Bolts are mechanical fasteners that must be tightened using a specific amount of turning force to ensure that the joint created by the bolt remain secure. Using too little turning force will cause the bolt to become loose, while using too much force may cause the bolt to stretch or even snap. The amount of turning force required to secure the bolt to the apropriate amount of clamp load is referred to as torque, and torque are measured in units of foot pounds.
In order to determine the correct torque that must be applied to a bolt, it is first necessary to identify the SAE grade of the bolt. The lines etched on the head of the bolt indicate the strength of the bolt; the more lines that is etched onto the bolt head, the stronger the bolt. For instance, a bolt that does not have any line etched into its head is a Grade 2 bolt.
Tighten Bolts with the Right Torque
Grade 2 bolts is made of low-carbon steel, and have a tensile strength of 74,000 PSI. A bolt that contains three radial lines etched into the head are a Grade 5 bolt; these bolts are made of medium carbon steel with a tensile strength of 120,000 PSI. Finally, bolts that has six radial lines etched into the head are Grade 8 bolts, which are made of alloy steel with a tensile strength of 150,000 PSI.
Each of these grade can be used in different applications, but you should of used the correct grade for the specific bolt and project. In order to determine the correct amount of torque that should be applied to a bolt of a specific size and grade, bolt manufacturer provide torque charts for bolts of different sizes (such as quarter-inch bolts or one-inch bolts). Additionally, these charts also provide different values for bolts with dry threads versus bolts with lubricated threads.
Lubricated threads will require less torque to be applied than bolts with dry threads due to the reduced friction between the threads of the bolts. For instance, a Grade 5 bolt may require 57 foot-pounds of torque for bolts with lubricated threads, but require more torque when the threads is dry. Thus, the torque specification must account for whether the threads are dry or lubricated.
Bolts have different type of threads. Coarse threads (UNC threads) have fewer threads per inch than fine threads (UNF threads), and therefore is easier to assemble quickly. Fine threads require more turning to be fully assembled due to the increased number of threads per inch.
Coarse threads are used for general repairs, but fine threads are used for projects that requires high degrees of precision. The friction between the threads of bolts play a crucial role in the amount of torque that is required to tighten bolts. For bolts with clean and dry steel threads, the friction factor is 0.20.
Zinc plating the threads will reduce the friction factor to 0.17; using oil or anti-seize on the threads will reduce the friction factor to 0.15. Thus, the friction factor of bolts must be taken into account when applying torque; less torque is required for bolts with higher friction factors. The mathematical equation for torque is torque = friction coefficient x bolt diameter x desired preload.
There are specific steps that must be taken in order to ensure that a bolt is properly torqued. First, the bolt must be cleaned of any rust or grit that may interfere with the proper measurement of the torque that are applied to the bolt. Second, the bolt must be inspected for damage or stretching of the metal.
Third, the bolt should be hand-started into the bolt hole in the project piece to avoid cross threading the bolt. Cross-threading can damage the bolt and the project piece. Finally, the bolt can be tightened with a standard wrench to initial torque, and then a calibrated torque tool can torque the bolt to the proper specification.
In order to apply torque to a bolt, appropriate tools must be used. Beam wrenches are useful in setting torque values up to 150 foot-pounds. Click-type torque wrenches must be unloaded after each use.
Digital wrenches are the most precise in their measurements of torque. Inch-pound drivers is used for small bolts. Using the wrong type of wrench may result in inaccurate torque specifications being applied to the bolt.
When tightening multiple bolts, a specific tightening sequence must be used. For instance, bolts on a wheel or a flange must be tightened in a cross or star pattern to evenly distribute the pressure applied to the bolts. If bolts are tightened in a circular pattern instead of a cross pattern, the metal parts may warp from the uneven pressure.
Using a cross tightening pattern will keep the metal parts flat, and ensure they remains evenly secured.
