ERAG / CAT Concentricity Alignment Tool
ERAG rental from ACQUIP, makes moving the instrument between two adjacent surfaces a breeze.
ERAG ELECTRONIC RADIAL ALIGNMENT GAUGE
Electronic Radial Alignment Gauge (ERAG) or Concentricity Alignment Tool (CAT) negates the need for tight wire and/or laser alignment. This tool can directly measure the amount of diaphragm misalignment with the rotor in place. This tool have a proven alignment method through thousands of hours of use.
ERAG/CAT Concentricity Alignment Tool features:
• Directly measures the amount of diaphragm misalignment with the rotor in place.
• Alignment method proven through thousands of hours of use.
• Easy to use – direct readout.
• Accommodates a wide range of turbine sizes.
• Very accurate (resolution to .0001 inch – accuracy plus or minus.0005 inch maximum).
MICRO ERAG FEATURE
µERAG (micro CAT) was born from the need to measure diaphragm misalignment on new turbine models with a very small radial clearances. The new head fits on a width slot 0.500″ (same as SCAT) and allows you to measure radial clearances from 0.200″ to 0.250″. Please contact us for additional information on the µERAG (micro CAT) at info@acquip.com or call us at +1 305 538 7101.
How to Use the ERAG/CAT for my Project?
- Inserted into the empty packing groove, the Concentricity Alignment Tool (CAT) allows annular space variations to be displayed on a LCD display as it is pulled through the packing groove as shown.
- The head “rub bars” ride on the turbine shaft and the head lever arm rides on the ID of the diaphragm. As the head is pulled through the packing groove, the lever arm follows the contour of the diaphragm ID and the dimensional variation is displayed.
- ERAG have a main body to which there is pivotally coupled a U frame whose arms extend at an angle from its base, so that as the former is pivoted in one direction, the latter will correspondingly pivot in an opposite direction.
- Contact points are provided to the arms and the bottom surface of the body, and the base of the U frame is biased by a spring-loaded slidable element within the body, which in turn is connected to a transducer.
- An ideal way to accomplish alignment is with the rotor in place so the dimension from rotor to seal carrier can be measured directly. One of the best methods of measuring between the rotor and the carrier includes an ERAG.
For more information about the ERAG please contact us at info@acquip.com or call us at +855 ACQUIP1 you can also learn more about our alignment services in the field visiting our projects gallery.
For more information about the ERAG please contact us at info@acquip.com or call us at +855 ACQUIP1 you can also learn more about our alignment services in the field visiting our projects gallery.
What Is an ERAG (Electronic Radial Alignment Gauge)?
An ERAG (Electronic Radial Alignment Gauge) is a precision measurement instrument used to determine radial position and concentricity between rotating components, most commonly within turbine-generator assemblies and large rotating machinery. Unlike conventional mechanical gauges, the ERAG electronic radial alignment gauge uses digital displacement sensors to provide high-resolution radial alignment measurements relative to a defined reference centerline.
In turbine applications, ERAG systems are frequently used to measure:
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Rotor-to-stator concentricity
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Diaphragm or casing alignment
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Radial position of internal components relative to bearing centerlines
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Coupling hub runout and radial deviation
The ERAG replaces or supplements traditional mechanical methods by reducing human interpretation error and improving repeatability.
The term ERAG is often associated with specialized turbine alignment procedures, and in some industry contexts it is discussed alongside internal alignment tooling such as the drag cat system. While a drag cat is typically used to mechanically sweep and transfer reference centerlines through a turbine casing, an ERAG electronic radial alignment gauge focuses specifically on precise radial measurement using electronic sensing technology.
For large steam or gas turbines where radial tolerances are extremely tight, the ERAG provides a quantifiable method of verifying concentricity rather than relying solely on mechanical feel or indicator interpretation.
Measurement Principle
An ERAG electronic radial alignment gauge typically consists of:
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A rigid mounting fixture referenced to a known datum
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Electronic displacement probes (often LVDT-based or similar)
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Digital readout or data acquisition interface
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Calibration standards for zero reference
The device measures radial deviation relative to a baseline axis. This allows engineers to determine whether internal components are concentric with the turbine rotor centerline or require correction.
Because turbine internal clearances are often measured in thousandths of an inch (or hundredths of a millimeter), measurement stability and repeatability are critical.
ERAG vs Traditional Dial Indicator Methods
Before electronic systems became common, radial alignment and concentricity checks were typically performed using dial indicators mounted on sweep bars or custom fixtures. While effective, these traditional methods have limitations.
Traditional Dial Indicator Methods
Dial indicator-based radial alignment:
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Requires manual sweep of the component
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Depends heavily on technician consistency
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Is susceptible to cosine error and bracket deflection
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Often lacks digital documentation
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Can introduce interpretation variability between operators
In turbine internal alignment, dial indicators are frequently used in combination with mechanical centerline transfer tools such as drag cat systems. However, accuracy is influenced by setup rigidity and operator technique.
ERAG Electronic Radial Alignment Gauge
An ERAG electronic radial alignment gauge improves upon these limitations by:
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Providing high-resolution electronic displacement measurement
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Reducing subjective interpretation of analog readings
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Improving repeatability across multiple measurement points
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Allowing digital recording of radial deviation values
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Minimizing human-induced sweep inconsistencies
From a reliability perspective, the ERAG reduces uncertainty in radial alignment verification, particularly in high-value turbine overhauls where concentricity directly affects seal clearance, vibration behavior, and bearing loading.
Engineering Relevance in Turbine Internal Alignment
In steam and gas turbines, improper radial alignment can lead to:
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Uneven seal clearances
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Rotor-to-stator rub risk
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Elevated vibration at running speed
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Uneven thermal growth distribution
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Increased bearing reaction forces
Because turbine internal geometry must be referenced to a stable centerline, tools such as the drag cat and the ERAG electronic radial alignment gauge are often used together, one to establish or transfer centerline position, the other to verify radial concentricity with electronic precision.
For a detailed overview of turbine internal alignment procedures, see our turbine alignment procedures.