Choosing the right fasteners

Choosing the right fasteners

Fasteners 

Spend more to reduce material

Don’t base your selection on the ‘strength’ table that might be provided. Consider the severe conditions such as the loads will impact the joint.

As the grade of material (BMT - base metal thickness) increases, the smaller the fastener has to be. However, higher strength means the material is harder. Hard metals are prone to ‘hydrogen embrittlement’ which is where the metal becomes brittle and fractures. In addition to this, harder and tighter joints crush soft joints easily.

When selecting a bolt base on strength level, remember that smaller parts in higher grade will save space, inventory and weight.

In most cases, the bolt is much stronger than the material it is attaching.

 

Thick finishes

Fasteners today require some sort of protective finish. Due to longer anti-corrosion requirements, the finishes have become much thicker. This becomes problematic when selecting the screws as the zinc-based finishes are inconsistent in their thickness.

 

Dimensional tolerances

Fasteners are often chosen with little to no thought regarding dimensional tolerances. Low tech, third world manufacturers sell parts at a fraction of the cost, constantly persuading customers to buy and regularly in large quantities. Many prints do not specify thread allowances, tolerances and dimensional information. An example of this is an automobile manufacturer purchasing washers of the correct size and thickness, but received them all with the punched holes off-centre - no doubt this was a slight issue…

 

Consider the conditions

A common mistake that is continually made by builders and designers is that the conditions in the real world will generally have an effect on the joint performance. Designs always look great on paper, however when the joint actually is physically fastened, it doesn’t perform as well as expected.
Vibration is one of the central factors that affect joint performance. Both major impacts (a vehicle collision at speed) and minor impacts (small repetitive collisions such as a corrugated dirt road) can destruct the most secure joints in time.

In addition to vibration, chemical effects (fuel, cleaning agents etc) can also damage joints.

 

Construct with future in mind

Many joints are constructed without thinking about service and repair - and rightly so! When you’re renovating or even building from scratch, finishing the job is in the front of your mind and it is quite easy to forget about repairing joints in the future.

If the joints are installed so that they can be disassembled with ease, it makes the service and repairing jobs a lot easier.