Chapter hvac engineering Fundamentals: Part 1 Introduction

5.2.6
Mechanical and
structural considerations
The mounting and driving mechanism for the fan wheel entails many
mechanical and structural considerations. The bearings which support
Design Procedures: Part 3
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104
Chapter Five
Figure 5.11
Typical normalized pressure-volume curve-inlet vane control for a centrif-
ugal fan. (
Reprinted from AMCA Publication 201-90,
Fans and Systems
with written
permission from Air Movement and Control Association, International, Inc.
)
the shaft come in many kinds, depending on the speed of rotation,
weight of the fan wheel, belt tension, power transmitted, and whether
the fan wheel is overhung (Fig. 5.12) or supported between bearings
(Fig. 5.13). Sometimes three bearings are used; then alignment must
be precise. Bearing supports must be strong enough to support the
bearings without flexing. The drive shaft must be strong, true, and
rigid enough to support the fan wheel between the bearings and to
transmit the required power without undue flexing over a specified
rotational speed. All rotating shafts have a critical speed at which
excessive vibration, noise, and possible failure will occur. Many shafts
have two or more critical speeds. Sometimes the lower critical speed
is less than the normal speed range of the fan. This is satisfactory
when the fan is accelerated quickly through the critical speed. It will
not be satisfactory if the fan is to be used in a speed-controlled VAV
application.
Design Procedures: Part 3
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Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.

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