TA AR-G2, Руководство по началу работы

Страница: 24 / 58

Page 24 AR-G2/AR 2000ex/AR 1500ex Getting Started Guide
Air Bearing (AR 2000ex/AR 1500ex)
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 consumption, a small clearance (<10 µm) is needed. How-
ever, as air has a finite viscosity (0.0018 mPa.s), small gaps give rise to high shear rates and, correspond-
ingly, 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 uses a mixture of proven bearing techniques with novel materials. The surfaces can be
easily machined to tolerances of less than 1 µm, providing an extremely smooth finish.
A schematic of the air bearing and the other main components of the rheometer head is shown in the figure
below. The bearing is designed to be virtually friction- free, so that it moves under the smallest of forces.
Even extremely small manufacturing variations in the bearing can be sufficient to make it rotate. There-
fore, to ensure that the bearing rotation is steady throughout a full 360°, a process called
Rotational Map-
ping , which is explained in the next section, is carried out.
Figure 5 AR 2000ex Rheometer head.
Ultra hi-res optical encoder
Air bearing (radial)
Air bearing (thrust)
Drag cup
Motor windings
Air bearing (radial)
Drive shaft