Archive for March 7th, 2010

Vibration Analysis in Mechanical Engineering, Part I

Sunday, March 7th, 2010

     Last week we wrapped up our discussion on heat transfer.  This week we’ll turn to a discussion on vibration analysis. 

     Vibration occurs when there is physical movement of a machine part when compared to a point of reference.  This physical movement can manifest in a straight line, a circle, or any way imaginable in three dimensions.  The point of reference can be a guideway for a sliding part or a shaft for a rotating part.

     As discussed in kinetics, machine parts have mass, and when mass moves it contains kinetic energy, so it makes sense that when a part moves within a machine, the energy created can result in forces that act upon the machine as a whole.  The net result is machine vibration which occurs in “sympathy” with the movement of the original mechanical part under discussion.

     Let’s talk about examples of vibration caused by straight line and circular motion.  And what figure conjures up a better image of up-and-down straight line vibration then a jackhammer, as shown in Figure 1.  This tool has a chisel which is attached to an air piston that moves up and down in a straight line, and the chisel and piston each have mass.  It is the rapid up and down movement of the total mass of the device that results in concrete-breaking force.  Unfortunately, those forces also vibrate back up the jackhammers shaft into the hands, arms and body of the construction worker operating it.

Figure 1 – A Jackhammer


     An example of vibration caused by circular movement can be found in your washing machine.  Ever notice what happens when you throw one heavy object, say a throw rug, into it, and it begins the spin cycle?  That “THUMP-THUMP-THUMP” sound that just won’t quit is due to the rug, now wet and congealed into a single heavy lump to one side of the agitator.  It continues to spin about the center of the agitator, creating an unbalanced outward centrifugal force that keeps changing direction around its central pivot point, the agitator.  This force makes the washing machine want to rock from side to side and front to back. Now imagine this situation taking place inside your washer day after day, hour after hour, every time you put a load to wash.  How long do you think your washer would last under this usage?  As this example illustrates, vibrations must be considered in machine design because if they are severe enough, they can cause machine parts to prematurely wear and fail.

     Speaking of wear and failure, you might have discovered how the tires on your car wore out prematurely and a mechanic said this happened because your shock absorbers or struts are bad.  Shock absorbers and struts help to safely control, or “dampen,” vibrations in the suspension system that result when your car’s wheels roll down the highway.  Without proper damping, the vibration forces can cause your tire to literally bounce down the road and grind on the pavement with each landing.  Oh, yes, if the vibration is strong enough, you can even lose control of your car and end up in a crash.

     Unfortunately, strong vibrations do not only affect machinery, but the people that come into contact with them, often causing physical discomfort and injury.  Ever heard of “white finger syndrome,” otherwise known as Raynaud’s syndrome?  This painful condition, which results when vibrations impair blood flow to the fingers, causes them to tingle, feel numb, and then turn white.  The longer a person uses a vibrating tool, and the faster the tool vibrates, the greater the risk.

     Well that’s our initial plunge into the world of vibrations.  Now that we know the basics of what vibrations are and what they can do, next time we can explore how to resolve a straightforward vibration problem which often presents itself in rotating mechanisms.