Bolt Science: What is the Jost Effect

On March 31, 2015 – Machine Design published a post titled, Demystifying Bolt Preload.

The article begins by highlighting a study for the National Transportation Safety Board (NTSB) which found the “… the leading causes of wheel separations from medium and heavy duty trucks are improper tightening of wheel fasteners…”

Then, on June 16, 2016, Stephen Mraz published a post titled, Force Calculations for Bolted Joints, in which he shared an illustration of how “… bolt preload that produces a given non-zero clamping force can be determined …” using the graph below in combination with the following equation:

Fe/FPR = (1-b)(Fe/FPR)0

Where b = |Fp/Fi|

And the corresponding bolt load can be computed from:

Fb/FPR = (Fe/FPR)0Fi/FPR)(b/r)

Machine Design - Force Calcuate on Jointed Bolts

These articles deliver answers on how to properly use and tighten bolts, but, also open the door to questions inside an engineers head.

Seriously, think about it.

Have you ever wondered about the science behind a bolt?

Actually, most things in life that we take for granted have science behind how they work.

Bolts are no different.

Most people think a bolt and nut are simple things that hold stuff together, but we both know there is more to it than that.

Here is some fun bolt science you should know:

How does a Bolt Work?

A bolt is specially designed to create friction.

This friction causes the bolt to state tight and pre-loaded.

Friction on the head and the thread helps keep a bolt from loosening.

The friction in a bolt is caused by the elasticity of the bolt.

Did you know bolts were elastic?

You probably never knew that a bolt’s thread is elastic much like a spring.

When the friction is put on the threads, the threads stretch preventing the bolt from loosening.

The main point of a bolt is to clamp machine parts together with the exact amount of force needed that it won’t loosen while working.

However, there are times that you need to loosen a bolt.

That is when the Jost Effect comes into play.

What is the Jost Effect?

The term “Jost Effect” was derived from the Jost Institute of Tribotechnology at the University of Central Lancashire.

They described the Jost Effect as “a reduction in friction in order to loosen the bolt.”

This effect is used in many things including removing corks from bottles.

If you rotate the cork, first, it is much easier to pull it out of the bottle.

Therefore, the Jost Effect states that “the reduction of friction causes a bolt to loosen or creates it to fail”.

Basically, the Jost Effect is turning the bolt in the opposite direction than you did in tightening it so that you can release the friction and loosen the bolt.

When do you Need to Use the Jost Effect:

Typically, you don’t want your bolts to come loose, but there are times you do want they to come loose.

In these cases, the Jost Effect is applied.

Here are a few examples and reasons you’d want to loosen a bolt:

  • When you need to change a tire:

You may need to loosen your bolts on your wheel to change and remove your tire.

If you are a heavy equipment service provider, you would most likely use a pneumatic torque wrench like a TTP TorcGun or JetYD to remove the bolts/lugs from the tire of say a CAT 775G.

However, the average Joe will not carry around a pneumatic torque wrench in his vehicle.

Instead, he utilizes a lug wrench / breaker bar which is torqued manually via application of the “Jost Effect”.

  • To open a bottle of wine:

We lightly covered this one earlier, when a cork is popped into a bottle, the friction between the cork and bottle makes it hard to open.

Depending on the bottles design, you can use a corkscrew, or your hand to pop open a bottle of wine/ champagne.

Either way, you will need to apply the Jost Effect of twisting the cork first, to loosen the friction and open the bottle secondly.

  • When one part of a bolted joint is broken:

Often, two parts bolted together will break or need repaired.

Reasons for breakage can be a result of excessive tightness, misuse of the item, intense vibration, old and tensile strength limited, among other things.

The loosing of a bolted joint often requires fixed pressure on a bolt head, and application of the Jost Effect to force rotation of the nut away from the bolt stud [the threaded portion of a bolt].

Once, the bolted joint is loosed, you fix/replace what you must, and securely fasten the joint again using an applied force called “torque”.

The Science Behind Loosing Bolts Is Fun

So, there you have it, when it comes to science — there is a little science in everything, and science is fun.

What did you learn about the Jost Effect that you didn’t know before?

What other areas of your life can you apply science?

We would love to know in the comments below.