CUSHION MY DELICATE APPARATUS

When an engineer’s efforts are thwarted by vibration, when vibration interferes with his project, he reduces (attenuates, lessens) the troublesome vibration. Alternately, he isolates or protects or cushions his activities from the troublesome vibration.

“Controls” is wrong verb
I don’t like to say that he controls the unwanted vibration, even though some engineers use that phrase. Our editor used it, early in the discussion that led to this article. She amended her phrase, wanting me (for some reason) to discuss protecting a clean room. Clean rooms can be damaged by building vibration. You want to protect your new clean room? That’s feasible in a to-be-constructed building. Just erect your clean room upon a very expensive isolated platform similar to Figure 1. That platform is supported on numerous isolators, pistons inside the air-filled cylinders of Figure 2. Such a clean room support would be even more expensive when placed into an existing building.
Amended request
When our editor saw Figures 1 and 2 she amended her request to this: protect a delicate apparatus that can’t be floor mounted because of building vibrations. So that’s what this article’s about.

Do we ever control vibration? Yes, indeed. When we’re performing vibration tests, using a shaker, we control the shaker’s vibratory force and frequency. I want to save the word “control” for that application. It’s not appropriate for this discussion.

Cushion your experiment
You want to isolate, to protect, your delicate assembly or delicate experiment from building vibrations that are caused by nearby foot traffic, by air conditioner pumps and blowers, by a furnace, by production machinery, by office printers, etc.? Also by aircraft flyovers, by nearby railroad and highway traffic? Rather than “hard mount” your delicate assembly or delicate experiment onto an ordinary lab floor, cushion it. Mount it on the “sprung platform” I’m going to describe. (The idea is oversimplified by Figure 3).

1

Figure 3 Protecting a delicate assembly or experiment from building vibrations
Sprung platform
Build a rather heavy “sprung”platform (not as massive as the Figures 1 and 2 platform, but dimensionally appropriate for attaching your apparatus). Position the platform on four or six compression springs. Much heavier springs, of appropriate total stiffness K, but shaped like those of Figure 4 (flat ends).

2Courtesy MW Industries

Figure 4 Compression springs

How stiff is Figure 3’s K in pounds/inch? That depends upon the total load W on the springs, in pounds, divided by the number of springs. It also depends on the natural frequency fn you want, about half the lowest frequency ff that’s disturbing you. Here’s a useful equation:

fn = 3.13/√δ

δ is the static deflection (change in length) of each spring, assuming they’re equally loaded. From δ = W/K, calculate the total K. Go purchase 4 or 6 appropriate stiffness springs. You probably need not worry about Figure 3’s damper C; your compression springs probably have adequate friction.

Some of Wayne Tustin’s students direct environmental test labs, some run “shakers,” more formally called vibration exciters, some design fixtures for attaching specimens to shakers, etc. Collectively, they perform dynamic environmental tests, making sure that electronic and other hardware will function in locations and on vehicles (rockets, for instance) where vibrations are very severe. Relating to this article, shakers are often isolated to protect the buildings in which they function. Wayne’s school is Equipment Reliability Institute, www.equipment-reliability.com. tustin@equipment-reliability.com. 805/564-1260. wayne.tustin on Skype. April 7-11 at Detroit. June 3-5 at Boxborough, MA. His vibration and shock test iBooks are offered by Apple Bookstore; see http://goo.gl/c3gPdv. iBook 5 enlarges upon this article.