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Even the smallest errors in aerospace projects can prove expensive, or worse, fatal. Overtorquing or undertorquing can easily lead to malfunctions and failure, and high shaker forces caused by overtesting can damage expensive payloads.
After the fatal Challenger explosion in 1986, engineers and spacecraft manufacturers were faced with finding better ways to design, build and communicate when it came to spacecraft assembly. Incidents since then have strengthened the need for more careful planning, inspection, process controls, communication, documentation and re-inspection. Two examples of improvements in these areas follow.
Torque Standard Tightening
One of the many ways to ensure optimal assembly of mission critical devices involves getting torques of bolted components precisely on target. Overtorquing or undertorquing can easily lead to malfunctions and failure because metal, as well as engineered polymers, can creep and distort.
For satellites, the temperature extremes and vibration in the presence of high forces require that torque be precisely on target.
Boeing’s Satellite Development Center (SDC), the world’s largest satellite manufacturer, in El Segundo, Calif., “has more than a dozen satellites under construction at any given time,” according to a recent case study from torque tool specialist Mountz Inc.:
Prelaunch testing includes subjecting them to spin rates of 30 to 100 rpm, as well as vibrations and sound similar to what they would experience during launch — up to 50,000 pounds of force and 165 decibels. To test their ability to withstand the rigors of space, satellites are placed in a thermal vacuum chamber and subjected to temperatures of minus-320 to plus-250 degrees Fahrenheit while powered up and with instruments operating.
“I had a task to improve the quality of the torque tools that were causing overtorque,” Six Sigma black belt Vu D. Pham of Boeing explained. “To achieve this, we first had to conduct a study of root causes using standard Six Sigma tools. This entailed looking at the people, systems, processes and methods, as well as the torque tools themselves.”
Conducting a study of repeatability and reproducibility, Pham’s team concluded that some of the 3,500 tools were out of calibration, “some by as much as a factor of 1.5 or 2, despite being sent to a third party for calibration.” Although they were within tolerance, they were still subject to operator error.
Then there was the issue of usability of the tools. “With some of our tools we could not change the torque values quickly enough so we use different tools for different torque values,” the Six Sigma black belt said.
Based on the initial evaluation of problems with the existing torque system, Pham and his team initiated a three-phase project to address the issues successfully:
Phase 1: Locate tools that were more accurate and consistent than what they had been using.
They investigated several manufacturers to find the tool that could produce repeatability and reproducibility. The team tried the tools with different operators and different values to find those that “came very close to Six Sigma values.”
Phase 2: Ensuring the tool was accurate every time, not just after the annual calibrations.
Pham challenged the vendor to come up with a portable calibration system that the team could bring to the point of use and test the tool prior to using it. “That way we will know if the values have shifted,” he said.
Phase 3: People and processes.
Using the above calibration system, the tools were verified right on the shop floor every time they were used. The portable system (PDA software) also recorded the calibration test and the information on the torque being performed, so the data were there for audit and verification purposes.
In the end, the process was validated as a replacement for sending out all the tools for calibration and having an inspector witness every torque.
“The main reason for it being on the floor is to have confidence in the level of torque before the torque is done,” according to senior calibration engineer William K. Jinbo in the recent study. “It makes it easier for the technicians and they don't have to worry about broken bolts from overtorquing or not applying enough torque.”
Overtesting Can Damage Expensive Payloads
As with ensuring precise torques for bolts, devices destined for space communication and research must withstand the shaking associated with liftoff. If the vibration testing is too weak, the device could fail due to vibration during the launch, leading to limited performance. On the other hand, if the shaking is too aggressive, the testers could damage the device, requiring hugely expensive repair or replacement.
Getting the simulated vibration optimized isn’t easy, though.
To alleviate the problem of overtesting, it has become common practice to notch the input acceleration, in order to limit the responses in test to those predicted for launch into space. In simple terms, to notch means to reduce the amplitude of the shaker input near the resonant frequencies of the test item.
“Since each location [on the device] will have a different resonant frequency and exceed the control limits at some point during the test, it is often difficult to determine from which accelerometer the test engineer should control the shaker,” Quality Magazine noted. Plus, many locations on the device can be inaccessible.
Force Limited Vibration (FLV) testing is an alternative that “improves the vibration testing approach based on measuring and limiting reaction force between shaker and test item,” according to Metz. By using this method, he wrote, the acceleration input to the test item is automatically notched — reducing the amplitude of the shaker input near the resonant frequencies of the test item — at the equipment resonances by limiting shaker force values to those predicted for actual flight.
Such notching is based on the premise that “mechanical impedance of payloads and the mounting structures are typically comparable for lightweight aerospace structures.” FLV testing provides the test engineer with an opportunity to mirror structural response as seen during actual flight hardware mounting applications.
Tightening Up Torque Standards
Mountz Inc., 2007
The End of Overtesting
by Bob Metz
Quality Magazine, July 16, 2007
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