After reading this article you will:

• Understand the term 'machine vibration'
• Cite some common causes of machine vibration
• Explain the reasons for monitoring machine vibration
• Understand how monitoring machine vibration saves money

What is Machine Vibration?

Most of us are familiar with vibration; a vibrating object moves to and fro, back and forth. A vibrating object oscillates.

We experience many examples of vibration in our daily lives. A pendulum set in motion vibrates. A plucked guitar string vibrates. Vehicles driven on rough terrain vibrate, and geological activity can cause massive vibrations in the form of earthquakes.

There are various ways we can tell that something is vibrating. We can touch a vibrating object and feel the vibration. We may also see the back-and-forth movement of a vibrating object. Sometimes vibration creates sounds that we can hear or heat that we can sense. To observe how vibration can create sound and heat, rub your feet back and forth on a carpet.

In industrial plants there is the kind of vibration we are concerned about: machine vibration.

What is machine vibration? Machine vibration is simply the back and forth movement of machines or machine components. Any component that moves back and forth or oscillates is vibrating.

Machine vibration can take various forms. A machine component may vibrate over large or small distances, quickly or slowly, and with or without perceptible sound or heat. Machine vibration can often be intentionally designed and so have a functional purpose. (Not all kinds of machine vibration are undesirable. For example, vibratory feeders, conveyors, hoppers, sieves, surface finishers and compactors are often used in industry.)

At other times machine vibration can be unintended and lead to machine damage. Most times machine vibration is unintended and undesirable. This article is about the monitoring of undesirable machine vibration.

Shown below are some examples of undesirable machine vibration.

What Causes Machine Vibration?

Almost all machine vibration is due to one or more of these causes:

(a) Repeating forces

(b) Looseness

(c) Resonance

(a) Repeating Forces

Imagine a boat anchored in a bay. Waves are slapping the sides of the boat, and as long as the waves continue to act on the boat we would expect the boat to rock.

The boat would be rocking because the waves would be exerting a repeating force on the boat - a force of a pattern repeated over and over again.

Most machine vibration is due to repeating forces similar to those causing the boat to rock. Repeating forces such as these act on machine components and cause the machine to vibrate. Where do the repeating forces that cause machine vibration come from?

Repeating forces in machines are mostly due to the rotation of imbalanced, misaligned, worn, or improperly driven machine components. Examples of these four types of repeating forces are shown below.

(b) Looseness

Looseness of machine parts causes a machine to vibrate. If parts become loose, vibration that is normally of tolerable levels may become unrestrained and excessive.

(c) Resonance

Imagine a child swinging freely on a swing, that is, without the child propelling himself or anyone pushing him. If we observe the motion closely we will see the child swinging at a particular rate. For example, we may see that it consistently takes him three seconds to complete one cycle of swinging.

The rate of the child's free-swinging is in fact a physical property of the child-swing system - much as the weight of the child is a physical property of the child. It is the rate at which the child will tend to swing while seated on that particular swing. It is the child's most natural swinging rate on the swing, and the only way he can change it is to interfere with the natural swinging by propelling himself with his feet, changing his posture, rubbing his feet on the ground and so on.

Machines also tend to vibrate at certain oscillation rates. The oscillation rate at which a machine tends to vibrate is called its natural oscillation rate. The natural oscillation rate of a machine is the vibration rate most natural to the machine, that is, the rate at which the machine 'prefers' to vibrate.

A machine left to vibrate freely will tend to vibrate at its natural oscillation rate. Most machines have more than one natural oscillation rate. For example, a machine comprising two substructures of different natural oscillation rates will exhibit at least two natural oscillation rates. In general, the more complex the machine, the more natural oscillation rates it has.

Now consider again the child on the swing. If we aided the swinging motion by repeatedly pushing the child, we would expect the child to swing higher and higher over time.

We would however only cause the child to swing higher and higher if we pushed with the right rhythm. If our pushing rhythm is such that he is sometimes pushed down while he is ascending, we would not expect him to swing properly. To make him swing higher and higher, our pushing rhythm would in fact need to be in harmony with his natural oscillation rate.

For example, we could push him every time - or every alternate time - he reaches his highest point. Only by pushing the child at a rate which is in harmony with his natural or preferred oscillation rate can we cause him to quickly swing higher and higher.

What happens if a machine is 'pushed' by a repeating force with a rhythm matching the natural oscillation rate of the machine? A similar situation will arise - the machine will vibrate more and more strongly due to the repeating force encouraging the machine to vibrate at a rate it is most natural with. The machine will vibrate vigorously and excessively, not only because it is doing so at a rate it 'prefers' but also because it is receiving external aid to do so. A machine vibrating in such a manner is said to be experiencing resonance.

