Vibration Monitoring of Mill Granulators


Mill Granulators are used in process plants for reducing raw coal or rock ore into fine power for burning or further processing. They come in many designs but usually are rotary mills with rings. Coal crushers of all types, such as ring granulators can often cause significant and unacceptable vibrations in their supporting structures. The source of the vibration from a mill granulator is usually imbalance. This may result from normal wear and tear or from an emergency situation, for example, a broken hammer.

Most machines (with speeds ranging typically from 600 to 720 RPM) will pass through resonance during start-up and shutdown, resulting in large amplitudes in the tall structures that often support them.

Imbalance forces in rotary breakers are usually low. However, the crushing process itself and the drive system produce random vibrations over a range of frequencies. The loss of a Mill Granulator will significantly affect production causing financial losses. A continuous vibration monitoring system will give timely warning to prevent premature plant failure or unscheduled outage.

A Mill's power consumption can account major share electricity used. It is a well recorded fact that an out of balance rotor or misaligned shaft will consume or waste up to 15% more energy. 


After taking care of initial installation set up the likely faults thereafter are mainly restricted to unbalance due to wear and build-up of deposits. Analysing and designing a Vibration Monitoring System must take into account the following:

  • Unbalance
  • Misalignment
  • Bed bearings-antifriction type
  • Mechanical looseness
  • Hazardous pulverised coal dust

Typical Vibration Sensor Measurement Positions - Diagnostics and On-Line

The sensor orientations are summarised as follows:

  • HORIZONTAL – Inductive velocity sensor on two motor bearings (A & B) to measure absolute casing vibration in velocity (Peak or RMS) and Piezo Electric Sensors on two mill bearings (C&D) to measures absolute casing vibration displacement microns pk-pk to detect unbalance and structural rigidity & foundation problems.
  • VERTICAL - Inductive velocity sensor on motor bearings (A) to measure absolute casing vibration in velocity (Peak or RMS) and Piezo Electric Sensors on mil bearings (D) to measures absolute casing vibration displacement microns pk-pk to detect unbalance and structural rigidity & foundation problems.
  • AXIAL - Inductive velocity sensor on motor bearings (B) and Piezo Electric Sensors on mil bearings (C) to detect misalignment or bent shaft 

Vibration Diagnostic Chart - Faults Common to Mill Granulator

S No. Fault Amplitude Frequency Phase Remarks
 1Unbalance Proportional to unbalance in radial direction1 x RPM

Single steady reference mark Most common cause of vibration is due to mill liner wear, deposit build & sudden loss of part such hammer s fracturing off.  
 2Misalignment of couplings or bearings. Axial vibration 50% of more of the radial level1 x RPM common but often x 2 or 3 RPM Single double or triple reference mark Best identified by dominant axial vibration. Confirm with phase measurement or dial gauges. If sleeve bearing with no coupling misalignment balance the rotor. Use strobe to optimise gear meshing. 
3Antifriction Bearings Use Spike Energy™ (gSE), g or Envel. Demodulation
High frequency 35K to 55K CPM not related to RPM. ErraticBearing responsible most likely the one nearest point of largest high frequency vibration
4Mechanical Looseness Often highest in vertical direction 2xRPM 1 or 2 reference marks depending on frequency.  Usually accompanied by unbalance and/or misalignment. Mills on elevated structures are prone to vibration and resonance
5Worn or damaged pinion (In case of gear driven set) Unsteady. Use velocity measurement if possible 1xRPM, Gear mesh frequency & harmonics

Usually unsteady

Single or multiple marks. May be erratic 
 Inspect Gear drive. Check for gear tooth wear or damage with strobe light to view gear meshing. 
6Bad belts (in case of belt driven set) Erratic or pulsating 1, 2, 3, & 4 x RPM of belts One or two reference mark depending on frequency. Usually unsteady.Strobe light best tool to freeze faulty belts .Look for worn or damaged belts or pulleys  


A comprehensive vibration protection system would cover sensors mounted horizontal and vertical the Mill Motor and Mill bearings respectively. The inductive velocity sensors for the drive motor and low frequency piezo electric sensors on the mill (detects bearing condition, mill unbalance looseness, structural rigidity & foundations). 

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