sensors & systems - Industrial Technology Magazine

POWER TRANSMISSION
Brakes, Clutches & Couplings
Couplings are soft on pumps
Pumping applications place stringent demands on couplings, but
rubber-in-compression products are adept at rising to the task
Rubber-in-compression couplings, from Renold
Hi-Tec, are perfectly suited to pumping
applications, particularly when large
particulates such as sand or gravel are being
pumped. The rubber blocks in the couplings eliminate
torsional vibration, isolate resonance and dampen
vibrations in the system that would otherwise cause
early failures.
Torsional vibration is caused by the almost
imperceptible pulses in torque that are inherent to
diesel engines and variable frequency drives, which are
commonly used to power pumps. Each power stroke of
the engine produces a pulse, or peak in torque that
causes an indiscernible twisting of the drive shaft that
will eventually destroy it if
left unchecked.
Each diesel
engine also has
its own natural
resonance, a
bit like the
note of a
ringing bell
or the sound
of a vibrating
guitar string.
If the engine’s
resonance coincides
with the natural
frequency of the pump then
the results can be catastrophic. The
modern solution is to fit a torsionally soft
coupling in-between the diesel engine and the drive
shaft to isolate the engine’s harmonics and torsional
vibration from the rest of the system. Rubber-incompression
couplings are frequently selected due to
their failsafe design, which is a critical requirement in
many pumping applications like fire fighting for
example.
Rubber-in-compression couplings are made up of
two round metal sections with what looks like the
paddles of a paddle steamer projecting inwards from the
outer section and outwards from the inner. Rubber
blocks are placed in the spaces in-between the
paddles, and, as the outer section is turned by
the engine, the inner section is driven through
the rubber blocks. As this happens the
rubber is compressed and hence the term,
rubber-in-compression. The couplings are
failsafe because even if the rubber blocks
failed the strong metal paddles would
come together and continue to drive the
pump.
Rubber-in-compression couplings are
also maintenance free and do not require
lubrication or adjustment of any kind,
which means they provide the lowest
lifetime cost on all pump applications. The
only serviceable item are the rubber blocks,
which in most cases are good for over
ten years of service.
As well as pumping applications,
the rubber-in-compression couplings
have also proved their worth in
other demanding areas, such
as their use in a
strip steel
mill in
the USA
where they have
cut torque amplification
by a staggering 23 percent.
The couplings were specified during
the refurbishment of the plant to protect the
drivelines of two reversing roughing stands and one
finishing stand. One of the main causes of torque
amplification is backlash across gears and couplings.
The Renold Hi-Tec coupling is backlash free by virtue of
the precompressed rubber blocks which not only
eliminate clearance but incorporate hysteresis damping
as the rubber deflects. Renold’s engineers were able to
use in-house software to analyse the dynamics of the
system and carefully select the correct type of
coupling and rubber blocks to change the
natural frequency of the system and avoid
resonant frequencies.
Three rubber-in-compression couplings
were specified delivering up to 8MW of power
each, and operating at speeds of up to
725rpm. The new couplings dramatically
increased the life of the plant and cut the
ongoing cost of ownership as rubber-in
compression couplings are also maintenance
free. Capital investment was also reduced.
MORE INFORMATION:
Enter D225 on the card, or visit ‘latest issue stories’ at
www.industrialtechnology.co.uk for further details from
Renold Hi-Tec and more news couplings
22
More details: Write in 220 on the free information card