Hoffman D.M., Singh B., Thomas J.H. (Eds). Handbook of Vacuum Science and Technology
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2.6.1 Turbomolecular Pumps [TMP]
187
the backing pump and using the values
5,,,^^
and A',,,,,^ for the first disk at the fore-
vacuum side, the real pumping speed of this disk can be calculated. The formula
can be used as a recurrence formula to calculate step by step the pumping speed
of the whole pump in the molecular flow. Then the pumping speed of the whole
TMP depends on the compression and the pumping speed S^ of the backing
pump, as given in Equation (3).
From there it can be derived that if
K„j„^
for a gas is small, the pumping speed
for this gas is a function of the ratio S/S^- For practical purposes, a TMP with low
K,jja;^
for H2 needs a larger backing pump to pump H2 effectively (low S/S^ ratio).
The inlet conductance of the blade area of the disk limits the pumping speed,
and the actual pumping speed 5 of a TMP becomes a function of the ratio v/u [7],
and therefore S depends on the molecular weight of the gas pumped.
The pumping speed and the compression of a TMP decrease at pressures of
above
10 ~^
torr, caused by the interaction of the particles with one another, as the
mean free path becomes smaller than the blade distance and the blades of the
rotor disks no longer are in the molecular flow range. This value corresponds to
a foreline pressure of
10
"^ torr to
10 ~ ^
torr.
As for any vacuum pump, the ultimate pressure
PQ
a TMP can attain can be cal-
culated from the pressure p^ at the outlet side of the pump by dividing it by the
maximum compression
K,„ax'
PQ
- Py,/K„
(4)
The increase of
K,„^^
with the molecular weight M means that heavy gases with
heavy molecules are highly compressed and have a low backflow probability; this
is the reason for the "clean" vacuum without contamination by oil vapors and
hydrocarbons (Figure 2). The smaller compression for light gases is the reason
Fig.
2.
Residual gas spectrum of
a
dean TMP.
2
M = relative molecular mass
N = electric charge numl)er
Ptot = 7-10-i0mbar
* Ordinate in arbitrary units [12]
188
Chapter 2.6: Turbomolecular Pumps
that the residual gas atmosphere of a TMP consists mainly of
H2.
This, however,
holds true only for a "clean" system with metallic flange seals. In the case of
viton or rubber seals, the ultimate pressure and the residual gas composition look
different [8].
2.6.1.3 Typical Vacuum Data of Commercial TMPs
The differences in the data of the different manufacturers depend on their actual
design (e.g., blade design, rotor staging) of the TMP.
COMPRESSION
Figure 3 shows the maximum compression
Ar„,^^.
of a TMP for different gases as
a function of the pressure. Table 1 shows typical compression values for TMPs
from the catalogues of different manufacturers. The differences in the indicated
values depend on the overall design of the TMP (disks, number of disks, staging
of disks).
Fig.
3.
Kmax for different gases as a function of the bacl(ing pressure.
[12]
N2 nitrogen
He helium
H2
hydrogen
'10'
lO-'mbar W"
P
2.6.1 Turbomolecular Pumps [TMP]
189
Table
1
Typical Compression Values for TMPs from the Catalogues of Different Manufacturers
Gas -
Pumped
[9]
[10]
H2 3X10^..2X103
He 3X10^..2X10^
N2 8X10^..1X10^
1X10\..2X10^
2X10^..1X10^
1X10^..1X10^
Manufacturer
[11]
[12]
[13]
8X 102...1 X 10^
6X 102...1 X 10''
5X10^..1X10^
IX 10^...IX lO'*
5X 10'...6X 10^
2x 10^..IX lO'o
4X IO2...8X 10^
4X 10^..3X 10^
5X10^..1X10'0
PUMPING SPEED
Figure 4 demonstrates the pumping speed of a TMP for different gases.
The product name of a TMP often includes the pumping speed for nitrogen,
which is used as the 100% value. This value is compared in Table 2 with the pump-
ing speed data for H2 and He from the catalogues of different manufacturers.
