Materials and stresses

Whilst the fan industry has been characterised throughout this book as “mature”, there has nevertheless been a revolution over the last few years in its use and selection of materials. The axial flow fan owes its increasing popularity to the availability of lighter materials which have reduced the centrifugal stresses to acceptable levels. Invented at the beginning of the nineteenth century, it did not prove a manufacturing success until after the 2nd World War. The aircraft industry had developed the aluminium alloys which were just what the fan industry wanted! This has been followed by the increasing use of engineering plastics.

For centrifugal fans pre-galvanised sheet has become an accepted norm for light duty fan casings, often of lock-formed construction. Aluminium alloys and even plastics have been introduced for impellers. At the other end of the duty scale, nickel and titanium alloys have extended the peripheral speeds and hence pressures that fans are able to achieve.

This Chapter does not seek to be a comprehensive textbook on materials. Rather it seeks to point those interested to the right sources of information. The stresses induced in the various parts of a fan can be subject to mathematical analysis and an introduction is given to the methods used. With the advent of specialised computer programmes, however, some readers may be tempted to think that a knowledge of first principles is unnecessary. It is hoped that these paragraphs will disabuse them of such thoughts!

The modern fan consists of many parts which may be made from a number of materials. The choice of these will be deter­mined by their cost, ease of manufacture and mechanical at­tributes. Increasingly, also, appearance may have some effect

— especially where the fan is in the public eye.

Whilst the rotating parts of all fans will be subject to centrifugal forces, the resultant stresses may determine the thickness or scantlings of their components.

At the present time 3 material groups are in the ascendant:

• Sheet steels and cast irons

• Sheet and cast aluminium alloys

• Engineering plastics and composites

Forthe sake of analysis, however, we may make a more coarse definition of metals or non-metals and these are discussed in Section 7.3.

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