Stainless steel bolts are popular fasteners because of their strength and resistance to corrosion. While in many households bolts are tightened until they feel tight enough, industries and car mechanics require specific torque values for bolts to ensure that they are tightened enough. Depending on the bolt size, construction, fabric, and lubrication, torque specifications vary from bolt to bolt and situation to situation, and should be followed precisely.
Sizes and Threads
Bolts are identified by their sizes and threads. Screws can be as small as the number one screw, which is 1/16 of an inch or as large as the number 24, which is 3/8 of an inch for the most common screw sizes. Their threads are listed by distance between the threads and number per screw, and are listed like this: 1/4 x 20, meaning that the screw has a diameter of 1/4 of an inch and has 20 threads per inch.
Stainless steel bolts come in numerous types, the most common of which are called 18-8 and 316. The 18-8 stainless steel bolt is composed of about 18 per cent chromium and 8 per cent nickel. These bolts are extremely corrosion resistant. The 316 stainless steel bolt, designed for industrial environments, has a higher nickel content and is austenitic (tempered for strength at low temperatures) and non-magnetic. These stainless steel bolts hold up under extreme stress and are used in heavy industry and as a part of surgical implants for humans and animals.
The 2-56 bolts are two inches in diameter with 56 threads per inch, torque to 2.5 inch-pounds for 18-8 stainless steel and to 2.6 inch-pounds for 316 stainless steel. Bolts that are 4 inches in diameter and have 40 threads per inch require 5.2 inch-pounds of torque if they are 18-8 stainless steel and 5.5 inch-pounds of torque if they are 316 stainless steel. For 18-8 stainless steel bolts with a size of 6-32, torque to 9.6 inch-pounds; for 316 stainless steel bolts of the same size, torque to 10.0 inch-pounds. The 1-14 bolts made of 18-8 stainless steel torque to 3110 inch-pounds, and those made of 316 stainless steel torque to 3250 inch-pounds.
These and other torque specifications for bolts are a starting point, but variation exists depending on the type and amount of lubrication used. If you'd like to calculate your own torque, you can apply the following formula: Torque = 1.33 times the coefficient of friction times the diameter times the necessary preload, or T = K x U x D x P. Use 0.2 as the coefficient of friction for dry, or un-lubed fasteners and 0.09 for lubed ones. These are not precise values, but are acceptable averages.
To determine preload, take the established ultimate strength of your fastener (this information will be available at your hardware store), and multiply it by 2/3 to determine yield strength. Multiply the bolt's thread area by the full yield strength and 2/3 to determine its preload. Once you've filled in these values, you can determine your bolt's required torque.