Page 284 - Steam Turbines Design, Applications, and Rerating

Transmission Elements for High-Speed Turbomachinery 263

machine. Because there is no metal-to-metal contact between primary
and secondary wheels, there is no wear. Hydraulic oils with additives
are used for power transmission. The amount of oil in the coupling can
be varied during operation using the scoop tube. This in turn regulates
the power-transmitting capability of the coupling and provides stepless
speed control dependent on load of the driven machine.
The coupling has a regulating range of 4:1 to 5:1 for driven machines
with increasing parabolic torque load characteristics such as centrifu-
gal pumps and fans. For machines with approximately constant torque
load characteristics, the regulating range is 3:1. With centrifugal
machines, this method of speed regulation is much more efficient than
throttling the machine output, giving considerable power savings. The
motor is started under no-load conditions with the coupling drained.
When running the load on, the motor can be controlled by the coupling.
Moreover by draining the fluid coupling, the prime mover can be
disconnected from the driven machine while the prime mover is still
running.
A mechanically driven oil pump on the primary side of the coupling
pumps oil from the reservoir underneath the coupling through a con-
trol valve into the working chamber of the turbocoupling. The level
of the oil in the working chamber and therefore the power that the
turbocoupling can transmit depends on the radial position of the ad-
justable sliding scoop tube. The scoop tube can pick up more oil than
the pump can deliver. The oil picked up by the scoop tube passes
through an oil cooler/heat exchanger and control valve back to the
working chamber and/or the oil reservoir. The heat exchanger dissi-
pates the heat originating from the slip of the turbocoupling. The
scoop tube actuator can be operated either electrically, hydraulically,
or pneumatically.
Geared variable-speed turbocouplings are connected to the driving
and driven machines through torsionally flexible couplings (gear cou-
plings lubricated by circulating oil or diaphragm couplings). All rotat-
ing parts are housed in an oil-tight casing. A mechanical gear stage on
the input side of the variable-speed turbocoupling matches the speed of
the high-speed driven machine to that of a standard two- or four-pole
squirrel cage motor. The compact space-saving design is a characteris-
tic feature of this gear stage and turbocoupling combination.
The rotating parts of the variable-speed turbocoupling—primary
wheel, secondary wheel, and casing—are usually made of cast steel or
special steel. All Voith variable-speed turbocouplings have plain bear-
ings. The highly loaded radial and thrust bearings are white metal
lined. Remote thermometers monitor the temperature of the bearings.
The housing is split along the shaft center-line giving ready access to