up against a standard nut to lock the two in place. It is also used in situations where a standard nut would not fit. The term "jam nut" can also refer to any nut that is used in the same function (even a standard nut used for the jamming purpose). Jam nuts, other types of

lock washer A washer is a thin plate (typically disk-shaped, but sometimes square) with a hole (typically in the middle) that is normally used to distribute the load of a threaded fastener, such as a bolt or nut. Other uses are as a spacer, spring (Bellevi ...

s, and

thread-locking fluid Thread-locking fluid or threadlocker is a single-component adhesive, applied to the threads of fasteners such as screws and bolts to prevent loosening, leakage, and corrosion. Most thread-locking formulas are methacrylate-based and rely on t ...

are ways to prevent vibration from loosening a

bolted joint A bolted joint is one of the most common elements in construction and machine design. It consists of a male threaded fastener (e. g., a bolt) that captures and joins other parts, secured with a matching female screw thread. There are two main t ...

Use of two nuts to prevent self-loosening

In normal use, a nut-and-bolt joint holds together because the bolt is under a constant

tensile stress In continuum mechanics, stress is a physical quantity. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...

called the ''preload''. The preload pulls the nut threads against the bolt threads, and the nut face against the bearing surface, with a constant

force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...

, so that the nut cannot rotate without overcoming the

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...

between these surfaces. If the joint is subjected to

vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic function, periodic, such as the motion of a pendulum ...

, however, the preload increases and decreases with each cycle of movement. If the minimum preload during the vibration cycle is not enough to hold the nut firmly in contact with the bolt and the bearing surface, then the nut is likely to become loose. Specialized

locking nut A locknut, also known as a lock nut, locking nut, self-locking nut, prevailing torque nut,. stiff nut or elastic stop nut, is a nut that resists loosening under vibrations and torque. Prevailing torque nuts have some portion of the nut that de ...

s exist to prevent this problem, but sometimes it is sufficient to add a second nut. For this technique to be reliable, each nut must be tightened to the correct

torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...

. The inner nut is tightened to about a quarter to a half of the torque of the outer nut. It is then held in place by a

wrench A wrench or spanner is a tool used to provide grip and mechanical advantage in applying torque to turn objects—usually rotary fasteners, such as nuts and bolts—or keep them from turning. In the UK, Ireland, Australia, and New Zealan ...

while the outer nut is tightened on top using the full torque. This arrangement causes the two nuts to push against each other, creating a tensile stress in the short section of the bolt that lies between them. Even when the main joint is vibrated, the stress between the two nuts remains constant, thus holding the nut threads in constant contact with the bolt threads and preventing self-loosening. When the joint is assembled correctly, the outer nut bears the full tension of the joint. The inner nut functions merely to add a small additional force to the outer nut and does not need to be as strong, so a thin nut can be used. The jam nut essentially acts as the "other object", as the two nuts are tightened against each other. They can also be used to secure an item on a fastener without applying force to that object. This is achieved by first tightening one of the nuts onto the item. Then the other nut is screwed down on top of the first nut. The inner nut is then slackened back and tightened against the outer nut. Jam nuts can also be used in situations where a threaded rod must be rotated. Since threaded rods have no bolt heads, it is difficult or impossible to apply torque to a threaded rod. A pair of jam nuts is used to create a point where a wrench may be used. Jam nuts can be unreliable under significant loads. If the inner nut is

d more than the outer nut, the outer nut may yield. If the outer nut is torqued more than the inner nut, the inner nut may loosen up.

References

Nuts (hardware) Kontermutter {{Mech-engineering-stub