Glossary of Shaft Alignment Terms

Air Gap The internal gap between a motor’s rotor and stator. Excessive misalignment forces can change this gap, causing excessive vibration and heat resulting in premature electrical failure of the motor.
Alignment The placement of two or more machines, such that when complete, the shaft centerlines of the machines are collinear.
Angularity Angular relationship between movable and stationary shafts. Typically expressed as a slope, or rise over run, relationship. Imperial units are mils/in. Metric units are mm/100 mm.
Axial The positioning of machine components along an axis, or centerline.
Axial Runout 1) The measurement of machine component eccentricity, due to machining errors, bent shafts, or other damage.2) The measurement of endplay or deflection along the axis or rotation of a shaft or rotating component
Axial Spacing The distance between two coupling hubs which is required for proper coupling operation, minimizing thrust forces on bearings, and maintaining proper magnetic centering of the motor.
Backlash Looseness in coupling that allows for torsional/twisting play between shafts.
Base-Bound A problem that arises when the piece of equipment you wish to move is sitting too high and there are not enough shims to remove to lower it.
Baseplate A heavy, flat metal plate, normally grouted into concrete, onto which machine components are mounted.
Belt An elastomeric band which is attached to two or more machines, used to transmit power by rotation.
Belt Alignment The positioning of shafts, sheaves, and/or pulleys so that excessive radial or axial forces are minimized.
Bolt-Bound A problem that arises when the piece of equipment you wish to move cannot be adjusted any further horizontally because the foot is hitting the hold down bolt.
Bracket Sag Sagging of dial indicator brackets due to the overhung weight of the bracket and dial indicator, which tends to bend the bracket toward earth due to gravity. The amount of bracket sag must be measured and compensated for when using dial indicators for shaft alignment.
Bucking In A process used to rough in measuring tools. The process consists of making rough adjustments to the measuring device until adequate range is obtained to complete a measurement. Similar to roughing in.
Cardan Shaft A shaft that is intentionally offset (inches or feet) by means of a universal joint at either end of the drive shaft. Typically used when the driven element must move during operation (a roll moving in and out of nip, etc.).
C-face Flange A machined flange used to mount a motor onto a machine which is being driven.
Chain Coupling A coupling which uses a short segment of double chain to connect two coupling hubs. Slight misalignment is compensated by the sliding motion between the chain links and hub teeth.
Charge Coupled Device A device which converts light into a digital form, normally used when extreme accuracy is required.
Cogged Belt A belt that has teeth molded directly into its surface, which mesh with corresponding teeth in pulleys. A cogged belt is designed to avoid slipping and is also known as a timing belt.
Collinear Positioned on one line. The purpose of shaft alignment is to position the centerlines of rotation of two or more shafts so they act as one. All centerlines should rotate on one line.
Coning A method of centering a laser device by rotating the transmitter(s) to form narrower “cones” of the beam, thereby minimizing the base of the cone.
Coplanar A set of points existing in space so that all points exist in the same plane. When two or more shaft centerlines are in alignment, all points about the shafts exist in the same plane.
Coupling A device for connecting two rotating shafts for the purpose of transmitting power.
Coupling Lockup When severe coupling misalignment causes coupling clearances to be essentially zero, making the coupling no longer flexible. Normally results in high vibration and damage to the coupling. The high vibration is often transmitted to the bearings and seals causing them to fail prematurely.
DBSE Acronym for Distance Between Shaft Ends. Axial spacing between shafts to allow for expansion during operation.
Dial Indicator Measurement device consisting of a spring-loaded plunger and graduated scale with needle. Used to measure relative positions along a surface. As plunger is pressed in, measured value increases. As plunger is released, measured value decreases.
Disc Coupling A coupling which uses thin steel discs, bolts, and elastic bushings to connect two coupling hubs. Slight misalignment is compensated by slight bending of the discs and by elasticity of the bushings.
Double Radial Alignment Method Dial indicator alignment method using two indicators to read two radial positions on the same shaft. This method requires good separation between the near and far indicators, and very careful compensation for bracket sag.
Drive Shaft A shaft used to connect the driving and driven machines; most commonly used with cardan or universal joints.
Eccentricity The amount a round object deviates from its centerline. In shaft alignment, eccentricity normally denotes being bored out of center, or other machining errors.
Elastomeric Coupling A coupling which uses an elastomer, such as rubber or neoprene, as the flexible element. Slight misalignment is compensated by the elasticity of flexible element.
