Motor Controls

Mechanical Maintenance for Motor Drive Systems 349

Mechanical Maintenance for Motor Drive Systems provides an overview of the most common mechanical faults found in motor drive systems and describes typical inspection methods for mechanical maintenance. Vibration is a major issue that can have very damaging effects. Shaft misalignment, shaft imbalance, looseness, and bearing issues are the four most common causes of vibration. Vibration inspection helps identify and correct the causes of vibration. Thermal inspection, ultrasound analysis, and oil analysis are also used during mechanical maintenance.

Motor drive systems are widely used to power industrial machinery. For these systems to operate safely and efficiently, their mechanical components must be in good working order. This class provides information that helps users understand major mechanical faults and how to identify and fix them. This information helps reduce unplanned downtime and expenses.

  • Difficulty Advanced

  • Format Online

  • Number of Lessons 17

  • Language English


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Course Outline
  • Mechanical Components
  • Mechanical Faults
  • Vibration Signatures
  • Vibration Inspection
  • Vibration Screening
  • Vibration Testing
  • Mechanical Faults and Vibration Inspection Review
  • Misalignment
  • Correcting Misalignment
  • Shaft Imbalance
  • Looseness
  • Bearing Issues
  • Vibration Faults Review
  • Thermal Inspection
  • Ultrasound Analysis
  • Oil Analysis
  • Mechanical Inspection Methods Review
  • Describe mechanical components of motor drive systems.
  • Identify common mechanical faults that occur in motor drive systems and their symptoms.
  • Describe vibration signatures.
  • Describe vibration inspection.
  • Describe vibration screening.
  • Describe vibration testing.
  • Describe misalignment in motor drive systems.
  • Explain how to correct misalignment in motor drive systems.
  • Describe shaft imbalance in motor drive systems.
  • Describe looseness in motor drive systems.
  • Describe bearing issues in motor drive systems.
  • Describe thermal inspection.
  • Describe ultrasound analysis.
  • Describe oil analysis.
Vocabulary Term


A sensor that detects movement and speed. Accelerometers are used to detect and measure vibration.


The property that describes the level of acid in a substance. High acidity in lubricants can cause mechanical issues in motor drive systems.


A measurement of the size or severity of a vibration signal wave. Amplitude describes the displacement of a component caused by the vibration signal.

angular misalignment

A condition in which the shafts of a motor and driven unit meet at an angle. Angular misalignment causes a gap where the two shafts meet.

apparent mechanical load

The actual opposition or force a motor works against during operation. The apparent mechanical load includes any additional opposition or force caused by faults such as misalignment, as well as the intended load.


The process of fitting components together into a larger or completed part. Assembly also refers to the state of the components once they are fitted together.


Moving along a central axis. Axial vibration is parallel to the shaft.


An imaginary straight line that passes through the center of an object. The axis of a motor shaft serves as the point around which the shaft rotates.

balance weights

An object used to even out the mass on a shaft. Balance weights can be placed on external or internal motor components.


Initial data collected while a system or component is in good working order. Baseline readings are compared to future readings in order to detect potential faults.

bearing impacting

Intermittent forces that the balls in a bearing apply directly to the races of the bearing. Bearing impacting is caused by bearing wear.


A friction-reducing device that allows one moving part to glide past or rotate within another moving part. Bearings use a sliding or rolling mechanism to reduce friction.


Bent in a U-shape that resembles a bow. With a bowed shaft, the middle of the shaft is not in line with the shaft's axis or ends.


A manufacturing process that involves pouring a liquid material into a hollow mold until the material cools into a solidified shape. Casting can create flaws that lead to shaft imbalance.

centrifugal force

A force directed outward and away from the center of a rotating object. Centrifugal force attempts to pull a rotating shaft away from the axis.


An amount of space or distance between two objects. Excess clearance between components causes looseness.


Occupy the same space or position. The axes of properly aligned shafts coincide with each other.


Lying on the same straight line. Collinear shafts are positioned with their axes on the same line.

compound misalignment

A condition in which the shafts of a motor and driven unit are both angled and offset. Compound misalignment is the most common form of misalignment.


Damaging foreign material, such as dirt or debris, that causes wear to machine components. Contaminants can cause bearing damage.


The presence of foreign materials, such as dirt or debris, in a substance. Contamination in lubricant can damage machinery.


A component that connects two shafts in a mechanical system in order to transfer motion. Couplings may be used to attach the motor and driven unit shafts in a motor drive system.

crest factor

A ratio of a vibration signal's peak amplitude to its root mean square amplitude. Crest factor indicates the amount of impacting and wear in a bearing.


dB. A unit of measurement that describes the intensity of a sound. The decibel level produced by a machine component indicates the condition of the component.


