TA Instruments AR 2000, Operator'S Manual, Page: 25 / 144

AR 2000 Operator's Manual
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Chapter 3
Technical Descriptions
Overview
In order to fully utilize the advanced capabilities available with the AR 2000 Rheometer, some of the important
components require a more detailed explanation. This chapter describes in detail the design and functions of
the:
• Air bearing
• Auto gap set device
• Smart Swap™
• Peltier plate
• Normal force transducer.
The Air Bearing
As its name suggests, an air bearing uses air as the lubricating medium. This allows virtually friction-free
application of torque.
The design of an air bearing is a compromise between several characteristics such as air consumption, friction,
stiffness, and tolerance to contamination and misuse.
The amount of air consumed is
related to the pressurized bearing
clearance. To minimize air con-
sumption, a small clearance
(<10µm) is needed. However, as air
has a finite viscosity (0.0018
mPa.s), small gaps give rise to high
shear rates and, correspondingly,
the friction increases.
If large gaps are used, the shear rate
is lowered and friction is reduced,
but the stiffness of the air bearing is
also reduced.
Thus, a compromise in the design
of an air bearing is needed for
optimal performance.
The air bearing used in the AR
2000 Rheometer uses a mixture of
proven bearing techniques with
novel materials. The surfaces can
be easily machined to tolerances of
less than 1µm, providing an
extremely smooth finish.
A schematic of the Air Bearing and
the other main components of the rheometer head is shown in Figure 3.1 above.
Figure 3.1
The Rheometer Head