We are bolting a lifting eye to a machine for transport. After transport the lifting eye is re-used for the next machine. Force is transferred by friction only. Now we are looking at ASME BTH-1 (2008) to check the strength. (I saw there is a 2011 version and ordered it, but this is typically going to take a while before I receive it)
From the paragraph of bolted connections:
Allowable tensile stress of the bolt: Ft = Fu / (1.2 * Nd)
Presumably Fu is the ultimate strength of the bolt. Nd is a safety factor. 1.2 is an additional safety factor (because of the stress concentration factor in threads?).
Allowable shear stress of the bolt: Fv = 0.62 * Fu / (1.2 * Nd)
The 0.62 is a rule of thumb to get from tensile allowable to shear allowable, taken form Kulak et al 1987. (I would have expected this to be 1/SQRT(3) = 0.58)
Allowable bearing stress of the connected part on the projected area of the bolt: Fp = 2.4 * Fu / (1.2 * Nd)
"where Fu = ultimate tensile strength of the connected part".
This is where I'm starting to get confused. It is not clear if that last statement is valid only for this equation, or also for the previous equations. I would have expected a different symbol in this case, for example "Fu,clamped" to indicate that this is a different ultimate strength than earlier.
The 2.4 is presumably a rule of thumb to get from tensile allowable to bearing allowable.
Allowable tensile stress for a bolt subjected to combined tension and shear stress:
Ft' = SQRT( Ft^2 - 2.6 * fv^2 )
The Ft was defined above as allowable pure-tension stress . The fv is presumably the actual shear stress in the bolt which will decrease the allowable tension stress? Instead of the factor of 2.6 I would have expected a factor of 3, but it is consistent with the 0.62 above ( 1/SQRT(2.6) = 0.62 ).
Allowable shear capacity of a bolt in a slip-critical connection for clean surfaces:
Ps = m * 0.26 * As * Fu / (1.2 * Nd)
"where As = tensile stress area and m = number of slip planes"
Big question is whether Fu is the ultimate allowable of the bolt or of the clamped parts? I would think the friction force that can be transferred is dependent on the clamping force, which is dependent on the bolt force, so I think it is the bolt allowable in this case. The 0.26 then seems to be some combination of friction coefficient and stress in bolt relative to ultimate allowable. "Bolts in slip-critical connections shall be tightened during installation to provide an initial tension of at least 70% of the specified minimum tensile strength of the bolt". That means the minimum coefficient of friction is 0.26/0.7 = 0.37 which seems kind of high. The text says "equation is based on a mean slip coefficient of 0.33 and a confidence level of 90% based on a calibrated wrench installation (Kulak et al 1987)". So I guess that with a bit of statistics one could arrive at 0.37 friction coefficient.
Do you people think I am interpreting all this correctly?
Does ASME have an email address that I can use to communicate technical questions about their standards?
From the paragraph of bolted connections:
Allowable tensile stress of the bolt: Ft = Fu / (1.2 * Nd)
Presumably Fu is the ultimate strength of the bolt. Nd is a safety factor. 1.2 is an additional safety factor (because of the stress concentration factor in threads?).
Allowable shear stress of the bolt: Fv = 0.62 * Fu / (1.2 * Nd)
The 0.62 is a rule of thumb to get from tensile allowable to shear allowable, taken form Kulak et al 1987. (I would have expected this to be 1/SQRT(3) = 0.58)
Allowable bearing stress of the connected part on the projected area of the bolt: Fp = 2.4 * Fu / (1.2 * Nd)
"where Fu = ultimate tensile strength of the connected part".
This is where I'm starting to get confused. It is not clear if that last statement is valid only for this equation, or also for the previous equations. I would have expected a different symbol in this case, for example "Fu,clamped" to indicate that this is a different ultimate strength than earlier.
The 2.4 is presumably a rule of thumb to get from tensile allowable to bearing allowable.
Allowable tensile stress for a bolt subjected to combined tension and shear stress:
Ft' = SQRT( Ft^2 - 2.6 * fv^2 )
The Ft was defined above as allowable pure-tension stress . The fv is presumably the actual shear stress in the bolt which will decrease the allowable tension stress? Instead of the factor of 2.6 I would have expected a factor of 3, but it is consistent with the 0.62 above ( 1/SQRT(2.6) = 0.62 ).
Allowable shear capacity of a bolt in a slip-critical connection for clean surfaces:
Ps = m * 0.26 * As * Fu / (1.2 * Nd)
"where As = tensile stress area and m = number of slip planes"
Big question is whether Fu is the ultimate allowable of the bolt or of the clamped parts? I would think the friction force that can be transferred is dependent on the clamping force, which is dependent on the bolt force, so I think it is the bolt allowable in this case. The 0.26 then seems to be some combination of friction coefficient and stress in bolt relative to ultimate allowable. "Bolts in slip-critical connections shall be tightened during installation to provide an initial tension of at least 70% of the specified minimum tensile strength of the bolt". That means the minimum coefficient of friction is 0.26/0.7 = 0.37 which seems kind of high. The text says "equation is based on a mean slip coefficient of 0.33 and a confidence level of 90% based on a calibrated wrench installation (Kulak et al 1987)". So I guess that with a bit of statistics one could arrive at 0.37 friction coefficient.
Do you people think I am interpreting all this correctly?
Does ASME have an email address that I can use to communicate technical questions about their standards?
