Bolt Torque Basics

Bolt Torque Basics

Torque is used to create tension. Bolts are used to affix two components so that they can resist tensile (pulling apart) and shearing (sliding apart) forces. After the nut has been turned onto the bolt, additional torque causes the nut to turn and stretch the bolt.

Torque is the application of force acting at a radial distance and tending to cause rotation; this is also called torque loading. It's used to create tension in threaded fasteners. When the nut and bolt are tightened the two plates are clamped together.

It is so important to realize the difference between torque and clamping force.

Torque is the application of force acting at a radial distance and tending to cause rotation; this is also called torque loading. It’s used to create tension in threaded fasteners. When the nut and bolt are tightened the two plates are clamped together. The thread converts the applied Torque into tension in the bolt shank. This in turn is converted into a clamping force. The amount of tension created in the bolt is critical.

Bolted joints are an integral part of equipment and component reliability as they connect a majority of our process equipment by coupling shafts together, holding down equipment or holding together the critical components.

A significant advantage of a bolted joint over other joint types, such as welding and riveted joints is that they are capable of being dismantled.

Under-tightening. By definition, an under-tightened bolt is already loose and the joint does not have enough clamp force to hold the individual sections together. This can lead to sideways slippage between sections, placing unwanted shear stress on the bolt that could eventually cause it to break leading to secondary damage or catastrophic failure.

A good bolted joint will be designed with the proper size and type of bolt and nut, and specify the optimal amount of tension to achieve the clamping force required to maintain joint integrity.

Under tightening leads to unwanted shear stress and bolt breakage.

Bolts can fail if we apply too much torque.

A bolt is like a spring and it can be over-torque and stretched too much causing distortion and

possibly failure.

A hytorc is used in many situations to tighten bolts.

The pressure setting will be given to achieve a specified torque.

The torque is applied and we may assume the joint is now tight.

It is important to be aware even if the correct torque is applied we may not achieve the desired preload or clamping force.

Simply we can apply a specified torque and there is a possibility the bolt and nut may come loose.

WHY?

First, we must understand the effect of friction and the use of a lubricant on the bolt and nut faces as well as the threads.

Lubrication Reduces Friction

IMPORTANT

When lubrication is indicated for a specified torque it must be used and remember to lubricate under the nut and bolt head faces achieving the desired clamping force.

It should be noted at times a torque setting may be stated without bolt lubrication. It is important to check this information.

Notice in the table, additional torque is required for a bolt and nut that have not been lubricated. Also, different grade bolts require different torque so it is critical only specified bolts must be used.

Importance of bolt markings.

Except for the very lowest grades, metric nuts and bolts all carry identification markings that indicate their strength. On the head of each bolt are two numbers separated by a decimal point.

Never use a bolt that is not marked.

Is an impact wrench the same as a torque wrench?

No. The main difference is that an impact wrench is a power tool that can exert a high impact force to tighten and loosen stubborn fasteners. On the other hand, a torque wrench is a hand or hydraulic tool with torque control that enables you to accurately tighten the nuts and bolts.

Torque Wrench or Tool has precise Torque Control

We do not have this control when we use an Impact Wrench. Never use an impact wrench if torque is specified.