Face to Face Alignment Method A method of measuring misalignment, when a spacer shaft is used. This method uses dial indicators the measure the amount of radial misalignment across two flexible couplings simultaneously and mathematically calculating the correction values.
Feeler Gauge Uniform stainless steel strips, normally in sets, graduated in thousandths of an inch (mils) for measuring the gap between machinery components. The thickness of the gauge inserted is the gap measurement. See also taper gauge.
Flat 1) In machinery installation, “flat” refers to the supporting feet of machinery existing in one plane. “Flat” is often confused with “level”, but these terms are distinct. A machine base can be flat without being level.2) In shaft alignment, flatness refers to all feet of the machine in question residing in one plane. Most machines can operate well even slightly out of level, but precision alignment requires a reasonably high degree of flatness.
Flat Belt A belt used in power transmission. The corresponding pulley is also flat or slightly crowned. Typically used where power transmission must take place over longer distances and is usually associated with linseshaft transmission systems.
Flexible Coupling Generic term for a non-rigid coupling, which compensates for slight misalignment by its ability to flex, bend, slide, and/or compress.
Fluid Drive Coupling A coupling which uses two non-contacting impellor-like turbines and a hydraulic fluid to transmit power.
Foot Bolt Bolts used to connect machinery feet to the base.
Gap 1) Term used to describe clearance between coupling hubs. Also see DBSE.2) Term used to describe the angle of the coupling faces–as in I measured a 7 mil gap difference between the top and the bottom.
Gear Coupling A coupling which uses two geared hubs, which are connected by a splined shell to transmit power. Slight misalignment is compensated by the sliding motion between the gear teeth.
Grout Material such as concrete or epoxy that serves as filler between the bottom of the machine base and the top of the concrete foundation or metal equipment skid.
Horizontal Parallel to earth.
Horizontal Shaft Alignment An alignment of of two horizontally-mounted machines, coupled to each other, where the corrections are made at two sets of feet–in general.
Hot Alignment 1) Measuring for misalignment correction values on machines that are at or near their operating temperatures. This method serves to minimize the amount of thermal growth calculation required by measuring the machines while hot.2) The act of aligning a machine while hot
Inertia Block A concrete block which serves as a base for mechanical equipment such as fans or pumps; the block is mounted on a resilient support to reduce the transmission of vibration to the building structure.
Jack Shaft A shaft that serves to connect the driver and the driven elements over large spans. Typically found in cooling towers, line shafts and other larger equipment.
Level 1) A measurement in machinery installation defined as being horizontal in relationship to earth. Often confused with “flat”, a machine base can be level and not “flat”.2) A device used to measure the degree of level.
Machine Train Three or more machines coupled together acting as one unit.
Magnetic Center The balance in a magnetic field between a rotor and stator. An electric motor tends to seek the balance by moving the rotor within the magnetic field. Most often associated with motors utilizing sleeve bearings.
Magnetic Coupling A coupling which uses two magnetically-induced fields to transmit power.
Mechanical Packing Generally flexible material used to seal fluids, such as oil, grease, or water, between segments of a machine.
Mechanical Seal A mechanical device used to seal fluids, such as oil, grease, or water, between segments of a machine. More rigid and susceptible to damage than packing.
Mil Standard imperial unit for measuring shaft misalignment equal to one thousandth on an inch. (1.0 mil = 0.001”)
Misalignment A measurement of the degree of non-collinearity between two or more rotating shafts.
Movable The machine you intend to focus your correction efforts on. The movable machine will be positioned to become collinear with the stationary machine
Offline to Running (OL2R) A method used to measure the changes in relative position between two or more machines between cold and hot states which includes both static and dynamic changes to relative position.
Offset Parallel relationship between movable and stationary shaft centerlines of rotation
Packing Gland A type of gland seal used to prevent leakage of fluid, such as water or steam, between sliding or turning parts. See “Stuffing Box”.
Partial Shim A shim for aligning machinery which is cut to smaller than normal size. Used to correct angular contact problems between a machine foot and its base.
Pipe Strain The strain caused on a piece of equipment due to the piping having to be pulled into position.
Plumb A measurement relating to verticality, or the positioning of machinery 90 degrees from earth horizontal.
