Torque is an measurement of the rotational force that is use to create a clamp load on the bolt. The clamp load is the load that hold the two parts of the bolt together. If too much torque is applied to a bolt, that bolt will stretch beyond it’s proof load and the bolt will permanent yield.
Once a bolt permanent yields, the bolt will no longer maintain a clamp load. The grade of the bolt determine the amount of torque that the bolt can handle before it will fail. Grade 2 bolts is low-carbon bolts that have no markings on the bolt head, and those bolts have a tensile strength of 74,000 psi.
How to Tighten Bolts Correctly
Grade 5 bolts has three radial lines on the bolt head, and those bolts can have a tensile strength of 120,000 psi. Grade 8 bolts have six radial lines on the bolt head, and those bolts has a tensile strength of 150,000 psi. It is important to identify the bolt grade to ensure that a Grade 2 bolt is not being used in an application that may experience high levels of stresses; in such an instance, the bolt will shear.
The degree of lubrication of the bolt will impact the amount of torque that are required to achieve the necessary clamp load. Bolts with dry threads will require more torque to achieve the same clamp load as bolts with lubricated threads. Adding lubrication, such as engine oil or anti-seize lubricants will decrease the amount of torque that is required to achieve the same clamp load as dry threads.
For instance, using grease will reduce the required torque to 70% of the dry torque value; using anti-seize will reduce the required torque to 65% of the dry torque value. Using the torque value for dry threads on a bolt that is lubricated will over-clamp the bolt, which can lead to the bolt stretching beyond its elastic limit. Stainless steel bolts requires lubrication during installation; stainless steel bolts are known to experience galling at the bolt threads.
In instances in which multiple bolts is being installed in a component, it is essential that the bolts are tightened in a specific sequence. On flanges that has four bolts, the bolts should be tightened in a criss cross pattern. The bolts should be tightened in stages to the target torque; one-third of the target torque, two-thirds of the target torque, and then the full target torque.
On components with six bolts arranged in a circle, the bolts should be tightened in a star pattern. In instances in which the neighboring bolts are tightened first, the flange may warp; a warped flange can lead to fluid leaks at that component. Different components of automobiles require different torque values and procedures for installation.
For instance, the lug nuts of a compact car may require 80 to 100 foot-pound of torque, but the lug nuts of a truck may require 150 foot-pounds of torque. In either instance, the lug nuts should be tightened in a criss cross pattern to ensure the lug wheel is centered on the compact car’s hub. Spark plugs require 15 to 20 foot-pounds of torque when anti-seize lubricant is use on the threads of the spark plug.
Intake manifolds require 18 to 25 foot-pounds of torque because the intake manifolds are made of aluminum, which is a material that yield easily. Exhaust manifolds may require the bolts to be retorqued after the engine has been started and heated up to allow the gaskets to settle into the exhaust manifold. Many moddern engines use torque-to-yield bolts for components like intake manifolds.
Torque-to-yield bolts is designed to permanently stretch the bolts to increase the clamp load. These bolts should of been replaced each time they are removed from the engine. The tools that are use to install bolts will impact the accuracy of the clamp load that is created by those bolts.
Click wrenches are available in different torque settings, and the wrench will provide accuracy in the clamp load when it “clicks” to the desired torque setting. Click wrenches provide accuracy of within 4% of the desired torque value. Beam-type torque wrenches are reliable because the needle against the scale provide accurate readings; there is no need for regular calibration of the tool.
Digital torque wrenches are often used by individuals who require precision in the clamp load that is created by the bolt. Digital torque wrenches provide the ability to log the peak torque value that was created by the bolt with the tool. Torque sticks are used for lug nuts; however, they are less accurately than click wrenches.
Therefore, torque sticks should never be used on engine components. A breaker bar is used to loosen bolts; however, a torque wrench should be used to tighten the bolts to the proper torque value. Inaccuracies in the installation of bolts can lead to the failure of the bolted components.
Using impact guns can apply a torque that is much higher than the target torque for the bolts; therefore, these bolts must be checked with a torque wrench after using an impact gun. Using dirty threads will increase the friction between the bolt and the components; this can fool the worker into thinking that the proper torque has been applied. Thread locker can be used to prevent bolts from loosening due to vibration.
Blue thread locker is used for bolts that are to be removed later; red thread locker is used for applications that are to be permanent, and that will require the use of heat to remove the bolts. Finally, each component of an automobile will have a torque specification in the owner’s manual for that make and model of automobile; using the wrong torque specification will lead to the failure of the component.
