vbBalancer Setting up the Tachometer
A tachometer collects information on a machine's rotation speed, providing you with an exact reading of machine speed as a measurement is taken. This is more accurate than using a default RPM as a machine's speed can vary significantly under different loads.
The tachometer also provides information on the angle at which the rotor is vibrating. The angle is measured from a fixed reference mark on the rotor and is called the ‘phase angle'. To balance a rotor it is necessary to consider not just the amplitude of vibration but also the phase angle. The amplitude shows the severity of the imbalance and the phase angle indicates the geometry of the imbalance.
Note: Instruments with serial numbers greater than 40800 (equipped with a second USB port for flash drives) use a low voltage 5 Volt tachometer output and must use the low voltage MiniVLS211/N Ex rated laser (Commtest product code LASA0315). Instruments with serial numbers lower than 40800 (those with only a single USB port) output the full battery voltage of 6.5 - 8.4 Volts and should be used with the MiniVLS213 laser (Commtest product code LASA0062).
The low-voltage MiniVLS211/N laser tachometer will work with the older instruments, but will be over voltage. We therefore do not recommend this practice.
Warnings: The laser tachometer is a class 2 laser product. Do not stare into the laser beam.
To comply with CSA Class I, Division 2 Hazardous Locations requirements, all instruments bearing the CSA logo MUST use the low voltage MiniVLS211/N laser tachometers.
Setting up the tachometer
- Screw the tachometer block's grip extension into the end of the mounting base articulated arm.
- Plug the cable socket into the tachometer connecting pins then turn the locking ring to make a secure connection.
- Locate the TACH input on the instrument and attach the other end of the cable to this.
- Stop the rotor.
- Cut out a small strip of the supplied reflective tape, approximately 5 mm x 15 mm (0.2" x 0.5").
- Stick the reflective tape to a machine part that rotates at the rotor speed e.g. the shaft. This trigger spot should provide a pronounced increase in reflection as it passes under the tachometer light beam. The tape width must be at least double the diameter of the light beam spot.
- Mount the tachometer magnetic base to a stationary portion of the machine, convenient to the trigger spot.
- Position the tachometer slightly away from the centerline of the rotating machine part in order that it is not 'blinded' by reflections from the surface of the machine part. The sensor
should be positioned within the measurement range shown in the following diagram. - Start the rotor and wait till it is at normal running speed.
- Now test if the tachometer will trigger. From the Main Menu press
Measure Vibration. - Press
Tach. The screen will display the rotor running speed in RPM and Hz. These values will continuously update but should stabilize, indicating that the tachometer is triggering reliably. If the RPM does not stabilize adjust the position of the tachometer.
Tachometer Signal from a Keyphasor®
If your machine has a proximity-probe based tachometer sensor (e.g. Keyphasor®), this can be used as the tach signal for your vbBalancer+ instrument. The sensor should have a buffered front-panel BNC output, to ensure the instrument's measurements can't interfere with pre-existing speed measurements. Keyphasor LEMO connector tach cables can be purchased through your local Commtest reseller for this purpose.
The Keyphasor proximity sensor gap should be adjusted so the driver box output is -8 V +/- 2 V (i.e. -6 V to -10 V) when the sensor is over the shaft. This is the normal mid-range position for these sensors. When the sensor is over the keyway its output will be -14 V to -22 V, depending on its make and model.
Last Reviewed: 11/Jun/2009 Source: vbBalancer Reference Guide Revision 24 March 2009 - pages 33 - 36