Screw locking methods are used to prevent fasteners loosening under vibration, thermal expansion, or dynamic loads. This guide explains the different types of locking devices used in engineering, including friction-based and positive locking systems such as lock nuts, washers, wire locking, and adhesives.
The purpose of a locking device is to prevent the loosening of mating components
which may be operating in conditions of varying stress, temperature or vibration.
The effectiveness of the locking device is often critical to the safety of the system..
The justification for a locking device is often not clear cut because there is a cost penalty ...
For a preloaded screw with a dry thread under a condition of low vibration there is a low
risk of loosening. The induced tightening torque may be relieved but this does not necessarily
result in the screw/nut loosening. Also when a machine has a large number of screws retaining
non-critical items which are regularly used,such as inspection covers, there may be a case
for not including locking devices. Additionally some threads my require regular
checking of the applied torque to counter the effects of creep or stress relaxation.
This function may be inhibited by mechanical screw locking devices..
Many locking devices are single use items and there is always the risk that during an
activity requiring screw removal and replacement the maintenance department may not have
the same replacement locking device in stock. They may then try to re-use the existing items,
or use available replacement items, or even not replace the locking device. This
will result is the screws being re-assembled such that they are not firmly locked ..
A clear case for locking can always be made for the following items
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Friction /Positive Locking
There are two main principles used for locking devices friction and positive locking.
It should be noted that locking systems using friction should be selected with care
because under high vibration friction between adjacent surfaces can be significantly reduced.
With positive locking device e.g. wire locking very high reliability results but only when
the locking device used correctly.
The overall safety and reliability of a machine is significantly affected by the judicious selection of appropriate locking devices.
Note: A number of the locking methods fix the nut or bolt head to the adjacent surface.
If these are used on nut + bolt applications please note that locking the nut /or bolt head does
not necessarily lock the other component (bolt /or nut)
It should be noticed that the thin lock_nut /jam nut is below the normal nut..
The proper connection is made by tightening the jam nut snugly first, then tightening the
upper nut so tightly that the stress on the jam nut is reversed as the bolt strains.
The locknut principle can also be used to fix the nut in any position on the male screw thread and therefore
create a shoulder..
This is a nut with a plastic or fibre collar set into the nut which is an interference fit on the male thread. On assembly the male thread forces its way through the collar and the resultant friction restricts the tendency to unscrew. These nuts are not as effective for locking if used more than once.
These nuts have slots in the top face. The nuts are fully tightened and a hole is drilled through the male thread to align with one of the slots. Split cotter pins are then inserted through the nut and the male thread and bent to hold it in position. This is a very effective and positive locking device but is expensive to install. It can also be difficult to install due to poor accessibility.
These are low cost items with questionable reliability generally considered only
suitable for non-critical consumer
items.
These washers are generally made from spring steel with serrations either on the inside
or the outside diameter. They are low cost items very suitable for low cost
domestic items such a lawn mowers , washing machines etc. These items can result in damage
to the adjacent surface because when the nut is tightened the serrations tend to dig into
the surface. It is therefore important when using these items that the provisions
for preventing corrosion are considered .
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These are low cost items with questionable reliability generally considered only suitable for non-critical consumer
items.
These are double or single coils of rectangular section spring steel.
These washers are used in place of plain washers. The washers prevent
rotation of the nut or bolt by the two ends digging into the surface of the two
adjacent faces. The free height of the coil washer is about 5 times the
compressed height.
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Wire locking is a very low cost and entirely effective method locking nuts
or hex headed screws and bolts screwed into surfaces. It is mainly
used when a number of screws have to be locked. Holes are predrilled
in the items being locked and when all of the items have been fully tightened
the wire is threaded through the holes and the ends are twisted to prevent loosening..
The routing of the wire is such that it prevents the screws
from unscrewing.
The wire is selected as non-corrodible ductile steel or brass wire of suitable
small gauge is used for this purpose.
The method is labour intensive and inhibits maintenance activities...
Tab washers are thin metal washers designed with two or more tabs which project from the outside diameter. The washer is placed below the head of the bolt or the nut and following tightening one or two tabs are bent upwards against the flats of the bolt/nut head. The remaining tab is bent down into a suitable hole in the surface being fastened or over a local edge, if one is available.
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Locking plates are manufacture items made from thin plates with with hexagon shaped holes sized to fit snugly over the hex heads to be locked. The plates have holes used for fastening the plate in place onto the surface being clamped using small screws which are locked..
A slot is machined into a nut and the nut is deformed to compress the slot as shown below. When the nut is tightened onto the male thread it forces the nut back to its original geometry. The thread system is locked by the built in friction..
A method of locking screw threads is available based on using a special thread form on
the female part of the system. This provides for very localised interference
such that the thread works normally but is self locking. This system can be provided
in the form of nuts or threaded inserts. The threaded inserts are based
on the Helicoil principle. .This is a modern system with significant advantages.
A modern trend is to use engineered adhesives. These are simply applied to the threaded component prior to assembly. The type of adhesive selected will depend on the need for a permanent assembly and an assembly which requires dismantling.. The most well known manufacturer of these systems are Henkel Loctite.. There are a significant number of advantages to this method and few disadvantages. The disadvantages include.
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There are a large number of other methods not discussed above. Most of these methods are based on the principles identified. Locking systems not yet listed in these notes include..
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| Method | Type | Reliability | Reuse | Typical Use |
|---|---|---|---|---|
| Nylock Nut | Friction | Medium | Limited | General applications |
| Wire Locking | Positive | High | Reusable | Aerospace |
| Tab Washer | Positive | High | Single-use | Bearing nuts |
| Threadlocker | Friction/Chemical | High | Depends | General engineering |
The best method depends on the application. For high vibration, positive locking methods like wire locking or tab washers are preferred.
Spring washers are generally considered unreliable for critical applications and are best used in low-load environments.
Threadlockers provide excellent vibration resistance and are widely used, but mechanical locking is preferred for safety-critical systems.
