Machine screws are as diverse as the machines being held together by them. There are thousands upon thousands of types of machine screws, each with its own intrinsic torque requirements. Complicating the task of determining torque requirements is that machine screws are made of different materials, and each material may have its own subcategory (such as steel grade 3 or steel grade 5). Compounding things even further, there are different specifications for each threads-per-inch count. Realising this, engineers have developed extensive charts specifying torques for each type of screw made of each material.
There are eight grades for steel bolts, 0 to 8. The higher the grade number, the stronger the bolt. To read a torque chart, find the size (such as 1/4-20), then the grade stamped on the head. The head stamping may be lines instead of a number (grade 8 is six lines). Then read across the chart to find the recommended torque for your bolt. For example, a 1/4-20 grade 5 bolt's torque is 10 foot-pounds. Bear in mind that this is a dry torque, with no lubrication on the bolt. Therefore, make sure there is no oil on the bolt prior to setting the torque.
Many screws and bolts are made of non-steel materials such as stainless steel (used in marine applications) or aluminium (for aviation non-stress-area applications). To read the chart, find the material first. Then find the size you are using, then the threads per inch. For example, a 1/4-20 screw made of brass will have a recommended torque of 61.5 inch-pounds.
Metric machine screw charts are read pretty much the same way, except that steel bolts have their own metric grading system. The grades are called a standard, and these vary from standard 5D to 12K, weakest to strongest. To read a metric chart, first find the standard of your bolt, then read across the chart to find the recommended torque. For example, a 10 mm-1.25 standard 8G has a recommended torque of 40 foot-pounds dry torque.
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- Engineers Handbook: Reference Tables -- Steel Bolt Torque Specifications
- Engineers Handbook: Reference Tables -- Metric Steel Bolt Torques
- Engineers Handbook: Reference Tables -- Hex Cap Screws Torque Specifications
- Engineers Handbook: Reference Tables -- Bolt Torques - Stainless Steel, Brass Aluminum Bolts
- Engineers Handbook: Reference Tables -- Whitworth Steel Bolt Torques