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TORQUE VS. CLAMP – AND WHAT IT MEANS FOR JOINT INTEGRITY AND VIBRATION RESISTANCE


For most heavy equipment applications, design engineers have focused on a stable, consistently tight joint for long-term durability and vibration resistance. At the same time, torque has long been the standard by which the tightness, and maybe even the overall integrity, of a joint were measured. This conventional thinking was based on the idea that the more power applied to a nut and bolt assembly in the form of torque, the tighter, more secure, and vibration resistant the joint. And as it turns out, like much conventional thinking, the belief that higher torque resulted in a more secure and durable joint was wrong.
 
Torque – An Explanation
 
Torque, as it relates to fasteners, is the twisting force required to spin a nut along the thread of a bolt. The basic formula for torque is T= (KDP)/12, where T= Torque in Foot Pounds, D= Nominal Diameter in Inches, P= Desire Clamp Load or Tension in Pounds, and K= Coefficient of Friction. The problem with this equation is that K is extremely hard to predict or measure, since it is impacted by a wide range of variables such as surface texture, oil, rust, debris, type of thread, material, and even humidity.
 
Another element in the joining equation, tension, is the stretch or elongation of a bolt that provides the clamp on the joint. A further explanation would define tension as the load on the joint brought about by drawing the fastener components together. Critical to the joining process, tension, is for the most part unrelated to torque. At best, torque is an indirect measurement of the tension applied to the bolt.
 
Clamp – The Real Measure of Joint Integrity

Clamp, defined as the load on the joint brought about by the drawing together of the fastener components, is virtually unrelated to torque. In fact, ongoing studies of clamp using sophisticated Skidmore-Wilhelm pre-load test equipment have clearly established that even when torque between two or more nuts and bolts is absolutely the same, the clamp value of these fasteners can vary more than 30% (Chart 1). By comparison, a HuckBolt® lockbolt will vary 5% or less. This is an important point, given the fact that clamp is the critical element in joint integrity and durability.