Glossary of Useful Machinery Alignment Terms

January 29, 2013

Alignment – Positioning two or more machines so that the rotational centerlines of their shafts are colinear at the power of transmission point under operating conditions.

Alignment specifications or targets – Desired intentional misalignment of MTBM (Machine To Be Moved) to compensate for thermal growth and/or dynamic loads. Most commonly specified as coupling target.

Angularity – The angle between two machine shaft centerlines. For spacers, the angle between each shaft and the connecting element (shaft) at each flex plane.

Bent foot – A type of soft foot characterized by a wedge shaped air space between the underside of the machine foot and the baseplate contact area.

Colinear – Two or more lines positioned in space with no offset or angularity between them. (This is the objective of an alignment job). In effect, all colinear lines are the same line.

Coplanar – Lying or acting in the same plane.

Chock – For alignment purposes, a thick shim, usually ¼” or thicker.

Hold-down bolts – The bolts anchoring or holding the machine to the base or foundation. Also called base bolts or anchor bolts.

Induced soft foot – A type of soft foot that is caused by external forces (coupling strain, pipe strain, etc.) acting on a machine independent of the foot to baseplate connection.

Jackscrew or jackbolt – A bolt or screw attached to the base or foundation used to move or position the MTBM (Machine To Be Moved) horizontally.

Jackshaft – A long shaft used as a spacer between 2 machines. (Generally used to describe connecting shafts more than a few feet in length.)

Offset – Distance between rotational centerlines at any given normal plane. For short coupling (short flex) usually displayed at the coupling center.

Parallel air gap – A type of soft foot characterized by parallel air space between underside of the foot and the baseplate contact area.

Repeatability – The consistency (or variation) of readings and results between consecutive sets of measurements.

Resolution – The smallest change or amount that a measurement system can detect.

Shim – A thin piece of metal material inserted between the base and machine feet to produce precise vertical adjustments of the machine centerline.

Soft foot – A term used to describe any condition where tightening or loosening the bolt(s) of a single foot distorts the machine frame. Must be corrected prior to final alignment.

Spacers – A generic term for any coupling characterized by two flex planes separated by a connecting shaft without bearings or other supports between the flex points.

Squishy foot – A type of soft foot characterized by material (could be shims, paint, rust, grease, oil, dirt, etc.) acting like a spring between the underside of the machine foot and the baseplate contact area.

STAT – A short form of “stationary machine”. The stationary machine’s centerline is used as the reference line to measure the misalignment of MTBM (Machine To Be Moved).

Step shim – Use of several shims to fill the wedge shaped gap of a bent foot. Each shim is inserted to a different depth so that a stair-step shaped support is built to better support the entire foot. All of the steps together are called a step shim.

Thermal growth – Movement of shaft centerlines associated with (or due to) a change in machinery temperature between the static and operating condition.

Tolerance – The maximum permissible deviation from a specified alignment position, defining the limits of offset and angularity.

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