While a doctor might ask, “Where does it hurt?”, the question for experimental test engineers like Garrett Smith is, “Where does it wiggle?” Smith is the president of Seattle’s Ball Spin Engineering, a consultancy specializing in modal analysis, a method used to find the causes and the remedies of unwanted vibrations in mechanical systems.
Like other professionals who assess the health of mechanical systems – finite element analysts, structural failure investigators, or flow measurement specialists – Smith has discovered a new tool for his work. EOS Systems' PhotoModeler software is a PC-based application that captures accurate dimensions and creates fully scaled 3D models of complex machinery through ordinary photographs. Like an EKG or CT scan for factory equipment, the tool allows specialists to visualize the problem in detail, and apply scientific analysis to the as-built 3D model to come to an accurate prognosis.
Here, Smith explains how he hunts down the source of mysterious vibrations, and how he finds a solution.
Q: What is modal analysis?
Smith: All objects vibrate to some extent. When energy passes through a structure at the same rate as what’s known as the structure’s ‘natural frequency, the structure may vibrate quite a bit more, which is a condition called resonance. Where there are problems with excessive vibration in machinery and equipment, the cause can be due to a resonance. An object vibrating excessively due to resonance moves in a specific dynamic manner. The motion at a particular frequency is described as the mode shape. A mode shape at a natural frequency might be a side-to-side sway, or rotation, or some other motion. Finding the mode shape and related properties of a structure is called “modal analysis.”
Q: When is vibration a problem?
Smith: Vibration analysis can be included as a part of a design process. Organizations like Boeing, NASA, and automakers invest a lot of effort into modal analysis to study how vibration is affecting their products and how this might alter performance.
Other times, designs are completed without considering dynamic response, so correcting vibration is performed as an afterthought, typically after undesirable shaking is discovered. In most cases, vibration is merely an annoyance; however, vibration can affect sensitive equipment, shorten product life through fatigue, and can even cause unanticipated failures on a grand scale.
Q:When a machine is shaking too much, most people’s intuition tells them to simply brace it down better. What you’re saying with resonance is that this doesn’t always work – that the vibration is often internal to the system?
Smith: Yes, that’s true. Foundation modification might work if it counters the mode shape properly, but if it doesn’t, the modification will have no effect on the equipment’s behavior. Similarly, if a mode shape of an object is twisting and you add stiffening down the center to counteract bending, the attempted fix will have no effect on the motion.
In many cases, when faced with a structural vibration problem, there can be a lot of speculation as to what’s going on and how to fix it. But once a mode-shape analysis is performed and it is shown how the machine is moving in an animated model, much of the speculation about what’s going on is eliminated. The question of how to fix it still remains, but there are methodical ways to go about that as well.
Q: Can you give an example?
Smith: There was a local project recently that involved a new pump installed to move water from a river up into a city’s supply system. The ultimate configuration was a 14-foot tall tower of machinery – a motor stacked on top of a clutch, stacked on top of the pump. When the pump was turned on, there was quite a lot of vibration, so much so that the installation did not meet spec.
The contractor and other experts tried all sorts of remedies to get rid of the vibration. Workers drilled extra holes in the base and put more bolts into the floor – a stiffness modification. They piled about 50 sandbags all around the base of the pump. None of this seemed to cure the vibration.
Q: How did you proceed on the problem?
Smith: On site, I took measurements with some instruments. I used an impulse hammer to determine the natural frequencies of the major components of the pump equipment. Then I took a few photographs.
Q: Why photographs?
Smith: To build a computer model of the pump. I use a technique called photogrammetry, which literally means ‘measurement from photographs.’ I use it simply as a fast way to capture the 3D object to scale. The photos are loaded into PhotoModeler to generate the 3D model of the machinery.
With PhotoModeler I can capture complex machines that would have been very tedious to measure any other way. If I had to take measurements with a tape measure on this 14-foot-high configuration, it would have taken several hours to get something close to accurate, and it would be rather unsafe, as well. I’m often exposed to dangerous industrial areas, with rotating machinery, hot surfaces, open pits, confined spaces.
Q: How long does it take you to create the 3D model?
Smith: Only about three hours, including loading the photos and editing the model to account for the parts I could not get in camera view. PhotoModeler exports the shapes in a DXF file, which then I import into Vibrant Systems' ME’ScopeVES, a software designed for modal analysis. After importing the frequency data I collected on site, I use the ME’Scope software to animate the model according to the mode shapes for each of the parts – the pump, the motor, and the drive. I can immediately see, for instance, that the pump base has a bending mode in the middle as its mode shape, similar to a person using a ‘hoola-hoop.’ Once I see the animation, I get an immediate sense of where the problem is, and how it can be best mitigated. I found in this case that it was a flange between the coupling and the motor.
Q: How did you remedy the vibration?
Smith: The next morning I went into the pump station, and I tried a temporary fix – placing some clamps and shims around the motor. The area on the machine was located about two meters above the floor. The workers thought I was nuts because they had spent so much time and money on the pump foundation and the floor. Once I put the shims and clamps in there, there was no longer any vibration in the machine. For a permanent solution, the motor mounting flange was re-designed with heavier components. The problem disappeared and the pump operated safely within specified vibration limits.
Q: So the entire analysis didn’t take long to find a solution.
Smith: No, the analysis itself took only about 5 hours. The new 3D technology makes these types of solutions much faster than they used to be. PhotoModeler allows me to collect the basic geometries about four to six times faster than manual measurement, and the visualization that comes from 3D model animation often gives immediate clarity to the source of the resonance.