The Importance of Knowing the Natural Frequency of your Design
• # The Importance of Knowing the Natural Frequency of your Design

Following up on our last article on buckling, here is one more design consideration which is highly important but does not seem to be studies as much – and that is the Natural Frequency. Natural frequency is the frequency at which a system tends to oscillate in the absence of any driving or damping force. When an object vibrates at a frequency equivalents to its Natural Frequency, its vibration amplitude increases significantly which could lead to irreparable damage!

This Helicopter never stood a chance against Natural Frequency!

We may not think about it much but we can sense it everyday and vibration is important for certain application. Do you hear your handphone vibrate when its on a pillow? Vibration is very useful there. How about anything that produces sound to communicate or signal? Vibration is the key. Which is why it is very important to know the natural frequency of your product. Vibration is bound to happen if there is a vibrating element (such as a motor or actuator). A safe design would then need to have a natural frequency which is way way different from the frequency of the vibrating element.

Let’s take a very common example which is a car. When a car is at a cruising speed, you can feel some vibration but it isn’t uncomfortable at all. Sometimes you barely feel it. However when you start to move at top speeds, you can feel the entire car vibrating much more, like the car is about to break apart. This is resonance at work (when the vibration equals to the natural frequency). So as an engineer, what do you do?

Design done, but will it work?

Usually we would know what the RPM of the motor is. We can then change this value into Hz. So for example if we use a 2000RPM motor, its vibration will be at 2000/60= 33.3 Hz.

Using SOLIDWORKS Simulation, we can then run a Frequency Analysis on our assembly, which we can then find the natural frequency at different modes such as below:

Here we see that for this simple set-up, the natural frequency is at 136 Hz, which is about 4 times higher than the vibrating frequency. Now we can say that the design is safe from resonance!

Sometimes we may also want to see the mode shapes of the vibration such as below:

By knowing the mode shapes, we may know how our design will behave and we can add stiffners to fortify our design for better operation.

But what if we find the natural frequency to near the vibrating frequency? Well we can make some design changes and re-run the study. As the natural frequency is dependent on the stiff of the material and the mass, we can change the:

1. Material of structure
2. Weight of the assembly

To control the natural frequency of the assembly.

If you would like to know more about natural frequency studies and how it can help you with your design choices, you may call us at +65 6372 1416 or email support@seacadtech.com.