Stress Indicators Inc.
Edited by Lawrence Kren
Condition-based maintenance (CBM) greatly improved reliability and readiness of Apache helicopters, according to a recent study. So the U.S. Army has decided to apply the technology to its entire fleet of Apache, Blackhawk, and Chinook choppers. Before CBM, aircraft technicians performed scheduled maintenance based only on flight hours or time intervals. They torqued fasteners to ensure proper tightness, and components were routinely replaced that still had usable life. All this added to aircraft downtime.
CBM, in contrast, subscribes to the philosophy, “If it ain’t broke, don’t fix it.” Only those components needing attention are checked or replaced, greatly reducing downtime. To accomplish this, most vital components are instrumented or contain sensors that collect data about vibration modes and performance. Onboard health and usage-monitoring systems (HUMS) continuously acquire and record data. After a mission, the data are downloaded to a computer on the flight line where a technician checks for discrepancies. However, some components such as fasteners do not lend themselves to HUMS. But a built-in monitoring device that provides visual indication of out-ofspec conditions is another viable approach, and one that led to development of the HR (high-resolution) SmartBolt.
Smar tBol ts work on the principle of Hooke’s Law, which states that a material in its elastic range linearly elongates a certain amount for a given stress. In this case, tightening the bolt to its design torque stretches it about 1 to 5 mils. This deflection is compared to an unstressed gage pin inside the bolt and fastened only at its lower end. A flexible transparent envelope (microindicator) between a window in the bolt head and the brightly colored pin head contains a tiny drop of fluid that absorbs light. Tightening the bolt linearly moves the gage pin away from the window, so there is a thicker layer of fluid between it and the pin head. The fluid absorbs incident light of intensity, Io, according to the Beer-Lambert Law of Absorption,
which states that the amount of radiation, I, absorbed in a fluid layer is inversely exponential with the fluid layer thickness, t, and is therefore linear on a semi-log plot. The term, k, quantifies a fluid’s absorption properties.
The above equation describes the optical principle behind the DTI SmartBolt, which displays a continuous change of color proportional to bolt tension. The Army’s CBM program accuracy spec of 5% or better required a redesign of the stress-indicating mechanism that introduces a measured amount of lost motion before the microindicator actuates. The upgrade increased optical density of the indicator fluid. This has the effect of concentrating the complete color change into the final 10% or so of the tightening range, typically the region of greatest interest.
An unloaded HR SmartBolt displays a bright yellow indication. It remains that way until the bolt is tightened to about 90% of design tension. At that point the indicator turns slightly green. At 100% design tension, the indicator turns “grass green.” Should the bolt be overtightened, the indicator color darkens to nearly black. A bolt loosened by about 5% is visibly discernable, while a bolt loosened by 10% reverts back to the original bright-yellow. Each HR SmartBolt is factory calibrated at full design tension to ensure accuracy.
The tension-indicating mechanism has no moving parts in the usual sense, operating entirely by elastic flexure and fluid migration. As a result, the indication is completely reversible and may be operated innumerable times without wear or degradation. This feature also helps the bolts resist damage from the severe vibrations common in helicopters. So far the bolts have passed the Army’s demanding shock and vibration analysis tests and are continuing in the process leading to airworthiness certification.
Stress Indicators Inc., www.smartbolts.com
How HR SmartBolts work
The indicator portion of an unloaded bolt has the gage pin head pressing up against a transparent window. The window is free to move axially, but is biased towards the pin head by a resilient ring. Between the window and pin head, a flexible transparent envelope (microindicator) contains a tiny drop of a proprietary light-absorbing fluid. The window pressing against the pin head squeezes the fluid from the gap. With no fluid in the gap, the color of the pin head (bright yellow) may be seen through the window. Tightening the bolt retracts the pin and the window follows along because it is urged downward by the resilient ring. The result is no color change with initial tightening.
Additional tightening makes the window hit a stop, thereby preventing further downward movement.
Further tightening of the bolt lets the fluid enter the gap between pin head and window, triggering a dramatic color change from yellow to green. The entire color change takes place in the final 10 to 15% of the tightening sequence, boosting sensitivity and tension resolution.
Consider a -13 3.5 Grade 8 HR SmartBolt with a proof load of 17,000 lbf. Assume a design load of 90% of proof, or 15,300 lbf. Using a 2-in.-long gage pin, retraction of the pin head at the design load is 4.7 mils, for a spring constant of 3,260 lbf/mil. The HR microindicator goes from yellow to grass green in 0.45 mil, which infers that it begins turning from yellow at 4.25-mils retraction, corresponding to 13,800 lbf, and goes to grass green at 15,300 lbf. In other words, the indicator resolves bolt elongation to 0.06 mils or 200 lbf. Assuming a 50% leeway in color estimation, users can set bolt tension with a precision of 300 lbf, or 2% of design tension.