Understanding Vibration – #5 Successful Vibration Control

Successful vibration control depends on common sense and best practices. The best place to begin for successful vibration control is by finding the goal isolation efficiency at operating speed or frequency. In a theatre or hospital, you’ll be aiming for 98-99%; in a basement, 80% may be acceptable

Generally, the closer proximity to people, and the height above ground, will each increase the required efficiency of an isolation system. You can consult documents like the International Organisation for Standardisation (ISO) 20816-1:2016, which provides general conditions and procedures for measuring and evaluating vibration of complete machines and find suitable efficiency from there. Your clients may also have higher expectations than are required by local codes, so liaise with them in case their requirements are different.

You’ll also want to determine the lowest operating frequency of the unit, as this will indicate the lower limit and whether the spring or mount is likely to resonate at this lower bound. Low frequencies tend to have the highest risk of resonance. Think of a washing machine on the slowest part of its spin cycle – that’s when it’s rattling around the most and trying to walk out the door.

From here, identify the weight of the equipment so you can find the isolator with the ideal deflection for the needed isolation efficiency. The underlying goal is that the isolators will compress to the right amount that will result in the goal isolation efficiency.

As you could see in the graph in the last article, the greater the deflection of a spring unit, the better the isolation efficiency. So you’ll want to find an isolator that can compress as much as possible without compressing entirely, at which point it acts like a solid and fails to isolate at all.

As an example, you have a 1000kg unit and a goal isolation of 98%. You might allow six springs that are designed to handle 200kg each and together isolate 98% or more. Or you could allow 10 springs at 150kg each. And so on. Here you’ll need to ask questions about what’s going to be cost-effective, what’s commercially available and what the equipment is going to be best suited to.

A related point is the necessity of balancing the load points, especially on a platform or isolation block. Most designers will pick one grade of isolator across four or six load points, assuming that it’s a balanced weight – but that’s an amateur mistake, and when the structure or unit rattles apart, someone’s going to get the blame for a ‘faulty product’. Understand the weight balances first, as this may necessitate different types supporting different ends of a unit – you may need two at one end at 200kg each, two at the other end at 150kg, and so on.

The above and the magnitude and nature of the vibration forces will point us to the right type of isolator (something we’ll cover more of soon). Whether you choose seismic springs or rubber blocks depends on the above, as each isolator has an ideal band of amplitudes, displacements and frequencies that it’s best suited to.

Next it’s necessary to balance the centre of mass, which builds from balancing the load points but also takes into consideration the location of the equipment within the structure and the goal isolation efficiency. You may find it necessary to use inertia blocks or brackets to lower the centre of mass to limit the torque created by a vibration. This is especially true in pumptype applications, where you’ll need a robust and solid base to fix the equipment to and keep its centre of mass low.

All of this will suggest a spring or mount that you can pick from a catalogue. Typically each isolator will have a range of suitable weights, so some quick calculations of weight distribution will tell you whether it’s a good potential fit.

In the next article, we’ll look at some common types and what sort of applications they might be suited to.

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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