The weakening or diminishing of a substance. Deterioration in lubricant occurs naturally over time, but contamination and other factors can cause premature deterioration.

dial indicators

A measuring device used to indicate linear movement. Dial indicators use needles that move to different graduations to display measurements.

direct coupled drives

A drive train system that uses a flexible coupling to connect a motor shaft to a driven unit shaft. Direct coupled drives do not require precise shaft alignment and are commonly used for pumps.

direct drive

A drive train system that uses a straight shaft to connect the motor immediately to the driven unit. Direct drives require precise, rigid alignment.


The actual distance that vibration forces a component to physically move. Displacement measurements indicate the amplitude, or severity, of vibration.


A period of time when production stops, often due to mechanical failure or maintenance needs. Downtime can be planned or unplanned.


The resistance to flow or movement. Drag counteracts rotation and increases as friction increases.

drive shaft

The rotating component that transfers power from a motor to a driven unit. The drive shaft attaches to the driven shaft.

drive train

A group of mechanical components used to transmit power from a motor to the driven unit. Drive trains connect the shaft of the motor to the shaft of the driven unit.

driven shaft

The rotating component that receives power from a motor's shaft. The driven shaft turns the driven unit.

driven unit

The machine or device powered by a motor drive system. The driven unit may also be called the load because it provides resistance to the motor.


A measure of a system's work output compared to the total work input. Efficiency indicates how much supplied energy is converted into the intended useful work.


The wearing away or destruction of a material. Erosion can be caused by friction, corrosion, or other factors.


Progressive structural damage to an object that is subjected to repeated or excessive loads. Fatigue leads to failure.


A flat projection that protrudes from the bottom of a motor's housing. The foot of the motor rests or mounts on the foundation.


The flat surface upon which a motor sits. The motor's feet may simply rest on the foundation or be mounted to it.


The rate at which movement caused by vibration occurs. Frequency measures the number of oscillations per second in hertz (Hz).


A force that resists motion between two objects that are in contact with each other. Friction generates heat and increases wear between components.


A semisolid substance composed of oil and a chemical soap or other additive. Grease is commonly used as an industrial lubricant.


Parallel to the horizon or ground. A horizontal axis runs side-to-side rather than up-and-down.


hp. A unit used to measure power in large electrical devices such as motors. One horsepower equals 33,000 foot-pounds (44,748 joules) of work per minute or 746 watts.


A protective cover designed to contain or support a component. The housing of a bearing encloses the races and balls.


Foreign elements or substances contained within a material. Impurities in lubricants include contaminants and particulates of wear debris.

inches per second

in./s. A unit used to measure the linear distance a component moves back and forth in one second. Inches per second indicates the rate of vibration.

indirect drive

A drive train system that uses additional components such as gears, belts, and chains to connect the motor and driven unit shafts. Indirect drives offset the shafts and may also use flexible couplings.

induced shaft voltages

An unwanted electrical charge in a motor's shaft. Induced shaft voltages can be caused by leakage currents and magnetic fields in a motor.


IR. Invisible rays of light energy emitted by heated objects. Infrared light can be used to capture a visual representation of an object's thermal signature.

infrared cameras

A thermal inspection device that uses infrared technology to produce a visual representation of temperature conditions. Infrared cameras are also called thermal imagers.

infrared thermometers

A thermal inspection device that uses infrared technology to measure temperature. Infrared thermometers provide a numerical reading of an object's thermal signature.

laser alignment tools

A corrective instrument that provides visual guidelines and instructions for how to re-align misaligned shafts. A laser alignment tool consists of two sensor modules and a display unit.


The opposition to an applied force, such as a weight or resistance. The load may also refer to the driven unit since it provides resistance to the motor.


Excessive clearance between mechanical components. Looseness can occur between rotating and/or stationary parts.


A substance used to reduce friction between two surfaces in relative motion. Common industrial lubricants include oil and grease.


The application of lubricants, such as grease or oil, to machines. Lubrication reduces friction and wear between mechanical components.


The property that describes a lubricant's effectiveness at diminishing friction. High-lubricity lubricants have increased smoothness and slipperiness.


The amount of matter in an object. Mass gives an object weight when it is acted upon by gravity.

mechanical failure

A condition in which a device loses its capacity to carry a load. Mechanical failure can be the result of gradual wear, insufficient lubrication, or overload.


A condition in which the shafts of a motor and a driven unit are not collinear. Misalignment can be angular, parallel, or compound.

motor drive system

The combination of an electric drive and the motor it controls. Motor drive systems typically use variable frequency drives and three-phase AC motors.

non-rotating looseness

Excessive clearance between two stationary parts. Non-rotating looseness may occur between a machine's foot and foundation.


The distance between the axes of two misaligned shafts. Offset indicates the degree of parallel misalignment.


A slippery fluid commonly used as a lubricant. Both natural and synthetic oils are used in industrial applications.

oil analysis

A machine testing method that involves inspecting a sample of a machine's lubricant. Oil analysis measures lubricant properties, contaminants, and wear debris.


Back and forth movement. The rate of oscillation depends on the frequency of a vibration signal.


