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Classification of Machines
Last Updated: 12/29/2011
Field Application Note courtesy of STI Vibration Monitoring Inc. Please visit STI at

Classification of Machines

This application note will provide a basis for selection of the machinery list of a Predictive Maintenance (PM) data collector program.

Creating a PM machine list is a logical process that will provide a smooth flow for data collection. Selection of the eligible machines will determine the size of the database. The measurement points for each machine will define the locations at which data will be collected. Measurement parameters will characterize how the data will be collected and trended.

Classification Criteria

Generating, the list of machines to include in the PM program should get considerable attention. The immediate decision might be to include everything in the plant. Indiscriminately including all machines in the plant could result in unnecessary data collection. Certain machines may not require data collection if they are only operated infrequently such as stepper motors or standby equipment. These machines should be monitored using specially developed testing programs.

Any machine which is operating continuously should be a candidate for the machine list. The machine list, which will most likely begin on paper, should include the machine name, horsepower, bearing type(s), plant location, and operating speed. Optionally, other items may be added to this list, such as bearing operating temperatures, fluid flow rate, pressures, etc. If available, the maintenance and economic history of each machine should be gathered.

After this information has been assembled the machine list can be sub-divided into three categories. These categories are critical, essential and general purpose.

Critical should be candidates for permanent instrumentation due to their production impact should they fail. Economic factors, such as replacement cost or maintenance expenses, should be considered to justify permanent instrumentation. These machine should always be monitored.

Essential machines usually provide major support for the production process and may be partially spared. These machines should be monitored, but due to their support function may be candidates for permanent instrumentation. If they are not permanently monitored they are candidates for the machine list.

General purpose machinery will include all other machinery in the plant. This category will most likely be a much larger list than the other categories and quite possibly consume the bulk of the maintenance budget. These machines are definitely candidates for the machine list.

Each category of the machine list can be further sub-divided into machine which operate nominally and those which are troublesome. The troublesome machines should be immediate candidates for the final machine list. They are good candidates to "cut your teeth" on, since the probability of improvements in operation are quite high once corrective action is taken that is based upon collected data. These machines should occupy the majority of the newly started PM program with weekly or bi-weekly monitoring cycles.

General purpose machines which are not troublesome and essential machines, along with the unmonitored critical machines, can be listed for routine monitoring on a monthly cycle. When the troublesome machines are operating nominally they can be monitored on a monthly cycle.

After the final list has been generated the next step is to decide upon the measurement points for each machine.

Measurement Points

Measurement points are the actual locations at which data is to be collected. Each machine should have all of its bearings included in the list of points. Generally, each bearing should have vibration data collected in three directions: vertical, horizontal, and axial. Due to machine construction and other considerations, the axial vibration data may not need to be collected at each bearing.

An often overlooked supplementary input which can be incorporated into many databases is inspection codes. The human senses of the data collector operator should not be overlooked, since the operator is working at the machine. An operator will observe certain situations which will not be identified by a vibration measurement. They are usually entered manually during the data collection process. Inspection codes can reveal important information about the condition of the machine which the vibration data could overlook.

Finally, now that the various machines and the measurement points are defined the only remaining bit of information needed is how each machine will be analyzed. This involves defining the measurement parameters for each measurement point.

Measurement Parameters

At each measurement point certain types of data must be collected. Vibration data may be collected in displacement, velocity, acceleration, or spike energy units. Machine construction and operating conditions will dictate which type to include.

Most data collector based PM programs are implemented on machinery with rolling element bearings. These bearings will generate specific frequencies related to the bearing condition. Many data collectors offer a measurement parameter called spike energy which can provide early detection of bearing defects. Thus, machines with rolling element bearings should have spike energy data collected along with at least one of the other vibration data types.

Certain machines have couplings which transmit axial vibration from one case to another and may not need to have axial vibration data collected at each bearing. Process variables, such as temperature, pressure, or flow rate, may be added to the measurement parameters list where provisions have been made that allow the data to be input automatically by the data collector.

In addition to bearing condition, proper measurement parameter selection will provide information about the balance state, alignment condition, and the general condition of the machinery internals.

Criteria Checklist

  1. Machine Classification
  2. Measurement Points
  3. Measurement Parameters

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