GAME OVER

There are many applications in which it is crucial that a proper bolt preload be applied, usually involving critical safety and reliability issues. One company with such an application – a leading company who designs and fabricates flight simulators for commercial and military aircraft personnel training – discovered the ideal solution for obtaining and monitoring optimum bolt clamp load in the form of an innovative bolt containing a Direct Tension Indicator (DTI).

"A torque wrench is not always an accurate means of measuring preload, as you have to overcome friction and other factors," says a senior mechanical engineer with the company. "Additionally, our hardware is in locations that are difficult to access." The method in which the bolts are mounted allows access to the bolt head but not the nut. In order to apply even the inaccurate measure of clamp load with a torque wrench, the nut must be accessible as well. Hence, without a visual inspection method, one can only hope the bolt is maintaining its optimum preload.

The solution came in the form of DTI SmartBolts, invented and developed by Stress Indicators (Bethesda, MD). This unique innovation contains a visual indicator on the bolt head which changes color when optimum preload is reached. Before tightening, the bolt is bright red, and when it reaches the specified preload, the indicator becomes black. Not only is the bolt at its correct tensile state when first applied, but it can be constantly inspected thereafter and can be instantly replaced or re-tightened if it becomes loose.

As one might expect, the company's flight simulators have strict hardware torque requirements. A simulator consists of a cockpit, which from the pilot's perspective is identical to the aircraft. The cockpit's visual system is mounted on a frame, which in turn is mounted on top of a motion system, consisting of six hydraulic rams. The rams provide all six degrees of freedom of motion – pitch, roll, and yaw translation in either of the three major axes, or combinations thereof. Each pair of hydraulic rams come together in what is called a knuckle pad, and each knuckle pad contains eight SmartBolts.

The simulator is an extremely accurate duplication of the operation and sensation of aircraft operation. "If a pilot is ‘flying' the aircraft, if the pilot pitches down, for example the visual scene gives him the visual cue that he's pitching down," explains the senior mechanical engineer. "The motion system physically moves him to a pitch down position so he feels the accelerations that he would feel in transitioning from level flight to a pitch. He would also change in orientation so he would know that he's moved and he's actually looking down. And all of this is in response to what the pilot's doing." An audio system also provides sounds such as engine noises or the chirp of tires when the engine hits the runway.

The reason the simulators must be so realistic is that airlines will utilize these simulators to train pilots, and with today's simulators a pilot can actually be certified without ever having been off the ground. Simulators must meet strict FAA (Federal Aviation Administration) requirements in order to be used for such training. Simulators offer training advantages that could not be offered in real aircraft. For example, an instructor can program a strong side wind or a downdraft when a pilot is coming in for a landing, or can actually have a tire blow or an engine fail. Obviously, with a real aircraft such situations contain a high level of danger, but with a simulator, if a pilot doesn't perform, the instructor simply resets the program and begins again.

Since simulators cost less to operate than an actual aircraft, they are also utilized to keep pilots' airtime up both in commercial and military applications. They are also used for military mission training so that a pilot will be familiar with the flight of a mission before it even takes place. It comes as no surprise, then, that obtaining and maintaining accurate preloads of the bolts within the motion systems is extremely vital. Utilizing SmartBolts within the simulators means that accurate preload is always maintained, and can be visually verified. Additionally, the bolts can be constantly monitored after the simulator has shipped by the customer's maintenance crew.

The new bolt design is the brainchild of Chuck Popenoe, technical director with Stress Indicators, who began seeking a better way than torque wrenches to accurately gauge bolt preloads all the way back in the seventies. "There's always been an issue with using torque to try and measure the actual tension on the bolt," says Charles Popenoe, Chuck's son and president of the company. "The tension is what matters. It holds the joint together, but the torque is about the only way to measure without fancy gauges and instruments. But with the torque, you still have to overcome all the friction and then try and separate that out from the tension."

Popenoe continues, "With the SmartBolt, the torque and the friction factors are taken out of the equation and it just actually measures the actual tension on the fastener. You can visually verify that the proper preload has been reached." Applications that benefit from the bolt are those where it is critical that the proper preload be applied and continually verified. Such applications usually also involve high safety and liability factors. Some examples are manufacturing equipment, large presses and rolling equipment, and heavy construction vehicles as well, such as earthmovers, bulldozers and cranes.

It is not always components in motion that may benefit, however. General Electric utilizes the bolts to ensure that electrical busway joints are properly tensioned, which is a static application. A loose bolt in that situation could mean a heat buildup or loss of current. An additional benefit comes to facilities may not even possess equipment such as torque wrenches to check or even install the bolts. SmartBolts can be applied with ordinary wrenches, and can then be visually inspected thereafter.

Bruce Boyers is a freelance writer based in Glendale, CA.

Stress Indicators, Inc., 6307 Wiscasset Road, Bethesda, MD 20816, 240-631-7246, www.smartbolts.com.