Vacuum Technology Know How - Triumf

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2.6.1 Design / Operating principle
Roots vacuum pumps belong to the category of technically dry-running rotary displacement
vacuum pumps. They are also termed Roots pumps or Roots blowers.
Operating principle
In a Roots pump, two synchronously counter-rotating rotors (4) rotate contactlessly in a
single housing (Figure 2.12). The rotors have a figure-eight configuration and are separated
from one another by a narrow gap. Their operating principle is analogous to that of a gear
pump having one two-tooth gear each that pumps the gas from the inlet port (3) to the outlet
port (12). One shaft is driven by a motor (1). The other shaft is synchronized by means of
a pair of gears (6) in the gear chamber. Lubrication is limited to the two bearing and gear
chambers, which are sealed off from the suction chamber (8) by labyrinth seals (5). Because
there is no friction in the suction chamber, a Roots vacuum pump can be operated at high
rotary speeds (1,500 - 3,000 rpm). The absence of reciprocating masses also affords troublefree
dynamic balancing, which means that Roots vacuum pumps operate extremely quietly
in spite of their high speeds.
Design
The rotor shaft bearings are arranged in the two side pieces. They are designed as fixed bearings
on one side and as sliding internal rings on the other in order to enable unequal thermal
expansion between housing and piston. The bearings are lubricated with oil that is displaced
to the bearings and gears by splash disks. The driveshaft feedthrough to the outside is sealed
with radial shaft seal rings made of FPM that are immersed in sealing oil. To protect the shaft,
the sealing rings run on a protective sleeve that can be replaced when worn. If a hermetic
seal to the outside is required, the pump can also be driven by means of a permanent-magnet
coupling with can. This design affords leakage rates Q l of less than 10 -5 mbar . l / s.
Pump properties, heat-up
Since Roots pumps do not have internal compression or an outlet valve, when the suction
chamber is opened its gas volume surges back into the suction chamber and must then be
re-discharged against the outlet pressure. As a result of this effect, particularly in the presence
of a high pressure differential between inlet and outlet, a high level of energy dissipation
is generated, which results in significant heat-up of the pump at low gas flows, which in and
of itself transports low quantities of heat.
The rotating Roots pistons can only be provided with relatively weak cooling by comparison
with the housing, as there are no contacting surfaces other than the front side. Consequently,
they expand more than the housing. To prevent contact or seizing, the maximum possible
pressure differential, and thus dissipated energy, is limited by an overflow valve (7). It is
connected to the inlet side and the pressure side of the pump-through channels. A weightloaded
valve plate opens when the maximum pressure differential is exceeded and allows
a greater or lesser portion of the intake gas to flow back from the pressure side to the inlet
side, depending upon the volume of gas encountered. Due to the limited pressure differential,
simple Roots pumps cannot discharge against atmosphere and require a backing pump.
However Roots vacuum pumps with overflow valves can be switched on together with the
backing pump, even at atmospheric pressure, thus increasing their pumping speed right
from the beginning. This shortens evacuation times.
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