SECTION 3 CONSTRUCTION AND PRINCIPLE
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3.1 Pump Construction
Fig. 3-1 is a sectional drawing of the TMP-3203LM and TMP-3203LMC and TMP-3403LMC
magnetic bearing type turbo molecular pump. The built-in high frequency motor (1) is accelerated
to the specified revolutions (speed) by the high frequency power supply unit. Rotor blades (4) are
fitted onto the drive shaft (3) and the stator blades (5) are arranged in between the rotor blades.
A positioning spacer (6) is inserted between the stator blades. The configurations and profiles of
the stator blades and rotor blades are designed for high efficiencies in various applications. The
upper stages of the rotor blade and stator blade configurations are ideally designed for high gas
throughput. The compression ratio of the stator blades and rotor blades becomes higher as the
gas molecules converge into the lower stage configurations. The profiles of the stator blades and
rotor blades are matched for the desired function.
A radial magnetic bearing (7) is used at the top and bottom of the drive shaft. The axial
magnetic bearings (8) are used to levitate a disk attached to the drive shaft between the axial
magnetic bearings. Each magnetic bearing is provided with a gap sensor (10) to detect the rotor
position. Fig. 3-2 illustrates the outline of 5 - axes control. The rotor is levitated by the control of
these 5 axes allowing rotational freedom.
A touchdown bearing is used at the top and bottom of the casing for safety protection of the
rotor and pump should the magnetic bearings become damaged. The touchdown bearings are
dry and oil free.
The cooling water pipe (14) is provided to cool the pump. With the air-cooled turbo molecular
pumps TMP-3203M and TMP-3203MC, cooling fans are provided instead of the cooling water
pipe (14).
Fig. 1-1 through Fig. 1-3 show the outside dimensions of the turbo molecular pump.
3.2 Principle of Turbo Molecular Pumping
The principle of turbo molecular pumping assumes gas molecules collide with a surface plane
(the blade of the pump rotor) moving in a radial span of very high speed in a space with an
enlarged mean free gas path (generally a vacuum area of less pressure than 0.1 Pa). Assuming
no heat is exchanged between the gas molecule and the pump rotor blade, the speed of the
pump rotor blade is added to the speed of the gas molecule, converting the gas molecule's non-
oriented thermal motion to a motion with direction. Thus the gas molecule has received an
impulse in a desired flow direction.
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