Why is important to keep a bolted joint tight?
It is well understood that a fastener will fail if it is loaded above its capacity, as shown in example #1 below. What is less obvious is a fastener will also fail if it is loaded below its capacity but is subject to cyclical loading. This condition is shown in example #2 below. A properly tightened joint is shown in example #3 below.
For illustration purposes, a 3/4″ UNC, Grade 5 hex bolt and nut will be used with these standard specifications:
|Load Type||Force (lbs)||Note|
|Clamp||31,000||recommended as 75% of proof load|
|Proof||28,400||90-95% of yield|
|Yield||30,800||the force beyond which permanent deformation occurs|
|Ultimate||41,100||the force that will result in failure|
Calculate these values for any size and grade bolt using the Excel spreadsheet downloadable below.
Example #1: Failure due to over loading
Independent of how much clamp force (A) is created by tightening the bolted joint, the bolt/nut assembly will fail if the working load exceeds 40,100 lbs. This is fairly obvious.
The bolt and/or nut will also be permanently damaged if the working load exceeds the yield strength of 30,800 lbs.
Example #2: Fatigue failure due to cyclical loading
In this scenario:
- The working load does not exceed the proof load strength of the bolt
- The bolt/nut assembly is only tightened to 50% of the working load.
Bolted joint tightened to 10,000 lbs (A) of pretension. In real world installations it is common the see joints that are tightened below the working load. In this situation, the bolted joint initially appears tight but as the working load is applied, the clamp surface is separated (figure 3 below).
The bolted joint experiences loads (B) varying from 10,000 lbs (pretension) to 20,000 lbs at full load.
As the load exceeds 10,000 lbs, the plates separate, and the assembly (bolt, nut, and plates) do not experience the static and cyclical load as one solid piece. Under this condition, the full cyclical load is transferred to the bolt and the bolt is subject to probable fatigue failure. This failure will occur after a predictable number of cycles even though the proof load of the bolt was never exceeded.
Along with the possibility of fatigue failure in this condition, when the clamp surface is separated, the plates can move in the lateral direction causing addition shear forces on the bolt and possible loosening of the nut.
Example #3: No failure – properly loaded joint
In this scenario:
- The working load does not exceed the proof load strength of the bolt.
- The bolt/nut assembly is tightened to the recommend clamp force (A) of 21,300 lbs.
With the surfaces are held tightly together, the assembly (nut, bolt, and plates) experiences the static and cyclical load as one solid piece. In this case, the bolt only sees the proportion of the total load that the cross section represents. The bolt in this scenario are in what is known as a protected condition.
Lesson: Loose bolts fail while properly tightened ones do not.
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