introduction TO FRICTION

For every two surfaces that come in contact there is friction. Friction opposes the direction of motion, such as when you try to push a box across the floor the frictional force is what is opposing the motion. To calculate the friction between these surfaces you must determine their friction coefficient, which describes the resistance of the the two surfaces to slide across each other. For instances, the rougher the surfaces are the greater the coefficient of friction will be. Since this is based on a microscopic level of the two surfaces the amount of surface area touching between the two objects has no effect on the friction coefficient. However, the force at which the two surfaces are being pressed together does make a difference, this is call the normal force. The normal force is another name for the action reaction forces between two objects. So the force of friction is equal to the friction coefficient multiplied by the normal force.


Friction between the threads

Friction plays an important role in the functionality of bolts. As was discussed on the PHYSICS OF BOLTS page, the action reaction (normal) forces increase as the nut is tightened onto the bolt (see picture below). And as we just went over, the frictional force increases as the normal force increases. This is why the nut becomes more difficult to turn as it is tightened down and why it stays in place once it is tighten down. If there was no frictional force between the threads to oppose motion the nut could easily come loose. This is why people often add a lubricant to the threads when they are trying to loosen a bolt.

Diagram by Taylor Tharp

Friction and torque

This plays an especially important role when it comes to pretension bolts. Pretension bolts use a special tool for installation; as you may have noticed these bolts do not have a hex shaped head to hold the bolt while the nut is tightened down. Instead pretension bolts have a spline at the end of the shaft. The tool that is used grabs on the spline and the nut at the same time and turn them in opposite directions which tightens them up faster. As the Normal force between the threads increase, the frictional force also increases which in turn also increases the amount of torque (rotational force) needed to turn the nut onto the bolt. When the torque becomes so great that the nut and bolt can barely turn anymore the rotational force breaks off the spline along a groove between the spline and the shaft of the bolt called the shear plane. The shear plane is specifically designed to be the weakest point along the shaft so that it intentionally breaks off at this point and not somewhere else along the bolt.

Everything about these bolts is specifically designed so that the spline breaks off when it reaches a specific torque, which varies based on the diameter of the bolt. However, it is easier and more accurate to measure the normal force than the torque using a load cell which measures the compression caused by the action reaction forces. Below is a table from the AISC (American Institute of Steel Construction) Thirteenth Edition published in 2009 that specify the minimum normal force that the bolts must reach based on the bolt diameter.

From AISC Manual 2009 13 Edition