A repeating force causing resonance may be small and may originate from the motion of a good machine component. Such a mild repeating force would not be a problem until it begins to cause resonance. Resonance, however, should always be avoided as it causes rapid and severe damage. For example, whole bridges have collapsed due to their natural oscillation rates being excited by the mere rhythm of soldiers marching in unison across the bridges.

Why Monitor Machine Vibration?

To do a good job of monitoring machine vibration and to fully reap the benefits, we must understand the answers to the above question. Monitoring the vibration characteristics of a machine gives us an understanding of the 'health' condition of the machine. We can use this information to detect problems that might be developing.

Why be concerned about the condition of a machine? Why not just continue to run the machine until it breaks down and then repair it? Operating a machine until it breaks down might be acceptable if the machine were a 'disposable' one. Most machines, however, are not 'disposable' due to their cost.

If we regularly monitor the conditions of machines we will find any problems that might be developing, therefore we can correct the problems even as they arise.

In contrast, if we do not monitor machines to detect unwanted vibration the machines are more likely to be operated until they break down.

Because machine vibration monitoring finds potentially damaging vibration, we can prevent problems arising and this saves a lot of time, money, and frustration. How? Below we discuss some common problems that can be avoided by monitoring machine vibration. These problems are worth avoiding as the costs of dealing with them are large and far exceed the cost of reasonably priced machine vibration monitoring programs.

(a) Severe Machine Damage

Machine vibration that is not detected early enough will often lead to severe machine damage requiring costly repairs or even total machine replacement. However, if the condition of a machine is monitored regularly, potential problems can be detected - and corrected - at an early stage when the repair required is simpler, faster, and cheaper. This is similar to our own health. Regular visits to a doctor help us to detect problems early and so avoid the large costs of remedying severe health damage.

(b) High Power Consumption

A machine that is vibrating consumes more power. As well as the power required for the machine to perform its intended function, additional power is also required to sustain the vibration. We can minimize this problem by regularly monitoring and maintaining the machine.

(c) Machine Unavailability

Because an unmonitored machine is more likely to break down, it is more often out of action. However, the cost of procuring and operating a machine is normally justified by its availability to process goods efficiently, or by its availability to convert raw material into cash. A machine should be consistently available to generate the money to justify its investment. Regular monitoring helps ensure that a machine is always available to generate money.

(d) Delayed Shipments

Because an unmonitored machine is more likely to break down, it is also more likely to cause delays in the shipping of goods. Customers have to wait and their payment is delayed. Customers could also cancel their order and stop doing business with us.

(e) Accumulation of Unfinished Goods

Because an unmonitored machine is prone to breaking down it is often unavailable. Goods still in the making tend to get stuck at the input point of the machine. This leads to unnecessary wastage - the waiting goods run the risk of spoiling, occupy floor space, and tie up money.

(f) Unnecessary Maintenance

To constantly ensure proper machine condition, some companies stop running machines according to predetermined schedules to adjust and replace parts regardless of whether or not the machines are malfunctioning. As a result, machines are often stopped unnecessarily to replace parts that are still good and to correct problems that do not exist. We can avoid such waste if the machines are regularly monitored and repaired only when necessary.

(g) Quality Problems

Sometimes a machine can be running into trouble even though it appears to be functioning normally. This is a dangerous situation. If not caught early, the problem could lead to poor quality products being made, large yield losses, rework costs, or worse still, warranty returns by irate customers. A machine that is regularly monitored is less likely to lead to such problems.

(h) Bad Company Image

We noted above that machines that are not regularly monitored can lead to shipment delays and produce goods of poor quality. Just one incident of shipment delinquency or product defect is often enough to seriously strain or even end relationships with customers. A bad company image associated with shipment delays and poor quality is something to be avoided. For a relatively small cost, machine vibration monitoring can protect customer relations and thus profitability.

(i) Occupational Hazards

Due to the noise and shaking they create, vibrating machines can cause occupational hazards and human discomfort. Human discomfort results in a loss to the company as workers who feel unwell will not be fully productive. Also, unexpected machine breakdowns leave workers with no work, and production planners with frustration.

Summary

In this article we described machine vibration and discussed the benefits of monitoring machines on a regular basis.

Machine vibration is simply the back-and-forth movement of any machine or machine component, and is generally caused by repeating forces acting on the machine, loose machine parts, or resonance (a repeating force with a rhythm matching the natural oscillation rate of the machine).

We identified the reasons for monitoring machine vibration regularly, as well as the consequences of not doing so. By regularly monitoring the vibration characteristics of a machine we can detect and correct machine problems as they arise. By correcting machine problems early we avoid many unpleasant and costly problems, some of which involve customers. The cost of failing to monitor machine vibration far exceeds the cost of implementing a vibration monitoring program.

To find out how to set up your own machine vibration monitoring program, contact Commtest Instruments Ltd or one of our representatives for a demonstration of a vbSeries vibration monitoring system. For the address of your nearest representative please visit our website at www.commtest.com