When vibration analysis is done correctly, you can assess the health of your equipment. In order to minimize unplanned downtime, maintenance personnel must identify inherent failures before catastrophic failures occur. It’s the oscillation or back-and-forth motion of machines and components such as drive motors, driven equipment (pumps, compressors, etc.), bearings, shafts, etc., that causes vibration in powered equipment, gears, belts, and other elements that make up a mechanical system.
Vibration in industrial equipment can be both a symptom and a source of failure. Other times the vibration is just “going around” and should not be cause for undue concern as a normal part of machine operation. Vibration-reducing machines are the focus of this article.
1. When vibration is an issue
In fact, most industrial equipment is designed to run smoothly and avoid vibration, not generate it. In these machines, vibration may indicate a problem or damage to the equipment. Unwanted vibration itself can cause additional damage if the root cause is not corrected. For mechanical equipment, you can assess the degree of your mechanical vibration through mechanical shock testing machines. For products, you can evaluate your product’s ability to withstand vibration through vibration testing.
2. Vibrations in machines can be caused by a number of factors
Vibrations can be produced by a variety of conditions alone or in combination. Not only primary equipment can cause vibration problems, but secondary equipment as well. Below are some of the main causes of vibration.
When an unbalanced weight rotates around the machine’s axis, the “grave points” in the rotating parts cause vibrations, which create centrifugal force. Imbalances can be caused by manufacturing defects (machining errors, casting defects) or maintenance issues (distorted or dirty fan blades, missing balance weights). As machine speed increases, the effects of unbalance become greater. Imbalance can severely shorten bearing life and cause excessive machine vibration.
When the machine axes are not in a straight line, it can cause vibration. For example, angular misalignment occurs when the axes of the motor and pump are not parallel. When the axes are parallel but not perfectly aligned, this condition is called parallel misalignment. During assembly, misalignment may occur or develop over time as a result of thermal expansion, shifting of components, or improper reassembly after maintenance. Vibrations may occur radially, axially (perpendicular to the machine’s axis), or both.
3) Wear and tear
Vibration can be caused when components such as ball or roller bearings, drive belts or gears wear down. For example, when a roller bearing race is pitted, the bearing rollers cause vibration every time they pass over the damaged area. Badly broken or worn gear teeth or a damaged drive belt can also cause vibrations.
If a vibrating component’s bearings are loose or loosely secured to their mounts, vibrations that might otherwise go unnoticed can become noticeable and damaging. There may or may not be underlying vibrations causing this looseness. Whatever its cause, loosening leads to damage from any vibration, such as further bearing wear, wear and fatigue on equipment mounts and other components.
3. Effect of vibration
The effects of vibration can be severe. Uncontrolled machine vibration can accelerate the rate of wear (i.e. shorten bearing life) and damage equipment. Vibrating machinery creates noise, raises safety concerns, and worsens working conditions in factories. Vibration can cause machinery to consume excessive power and can compromise product quality. In worst-case scenarios, vibrations can severely damage equipment, causing equipment to stop functioning and stopping factory production.
However, machine vibration also has a positive side. Plant maintenance professionals can use vibration as an indicator of machine condition in preventive maintenance programs to take remedial action before disaster strikes if it is measured and analyzed properly.
4. Vibration characteristics
To understand how vibration behaves, please consider a simple rotating machine, such as an electric motor. The motor and shaft rotate around the shaft, which is supported by bearings at both ends. A key consideration in analyzing vibration is the direction of the vibration force. In our electric motors, vibration can occur as a radial (outward from the shaft) or axial (parallel to the shaft) applied force.
For example, an imbalance in a motor is likely to cause radial vibrations as the motor “heavy” spins, creating a centrifugal force that pulls the motor outward as the shaft rotates 360 degrees. Shaft misalignment can cause vibration in the axial direction (back and forth along the shaft axis) due to misalignment in the coupling arrangement.
Another key factor in vibration is amplitude or the force or severity involved in the vibration. The more unbalanced our motor is, the more it will vibrate. Other factors, such as rotational speed, also affect vibration amplitude. As the rotational speed increases, the unbalanced force increases significantly.
Frequency refers to the rate of oscillation of vibration or the speed at which a machine moves back and forth under the force of the conditions causing the vibration. Frequency is usually expressed in cycles per minute or hertz (CPM or Hz). 1 Hz equals 1 cycle per second or 60 cycles per minute.
In spite of the fact that we called our example motor “simple,” it is capable of exhibiting complex vibration signatures. In operation, it may vibrate in multiple directions (radial and axial) at various amplitudes and frequencies. Unbalanced vibrations, axial vibrations, vibrations caused by deteriorating roller bearings, etc. can all combine to form a complex vibration spectrum.
Vibration is a feature of nearly all industrial machinery. When vibration increases beyond normal levels, it may simply indicate normal wear, or it may indicate the need for further evaluation of the root cause or immediate maintenance action. The first step to preventing vibration from causing problems in your production environment is to understand why vibration occurs and how it manifests.