Fan selection – quick techniques to compare various tube-axial fan designs

The worldwide market for tube-axial fans is very large and, as a consequence, the number of tube-axial suppliers is significant. This article will focus on the techniques that can be used to quickly compare various tube-axial fan product offerings, in order to provide a quick screen and minimize the number of suppliers that are submitted to a detailed evaluation.

Supplier Classification

In simplistic terms, fan suppliers can be categorized as suppliers of: Low-Cost Fans, Competitive Fans, and Fans Sold Through Distribution.

Low-Cost Fans: Here, price is the main driver in determining the fan design, as well as the manufacturing process. Fans in this category generally utilize techniques that minimize cost. Plastic parts, when viewed under magnification, will have a high degree of tool marks present, indicative of a low degree of tool polishing (minimizing tool cost). These fans will also have a higher amount of parting line or ejector flash on their plastic parts. The circuit board design is simple and is dominated by low-cost components, with the final assembly having a large degree of solder flux present. Motor control or signal output options are typically not available.

Competitive Fans:This category is dominated by major electronic manufacturers, which typically build to order and may have minimum order requirements.

These fans utilize a more complex board design, incorporating various options. A third wire may be included to provide some type of signal output, such as fan rpm or if a locked rotor occurs. The circuit board design typically utilizes an integrated circuit as the main motor controller. Complete solder flux removal on the circuit board assembly is apparent. Plastic parts exhibit a high degree of tool polishing, as well as minimal parting line and ejector flash. In addition, more emphasis on aerodynamics in designing the frame and impeller is present (this leads to slightly more expensive tooling).

Fan Through Distribution: Fan design is similar to the Competitive Fans class. Manufacturers whose strategy is based on a large distribution focus dominate this category. As such, they concentrate on lower volume applications, where the distribution strategy is an advantage. Typically, these will be associated with a slightly higher price structure with no minimum order requirements.

Comparison Techniques

Fan testing is expensive and time consuming, which makes it worthwhile utilizing quick comparison techniques in order to identify the few suppliers that require follow-up evaluation. The following list identifies simple comparison techniques, with no particular emphasis on order.

Fan Pricing

Ball bearing designs are more expensive than sleeve bearings and, as a general rule, can add an extra dollar to the price. The issue of ball versus sleeve bearings is application dependent and will not be discussed here, as a multitude of articles have been written on the subject [4,5].

Fan Design

The method with which the impeller is attached to the frame is crucial, in order to ensure that the impeller does not come loose when exposed to shock conditions. Most designs utilize metal retention rings, which snap into a groove on the impeller shaft. Disassembly of the impeller from the frame provides a quick insight into the robustness of the fastening method of the impeller shaft to the frame.

With the impeller off, the stator/bobbin assembly is now accessible. Some designs utilize stacked lamina sprayed with a dielectric paint (called fluidizing), that enables it to provide a dual function as a bobbin. There are two inherent advantages with this approach. As the stacked lamina is coated, it is able to withstand a harsher environment before the steel lamina starts experiencing severe rusting. The other is that it provides the appropriate housing support, if a ball bearing design is used. The main disadvantage is cost and the increased potential for electrical shorts. This design is more expensive to manufacture and, if the dielectric is missing or weak, shorting of the windings can occur. Other designs utilize a plastic bobbin that has the stacked steel lamina attached to the periphery. As the bobbin is plastic, shorting of the coils is minimized. In fan designs incorporating ball bearings, the bobbin must also be able to provide the appropriate circumferential support to the ball bearing outer race. Bearing manufacturers usually specify a press-fit interference between the outer race of the bearing and the inner diameter of the housing. This is required in order to ensure the proper radial play, internal to the bearing, is present. The proper press-fit interference will result in a preferred running clearance with minimal radial run-out. Both of these are major factors in ensuring the maximum life of the ball bearing and, at the same time, provide a smooth and quiet operation.

Next, we examine the circuit board for any solder flux residual. Removal of the circuit board will determine whether it is a one or two-sided design and the type of component make-up (surface mount, thru-hole or a combination of both). The most expensive designs to manufacture are those that combine both surface mount and thru-hole technologies. Long-term availability of some thru-hole components may be an issue. Designs that are all surface mount on one side and with only the bobbin/stator assembly on the other, may have a manufacturing advantage, be it cost and/or quality.

Investigate if the fan has polarity protection, as the consequences for not having this are obvious (mis-wire the fan during the application and the results are disastrous).

Reliability

An understanding of the standard qualification testing can give an insight into the design robustness of a fan. Typically, the fan is subjected to some of the following types of environments: vibration, shock, high and low temperatures, humidity, and/or a combination. An ongoing life-testing program will provide a better picture of the long-term reliability of the design. Since there is no universal standard for fan life and qualification testing, a description of the environmental test conditions is required, as well as the criteria used to determine if failures occurred [6]. A good way to judge a supplier’s reliability program is to ask for actual test data, both on the qualification of the design as well as actual life testing.

References