Fans keep their cool with ceramics

DC type cooling fans are essential for many different types of cooling applications, but they do have a relatively short life expectancy when used on products that are designed to operate continuously in hot or hostile environments. One of their major limitations comes from the bearing system within the fan mechanism.

Generally, design engineers have two options of bearings that they can consider. Either sleeve type bearings, with an average life expectancy of 30,000 h, or a double ball bearing type with an average life expectancy of around 50,000 h.

In both cases these figures will drop if the fan is exposed to high ambient temperatures or corrosive environments.

Noise levels on these metal type bearing fans can also be a downfall, as it will increase as operational time gets larger.

It was, therefore, only a matter of time before better high tech materials were available to replace the common ball bearing.

Ceramic materials have come a long way since their discovery last century and the development costs have fallen dramatically due to special low cost techniques.

New production developments have allowed Ceramica to develop a process of injection moulding of ceramic sleeves using zirconia powder together with an adhesive to produce a very strong, flexible and durable ceramic sleeve that replaces the common metal bearing.

These new, low cost ceramic bearings are claimed to be far superior to the conventional ball bearing.

Ceramic bearings run more smoothly and are quieter than their steel equivalents, and their life expectancy is increased by a factor of six.

While the average life span of a ball bearing fan is 50,000 h, a ceramic equivalent model has a 300,000 h life expectancy, at a cost much less than any ball bearing.

Since 1975, considerable research, development and marketing efforts have been expended on zirconia or ceramic steel which offers the traditional ceramic benefits of hardness, wear resistance and corrosion resistance, without the characteristic ceramic property of absolute brittleness.

To use zirconia to its full potential, the properties of the oxide need to be modified extensively by adding stabilising oxides. This modifies the grain size, particle shape, agglomerate size and specific surface area by controlling precipitation and calcination. Applying zirconia to the slip casting, tape casting and mould injection, particle size and specific surface are important characteristics. Well-controlled precipitated zirconia powder is fairly uniform and fine.

These ceramic materials and their fabricating methods have unique properties of:

• High strength;
• High fracture toughness;
• High hardness;
• Wear resistance;
• Good frictional behaviour;
• Non-magnetic;
• Electrical insulation;
• Low thermal conductivity;
• Corrosion resistance to acids and alkalis;
• Modulus of elasticity similar to steel;
• Coefficient of thermal expansion similar to iron.

All these advantages are incorporated into the mould injection of the shaft and shaft sleeve of a fan to produce a strong sleeve.

Now there is a full range of DC fans using ceramic bearings.

A comparison test on both metal and ceramic bearings was performed after 8000 h of operation.

Figure 1 and Figure 2 show a magnified surface of a new ceramic shaft and a steel shaft.

Figure 3 shows the magnified surface of the ceramic shaft after 8000 h of operation. This compares with the steel type shaft after 1500 h of operation under the same conditions.

With these materials and their new low cost techniques, engineers now have the option to implement their new designs with a low cost fan able to withstand higher temperatures in hostile environments without compromising its life span.