In addition to the dependence of the indicated values on the internal design
parameters already given, other factors also influence these pumping speed ra-
tios.
Most commercial TMPs for a certain rotor/stator dimension come with inlet
flanges of different diameters, to economize on the accessories (e.g., valves) con-
nected to the inlet
flange.
The conductance of this
flange
lowers the effective pump-
ing speed for the heavier gas molecules, for which conductance is low. The max-
imum compression of the TMP also has some influence on pumping speed for
different gases —
see
Equation (3).
Fig.
4.
Pumping speed
for
different gases as
a
funaion
of
tfie iniet pressure.
[12]
N2 nitrogen
He helium
H2 hydrogen
AT
argon
•s'
700
600
500
400
300
200
100
1
a»
10^
10-' 10*
^He
mtx
r
2
<2
U
10
190 Chapter 2.6: Turbomolecular Pumps
Table 2
Pumping Speed Ratios from the Catalogues of Different Manufacturers
Pumping
Speed
Ratios
Gas
Manufacturer
Pumped [9] [10] [11] [12] [13]
H2 (%)
He
(%)
N2 (%)
34...
63
60...
80
100
96 ...133
100...
133
100
57
...
98
73...
110
100
79...
126
87
. .
.137
100
66...
115
88...
124
100
ULTIMATE PRESSURE
The ultimate pressure of a commercial TMP is generally between 10"^^ torr and
10 ~ ^
torr, using metal flange seals and a two-stage rotary backing pump. The par-
tial pressure of H2 above an oil-filled two-stage backing pump is approximately
5 X lO"*^ torr. Assuming a
K,,,ax
value for the TMP of
1.000,
and using Equa-
tion (4), an ultimate pressure for H2 above the TIVIP of 5 X 10"'^ torr could be
expected, which corresponds to the typical value of the ultimate pressure of this
pump combination.
The bakeout status of a TMP and the kind of sealing material used influence
the ultimate pressure and the composition of the residual gas atmosphere. To at-
tain the just-mentioned typical ultimate pressures, the TMP must be baked out,
and metal must be used as the sealing material at the high-vacuum side to reduce
the gas desorption from the internal surfaces and the sealing material.
2.6.1.4 Actual Design of Turbomolecular Pumps
ROTOR AND STATOR GEOMETRY
The pumping speed and the compression of a TMP depend strongly on the rotor
geometry and rotational speed. Starting from the 1958 original geometry of the
rotor and stator disks, new disk geometries have been developed. These geome-
tries,
together with increased rotor speeds, allow much smaller and lighter rotors
for the higher-speed range.
Because the gas throughput is constant (the product of pressure and pumping
speed) in each stage, the blades nearest to the inlet of the TMP are designed to
have a high pumping speed and a low compression, whereas the blades nearest to
2.6.1 Turbomolecular Pumps [TMP) 191
ng.5.
Rotor with different disk design.
Pumping
Speed Compression
Strong inclined blades on the HV side (40°
...
45°) High
Low
Less inclined blades in the transition region (35°
...
22°) Medium Medium
Low inclined blades on the discharge side (20°
... 10°) Low
High
Courtesy
of
Leybold Vacuum
the foreline port are designed for high compression and low pumping speed (Fig-
ure
5).
For
economic
reasons,
it would
be
impractical to make each stage different
from its neighbor. A compromise results in groups
of
two
to four different types
of blades, in which each is designed for
a
particular speed and compression ratio.
The methods
of
manufacturing
the
rotors
and
stators influence the pumping
speed and compression. Rotors can be made
of
individually machined disks that
are heat-shrunk to the rotor shaft, by machining complete groups of
disks
from a
single block of material or by manufacturing the rotors using spark erosion. The
individually machined
disks
offer
the
advantage of making them optically opaque,
maximizing them
for
the compression. The other methods
of
rotor production
yield disks with less opaqueness and lower compression, maximizing them
for
pumping speed.
192 Chapter 2.6: Turbomolecular Pumps
The stators are either manufactured from individually machined disks or from
stampings.
