Proposal for Preload Indicating Hardware for Bolted Joints

Proper bolt tension preload is vital to a joint’s performance, particularly in regard to its fatigue life. Unfortunately the installation of this vital critical characteristic is subject to numerous variables including lubrication, thread tolerances, friction coefficients and other factors.  Rarely is the actual bolt tension preload directly measured. Rather the preload is deduced based on the bolt torque applied. Bolt preload can reduce from initial installation values due to the following factors; The tightening of adjacent bolts reduces the preload on a given bolt,   Bolt tension preload relaxes as much as 10 – 20% immediately after tightening due to embedment relaxation. In helicopters especially, the vibration and loading of a flight can reduce bolt tension preload as the bolted assembly “settles in”. A method of easily verifying bolt tension preload, without significant disassembly or manpower effort is required. Dayton T Brown Inc., a leader in engineering and testing is proposing to verify bolt tension preload, without significant time consuming disassembly and bolt re-torqueing by using ultrasound to measure the change in bolt length due to the preload. Innovations in the design of the transducer and its application to the bolt will make this method easy to apply and will produce repeatable and accurate measurements.