Locking Devices

The problems associated with threaded devices and the effects of vibration on their security were discussed previously, when the use of stiffnuts and anchor nuts was considered.

In addition to using methods which increase the friction between threads, there are several other ways in which the integrity of a threaded joint can be assured.

Spring Washers

These washers are available in a variety of forms (refer to figure below). In some instances (particularly with light alloy assemblies), spring washers are assembled with plain facing washers between the spring washer and the component. This is done to prevent damage to the surface finish when the spring washer is compressed, although with steel assemblies, the plain washer is usually omitted.

It is good practice to renew spring washers during overhaul or repair. This procedure is most essential in engines and engine components as well as where units have reciprocating parts, such as in compressors or pumps. However, in normal circumstances, spring washers can be re-used if they have retained their ‘springiness’ and ‘sharpness’. Types of spring washers include:

• Single and Double Coil Washers: Manufactured from rectangular-sectioned steel sheet and formed into a portion of a helix, the single and double coil are the most common types of spring washer to be found on aircraft components.

FIGURE

• Crinkle Washers: Crinkle washers are usually manufactured from either copper alloy or corrosion resistant steel. They are often used in lightly loaded applications such as instruments and electrical installations.
• Cup Washers: Cup (or Belleville) washers are manufactured from spring steel and are ‘dished’ to form a spring of high rating. During tightening, the flattening of the washer exerts an axial load to the nut, which will resist any tendency of the nut to lose torque. Assembly should always be in accordance with the manufacturer’s instructions.

Shake-Proof Washers

Flat washers of this type (refer to figure below) are manufactured from steel or phosphor bronze and are used in place of spring washers. In some circumstances, conical shake-proof washers are used for locking countersunk screws.

Either the internal or the external diameters can be serrated, the serration being designed to bite into the component and nut to prevent rotation.

All shake-proof washers should be used only ONCE. It is rare for these washers to be specified in assemblies where an anti-corrosion treatment of the components has been specified, as this could damage the treatment.

FIGURE

Tab Washers

Tab washers (refer to figure below) are normally used on plain nuts. The washers are manufactured from thin metallic sheet material and have two or more tabs projecting from the external diameter. They can also be designed for locking two or more nuts.

When the washer is installed, one tab is bent against the component or inserted into a hole provided, whilst a second tab is bent against the flat (or flats) of the nut after it has been torqued down correctly.

Note: Multi-tab washers can be re-used until all tabs have been used once.

FIGURE

Lock Plates

In certain circumstances, the torque applied, the thread or the type of nut being used may not guarantee that the nut would not unwind in use (such as during vibration). Lock plates (refer to figure below) are used where positive retention of a nut is required.

FIGURE

The nut is torque loaded and then (only if necessary) turned a small amount (< 1/12 revolution) until its flats align with the hole in the lock plate. The plate usually has 12 facets to allow for this adjustment. The plate is then placed over the nut and the small setscrew fastened into the tapped hole adjacent to the nut. Removal of the nut simply involves removing the setscrew, lifting off the plate and unwinding the nut.

Note: A tab washer could be used to do the same task. The lock plate is used where the nut is frequently removed. The plate can be used indefinitely, providing it retains a good fit with the nut.

Split (Cotter) Pins

These pins (refer to figure) are usually manufactured from either cadmium-plated carbon steel or from corrosion-resistant steel. Their primary purpose is to lock slotted and castellated nuts as well as for securing clevis pins. The nuts are locked onto their bolts by passing the pin through the hole in the bolt and the nut castellations. The legs of the pin are spread in one of two methods. Whilst either of these methods will secure the nut to the bolt, different airworthiness authorities prefer one method to the other.

The pins are measured by diameter and length. It must be noted that the nuts must never be over-torqued to get the holes into line. If this is permitted, the nut must either be backed-off or washers added under the nut.

Often a stated torque value will be over a small range rather than a set figure. This allows very small movement of the nut to facilitate alignment of the locking pins. Details of the correct method for each task will be in the AMM.

FIGURE