The importance of using proper torque values for nuts and bolts

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BMW wheel torque wrench
Torque stripe half shaft
Torque stripe half shaft

Whether installing your lug bolts on your car after removing a tire, replacing spark plugs, or replacing a water pump,  each bolt or nut you tighten on your car has calculated torque values it should meet for reassembly.  Both under torquing and over torquing a bolt can have catastrophic failure, therefore it is important to tighten each bolt or nut to the manufacturers specified values.

What is torque?
Torque is defined as a twisting force that tends to cause rotation.  When a socket is applied to a nut and turned down on, the force rotates the bolt.  As it tightens the bolt has a tension stress acted on it which stretches the out bolt.  Over torquing the nut or bolt too much will cause the bolt to stretch until failure.  Under torquing a bolt or nut will cause it to rattle loose over time.  

How is torque measured?
Torque is measured in force multiplied by distance.  A lug nut for example may have a torque value of 75 foot pounds.  Imagine having a wrench 1 foot in length with a 75 pound weight at the end, this resulting force is equal to a torque of 75 ft lbs.  A 2 foot wrench with a 10 pound weight at the end would have a torque value of 20 ft lbs on the nut or bolt.  In real life applications your hand applies the force and length of the wrench creates the distance.  This is how torque is transferred to a nut or bolt, force multiplied by distance.

American standard uses foot pounds or inch pounds to indicate torque values.  In Europe a Newton is used as a measure of force and Meters for the distance.  You can find conversation charts or calculators online to convert between the two.

What tool measures torque?
Torque wrenches are mostly widely used in either digital or analog styles.  Torque wrenches come in different lengths as well as different drives: eighth inch, quarter inch and half inch drive.  Having all three sizes in your tool box will make sure your all torque ranges are covered when working on a car.  Small bolts or nuts will require inch pounds and large bolts require foot pounds.  

It is important not to use these wrenches as you would some of your other shop tools; like as a hammer for instance.  Inside the wrench sits a calibrated spring which determines the torque value, improper use will throw the wrench out of calibration and it will need to be sent it off to a calibration shop.  Dropping your wrench will also throw off the calibration so it is important to handle the torque wrench with care

These fragile wrenches should be properly stored in a non humid environment and in its case after every use.  Make sure to return the torque wrench to its lowest setting to ensure the spring inside does not stretch out over time.

What determines the proper torque value for a bolt or nut?
Each application of torque is unique, the torque required for a bolt of a given size has a different torque value of a nut  of the same size fitting on that bolt.  This is because a nut has less surface area in contact with the threads than a bolt shank does with the mating surface.  These differences in surface areas create different friction values which influence the final torque value.  The more friction present, the higher the torque required to overcome this friction.

Other factors of determining a torque values are considered such as the material of the bolt.  As previously stated, while torquing a nut or bolt the bolt stretches from the tension stress created.  Bolts have different metallic properties requirements for each location of use.  Exhaust bolts need to withstand high temperatures and are often made of a metallic alloy called inconel.  This bolt will stretch differently than a standard steel bolt for used as a lug.

The engineers designing the product you are working on factor in these considerations and more calculating a proper torque value or range. 

Where to find torque values.
Torque values can easily be found in tables in shop repair manuals such as Bently publications or even in your owners manual for routine maintenance type applications.  For critical situations such as an engine or transmission rebuild I highly recommend investing in a Bently repair manual to find every critical torque value.  

What is torque sealing?
Have you ever noticed some bolts or nuts have a colorful stripe crossing from either the head or nut to the mating surface?  Chances are this is a torque seal or torque stripe.  When the car is being assembled from the factory the mechanics tighten each nut and bolt per the engineering specifications and when the inspector comes along to check the mechanics work he will place a liquid stripe across the bolt indicating it has been torqued to the proper value.  

This torque stripe can withstand grease, oil, gasoline and other toxic chemicals to make sure it will not be removed over time.  Other than indicating proper torque has been applied, torque striping also allows for a visual indicator to know which bolts have been torqued and which have not.  If you are reinstalling a half shaft you may lose your place and accidentally miss a bolt or maybe even tighten a bolt twice.  Torque sealing each bolt after achieving torque will prevent this from happening.

Another advantage of applying torque seal is that it allows a visual to see if over time the bolt has rattled loose.  If the seem between the nut or bolt head and the mating surface area is broken you know the bolt has been loosened, either by someone else or through vibrations over time.  

You can buy  tubes of torque stripe in a wide selection of colors for only a few dollars.  I highly recommend picking up a few bottles and get into the practice of using it after torquing each bolt.  Not only is it good shop practice but it looks professional.

 


BMW M3 wheel lug torque wrench
torque stripe
torque stripe
torque stripe
torque stripe

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