Nuts

Aerospace standard nuts are made in a variety of shapes and sizes. They can be made of cadmium-plated carbon steel, stainless steel or anodised 2024–T aluminium alloy and can have right- or left-hand threads (refer to figure below).

As a general rule, nuts are manufactured from the same material as the bolt or screw to which they are attached, with the exception of high-tensile steel bolts with which mild steel nuts are used.

FIGURE

As they do not have any identifying marks or lettering, they are usually identified by their colour and their constructional features. Familiar types of nuts include:

• Castle Nuts: They are used with drilled shank hexagon-headed bolts or studs, eye-bolts and clevis bolts. They are fairly rugged and can withstand large tensile loads. The slots (castellation) are designed to accommodate a split (cotter) pin.
• Slotted Nuts: These are similar in construction to the castle nuts and are used in similar applications, except that they are normally used for engine use only.
• Plain Hexagon Nuts: They are of rugged construction and suitable for large tensile loads. Since they require an auxiliary locking device, their use on aircraft is limited.
• Light Hexagon Nuts: These are much lighter nuts and are used for miscellaneous light tensile requirements.
• Plain Check (or Lock) Nuts: These are employed as locking devices for plain nuts for threaded rod ends and for other devices.
• Wing Nuts: These are used where the desired tightness can be obtained merely with using the fingers and where the assembly is frequently removed.

Stiffnuts and Anchor Nuts

An ordinary standard nut will depend upon friction between the engaging threads to ensure its tightness. The enemy of this system is vibration, which can cause the nut to slacken off. In extreme cases, unwind itself completely from the bolt or screw. In areas where this might occur, locking devices are used. These either increase the frictional resistance between the threads or take the form of positive securities that prevent any movement of the nut once they have been applied.

Stiffnuts and anchor nuts (refer to figure below) employ various means of increasing the friction forces between the threaded devices and common types include:

• Nyloc: This looks like a standard hexagonal nut, but has a plastic insert in the counter-bored end. This insert is initially unthreaded and has an internal diameter slightly smaller than the nut thread, so that, as the nut is screwed on the bolt, the plastic insert is displaced and a high degree of friction is created. Another type of plastic ‘stop’ nut is named the ‘Capnut’. This type is completely sealed and is used in pressurised compartments and fuel and oil tanks etc.

Note: As the insert is nylon, this type of stiffnut should not be used in high or low temperature areas. A typical maximum temperature would be 120ºC. A similar type of stiffnut has a fibre insert instead of nylon, and is called a fibrelock nut. Neither nylon nor fibrelock stiffnuts should be re-used.

• Oddie: The top of this nut has a slotted end, consisting of six tongues, which form a circle slightly smaller than the bolt or stud diameter. As the nut is turned, a friction load is imparted onto the threaded device.
• Philidas: This nut has a circular crown, which is slotted horizontally in two places. The thread on the slotted part is slightly ‘out of phase’ with the rest of the thread, so that increased friction is achieved when the nut is turned.
• Aerotight: Similar to the Philidas in appearance, except that the slots are vertical. Its locking method is also similar.
• Lightweight: The locking section of this stiffnut is slightly oval in shape and so causes increased friction when the thread passes through it.

Note: Metal hexagonal type stiffnuts may be re-used, provided they are not being used in vital areas such as flying controls and they retain their friction effect. A recognised rule for serviceability is that they are discarded when they can be screwed all the way down on a new bolt using only the fingers.

• Anchor nuts and Stripnuts: Anchor nuts are supplied with single or double attachment points and may be either fixed or floating in a cage. The anchor nut may be a single unit stiffnut, integral with the base plate or it may be an assembly, comprising stiffnut, cage and base plate. Single attachment types are used in corners or where space is limited and have two adjacent fixing points. Double anchor nuts have a hole either side of the stiffnut. They are fitted to the structure by riveting. Where a number of anchor nuts are required to secure the panels, a number of stiffnuts may be fitted into metal strips for ease of securing. Stripnuts are usually of the floating variety.

FIGURE