Specifications

192
Turbo-V Pump Technical Notes
S
eff
reaches its maximun value “S” (nominal pumping speed)
when “R
p
” equals unity, and it is zero when the pressure ratio
R
p
has reached its maximum value “K”.
This linear dependence can be expressed by the following
relationship:
S
eff
= S / (1 - 1 / K + S / S
foreline
K) (1)
As it can be seen:
when
K >> S / Sforeline
and
K >> 1
then
S
eff
≅ S
when
K ≅ 1
then
S
eff
= S
foreline
The above formula (1) must be used to evaluate pumping
speed when operating at high pressure, especially with light
gases (low K).
Base Pressure
The base pressure of a turbomolecular pump is the
equilibrium pressure between outgassing of pump surfaces
exposed to high vacuum, including test dome, and the
pumping speed of the pump.
p
base
= Q
outgas
/ S
eff
In the case of ultimate operational pressure, as specified by
norms, the pressure is measured after 48 hours bakeout of
pump and dome (provided with metal gasket); therefore the
prevailing outgassing product is H
2
and equilibrium is
reached with hydrogen pumping speed.
p
base
= Q
H2
/ S
effH
2
When foreline pumps with relatively high base pressures are
used, base pressure is sometimes limited by the compression
ratio for H
2
O (or N
2
).
p
base
= p
forelineH
2
O
/ K
H
2
O
Vibration Level
Thanks to low vibration, focused design, and computer
assisted balancing tools, today turbomolecular pumps
generate very low levels of mechanical vibration. This is
mainly a result of the numerical modeling of the pump
rotodynamics (see Figure 3) and a specific vibration damping
system already built into the pump structure. Thanks to
both design features, today ceramic ball bearings pumps are
standard even in very high vibration applications like SEM
and Metrology Tools.
A typical vibration spectrum of a turbomolecular pump can
be seen in Figure 4:
Possible sources of vibration in a turbomolecular pump are
unbalanced rotor, high frequency motor or bearings. Rotor
unbalance can be reduced to a very low level through dynamic
balancing, which minimizes forces caused by a nonsymmetric
distribution of masses in relation to the rotational axis. As an
order of magnitude, the radial displacement on the pump HV
flange after balancing can be as low as 0.001 µm.
The vibrations from a high frequency motor are caused by
electromagnetic interactions between the motor stator and
rotor: their characteristic frequencies are multiples of the
motor driving frequency. Also, the rotor supports generate
both white noise and vibrations at specific frequencies of the
bearings’ moving parts (cage, balls and rotating ring, usually
the inner one).
Figure 3
Figure 4