The torque-control method: to assemble securely fastened joints time after time, engineers must understand the relationship between torque and tension

摘要

What torque should be used to tighten a threaded fastener? To answer this question, engineers must first understand the purpose of torque in installing threaded fasteners. Fasteners are typically used for three major reasons: to hold two or more components together, to allow for disassembly at a later time, and to provide a high level of clamp force. It is for this last reason, high clamp force, that threaded fasteners are typically selected over other fastening means. It surprises many people to learn that a *-inch diameter fastener can easily supply 3,000 to 4,000 pounds of clamp force. When tightening a threaded fastener, the goal is to produce enough clamp force to prevent joint failure. The clamp force that can be produced and maintained within a joint is influenced by the fastener design, joint design, assembly procedure and the environment in which the product is placed. Of these influences, the chief cause of inappropriate clamp force is the procedure used to tighten the fastener. Torque is the force applied to rotate the fastener into the joint. As the joint comes together, several events take place simultaneously: · The amount of torque needed to continue rotation increases; · The fastener starts to stretch under a tensile force, also known as the preload; and · The joint is squeezed together with an opposing clamp force. Due to this relationship, the total amount of clamp force on the joint is equal to the preload of all fasteners placed within it. What torque should be used to tighten a threaded fastener? The answer is the one that provides the correct preload on the fastener and thus the appropriate clamp force in the joint. Otherwise, just one incorrectly tightened bolt or screw can lead to the failure of the complete product. Consult a fastener specialist to determine the clamp force and seating torque your joint requires.