BOC Edwards QMB Mechanical Booster Pumps are compact and have high pumping speeds.
Low system pressures can be achieved by using two or more mechanical booster pumps in
series. You must use the QMB Mechanical Booster Pump with a suitable backing pump. The
maximum continuous inlet pressure is 20 mbar for the QMB250F and QMB500F Mechanical
Booster Pumps and 1 mbar for the QMB1200F Mechanical Booster Pumps.
The QMB250F and QMB500F pumps are air-cooled and the QMB1200F pump is water-cooled.
All the QMB pumps have enclosed, water-cooled motors and are therefore suitable for
applications in clean environments where fan cooling is unacceptable.
The pump coupling-cover is connected to the pump outlet and forms an integral part of the
vacuum system. The connecting pipelines have a filter which removes debris and so prevents
contamination of the lubricating oil and bearings. For an even cleaner system, the
coupling-cover and bearings can be evacuated by connection to the pump-inlet or to an external
Three motor-protection thermistors (one on each winding) are fitted to the motor of the QMB
pump. These thermistors are solid-state devices which have an electrical resistance of 100 to
500 W at normal pump-motor operational temperature. When the pump-motor is too hot, the
electrical resistance rises quickly to 3000 W.
The thermistors are connected in series and you can connect the thermistors to control
equipment to automatically shut down the system if a pump-motor is too hot.
The QMB pumps are positive displacement Roots vacuum pumps. The pump mechanism is
driven by a three-phase electric motor through a hydrokinetic fluid-coupling. The motor is
enclosed and is cooled by integral cooling-water coils.
The pump shafts and rotors are made of high-grade, corrosion-resistant, cast-iron. The internal
and external shaft-seals are made of polytetrafluoroethylene (PTFE) or fluoroelastomer.
The pump-bearings, gears and seals are lubricated by oil fed from reservoirs in the
coupling-cover. A series of seals stops the oil from reaching the vacuum side of the pump. The
coupling-cover is evacuated. You can inspect the oil-levels through sight-glasses which are
fitted to the coupling-cover. Oil-filler, oil-drain and external evacuation connections are
provided on the cover.
The QMB250F and QMB500F pumps have rear bearings which are lubricated by
perfluoropolyether (PFPE) grease. The QMB1200F pump gears are lubricated by oil in the
gear-cover. The gear-cover of the QMB1200F has oil-filler and oil-drain plugs and an oil-level
sight-glass. The QMB pumps are suitable for use only with PFPE oils.
Principle of operation
The motor-shaft drives one of the
rotors through the fluid-coupling in the QMB mechanical booster pump. The 1:1 gears inside the coupling-cover drive the second rotor
in the opposite direction inside the stator housing. A small, accurately gauged, clearance is
maintained between the rotors and between each rotor and the stator wall. This clearance allows
thepumpto operate at high speed without mechanical wear and without the need for lubrication
inside the swept volume.
The hydrokinetic fluid-coupling connects the electric-motor shaft to the rotor. This system is
configured so that when the gas-load is high the rotational speed of the rotors is reduced. As the
gas-load decreases, the rotors accelerate to full speed. This allows continuous operation of the
pump over the vacuum range without the risk of overloading the motor and removes the need
for bypass-valves and associated pipelines.
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