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Preload Indicating Hardware for Bolted Joints



OBJECTIVE: Develop a method of determining preload on aircraft bolted joints through a visual indication, or alternate means, which does not require physical measurement of torque via a torque wrench and does not require disassembly of any adjacent parts.

DESCRIPTION: Helicopters experience high amounts of vibration and as a result, the onboard equipment experiences different frequency resonances that cause bolted joints to loosen during operation. Torque checks are one of the regularly performed maintenance actions to make sure fasteners are still within designated torque values, and that no bolted joints have loosened over the operational life of the aircraft.Preload Indicating Hardware is hardware in a bolted joint assembly that gives the user a visual indication that proper tension/torque is applied. Examples in industry for industrial/commercial hardware applications range from one-time use washers with visual indicators to stress indicating bolts that contain a visual coloring or scale indication. Unlike torque checks where the clamping force is approximated via the resistance of the spinning bolt, this type of hardware directly measures clamping force in the bolted joint. Current available products would need to size down to accommodate aircraft application and show that the torque indication accuracy is within the desired parameters.Another method of determining preload on a bolted joint without the use of a torque wrench or other tool to turn the bolt is the use of ultrasound technology. Ultrasound technology generally involves a probe that emits ultrasonic waves into a material and analyzes the reflection of said waves to determine the characteristics of the material. The clamping force of a bolted joint can be determined through this method by analyzing the amount of strain a bolted joint is exerting on a part. This method of inspection can often require physical contact with the bolted joint, but compared to using a torque wrench, one only needs enough space for the probe to make contact versus space for a full wrench turn. This technology would have to be adopted to detect loss of preload on varying sized hardware to limit the need to manufacture multiple tools and reduce possible maintainer training.Current torque checking procedures require maintainers to expose the bolts to the degree that space permits free engagement of a torque wrench. After re-installation and the torque check, a vibration check and a functional check flight (FCF) are often required, which take multiple maintenance man-hours to complete. These maintenance man-hours exponentially accumulate in the event a torque check fails or subsequently doesn't pass the vibration check and/or FCF on the first round, affecting overall aircraft readiness. This is where preload indicating hardware could be a significant game changer in reducing maintenance man-hours for torque verification. This technology would give the maintainers a visual indication of whether or not a fastener is still exerting the required amount of preload in a bolted joint, and would eliminate a significant amount of non-mission capable hours (NMCH) and maintenance man-hours required as part of a physical torque verification process.A product (preload indicator) is needed that can be easily implemented onto existing Navy/Marine aircraft, without major modifications to any part of the structure or any other components on the aircraft. The preload indicator can be built into the bolt, the nut, the washer, or any combination of the three as long as the design does not hinder current torque check procedures, or can be a separate tool so long as it does not damage any surrounding components. The hardware should be able to accommodate bolts as small as 0.375-inches in diameter to as large as 1.6875-inches in diameter. Binary preload indication should be visible enough that maintainers can clearly see an out-of-torque and over-torqued bolt during night conditions typical of ship-based operations. It should be able to withstand flight conditions/loads without loss of preload in the bolted joint over the life of the aircraft for airframe application, or the predefined maintenance interval for dynamic components. The product should be able to endure corrosion prone environments typical of naval operations, vibrations, and accommodate temperatures typical of naval aircraft (details will be provided to Phase I awardees). Debris commonly found inside the aircraft (i.e., hydraulic fluid, gearbox fluid, etc.) should not affect preload indication. Handling and fall damage should not cause the product to lose accuracy. The preload of the current bolted joints should remain the same, as well as keeping to the currently implemented fastener standards. Additional installed hardware on the aircraft should be no more than a combined weight of two pounds. The hardware must have negligible effect (+/-5%) on the natural frequency of the fasteners as to not interfere with the existing health monitoring sensors. Implementation of this hardware should also not introduce new failure modes. Wireless inspection solutions of bolted joint preload must be consistent and accurate. Parts adjacent to the designated bolted joints should not require removal in order for the tool to properly inspect bolt preload.

PHASE I: Develop and design a preload indicator for bolted joints that provides a binary indication of torque value. Ensure that the selected indicator methods have no intrinsic limitations to scaling with the bolt sizes described in the Description. Demonstrate the feasibility of the indicator showing a path forward to meeting Phase II goals. The Phase I effort will include prototype plans to be developed under Phase II.

PHASE II: Develop and build a prototype that can successfully provide indication that a bolted joint, through a visual cue, a tool, or otherwise, has either lost preload, or been over torqued. Demonstrate non-destructive inspection of the bolted joint complies with Naval Aviation standards. Demonstrate that the prototype will withstand handling and fall damage without losing accuracy.

PHASE III: Verify and validate the viability of the prototype at a fleet maintenance facility and in the field. Transition the prototype into a final product for Navy/Marine Corp fleet application. Distribute the product, support equipment, and process specifications to maintainers. Commercial applications include structures (e.g., factories, bridges, and buildings), transportation equipment, and commercial aircraft.

KEYWORDS: Preload, Torque, Bolt, Hardware, Wireless, Inspection


1. Chambers, Jeffrey. “Preloaded Joint Analysis Methodology for Space Flight Systems.” National Aeronautics and Space Administration, 1995. 2. Chapman, I., Newnham, J., and Wallace, P. "The Tightening of Bolts to Yield and Their Performance Under Load." ASME. J. Vib., Acoust., Stress, and Reliab. April 1986, 108(2), pp. 213-221. 3. Chen, S. H., He, Z. G. & Egger, P. “Study of Hollow Friction Bolts In Rock By a Three Dimensional Composite Element Method.” International Society for Rock Mechanics and Rock Engineering, January 1, 2003. 4. “Fatigue Tests on High Strength Bolts and ‘Coronet’ Load Indicators.” TurnaSure, LLC.

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