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| Torque to Yield Bolts |
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The reason the auto manufacturers began using torque to yield bolts was in the
effort to reduce the weight of their vehicles. They also offer a better clamping
technique, they are able to use less bolts, and a reduction in material costs.
With a regular head bolt a technician could torque a bolt to its yield point and it
would return to its original form and length. This is because of its elasticity it is able
to stretch and return to its original form.
This is not so with a torque to yield bolt. There are two terms related to this
phenomenon. The first is elastic deformation and the second is plastic
deformation:
1. Elastic deformation: This is the amount of force or torque where you haven't
changed the shape or length of the bolt. At this point you are able to reuse the bolt
as it hasn't changed its shape.
2. Plastic deformation: This is the point past elastic deformation where the bolt
has changed its shape and length. At this point the bolt can't be reused and needs
to be replaced.
For example an aluminum cylinder head will be torqued in a sequence say one to
ten. The first pass would be a torque to say 36 foot pounds. This is the elastic
deformation point. The second pass would be a 90 degree angle or a quarter turn.
At this point the bolt is going into plastic deformation. The third pass would be
another 90 degree angle. At this point the bolt has changed its form and must be
replaced with a new torque to yield bolt.
So in summary you can use the old fashion head bolts over and over but when it
comes to torque to yield they need to be replaced with new ones.
effort to reduce the weight of their vehicles. They also offer a better clamping
technique, they are able to use less bolts, and a reduction in material costs.
With a regular head bolt a technician could torque a bolt to its yield point and it
would return to its original form and length. This is because of its elasticity it is able
to stretch and return to its original form.
This is not so with a torque to yield bolt. There are two terms related to this
phenomenon. The first is elastic deformation and the second is plastic
deformation:
1. Elastic deformation: This is the amount of force or torque where you haven't
changed the shape or length of the bolt. At this point you are able to reuse the bolt
as it hasn't changed its shape.
2. Plastic deformation: This is the point past elastic deformation where the bolt
has changed its shape and length. At this point the bolt can't be reused and needs
to be replaced.
For example an aluminum cylinder head will be torqued in a sequence say one to
ten. The first pass would be a torque to say 36 foot pounds. This is the elastic
deformation point. The second pass would be a 90 degree angle or a quarter turn.
At this point the bolt is going into plastic deformation. The third pass would be
another 90 degree angle. At this point the bolt has changed its form and must be
replaced with a new torque to yield bolt.
So in summary you can use the old fashion head bolts over and over but when it
comes to torque to yield they need to be replaced with new ones.