Ask The Expert: With Kevin Connolly, Earnest Machine’s Application Engineer.
Question: Why is the hardness of a Grade 8 Nut lower than the hardness of a Grade 8 Bolt?
Grade 8 bolts have a specified hardness of Rc 33/39.
Grade 8 nuts (in diameters up to 5/8”) have a specified hardness of Rc 24/32.
Answer:
A rule of thumb for fastener applications is that the strength of the nut should be equal to or stronger than the bolt. Since hardness has a direct correlation to strength, it is logical to assume that since the hardness of the nut is lower than the bolt, then the nut is not as strong.
There are two reasons why the nut is typically lower in hardness than the bolt of the same grade.
- The thread shear area of a nut is larger than the cross sectional area of the bolt so the nut does not have to be at the same hardness of the bolt in order to support the same clamping load.
- It is desirable for the threads inside the nut to deflect so that all the nut threads are supporting the bolt clamp load. The lower hardness of the nut threads ensures the nut threads deflect to conform to the bolt threads and support the load for the entire nut height.
The “strength” of the nut (called its Proof Load Strength) is controlled by two factors:
- the number of full threads that are in the nut (the nut thickness)
- the hardness of the nut
Even though the nut is not as hard as the bolt, a full thickness nut at a hardness of Rc 24 will be as “strong” as the bolt that is at Rc 39. The Nut will have a Proof Load Strength of 150,000 psi and the bolt will have a Tensile Strength of 150,000 psi.
Kevin Connolly is a 35 year veteran of the fastener industry having earned his Bachelor of Science Degree in Mechanical Engineering Technology from Cleveland State University. His experience in the fastener industry includes Product Design, Product Testing and Applications.
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