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How to Calculate Bolt Preload?

Bolt preload is the tension created in a bolt when it is tightened. It is the force that holds the two parts of a bolted joint together. The preload must be sufficient to prevent the joint from coming apart under normal operating loads, but it must not be so high that it damages the bolt or the parts being joined.

There are several methods for calculating bolt preload. The most common method is the tensile stress method. In this method, the preload is calculated by dividing the tensile strength of the bolt material by the safety factor. The safety factor is a number that accounts for the uncertainty in the calculation and the possibility of the bolt being damaged during bolt tightening.

The following formula is used to calculate bolt preload using the tensile stress method:

Preload = Tensile Strength of Bolt Material / Safety Factor

For example, if the tensile strength of the bolt material is 10,000 psi and the safety factor is 2, then the preload would be 5,000 psi.

The following are the steps on how to calculate bolt preload using the tensile stress method:

1. Determine the tensile strength of the bolt material. This information can be found in the bolt’s specifications.

2. Determine the safety factor. This is typically a number between 2 and 3.

3. Divide the tensile strength of the bolt material by the safety factor. This will give you the preload in psi.

For example, let’s say we have a bolt with a tensile strength of 10,000 psi and a safety factor of 2. The preload would be calculated as follows:

Preload = 10,000 psi / 2 = 5,000 psi

This means that the bolt must be tightened to a torque that will create a tension of 5,000 psi in the bolt.

The slip-critical method is another method for calculating bolt preload. In this method, the preload is calculated by dividing the coefficient of friction between the bolt and the nut by the pitch of the bolt thread. The coefficient of friction is a measure of the resistance to slippage between two surfaces. The pitch of the bolt thread is the distance between adjacent threads.

The following formula is used to calculate bolt preload using the slip-critical method:

Preload = Coefficient of Friction * Pitch of Bolt Thread

For example, if the coefficient of friction between the bolt and the nut is 0.3 and the pitch of the bolt thread is 1 inch, then the preload would be 0.3 inches.

The following are the steps on how to calculate bolt preload using the slip-critical method:

1. Determine the coefficient of friction between the bolt and the nut. This information can be found in engineering handbooks or by conducting a friction test.

2. Determine the pitch of the bolt thread. This information can be found in the bolt’s specifications.

3. Multiply the coefficient of friction by the pitch of the bolt thread. This will give you the preload in inches.

For example, let’s say we have a bolt with a coefficient of friction of 0.3 and a pitch of 1 inch. The preload would be calculated as follows:

Preload = 0.3 * 1 inch = 0.3 inches

This means that the bolt must be tightened to a torque that will create a displacement of 0.3 inches between the bolt head and the nut.

The choice of which method to use for calculating bolt preload depends on the specific application. The tensile stress method is more accurate, but it is also more difficult to use. The slip-critical method is less accurate, but it is easier to use.

In addition to the above, here are some other factors that can affect bolt preload:

• The type of bolt

• The type of nut

• The lubrication used

• The ambient temperature

• The environment in which the joint is operating

It is important to consider all of these factors when calculating and controlling bolt preload.

How to Calculate Bolt Preload