Pulley A wheel on an axle that is designed to support movement of a cable or belt along its circumference. Most often a pulley denotes mechanical power transmission by use of a flat belt. Often used to define a sheave.
Rabbet Fit A step-shaped recess cut along the edge or in the face of a machine component to the edge or tongue of another component. In shaft alignment, the term refers to the method used to connect a C-face motor to an adaptor for power transmission.
Radial In shaft alignment, radial refers to the measurement along the outside edge of a coupling.
Reverse Dial Indicator Method A method to measure shaft misalignment by using two dial indicators mounted on opposing shafts. The shafts are turned together, and the amounts of change on each indicator is recorded at the 3, 6, 9, and 12 o’clock positions. The amount of misalignment is then calculated based on these measurements.
Rigid Coupling Generic term for a non-flexible coupling. Alignment values must be near zero. Any misalignment forces are transmitted to bearings or shafts.
Rim and Face Indicator Method A method of measuring shaft misalignment where two dial indicators are used–one measures offset on the rim of the coupling, the other measures the angularity on the face of the coupling.
Rough Alignment Using a straight edge to bring the driving and driven machines closer to alignment.
Runout The relative movement of a shaft as it is rotated. Checked with a dial indicator, the base is fixed with the indicator stem touching the shaft. The shaft is turned underneath as the indicator measures how much the shaft is deflected or bent.
Shaft Seal A dynamic seal designed to retain or contain fluids and/or exclude foreign materials through the exertion of radial pressure.
Sheave A wheel on an axle that is designed to support movement of a cable or belt along its circumference. Most often a sheave denotes mechanical power transmission by use of a V-belt. Often used to define a pulley.
Shim Machined metal or plastic spacers used to correct the vertical misalignment. Shims are added to raise the equipment; shims are removed to lower the equipment.
Soft Foot Condition where uneven contact between equipment foot and base distorts the equipment. Distortion can lead to clearance issues (bearings, seals, impellers, motor air gap, etc.) and non-repeatable shaft movement.
Sole Plate A mounting plate onto which a motor is attached to provide a foundation to minimize vibration and movement.
Spacer Shaft See Jack Shaft.
Stationary A label for the machine that is typically more difficult to move and so becomes the reference for the alignment. The movable machine will be aligned to the stationary machine.
Straight Edge A straight piece of metal, like a metal ruler, used to measure rough alignment.
Stuffing Box A type of gland seal, used to prevent leakage of fluid, such as water or steam, between sliding or turning parts. See “Packing Gland”.
Taper Gauge Wedge-shaped stainless steel strips, normally in sets, graduated in thousandths of an inch (mils) for measuring the gap between machinery components. Gauges are inserted until resistance is felt. The gap is then read from the gauge. See also feeler gauge.
Target Values The desired ending position of the movable and stationary shaft centerlines.
Thermal Growth The amount equipment centerlines change, both vertically and horizontally, due to temperature changes.
Thermal Offset Target values entered to counteract the affects of thermal growth. Can be expressed as an angle and offset, TIR of dial indicator configurations or how to position the feet of the movable and/or stationary machines
Thrust Bearing A type of rotary bearing designed for high axial loads.
Timing Belt See “cogged belt”.
TIR Acronym for Total Indicator Reading.
Tolerance The acceptable variation from the target. In general, tolerances get tighter as the operational speed gets faster.
Undercut Bolt A bolt whose threads have been removed to reduce the diameter. The threads can be removed down to the shank without any effect on the bolt’s tensile strength. Its shear capacity, however, will be diminished slightly.
V Belt A belt used in power transmission. The V shape of the belt serves to wedge the belt into the sheaves to improve gripping power, minimize slippage, and improve efficiency.
Validity Rule Dial indicator usage rule that states when taking TIR values, the sum of any two values diametrically opposed (180° apart) must equal the sum of the diametric pair of readings perpendicular to the first set. Generally stated as “top plus bottom equals side plus side.”
Vertical Shaft Alignment An alignment of of two vertically-mounted machines, coupled to each other, where the corrections are made at the C-Face motor interface–in general.
Verti-Zontal™ A method of measuring and correcting shaft misalignment so that both the vertical and horizontal planes can be aligned with one measurement of the shafts.
Zeroing When using measuring tools, the act of setting the display/dial/gauge back to zero to measure the relative distance from that instance. For dial devices (i.e. dial indicators, dial calipers, etc.) the bezel on the dial is rotated to bring zero back to the needle.