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Screw Locking and Retaining Methods



Introduction

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

  • Guards and associated systems
  • Location stops and guides
  • Critical items under high vibration which are not accessible
  • Critical loaded items used for lifting
  • Bearing nuts
  • Threads which may be unscrewed as shafts are rotated.
  • Components which are safety related.

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.



Review of various Locking methods

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)

Locking Nut /Jam 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..

Nylock /Simmonds Nut

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.

Slotted /Castle Nuts

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.

Shakeproof Washers

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 .

Spring Washer

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.

Wire Locking

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

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.

Locking Plates

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..

Slotted Nut

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..

Contoured Thread

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.  

Threadlocking Adhesives

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.

  • Some dependency on surface preparation
  • Affected by extreme temperatures
  • Inspection confirmation difficult
Miscellaneous

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..

  • Bearing Locknuts
  • Plastic Inserts
  • Helicoil Inserts
  • Taper Thread


Sites Providing Information On Bolt Loading
  1. Henkel-Loctite..Industrial Adhesives - Useful for threadlocking and sealing
  2. Spiralock..A screw thread system which is intrinsically self locking and claims superior load distribution
  3. The Jam Nut..A short paper including notes on the proper way to use a Jam Nut
  4. Boltscience - Vibration Loosening..Some useful relevent notes

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Last Updated 24/01/2013