Bolted Connections

At present, bolts are being produced to at least 85 different industrial standards and the requirements for bolted connections vary for the different sectors depending on design, operational and maintenance requirements.

Achieving a stable bolted connection will therefore require a qualified evaluation of the following factors:    

  • Load design
  • Choice of materials with a view to mechanical properties and corrosion resistance
  • Where appropriate, use of lubricant
  • Pre-loading (pre-tensioning) and use of the correct torque equipment

The locking of bolts in order to secure against loss of pre-load (tension) is defined as secondary retention. Reasons why so many bolts and bolted connections fail:

Improper use, installation and handling of the bolt      (30%)
Vibrations                                                                              (20%)
Knocks                                                                                    (12%)
Loads (beyond design)                                                         (11%)
Wear                                                                                         (6%)
Corrosion                                                                                 (5%)

Several independent industry tests show that double nut, jam nut or dual nutting arrangements are not reliable methods of securing screwed / bolted connections and are particularly unsuitable for retaining loads in tensioned bolting. The practice of dimpling threads is also inadvisable.


Vibration test
The graph above showing the performance of Nord-lock products illustrates a typical comparison study of different retention methods.
These tests are designed to replicate anticipated dynamic loading on bolted connection.

To distinguish between bolt types and retention suitability, we have presented the following recommendations in two groups namely bolted connections where clamping force is critical and others where it is not

Reliable Securing of Bolted Connections

Flexing of bolted structures and vibration or shock loading in machinery can cause bolted joints to loosen. Thermal cycling may also cause nuts and bolts to become loose.

The majority of bolted connections in the oil and gas industry are subject to dynamic loading in some form. Loose nuts and bolts can lead to joint failure and dropped objects, resulting in avoidable incidents and un-scheduled downtime.

To prevent nuts and bolts from loosening, a reliable, tested and suitably approved secondary retention method should be used. This is prerequisite where maintaining the clamping force across the bolted connection is critical.

Note: Studies undertaken by many independent test organisations over the last forty years prove that many of the older methods of securing bolted joints are not reliable. These methods are now falling out of favour, and may be forbidden by some companies.

Reliable securing of bolted connections where maintaining the clamping force is critical

Several independent tests show that double nuts and so-called jam nuts are not a reliable method for securing screws / bolted connections. 

Here are some recommended methods:

Nord-lock washers

Nord-Lock secures the bolted connection using tension instead of friction. The system consists of a pair of washers that fit together, with cams on one side and radial teeth on the other. Since the cam angle ‘ ’ is greater than the thread pitch ‘ß’, a wedge effect is created by the cams, and this prevents the bolt from rotating.
Nord-Lock’s locking washers are certified by several independent institutes, such as ABS, DIBt, DNV and TÜV.
Each pair of washers has a unique control number, which ensures full traceability right down to the first installation.
Areas of use:
Almost unlimited, but particularly suitable for connections exposed to vibrations or other dynamic loads. nord-Lock’s washers are available in dimensions from 3 mm up to 130 mm in various materials.

Safe-lock (spiralock)

Spiralock is an all-metal lock nut / bolt with a specially designed threaded profile that locks when tightened and distributes the tension across the entire length of the thread. This provides better load distribution, which helps to improve the locking of the bolt connection.
Areas of use:
Most often used in cable tray systems, but can also be used in other installations.

Bondura bolt

BONDURA® has a unique design which uses expanding taper sleeves at both ends of the bolt to eliminate play.
The bolt also prevents play from recurring. Standard screws are used to push in the bolt tapers, and the bolt is fixed directly to the machine component using locking screws. This prevents the bolt from coming loose, falling out or turning in the hole. The bolt can be retightened as the equipment is exposed to wear.
Areas of use:
For example, as a replacement for clevis bolts in top drives, and hinge bolts on dollies, pipe handling equipment and cranes.

SuperboltMulti-JackBolt Tensioners (MJT)

Superbolt MJTs are available as nuts or bolts as replacements for conventional bolting elements. They only require hand tools for installation and removal, eliminating requirements for heavy tightening equipment


The concept of torque

Torque is the application of Force that creates Tension in the bolt. Tension creates a Clamping Force between the two parts.
Where Clamping Force is critical, consult with manufacturer, relevant design codes or industry specialists to determine the most appropriate retention methods.

Here we illustrate secondary retention for bolted connections typically used for securing of equipment components and other ancillary items.
The following methods are recommended for bolted connections where maintaining the clamping force is non-critical.

Nylon Insert Nut

Also known as Nyloc, this nut includes a nylon collar insert. The collar deforms elastically as it is applied to the bolt. This increases friction between both sets of threads creating the required purchase for the connection.
A versatile fastening for non-critical connections.
Re-use is not advised. May rotate and loosen when exposed to dynamic loading.
Areas of use:
This type of nut is often used for locking in connections where a certain degree of lost pre-tension can be accepted.


Metal Locking Unit

Metal locking nuts may be used on all bolt dimensions. This type of nut comes in various forms and may feature a deformed head, split neck or toothed collar ring. Purchase is created by friction, cutting into the thread or contact face. Friction grip relies upon high pre-load and correct torque.
A versatile fastening for non-critical connections.
Lubricating the threads my reduce function. May rotate and loosen when exposed to dynamic loading


Castle Nut and Split Pin

Castellated nuts provide a visual and reliable method for locking bolted connections. Generally used for bolted connections exposed to shear forces rather than tensile stresses. The nut has radial slots and is locked by non-corrosive split pins that are inserted through a hole in the bolt.
Used on connections or components that are disconnected frequently.
Castellated nut arrangements are not suitable for pre-loaded joints or connections where maintaining the clamping fore is critical.


Tab Washer / Tap plates

Tab washers can be used on all dimensions and in any application where the use of tab washers is appropriate. There are several types with different areas of application for locking either nuts or bolts. it is important to use the correct type for each purpose.
Areas of use:
Typically used on machinery where it is important to prevent the bolt from rotating.


Palnuts cut into the bolt threads when they are applied and tightened.
Not suitable for re-use. May loosen when exposed to dynamic loading. Low-grade palnuts may corrode in marine evironments
Areas of use:
This method is widely used as one of several alternatives as an extra barrier for locking of through bolts. Industry experience shows that self-locking contra nuts is a reliable method to use on bolted connections witch are not exposed to heavy and continuous vibrations.


Spring / Serrated / Tab Washers

There are several types of washers and tab plates – all with different designs and features. Always ensure the correct type has been applied in accordance with equipment manufacturer’s guidance / technical support.
Independent tetsts have shown that some washer types will loosen when exposed to dynamic loading. All washer selections should be appropriate to the application, environment and the criticality of the connection.

Prevent Dropped Objects