Understanding Vibration – #3 What’s the Issue with Vibration?

The reason we’re so concerned about vibration is that if it’s not properly understood – or not correctly handled – you’re setting up your structure for disaster. Oscillating stresses happen, from the environment down to pumps and fans, so it’s necessary to design with these vibrations in mind. The most common vibrations are due to non-uniform masses in motion – a motor with a tiny imbalance, say, operating in a basement or attached to a hospital wing. Since the machine vibrates, it’s going to transmit that to its environment. It causes stresses on structures and machine parts.

It shortens mechanical lifespans and increases the chance of early and potentially unpredictable failure. It can be a huge nuisance, especially if it’s in the range of human hearing. Even deep bass vibrations can affect users’ wellbeing in ways they may not be quick to identify, since they can feel it but not hear it.

The trick is then to minimise or isolate the vibration. Take the suspension off your car and drive across the country. With no dampening or isolation of the road vibration, you’d not only cause some serious damage to your car, but also be in for a pretty uncomfortable ride. That’s one risk of unchecked vibration.

There’s also a big risk of a phenomenon in your structure called resonance. If you’ve ever made a wine glass sing by running a wet fingertip around the rim of the bowl, you’ll understand resonance in action. Resonance is when external, applied frequencies align with the natural frequencies of an object and amplify each other. In this case, as your wet fingertip slips along the rim, it causes the bowl to vibrate back and forth at its natural frequency. If you were to see that glass in slow-motion, you’d see the whole bowl of the glass vibrating back and forth. And with enough amplitude or volume, the glass vibrates itself apart and you’ll end up with a mess on your dinner table.

There’s a classic example of the Tacoma Narrows Bridge that collapsed in 1940.

You’ve probably seen the black-and-white footage of cars trying to drive over ‘Galloping Gertie’ as the deck of the bridge swings and lifts wildly. The suspension bridge failed, in short, because the normal wind speeds in Washington State caused an ‘aerodynamic flutter’ – the bridge began to vibrate in the wind, which happened to also be at the bridge’s resonant frequency. Thankfully there were no fatalities. There were plenty of confused and embarrassed engineers though.

The trick, then, is not just understanding the resonant frequencies of your structures – which can sometimes be determined with computer-aided design software – but more importantly, understanding how to prevent the frequencies of separate mechanical vibrating components from affecting the structure and each other so that resonance, wear and noise can be prevented.

That’s why it’s necessary to isolate each vibration in turn. Good design is a matter of taking a massively complex problem and breaking it down into a series of smaller equations, and that’s the best practice here too. By identifying and isolating each vibration in turn, it’s possible to limit the risks of stress travelling from each individual component and into the structure or building.

That’s the subject of our next point: vibration isolation.

Unfortunately, vibration is often overlooked or oversimplified, even by some of the greatest architects and engineers. Download the ebook to make sure you're prepared for vibrations in any form they come.

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