Utilizing Fluid Couplings to Improve Conveyor Startup
 Bulk
material handling conveyors are becoming faster, wider and larger
-- demanding increased power to meet today's demand for higher productivity.
The ability of the drive system to start the conveyor slowly and
smoothly is crucial to your company's bottom line. If starting torques
are too high, the risk is increased for equipment damage, spillage,
downtime and unnecessary expense.
Importance of Soft Start
Across the line starting of AC motor systems can produce excessive belt tension and rapid
acceleration rates. In fact, AC motors often exceed 250 percent of full load torque during startup.
And, the motor current can remain at up to 600 percent of full load during acceleration. Ultimately,
high current builds up heat that can drastically shorten the motor's life expectancy.
With empty or partially loaded belts, the acceleration rate will be even faster and the excessive
high-torque shock loads during startup can eventually surpass the belt's design limits. Belts can be
damaged, splices can break and take-up problems may occur, not to mention the high potential of
material spillage.
In either case, with belts empty or full, the threat remains for lost profits associated with downtime
and belt breakage costs. Fluid couplings offer a "soft start" option specifically for conveyor drive
systems to prevent such damage and loss.
Under both loaded and unloaded starting conditions, fluid couplings are designed to provide smooth
conveyor acceleration by keeping initial starting torque as low as 40 percent of full load, controlling
acceleration, and dampening shock and vibration.
How Fluid Couplings Work
 Fluid
coupling operation is based on a hydrokinetic principle with the
power transmitted through hydraulic fluid. Inside the coupling are
two basic elements: an "impeller" acting as a centrifugal
pump and a "runner" acting as a hydraulic turbine. There
is no mechanical connection between driven and driving shafts.
When the AC motor is started, the driven impeller begins to centrifugally pump fluid within the
coupling to the stationary runner. As the runner torque increases to conveyor breakaway, the runner
starts rotating and begins to accelerate the driven load. Starting torque is gradually increased and
controlled as power is transmitted smoothly to the driven load.
The time it takes a fluid coupling to start and accelerate a load depends on several factors including
the resistive load torque, the driven inertia and the torque transmitted by the fluid coupling. With
most fluid couplings, typical belt conveyor acceleration times range from a few seconds to over 60
seconds, depending on the load conditions.
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Acceleration Time
On conveyor applications, most fluid couplings are designed with delay fill chambers that retain a
portion of the working circuit fluid during the initial start. Since the amount of fluid in the working
circuit determines the output torque, acceleration time can be varied by adjusting fluid levels. And,
initial starting torque can be lowered to meet conveyor needs.
After the AC motor is started and accelerates, fluid from the delay chamber is slowly metered into
the working circuit. Metering orifices should be changeable, allowing torque build up and
acceleration to be tailored to the conveyor needs.
Fluid Coupling Efficiency
At operating speed and full load, fluid couplings remain highly efficient. Since all the mechanical
components, i.e. shaft seals, bearings, runner, and impeller, are operating at virtually the same
speed, there are few mechanical losses and the output torque is essentially equal to the input torque.
A simple method for determining fluid coupling efficiency includes a comparison of output speed vs.
AC motor speed. With percentage differences normally ranging between 1 and 4 percent, full-load
running efficiencies between 99 percent and 96 percent are possible. AC motor power does not
need to be increased to take advantage of fluid coupling benefits.
Slow start up times and the ability to operate in the 100 percent "slip" mode also make fluid
couplings useful on conveyors incorporating two or more drives on the headshaft and secondary
pulleys. By sequence starting motors with 3 to 4 second delay, both loaded and empty starting times
can be further increased, minimizing line voltage drop problems caused by excessive inrush current.
Such configurations can also reduce excessive belt tension, which prolongs belt life.
Fluid couplings also provide a simple method to load balance two or more drives operating on the
same headshaft or secondary pulley. For instance, once full-load power on each drive is recorded,
the working fluid can be increased or decreased slightly ensuring each drive delivers its share of the
power.
Reducing Motor Demand
A standard Nema B design is usually all that is required to operate conveyors utilizing fluid
couplings, because, in effect, motors are started under virtually no load. As a result, motors and
switch gear can be properly sized for the running load without concerns of having enough
acceleration torque capability. High starting current duration is minimized which reduces heat
build-up and can significantly increase motor life.
Overload Protection
Fluid couplings protect beyond startup. They can also guard driven equipment in the event of high
overload or jam conditions. When a sudden jam occurs, fluid couplings remove the inertia effect of
the AC motor and act as shock absorbers, spreading the impact over a longer time period as
coupling slip increases.
If a gradual overload occurs, fluid coupling slip will again increase, reducing the coupling's output
speed and allowing the conveyor to stall safely. Of course, stalling or repeated full-load starts can
cause the coupling fluid temperature to rise to unacceptable operating levels.
To guard against excessive fluid temperatures, fusible plugs with fixed melting temperatures are often
installed for protection. In the event of overheating, hot fluid melts the plug and empties the fluid
from the coupling against protective guards, effectively disconnecting power to the driven shaft.
Thermal trip plugs with limit switches provide another safety option without the release of fluid. The
trip plug, preset to a determined temperature, operates a limit switch which shuts down the AC
motor in the event of fluid overheating. Either the plug or trip switch provide fluid couplings with
adequate overload protection.
Mounting Arrangements
Mounting fluid couplings is a simple procedure. Fluid couplings are installed in conjunction with
flexible shaft couplings to compensate for driving and driven shaft misalignment.
For easy-in and easy-out servicing without the need to move equipment, mechanical gear or
multi-disk couplings are most often used. They allow for easy shaft alignment, require minimum
maintenance and equalize the overhung load between the AC motor and the driven equipment
When equipment space is limited, hollow output shaft fluid couplings with flexible input shaft
couplings can reduce the distance between equipment shafts to an absolute minimum.
Naturally, the performance requirements of any fluid coupling will be dictated by your conveyor's
design parameters. But, fluid couplings offer a myriad of benefits for conveyor drives: controlled soft
smooth starts, mechanical/electrical overload protection, reduced heating of electric motors, and
effective load balancing, to name a few. Through reduced downtime and longer equipment life, fluid
couplings can help improve your bottom line.
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