The Importance of Frequency Analysis

Grandfather ClockHave you heard the buzz about running a frequency analysis on your assemblies? Actually’ if you’ve heard a buzz at all, frequency was the reason. Sounds are transmitted by an object that vibrates and Alarm Clockpushes the air that surrounds the object. Because the object moves back and forth when it vibrates, it pushes the air in waves. The number of waves of air that hit your ear per second (frequency) will determine the pitch of the sound. A sound with more waves per second (higher frequency) will have a higher pitch. To put this in perspective, the sharp sound of the bells on an old fashioned alarm clock have a higher pitch than the “gong” noise of a grandfather clock.

Wait a minute’ back up. How does this have anything to do with my assembly design, you say? Well, let me try to connect the dots. Does your assembly have a motor connected to it? Does your assembly connect to something else that has a motor attached to it? Does your assembly connect to anything that moves in a cyclic (back and forth) motion? Think of a swing set or either of the clocks above. If the answer is yes to any of the above, you should be concerned with frequency in your designs. Let me explain why.

Every object has at least one natural frequency associated with it. Oh no, more new terminology! Hang with me for one more second, I’ll explain this one quickly. A natural frequency is a state where you can shake something back and forth a small amount and it will respond by moving back and forth a larger and larger amount with every shake. With that in mind, if you want to make sure you assembly stays together, you want to design it so the assembly doesn’t have a natural frequency anywhere near the operating frequency of any motors that could be attached to it.

To help you understand how this can be a problem, answer this simple question: Have you ever been in a vehicle that was close to stalling out? If you have, you’ve probably noticed that the entire vehicle will start shaking more and more violently the longer it’s running that way. The reason for this is because the engine is spinning at a frequency that’s near, or equal to, a natural frequency of the vehicle in this low RPM state. If you want to determine if this will happen to your design, you can run a frequency analysis like in this engine mount assembly example:

Engine Frame

The engine has a crankshaft inside that spins around at some rotational velocity usually measured in RPM (revolutions per minute). Let’s say the engine operates at 2000 RPM. How can we find out if this will excite the natural frequency of the engine mount assembly? First we need to find out what frequency 2000 RPM equates to. Frequency is usually measured in Hz (Hertz). To convert an RPM value to Hz, just divide the RPM value by 60. The following calculation would look like:

CalculationImage

This means the operating frequency of the motor at 2000 RPM is 33.3 Hz. We want to make sure our engine mount assembly doesn’t have a natural frequency value around 33.3 Hz so that it doesn’t shake uncontrollably and break.

After running a frequency analysis on this frame it was found that the first 5 natural frequencies are 136, 143, 365, 671, and 716 Hz respectively.

Frequency Results

We can see that the first natural frequency of the engine mount assembly is 136 Hz. This value is well above the 33.3 Hz operating condition of the engine at 2000 RPM, so it would be considered safe. Converting 136 Hz back into RPM we find:

136 Hz x 60 = 8160 RPM

What this tells us is that this engine mount assembly would not be an acceptable design if our engine had an operating range that included 8160 RPM. If we did run this engine at 8160 RPM, the frame would start to shake violently and eventually break apart.

Like any computer aided simulation analysis, a few assumptions were made when running this study that could have an impact on the results. It’s very important to have a firm understanding of these assumptions before trying to run any computer aided simulation analysis. It’s also important to understand what can be done to increase or decrease the natural frequencies of your assembly designs to avoid these vibrational issues.

If you want to discuss these topics in more detail, please feel free to contact us and one of our Certified SolidWorks Professionals will be glad to help.