Minimum Permissible Bolt Stress, Sbmin

Let’s consider the Minimum Permissible Bolt Stress, Sbmin needed for the ASME PCC-1 Appendix O calculation. 

Unlike the Maximum Permissible Bolt Stress, Sbmax, which uses a percentage of the yield stress the Minimum Permissible Bolt Stress, Sbmin should be a hard floor of at least 20 ksi (140 MPa). This ensures that the bolts are preloaded enough to maintain their clamping force over time. The spring-like effects that make bolts such attractive fasteners are negated when they are not tight enough. Many times the Minimum Permissible Bolt Stress, Sbmin default values used by designers are based on 20% of the bolt yield stress, which may be TOO LOW or your application.

What considerations do I make when selecting the Minimum Permissible Bolt Stress, Sbmin? 

• How high is the Maximum Permissible Bolt Stress, Sbmax value? Anything above 65 ksi and I use 35 ksi with confidence. 

• What material are the bolts? Stainless steel bolts such as SA-193 B8 2 may have a Maximum Permissible Bolt Stress, Sbmax value as low as 35 ksi.  I like to use 20 ksi for stainless steel bolts with low Sbmax values so I have a useful range between the minimum and maximum.

• What value does the manufacturer suggest? Bolt manufacturers know more about bolts than you do. Just ask them.

Minimum Permissible Bolt Stress, Sbmin

Are poor assumptions being made in your torque tables? Metalmark Engineering loves reviewing torque tables to determine the source of leaky flanges.