Frequency Studies in SolidWorks Simulation Professional by Brian Zias
Intro
Every product is subject to real-world conditions that ultimately cause failure in one way or another. SolidWorks Simulation is a tool to computationally quantify the performance of our assemblies and components inside SolidWorks. Using the extended capabilities of SolidWorks Simulation Professional, a company's development team can model more real-world challenges and scenarios.
Typically designs are sensitive to cost, safety, durability, and quality. Making use of today's software leads toward higher product quality, lower costs, longer durability, and far less physical testing. This will propel company innovation.
How does software, such as Simulation, add value to a company? In other words: As engineers and designers, what is our value? Our value is our production of technical work. If we have tools with which we can create, publish, and validate our technical work more quickly, with fewer errors, higher quality, and greater consistency, how will this affect our top and bottom lines? These topics are exactly what we refer to when we say "SolidWorks helps you Design Better Products." We are ultimately going to increase the effectiveness and efficiency of your workflow by adding key capabilities such as SolidWorks Simulation Professional.
The example product is a medium-duty paint sprayer. This is a product I am sure we have seen in the home improvement store.
Frequency Study
To begin, consider how many man-made and naturally-occurring phenomenon in existence are periodic. Vibrations can be quantified in terms ranging from simple sinusoids to statistically-measured earthquakes. Products must endure the world in which they exist. Understanding the natural frequencies of our components and assemblies will help us avoid failure due to resonance, or dynamic amplification. This differs from linear static analysis, because when a structure is disturbed or excited in the time domain (no longer static or time-independent) the elastic forces no longer equilibrate the model. If we were to suddenly excite a structure and let is freely vibrate, we would see the elastic forces cause free, or natural, vibrations.
The results of the frequency simulation describe these natural frequencies (in Hz) and indicate the shape the structure as it oscillates at that natural frequency, fn. The paint sprayer is not particularly prone to earthquake loads, especially here in Minnesota, but inside the injection-molded casing we find another excitation source: a motor that operates at a constant 10,000 revolutions per minute (or 166.7 Hz). Based on the product geometry and mass characteristics, every design has natural frequencies. Simulation determines these frequencies so challenges such as ensuring that the motor does not operate at that same frequency (or range of frequencies) as the product becomes achievable in just a few minutes. Can you imagine a paint sprayer resonating in your hand as you attempt to precisely trim those shutters?
One trick when simulating hand-held products is to use no fixtures. This of course will result in six rigid body modes. Think of something floating in space -- the easiest way for it to oscillate is to simply translate or rotate in or about the three orthogonal directions. The results from the seventh mode demonstrate the first elastic natural vibration (designated f1). Here are results from the first four modes (the shadow underneath is the non-deformed geometry):
As the modes become associated with higher frequencies, they become less likely to be excited; the higher modes require more energy. Compare the frequencies of the casing and frame structure to that of the excitation, the motor. The fundamental, or first, frequency is 290 Hz, which is fairly far above the 166.7 Hz excitation. Thus we can conclude that the case will not be prone to resonance unless we increase the motor operation to 17,400 RPM (290 Hz). As the excitation frequency gets closer to the natural frequency, we enter the world of dynamic response and must solve for damping to see how much stress and displacement will actually result. This is precisely what the Linear Dynamics package can do in Simulation Premium -- information that is invaluable to any designer who is concerned with product quality, safety, and performance.
Look for our other Study examples elsewhere in our Blog on Thermal and Fatigue Optimization.