MTS civil actuator production
Photo courtesy of University of Minnesota
Civil actuators utilized in a Multi-Axial Subassemblage Testing (MAST) Laboratory
Why Any Actuator Won't Do For Civil Structural TestingMTS senior product manager Sterling Anderson discusses the specific actuator attributes that every civil engineering test lab should insist upon.
Q: How does civil structural testing differ from other types of mechanical testing?
Anderson: First, it’s a matter of scale. Testing of civil structures commonly involves huge specimens that can dwarf the test actuator in size. Massive loads have to be generated to simulate the real-world operating conditions for the test specimen, with significant reaction frameworks and masses in place to mitigate test rig strain.
Civil test specimens may also exhibit challenging behavior as they approach failure. Concrete civil engineering structures, for example, are extremely stiff and brittle; once initiated, specimen failure can proceed too quickly for meaningful evaluation. Therefore, tight control over the specimen failure rate is critical.
Additionally, the physical dynamics of an entire test rig can change considerably as a civil specimen proceeds through various levels of failure. For instance, applying force to a steel structure will likely cause it to twist, placing unique forces on the test system that were not present when the test began.
Q: How do these unique characteristics influence what is required from actuators?
Anderson: The key point is that massive forces must be controlled on the minutest of scales. This can only be achieved through precise servohydraulic actuation. However, more force means larger actuators which in turn require more flow capacity from its servovalves. High flow servovalves require precision manufacturing to achieve precise control particularly near zero speed where much of the testing occurs. Without using high precision servovalves, a host of dynamic control challenges can be expected. Therefore, the use of high-precision servovalves is critical to effectively and economically simulating real-world civil structural conditions.
During a typical civil structural test, the actuators can be subjected to significant off-set forces as specimens pass through various stages of failure. It is essential for an actuator to perform precisely and reliably under the extreme side loads that often occur as a test specimen fails.
Q: How is MTS able to successfully address these unique actuator requirements?
Anderson: Our actuators include several subtle but critical engineering innovations that combine to enhance actuator reliability, precision and energy efficiency.
First, we use our past actuator design experience to enhance robustness, including piston rods that are larger in diameter to handle extreme side loads. And unlike other actuator manufacturers, we manufacture our own high-flow servovalves, which allows us to exercise the highest quality control to ensure their superior performance, reliability, and efficiency.
To optimize actuator precision and durability, MTS 201 single ended actuator designs feature bolted connections between the piston rods and piston hubs, which resist fatigue better than the threaded connections commonly used by other actuator suppliers. Four fatigue-rated tie rods, one at each end cap corner, are pre-loaded on MTS actuators with a total force exceeding the rated load of the actuator. This creates the stiffness necessary to counter axial stresses and minimize the fatiguing effects of heavy cycling.
MTS actuators also integrate high-performance polymer bearings with relatively large surface areas. Theses withstand the high moments caused by cantilevering forces far better than the brass bearings used in other actuators. Low-friction seals provide ideal tolerance levels while introducing the minimum amount of friction into the system. For example, seal frictional characteristics for MTS Series 244 actuators are typically 1 percent of the rated load, compared to the 10 percent that is normal for industrial actuators. So while a 3000 psi-rated MTS test actuator requires only 30 psi to move its piston, a similarly rated industrial actuator would require 300 psi. This inefficiency would have to be made up by higher capacity hydraulic power units and distribution systems.
Lastly, MTS actuators are designed to 130 percent of their rated load to provide fatigue-rated performance. This means that structural failure has been successfully engineered out of the picture.
Q: How will civil engineering test labs benefit from using MTS actuators?
Anderson: It will give them absolute confidence in the reliability and precision of their test setup. By using MTS actuators, test labs will have fatigue resistant actuators capable of tolerating high side loads across generations of tests.
Specifying these high-performance actuators will also help to optimize cost-efficiency. MTS actuators might have a slightly higher purchase price than lower-quality actuators, but this differential can be recovered through increased uptime, extended life and productivity, and energy savings.
In addition, by choosing MTS as their test solutions partner, university and government test labs can tap into a vast reservoir of system integration expertise. We not only offer test-level expertise, but can also help test programs create the most efficient overall system design possible.
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