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POWER PRESS RELIABILITY

File: Press Inspection and Evaluation

Engineering: Power Press vibration diagnostics

Theodore A. Cannell

Vibration Measurement and Diagnostics

Another inspection technique used to evaluate and pinpoint problems in power press equipment is "Vibration Measurement". Vibration techniques are used either to pinpoint an existing problem or to evaluate the health of a system on a routine basis. Mechanical equipment produces vibrations which correspond to its inherent design, the condition of the equipment, and how the equipment is being used. By Qualifying and Quantifying the information with state of the art methods and instrumentation, reasonable levels of reliable evaluation and diagnosis are possible.

In Power Press equipment the system consists of the press structure and the foundation. If the press structure is separated from the foundation by isolators, then the press structure and the foundation are evaluated separately.

Mechanically the press structure consists of five basic elements. They are the frame, drive train, clutch/brake unit, motor/flywheel assembly, and the slide assembly. Although vibration measurement is useful in evaluating all these elements, it is most beneficial in evaluating rotating (or reciprocating) components in the drive train, clutch/brake assembly, motor/flywheel assembly.

Press System Failures:

Failures in press systems are typically caused by:

  • Excessive clearances.
  • Excessive wear.
  • Bearing Problems- both bushings and roller berings.
  • Out of balance conditions- drive system .
  • Improper foundation, mounting, isolation, or leveling.
  • Overloading- excessive tonnage requirements.
  • Operational errors- incorrect shut height, double parts in die, etc., usually resulting in catastrophic failure of the equipment.

Equipment Sensors:

The sensors used to measure these vibrations are accelerometers either permanently mounted or hand held. The signals from these sensors are sent to an instrument where the wave form is analyzed and interpreted. The instrument of choice now is the "fast Fourier Transform (FFT) Spectrum Analyzer". These signals may be evaluated on the spot or saved for future analysis.

Why use vibration equipment ?

There are various parameters used to detect problems in machinery. See table below:

Condition

Oil Temperature and equipment "hot spots"

Oil Pressure

Oil Flow

Oil Analysis

Vibration

Out of balance

       

X

Misalignment or bent shaft

X

     

X

Damaged Roller Bearings

X

   

X

X

Damaged Journal Bearings

X

X

X

X

X

Damaged or worn gears

     

X

X

Mechanical Looseness

       

X

Noise

       

X

Cracks in members

       

X


Note: Vibration analysis can be used as a diagnostics tool for all of the above conditions.











PM Program:

Typically the initial evaluation of a machine would be to locate areas susceptible to failure and mount sensors in these areas, then take readings, look for obvious problems, and if none are found store the information. Then routinely gather new readings and compare.

Vibration analysis:

The general approach would be to locate the source of vibration, determine the cause, and correct. Vibration analysis is a helpful tool that will assist in machinery assessment, but is most helpful when used in conjunction with a PM Program that would track not only the vibration characteristics of the equipment but also the additional items noted in the table above. Combined with a periodic press inspection, the frequency of catastrophic failures can be substantially reduced.

Bibliography

Wowk, Victor. Machinery Vibration. TJ177.W68, 1991
Heinz P. Bloch and Fred K. Geitner. Machinery Failure Analysis and Troubleshooting. Gulf Publishing Co., 1986.
John Piotrowski. Shaft Alignment Handbook, Marcell Dekker, New York, 1986.
J.D. Smith. Gears and their Vibration: A basic approach to understanding Gear Noise. Marcel Dekker, New York, 1983.
Simon Braun. MSA-Mechanical Signature Analysis. American Society of Mechanical Engineers (ASME), 1983.