[modern] Fastener Torques

Dave,

You mentioned in a previous note …

“If you torque a normal bolt to the point where it starts
to stretch you are over torque. On a Torque to yield bolt
the design and material is such that you are actually
expecting to put a certain degree of stretch into it. That
results in a spring like clamping action.”

I think I understand what you are trying to say … but I’m
not sure you described the situation correctly. Any
bolt/stud will stretch whenever a force (torque) is applied
to it. This is just the nature of materials … they will
stretch whenever resisting a force. The key to this
stretch situation is as follows:

If the force acting on the fastener does NOT exceed the
yield strength of the fastener material, then the bolt or
stud will stretch in an “elastic” manner. As long as the
fastener operates in this mode, there is NO permanent
deformation. What is important to understand is the
clamping force acting on the components is a linear
relationship in regards to fastener tension. Increase
fastener torque and you will increase clamping force.

If the force acting on the fastener EXCEEDS the yield
strength of the fastener material, then the bolt or stud
will stretch in an “plastic” manner. Once the yield
strength of the fastener is exceeded, there is NO
significant additional clamping force generated. Maximum
clamping force has been achieved and the fastener simply
continues to elongate when additional force (torque) is
applied until the point of failure.

The fasteners ability to withstand high tensile loads gives
it this “stretching or spring action” as you call it. The
tensile force acting on the fastener is converted to a
compressive force working to hold the affected components
together. I think you are referring to operation in
the “plastic” range when you call it “over torque”.

The “spring” analogy doesn’t completely fit the operation
of a fastener, but its sufficient to say they both work
under tensile loads.

John

There is a good description at
http://www.dnd-automotive.com/engine/torque-to-yield.htm

So if Jaguar says the bolts can be reused twice is that indicating that they are not torque-to-yeild bolts or that they
think they are strong enough to stretch three times?

3/7/02 8:58:00 PM, jpring@carolina.rr.com wrote:>Dave,

You mentioned in a previous note …

“If you torque a normal bolt to the point where it starts
to stretch you are over torque. On a Torque to yield bolt
the design and material is such that you are actually
expecting to put a certain degree of stretch into it. That
results in a spring like clamping action.”

I think I understand what you are trying to say … but I’m
not sure you described the situation correctly. Any
bolt/stud will stretch whenever a force (torque) is applied
to it. This is just the nature of materials … they will
stretch whenever resisting a force. The key to this
stretch situation is as follows:

If the force acting on the fastener does NOT exceed the
yield strength of the fastener material, then the bolt or
stud will stretch in an “elastic” manner. As long as the
fastener operates in this mode, there is NO permanent
deformation. What is important to understand is the
clamping force acting on the components is a linear
relationship in regards to fastener tension. Increase
fastener torque and you will increase clamping force.

If the force acting on the fastener EXCEEDS the yield
strength of the fastener material, then the bolt or stud
will stretch in an “plastic” manner. Once the yield
strength of the fastener is exceeded, there is NO
significant additional clamping force generated. Maximum
clamping force has been achieved and the fastener simply
continues to elongate when additional force (torque) is
applied until the point of failure.

The fasteners ability to withstand high tensile loads gives
it this “stretching or spring action” as you call it. The
tensile force acting on the fastener is converted to a
compressive force working to hold the affected components
together. I think you are referring to operation in
the “plastic” range when you call it “over torque”.

The “spring” analogy doesn’t completely fit the operation
of a fastener, but its sufficient to say they both work
under tensile loads.

John