The amount a bolt lengthens when tightened, which is proportional to its tension as long as it stays elastic. Measuring stretch with a stretch gauge, a micrometer over both ends or an ultrasonic bolt gauge sidesteps the friction uncertainty of torque control. Racing connecting rod bolts are specified this way, often a few thousandths of an inch, and critical bolting on presses and wind turbines is checked ultrasonically.
Rod bolts call for six and a half thou of stretch, so put the stretch gauge on them and forget the torque number.
Also heard as
- elongation
Related terms
Preload
The tension created in a bolt by tightening it, which clamps the joint and keeps it from slipping or opening.
Torque-to-yield
Tightening by torque plus angle until the bolt just yields, giving consistent high preload; the bolt is single-use.
Torque-tension
The relationship between the torque applied to a fastener and the preload it produces, dominated by friction.
Necked down
Describes a bolt visibly thinner in one spot because it was stretched past yield and started to neck.