Do bolts stretch when torqued?

A torque to yield fastener (TTY) or stretch bolt is a fastener which is torqued beyond the state of elasticity and therefore undergoes plastic deformation, causing it to become permanently elongated.

Do bolts stretch when tightened?

Even though bolts may seem perfectly rigid, steel is an elastic material. This means that when they are tightened in a bolted joint, they actually stretch a tiny amount.

What happens to bolts when they are torqued properly?

The Importance of Proper Torque

Typically, an under torqued bolt will deform and be unable to provide as much clamping force as needed. An over torqued bolt will break.

Do bolts stretch?

A bolt will stretch slightly as it is tightened. If you tighten the bolt too much, it will stretch beyond its elastic limit. If you measure a bolt’s overall length before you overtighten it, then again when you release the preload, the bolt will be slightly longer. This is similar to overstretching a coil spring.

Is when a bolt is stretched by applying tension or torque to a bolt?

“Torque” is simply a measurement of the twisting force required to spin the nut up along the threads of a bolt, whereas “Tension” is the stretch or elongation of a bolt that provides the clamping force of a joint.

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How do you tell if a bolt is a stretch bolt?

Like the others have said…if it gives you that “1/4 turn” line, it’s a stretch bolt. For example, the brake caliper bolts are listed as “always replace” in the Bently…but it’s just a regular bolt with the threadlocker stuff on it.

How much do bolts stretch?

The head is thought to stretch about half as much as the same volume of body would stretch. In order to use stretch control one must initially determine the amount of stretch each separate portion of the bolt contributes to the total stretch.

How many times can you use stretch bolts?

Bolts are designed to stretch as you torque them into place, and when you remove them, they snap back to their original position. Because of this, many standard head bolts are okay to use more than once, as long as they have not been stretched past their spring back point.

What happens if you don’t torque to spec?

Insufficient torque

Engines that are running, tend to vibrate. … In the case of insufficient torque, it’s the stud or the bolt itself that suffers all the impact, causing it to shear. Alternatively, the engine’s vibrations can cause the bolt or nut to come undone, displacing the tensioner.

Why do bolts stretch?

For a bolted joint to perform properly, the bolt must be stretched. As the bolt stretches it draws closer to the nut member thus compressing whatever is between these two components. … The more we can stretch the bolt (i.e. higher the preload) the higher the resulting clamp load.

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Why do bolts loosen?

Essentially, spontaneous loosening or rotational self-loosening occurs when a bolt rotates loose because of shock, vibration, or dynamic loads. Even a slight rotation can be enough for a bolted joint to lose all of its preload. This is the most typical cause of bolt loosening.

Why is bolt stretch important?

The stretch created in the bolt during tightening allows the fastener to clamp. This tension creates the clamping force, which holds our joint together. In critical joints, it is vital to ensure that the proper amount of preload is applied to the fastener and that the preload remains over the service life of the joint.

What should bolts be torqued to?

US Recommended Bolt Torque

Size Recommended Torque
Grade 2 Grade 5
7/16″ 24 41
1/2″ 37 64
9/16″ 53 91

How is bolt torque required?


The general equation for bolt load based on applied torque is T = KFiDb where K = torque coefficient. K is a function of the coefficients of friction and collar friction in the joint, and these are based on variables such as surface finish, coatings, and so on.