ISO 7919-1: Mechanical vibration – Evaluation of machine vibration by measurements on rotating shafts – Part 1: General guidelines

Summary

ISO 7919-1 is a key international standard that provides general guidelines for the measurement and evaluation of vibration on the rotating shafts of machinery. This standard is the counterpart to ISO 10816, which deals with vibration on non-rotating parts. ISO 7919 focuses on using non-contact proximity probes to directly measure the movement of the shaft relative to its bearings. This type of measurement is particularly important for large, critical machinery with fluid-film bearings, such as turbines, compressors, and large pumps, where understanding the rotor’s dynamic behavior is essential for safe operation.

Table of Contents (Conceptual Structure)

The standard provides a framework for setting up a shaft vibration measurement program and for interpreting the results:

1. 1. Scope and Measurement Principle:

   This initial section defines the standard’s scope, clarifying that it provides the general procedures for measuring and evaluating vibration on rotating shafts. It establishes the fundamental principle: this type of measurement focuses on the vibratory motion of the shaft itself, typically relative to the stationary bearing housing. This is a critical distinction from casing measurements (covered by ISO 10816). Shaft vibration is the preferred measurement for machines where the rotor is massive compared to the casing and is supported in fluid-film bearings. In these cases, significant shaft motion can occur within the bearing clearance that is not transmitted to the external casing. The primary goal is to assess the severity of this dynamic shaft motion to protect the machine from bearing damage or rotor-stator contact.

2. 2. Measurement Quantities:

   This chapter specifies the parameters that should be measured and evaluated. The primary quantity for overall vibration severity assessment is the S_p-p value, which is the peak-to-peak vibratory displacement of the shaft. This represents the total excursion of the shaft’s centerline as it moves within the bearing and is a crucial metric for machinery protection, as it can be compared directly to the physical bearing clearances. However, the standard also recognizes the value of other quantities for diagnostic purposes. It recommends that the measurement system also be capable of providing the shaft orbit (the path of the shaft centerline), which is essential for diagnosing issues like oil whirl or misalignment, and the average shaft centerline position, whose change can indicate changes in load or alignment. For some applications, filtered vibration values (e.g., at 1X running speed) are also used for evaluation.

3. 3. Instrumentation and Mounting:

   This chapter provides guidance on the hardware required for shaft vibration measurements. It specifies the use of non-contacting probe systems, which consist of three main components: the probe (sensor), an extension cable, and a driver (or proximitor). These components are calibrated as a system and are not interchangeable. The standard recommends mounting the probes in pairs at each bearing, arranged at 90 degrees to each other (an X-Y configuration). This allows the measurement system to capture the full two-dimensional motion of the shaft centerline and construct the shaft orbit. Proper installation is stressed as critical, requiring rigid mounting brackets, correct probe gapping, and ensuring the shaft’s “probe track” surface is smooth and free of any electrical or mechanical runout that could corrupt the signal.

4. 4. Evaluation Criteria and Zones:

   This section presents the framework for judging the severity of the measured vibration. It proposes two primary criteria. The first is an absolute criterion, which involves comparing the measured shaft vibration (S_p-p) against pre-defined limits. The standard suggests a four-zone model for this:

   • Zone A (Good): Vibration levels on newly commissioned machinery.
   • Zone B (Satisfactory): Acceptable for unrestricted long-term operation.
   • Zone C (Unsatisfactory): Indicates a potential problem; the machine should be investigated to determine the cause.
   • Zone D (Unacceptable): Vibration levels are considered damaging; immediate action is required.

   The second criterion is based on a change in vibration magnitude from a known baseline. A significant increase in vibration, even if it is still within the “Satisfactory” zone, can be an early indicator of a developing fault. This part of the standard (Part 1) provides the general framework; the specific numerical values for the zone boundaries are provided in the machine-specific parts of the ISO 7919 series.

5. 5. Guidance on Setting Alarms (Alert and Trip):

   This final section provides a practical framework for implementing the evaluation criteria into an automated machinery protection system. It recommends a two-tier alarm strategy. The first level is an Alert (or “alarm”) setpoint. This is typically set above the machine’s normal, stable operating baseline. If this level is breached, it should trigger a warning to the operator that the machine’s condition has changed and that an investigation is warranted. The second, higher level is a Trip (or “shutdown”) setpoint. This is an absolute limit set at a level where continued operation is likely to cause severe damage. If this level is breached, the system should trigger an automatic shutdown of the machine to prevent a catastrophic failure. The standard advises that these setpoints should be based on both the absolute zone boundaries (a Trip should not be set above the Zone C/D boundary) and on significant changes from the established baseline (e.g., an Alert could be triggered if vibration doubles, even if it’s still in Zone B).

Key Concepts

• Shaft vs. Casing Vibration: The core principle is that for certain machines (especially those with massive, stiff rotors and relatively flexible casings), the motion of the shaft itself is a much more direct and reliable indicator of the machine’s dynamic state than the vibration transmitted to the outside of the bearing housing.
• Machinery Protection: While the data is also used for diagnostics, the primary application of the ISO 7919 framework is in real-time machinery protection systems designed to prevent catastrophic failures.
• Importance of Relative Motion: By measuring the shaft’s movement relative to the bearing, analysts can directly assess the utilization of the bearing clearance and diagnose specific issues like oil whirl or excessive preload.

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