Application Report 11: 2-plane balancing during operation

  • Impact mills, also known as hammer mills, are used for crushing various and especially mixed granular materials. In doing so, a rotor, at whose outer circumference hammers are mounted, is rotating inside of a metal housing.

    In the present case, the rotor has been replaced after the rotor shaft was fractured, a new bearing has been installed and the mill has been equipped with new grinding plates. In order to ensure a low-wear operation, the rotor had to be rebalanced. Balancing the rotor was performed in installed condition with all accessories.

  • Overview

    Industry:

    Polymer Processing/Impact mill

    Equipment:

    Machine diagnostics VMSet-04-P

    Software:

    VibroMatrix with InnoBalancer Pro and InnoAnalyzer Speed Pro

    Application:

    Order analysis and balancing in 2-planes

  • Measurement planes
    Fig. 1 Measurement planes
  • Measuring setup

    With the InnoBeamer X2 of the VibroMatrix measurement kit for machine diagnostics VMSet-04-P measurements take place simultaneously on 2 analog inputs for acceleration and one digital input for speed measurement. The vibration measurement on the mill has been performed on two planes (plane 1 above, plane 2 below). The accelerometers (type KS80D) have each been mounted as high as the bearing of the rotor (Fig. 1).  For mounting clamping magnets were used.

    Fig. 2 Speed measurement

    The speed of the rotor is detected optically by a photoelectric reflex switch. Therefore, a piece of polarising reflection foil has been attached to the shaft of the rotor (Fig. 2). The switch is connected to the digital input Sys 1 of the InnoBeamer X2. The power supply of all components takes place by the InnoBeamer X2 directly via the USB port of the PC, without any additional mains adapter.

Run-up
Fig. 3 Run-up

Preliminary assessment

For the entire rotor a maximum speed of 4700 1/min had to be achieved. That are almost 80 Hz. In order to learn about the vibration behaviour of the rotor with its stacked grinding plates (Fig. 4), run-up and coast-down analysis were performed as preliminary assessment of the rotor.

This way, possible natural frequencies and their excitation by the imbalance of the rotor could be determined. The instrument InnoAnalyzer Speed Pro of the VibroMatrix system is used here. It offers an order analysis with free adjustable parameters for both measurement channels simultaneously and operates in real time as all VibroMatrix instruments.

  • Construction of a rotor
    Fig. 4 Construction of a rotor
    Balancing run
    Fig. 5 Balancing run
  • Prior to reaching the requested operational speed, vibration severities of up to 30 mm/s have been attained. At this point the run-up was canceled so that the rotor was not strained any further.

    The InnoAnalyzer Speed displays the natural frequency precisely (Fig. 3). A rotational speed of 34 Hz has been selected for balancing, which is well below the first natural frequency of 39 Hz.

     

    Balancing

    The construction of the rotor (Fig. 4) is subdivided top down into 4 planes with 12 grinding plates each. These are mounted in a radius of 120 mm. Therefore, the following settings have been made for the balancing software InnoBalancer Pro:

      Number of Balancing planes (1 & 4):
    2
      Balancing radius for mounting
      balancing and test masses:
    120 mm
      Number of fixed places:
    12
      Test mass to be set:
    20 g
      Indicator vibration velocity:
    in mm/s
      Tolerance limit:
    1 mm/s

    Washers divisions into shares of 2.8 gr have been used as balancing masses on the fixation bolts of the grinding plates.

Fig. 6 Run-up after balancing
Fig. 6 Run-up after balancing

Result

Figure 5 shows the whole balancing process. The vibration values werde reduced by 95% on both planes. Values of 0,080 mm/s on plane A above and 0,217 mm/s on plane B below remained.

At the end a new run-up analysis was performed

Now that the vibration cause of the imbalance has been reduced significantly, a run-up analysis up to maximum operational speed could be performed (Fig. 6). During the first attempt (see above) speed frequencies around 45 Hz led already to vibration velocities of more than 30 mm/s. After balancing, the vibration velocity was not more than 5 mm/s for the entire speed range and the operational speed even amounted to 2.1 mm/s only.

A complete succes has been achieved. The running smoothness ensures a significantly increased lifetime of the hammer mill.