Bolts are an essential item in a bolted connection, and whilst that sounds like the ultimate obvious statement, you would not believe how many missing bolts we discover in our structural inspections. Some (due to misaligned holes) were NEVER installed.
When we calculate bolt group strength, and we need 5.3 bolts, we use 6. Not 8, just in case a couple fall out or don’t make it into the hole before knock-off.
So it’s pretty important that bolts, once installed, stay there for the life of the structure.
When choosing a method for preventing bolts from loosening in an assembly there are many different options, each having a specific purpose.
Personally, I’ve seen just a single nut and a flat washer in structural connections as per standard drawing notes. For tensioned bolts, this is usually fine, unless it’s not tensioned correctly.
Where vibration exists, there should be a greater emphasis on the use of proprietary systems such as those by Nord-Lock Group. Whilst they cost more than a flat washer, they pay for themselves in reducing unplanned shutdowns.
How many of the following methods have you seen employed?
Friction locking methods are based on preventing surfaces from sliding against each other by increasing the resistance (friction) between these contact surfaces. Increased friction between the male and female threads, or between the fastener and contact surface, creates resistance to loosening.
Common friction-based bolt locking products:
- Split ring washers
- Serrated washers
- Tooth lock washers
- Nylon insert nuts
- Double nuts
Split ring washer
Mechanical locking utilises a physical barrier with the aim of preventing the fastener from rotating. Mechanical locking devices include tab washers, which have a side tab that can be bent upwards to lock the nut in place, and locking wire, which can be threaded through a hole in the bolt head/nut and tightened to another fastener close by.
Both can prevent the fastener from rotating fully, but it should be noted that a proportion of the preload can be lost over time under dynamic load conditions.
Common mechanical bolt locking products:
- Tab washers
- Locking wire
bolt locking wire
This method uses a liquid, known as thread-locking adhesive, to prevent a bolted joint from loosening. The adhesive is applied to the thread of the fastener.
Adhesives are not restricted by the size of the fastener used; however, they do pose a risk of chemical exposure and may be hazardous to health. They can also have a significant adverse effect upon the torque/load ratio, resulting in uncontrolled preload during tightening.
Applying thread-locking adhesive
The geometry method utilizes the shape of the fastener to secure the bolted joint in place when subjected to vibration and dynamic loads. The most common system, wedge-locking washer, is composed of a pair of washers with cams on one side and serrations on the other.
The serration forces any rotational movement to occur between the washers and the geometry of the cams introduces a need for an increase in tension in the bolt if the bolts want to rotate loose. This safely secures the joint.
How the Nord-Lock Washer works
There are some products that do not have a locking function. They might be commonly used due to ease of procurement, but the main purpose is mostly to protect the mating surface of the application.
Common non-locking products:
- Plain washers
- Disc spring washers
Disc spring & flat washer
Perhaps it’s time to consider the method of fastening bolted joints, especially those in environments where vibration and cyclic loads can (and will) loosen bolts over time. If nothing is specified, you’ll get a flat washer and your structure will ultimately suffer.
Need help specifying the most appropriate method? We can help.
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Book a meeting with us and we will assist you with the structural design and analysis of your structure to help you produce a structure that is fit-for-purpose and capable of resisting all applied loads without failure during its intended life.
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