The first commercial TMPs were dual-flow
("horizontal")
pumps having a
double-ended rotor, pumping gas from a central inlet toward both sides and con-
ducting the gas
flow
in a conmion foreline. Single-flow
("vertical")
TMPs, using
single-ended rotors, became available in 1969 [14]. The double-ended rotor de-
sign allows
a
more stable bearing design, which is advantageous for easy balanc-
ing and lower vibration levels. The single-flow design has little conductance
losses between the inlet
flange
and the rotor, whereas the dual-flow design suffers
losses from
the
inlet to both
sides.
Today only
a
few models of conmiercial TMPs
still use the dual-flow design.
ROTOR SUSPENSION
The change in the size of the TMP has been quite spectacular. It was made pos-
sible by
an
increase of the circumferential (tip) speed from
150
m/s in 1958 to ap-
proximately 400 to 500 m/s today, and by changes of
the
rotor geometry as well.
These high tip speeds relate to high rotational speeds of the rotors, which exert
high loads on the rotor's suspension.
Mechanical Bearings
Today most conmiercial TMPs are equipped with lubricated mechanical rotor
bearings, or by a combination of permanent magnet bearing at the high-vacuum
side with a lubricated mechanical bearing at the forevacuum side. Depending on
the disk diameter, the rotational speed of the rotor goes up to 90,000 rpm. These
high rpm
have become
possible,
due to the
advances
in
bearing
and
balancing tech-
nology, without sacrificing the high reliability of
the TMP.
Today, high-precision
ball bearings are available that, when specially tuned to a certain TMP rotor, at
comparable radial and axial loads, have a longer lifetime, even at much higher
rpm, than bearings of older design.
"Ceramic" bearings (ceramic balls) are widely used today. The ceramic balls
exert lower centrifugal forces and lower stress on the races than metal balls, they
are harder and more temperature resistant and, therefore, have a stable spherical
shape and minimal wear on balls and
races.
Their surface is smoother, leading to
less
friction
and the pairing of different materials
(ceramic
balls/steel races) avoids
micropitting. Therefore these bearings are more reliable even under lubrication-
starved conditions.
Lubrication
of
Mechanical Rotor Bearings
Three main requirements have to be fulfilled by the lubricant used for mechani-
cal rotor bearings: (1) the lubricant has to cool the rotor bearings, because
2.6.1 Turbomolecular Pumps [TMP]
193
these are running under forevacuum conditions and a heat transfer from the inner
to the outer race is possible only by the lubricant, (2) a low vapor pressure, and
(3) good lubrication properties at high speed. Today most lubricants used have a
synthetic base.
Even at low rotor weights a minimum fluid flow through the bearings for heat
transfer is necessary. Many smaller TMPs use a wick lubrication system, while
most larger TMPs have pump systems circulating the oil. "Ceramic" bearings
with grease lubrication are also used.
Dry Bearings: Magnetic Suspension
After some unsuccessful experiences with commercial "gas-bearing" TMPs [15],
today commercial magnetic bearing TMPs are available in which one, two, three,
or all the possible five degrees of freedom of the rotor are actively controlled by
an electronic system (Figure 6). The position of the rotor spindle is monitored by
sensors, and the electronic circuit corrects the spindle position from the data sup-
plied by the sensors. There is no mechanical friction, and hence no wear. The
Fig.
6.
Magnetic suspension.
HV flange
pump case
driver shaft
venting connection
VV flange
magnetic bearing
stabilizer
7 water-cooling entry
8 axial sensor
9 splinter guard
10 touchdown bearing
11 rotor
12 stator disk
13 permanent magnet
bearing
14 purge gas entry
15 electric driver
(DC motor)
16 safety ball bearing
17 connector to stabilizer
18 axial sensor connector
[12]
194 Chapter 2.6: Turbomolecular Pumps
price of these magnetic pumps is still considerably higher than of standard ball-
bearing TMPs, limiting their general use.
BALANCING AND VIBRATION
The dynamic balancing of the rotor of a TMP is very important for minimizing
vibration and noise levels, which are related to the mechanical bearing lifetime.
