control functions as well as for indications of
equipment speed. Figure 5-32 shows the magnetic
speed pickup transducer used in an LM2500
power turbine.
The most important thing you should
remember when replacing a speed pickup is to
check the depth setting. You can measure the
depth clearance with either a special tool supplied
by the equipment manufacturer or a depth
micrometer. Set the proper clearance by using
either shims or locknuts. If you do not set the
clearance properly, the magnetic sensor will be
either too close to the toothed gear or too far from
it. If it is too close, it may be damaged by the
rotating gear. If it is too far from the gear, the
unit will fail to give the proper voltage signal.
Always refer to the appropriate manufacturer's
technical manual when you replace a magnetic
Figure 5-33.--Velocity vibration transducer.
speed sensor.
The vibration pickup is a linear velocity
VIBRATION SENSORS
transducer (transfers mechanical energy to
electrical energy). The magnetic circuit is closed
One of the first indications of internal damage
through the circular air gap between the pole piece
to gas turbines is high vibration. High-vibration
sleeve in the case and the pole pieces on the
conditions can be indications of failed bearings,
magnet assembly. The air gap is interrupted by
damaged blading, or a dirty compressor. The GTEs found
the coils. (The flux field in the air gap is
on Navy ships use velocity vibration and accelerometer-
normally aligned with the approximate midpoint
type pickups. Figure 5-33 shows a typical velocity
of the two coils by the centering action of the two
vibration pickup used on GTEs.
springs.) As the engine vibrates, the rigidly
Figure 5-32.--Magnetic speed pickup used on the LM2500 power turbine.
5-23