Equidistant to another plane or object. Parallel shaft movement is caused by axial vibration forces.

parallel misalignment

A condition in which the shafts of a motor and driven unit are parallel but on separate planes. Parallel misalignment creates an offset between the axes of the shafts.


Tiny pieces of solid materials. Particulates of metal that break off components during operation create wear debris.


Forming a 90° angle with another plane or object. Perpendicular shaft movement is caused by radial and tangential vibration forces.


Corrosion that forms small holes on the surface of metal components. Pitting in a bearing race can cause vibration.


An imaginary two-dimensional surface that is perfectly flat and level. Planes are used to help identify different types of shaft misalignment.

predictive maintenance

A maintenance approach that involves testing and monitoring each machine individually in order to predict failures. In predictive maintenance, periodic readings are compared to baseline readings in order to identify problems.


A measurable characteristic that describes the qualities or attributes of a specific material or substance. Properties used to describe lubricants include acidity, viscosity, and lubricity.


A metal ring that houses the steel balls in a bearing. Races have grooves along which the balls roll.


The groove that runs along the middle of a bearing race. Raceways guide the motion of the balls in the bearing.


Moving vertically outward from a central point or axis. Radial vibration is perpendicular to the shaft.


A numerical expression that describes how many times one number fits into another number. A ratio is expressed as a fraction.

reaction forces

A force that acts in a direction opposite that of an initial applied force. Reaction forces lead to friction, heat, and wear.

revolutions per minute

rpm. The number of times an object rotates 360 degrees, or in a complete circle, in one minute. Revolutions per minute is a measurement of speed.

root cause

The true origin of a problem, as opposed to any resulting symptoms or effects. Identifying the root cause of a problem is an important aspect of troubleshooting and maintenance.

root mean square

RMS. The square root of the arithmetic mean of a set of values. The root mean square indicates the effective average of the values.

rotating looseness

Excessive clearance between a rotating part and a non-rotating part. Rotating looseness may occur in a machine's bearing.


A device used to close an opening or form a closed joint between objects. Seals are used with bearings and other components to retain lubricant and prevent contamination.


A device that detects the presence or absence of an object, or certain properties of that object, and provides feedback. Sensors are used to detect vibration levels and characteristics during vibration inspection.

shaft imbalance

A condition in which a rotating part's center of mass does not lie on the axis of rotation. Shaft imbalance can never be completely eliminated, and it can cause mechanical issues if it is out of normal range.


A thin wooden wedge. Shims are used to move components up and down to correct misalignment.


Moving horizontally outward from a central point or axis. Tangential vibration is perpendicular to the shaft.

thermal expansion

An increase in a material's dimensions that occurs as temperature increases. Thermal expansion of machine components is undesirable and can lead to misalignment.

thermal imagers

A thermal inspection device that uses infrared technology to produce a visual representation of temperature conditions. Thermal imagers are also called infrared cameras.

thermal signatures

The specific temperatures produced by a component. Thermal signatures can be used to identify abnormal temperature conditions.


Developing in a general direction. Trending data helps indicate changes over time.

ultrasound analysis

A machine testing method that involves measuring the quality and intensity of sound produced by machine components. Ultrasound analysis uses sound as an indicator of component health.

ultrasound detector

A testing instrument that can be used to listen to and measure the sound produced by machine components. Ultrasound detectors are used to perform ultrasound analysis.


Perpendicular to the horizon or ground. A vertical axis runs up-and-down rather than side-to-side.


A rapid, continuous, repetitive motion in a machine or other structure. Vibration can negatively affect mechanical operations.

vibration meter

A screening tool that takes basic vibration and temperature measurements. Vibration meters determine whether or not problematic vibration is occurring.

vibration screening

The process of checking a system for problematic vibration. Vibration screening identifies issues that are then further investigated during vibration testing.

vibration signal

A mechanical impulse generated by a vibrating component that radiates outward through a system. A vibration signal has a unique signature that describes its specific amplitude, frequency, and direction.

vibration signature

The specific properties of a vibration signal. A vibration signature describes the direction, amplitude, and frequency of a vibration signal.

vibration tester

A diagnostic tool that indicates the location, severity, and frequency of vibration in a mechanical component. Vibration testers use sensors to measure machine vibration along three different axes.

vibration testing

The process of locating and diagnosing causes of vibration. Vibration testing investigates issues first identified by vibration screening.


The property that describes how easily a fluid moves or flows. Low viscosity indicates a substance that flows easily.


The shape of a wave produced by a vibration signal. The waveform of a vibration signal depends on its amplitude and frequency.


The gradual removal of material on a surface caused by contact and friction. Wear is both a cause and an effect of faults in motor drive systems.

wear debris

Unwanted particles of metal that break off a component as it wears. Wear debris can reduce the efficiency of lubricant.


Wire wrapped around a core or into a coil that is used to conduct current. Windings create the magnetic field in a motor that makes the motor work.