Due to the high rotational speed of TMP rotors, the centrifugal forces associated
with the residual imbalance (material inhomogeneities, radial bearing clearance,
geometric imperfections) attain considerable values and transmit vibrations to the
body of the pump.
The balancing process modifies the mass distribution of the rotor by adding or
taking off material to bring the rotational axis as close as possible to the principal
axis of inertia. To further reduce the influence of the residual imbalance, the me-
chanical rotor bearings within a TMP are held in elastic "antivibration" rings that
effectively dampen the residual unbalanced forces.
Dynamic multifrequency balancing of the rotor is generally done in several
balancing planes. This relative complex procedure in the last years has become
simplified by the use of computers and dedicated software. Modern TMPs have
very low residual vibration amplitudes, below 0.02 ^tm. These low values are re-
quired for the use of a TMP with vibration-sensitive instruments, such as mass
spectrometers and electron microscopes.
ROTOR MATERIALS
Most commercial TMPs use high-strength Al alloys as rotor material. Compared
to other high-strength materials, such as Ti and steel alloys, these Al alloys have
a lower specific weight, are much easier to machine, and have sufficient thermal
stability for the operational temperatures even at the typical TMP bakeout cycles.
The use of ceramics (Si3N4) has been reported for the use of TMPs in very strong
magnetic fields [16].
A typical maximum stress in the root of the rotor blades of a high-speed TMP
rotor at operating conditions is in the range between 50 N/mm^ (newton per nmi^)
and 150 N/nmi^, well below the 0.2% elongation limits of adequate Al alloys (e.g.,
AlZnMgCu 1,5 F50: 430 N/mm^).
DRIVE SYSTEMS FOR TMPS
The drive rotor of a TMP is an integral part of the TMP rotor and together with its
drive stator is located in the forevacuum area.
2.6.2 Molecular Drag Pumps [MDP]
195
Today three different motor systems
are
used:
DC
motor,
AC
motor,
and
hys-
teresis motor.
The
somewhat more expensive
DC
motor
has
lower energy con-
sumption and energy losses than the other motors. The motors are driven by solid-
state frequency converters. Some
of
these converters
can
operate
the
TMPs
at
variable rotor speeds.
For special applications (e.g., high radiation with particle accelerators), motor-
driven frequency converters
are
available.
2.6.2
MOLECULAR DRAG PUMPS (MDP)
The molecular drag pump (MDP) depends
on
the same principle as the
TMP.
Mo-
mentum
is
transferred from
a
rapidly moving rotor surface
to the
particles
to be
pumped. Therefore
the
theoretical considerations
are
very similar.
2.6.2,1 Design
Molecular drag pumps
are
designed
on the
basis
of the
Hoi week principle
[3].
These pumps include
two
different sections:
(1) an
entrance section with
a row
of TMP blades
for
maximum conductance
to
ensure
a
high pumping speed,
and
(2) subsequent molecular pumping stages
(up to 5)
having
in
principle
a
drum
design with
a
spiral groove structure
to
ensure efficiency
and a
high compression
(Figure 7).
2.6.2.2 Typical Performance
of
Commercial MDPs
Commercial MDPs have
a
high admissible foreline pressure, ranging from
1
torr
to 40 torr, depending
on
design and number
of
stages,
and therefore can use small
simple, dry, and inexpensive forevacuum pumps (e.g., membrane pumps).
Typical data
of
conmiercial MDPs
are
shown
in
Tables
3
and
4.
ULTIMATE PRESSURE
Depending
on
how many molecular pumping stages are used
in
series,
the
attain-
able ultimate pressure
is in
the range
of
10
"^ torr
to
10
"^ torr using
an
adequate
membrane backing pump.
196
Rg.7.
Chapter 2.6: Turbomolecular Pumps
Molecular drag pump.
1
2
3
4
5
6
7
8
9
10
HV flange
turbo disk for volume speed enhancement
threaded drag pump stator—five stages
pump rotor cyUnders
FV flange
socket for electric connector
lubricant reservoir
purge gas entry
permanent magnet bearing
safety ball bearing
Courtesy
of
Pfeiffer Vacuum