Variable Speed Machine Condition Monitoring (VSM)

Contact: Alex de Kiefte Technical Manager, Condition Monitoring

Flyer: PVSM Portable Variable Speed Monitoring system overview

VSM Overview

VSM is a vibration data collection and analysis system designed specifically for the demanding requirements of slow, variable speed and variable load machinery.

Normal machine vibrations arise from the contact of moving parts, e.g. gears meshing or rollers within bearings running over raceways. These vibrations are readily transmitted to the outer housing of the machine, e.g. a gearbox. Additional vibrations arise due to defects in components such as cracked or missing gear teeth, spalling (pitting) of bearing rollers or raceways, and misalignment and imbalance of rotating parts. Indications of defects can usually be attributed to the specific component from which they emanate because their frequency characteristics are directly related to the geometry of the component and the speed of rotation. The amplitude of this vibration can also be used as an indication of the severity of the defect.

Unfortunately, many defects are difficult to detect and diagnose because they are (i) obscured by "noise" from surrounding vibrations, (ii) their signature is "smeared" out due to the fact that the machine is not running at a constant speed or (iii) large variations in the load characteristics for the machine during a single cycle result in equally variable amplitude levels in the vibration signal. Slow moving machines such as shovels and draglines represent a further problem because a larger time interval of data is required to properly observe defect indications than is often possible using commercially available equipment.

Raw data collection

The VSM database stores all of the raw data allowing complete and flexible post processing and analysis of data. A number of parameters, selected as key indicators of various fault types are also extracted from the raw data and stored to allow trending of machine condition over time.

Audible data analysis

  Download an audio sample to listen to a survey of a dragline swing motor gearbox with a cracked gear tooth.

The VSM system allows technicians to listen to the acceleration data collected by each accelerometer. Audible vibration analysis is an important, but often overlooked component of a complete vibration survey.

Speed Normalisation

Speed normalisation is essential in order to extract defect frequencies from the raw data.

If a survey is made while a machine is varying speed, revolutions of that machine will take up different periods of time over the course of the survey. This results in a smeared spectrum, which can be impossible to extract run speed based frequency components from. To overcome this, speed normalization is performed. This operation compares the measured speed plot with the low frequency data plot, and skews the time axis such that each revolutions consists of the same number of data samples. Thus the time axis becomes a revolution axis.

After the data has been speed normalized, normal operations such as spectrum analysis can be performed as if the machine was at a steady speed. The spectrum will use a bottom axis of orders, indicating events per revolution, eliminating the regular time reliant scale of Hertz.

Portable VSM (PVSM)

PVSM is a portable 10-channel vibration monitoring system. It incorporates the VSM collection and analysis software on a portable industrial notebook computer. It was developed in house to allow CCIPope technicians to obtain an instant and complete picture of the vibration signatures of each machine.

Advanced built-in analysis tools allow for a quick and accurate diagnosis of machine condition at the point of data collection.

Download a PVSM Portable Variable Speed Monitoring system flyer.