Anatomy of the Silent Fan

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Nov 12, 2006 by Mike Chin

The electric, multi-bladed, axial fan has been around since the late 1800s, starting originally as a ceiling fan for well-to-do households in the U.S. Today, they are visibly ubiquitous as air-blowing devices for human comfort all over the world, especially in warmer climates and seasons. Electric axial fans are also employed in almost every sector of industry, mostly for gaseous transportation and cooling ventilation of machinery. Their first application in the personal computer was to provide cooling airflow in the power supply. Now, axial fans can be found on every type of computer component.


Not a reproduction of an early ceiling fan. This one is inspired by Wright... Wilbur and Orville. (Photo courtesy of www.ceilingfan.com)

The body of information about electric fans is voluminous. With a history that extends back to the first practical use of electricity, the scientific and engineering knowledge about electric fans seems endless. There is no desire or advantage in trying to reproduce or reference all of this extensive information. The focus in this article is on those aspects of DC axial fans that are particularly relevant for quiet operation in cooling computer components.

A simple way to think about an axial fan is that it is the marriage of a impeller (or propeller) and an electric motor. The impeller has blades extending from a hub that is spun by the motor. The electric motor has a shaft with a wire armature electromagnet that connects to the impeller's hub. A rotary switch or commutator reverses the direction of the electric current twice every cycle, to flow through the armature. The electromagnets push and pull against permanent magnets on the motor housing. As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet. During that instant of switching polarity, inertia keeps the motor going in the proper direction.

Two key mechanical parts in a DC axial fan are critical to its acoustics: The bearings that hold the shaft in place, and the design of the blades that interact with the air. Other factors include structural aspects (such as rigidity, dynamic balance, and the internal self-damping characteristics of the materials the fan is made of) and the commutator frequency. All of these factors will be touched upon in this article.

BEARINGS

I. Ball vs. Sleeve

The main types of bearings used in axial fans are ball bearings and sleeve bearings. When choosing between them, the engineer usually considers the following factors shown in the table below (from Ball vs. Sleeve: A Comparison In Bearing Performance (pdf) by Melody Williams of NMB Technologies):

Criteria Ball Bearing Sleeve Bearing
Fan Longevity Longer life Shorter life
Heat Endurance Higher Lower
Fan Mounting Options Vertical, Shaft Center Line Parallel, Perpendicular Vertical
Noise Emission Quieter at High Speeds Quieter at Low Speeds in Early Life
Parts Precision Non-precision
Lubricant Less Evaporation More Evaporation
Contact Point Line
Cost More Expensive Per Unit Less Expensive Per Unit

Ms. Williams favors the ball bearing for many reasons, the primary being long term reliability. For silent computing enthusiasts, the issue is not so cut and dry. Key highlights are cited from the comparison article.

"Temperature: The chart below uses the L10 method to illustrate fan performance at various temperatures. L10 refers to the point of time at which 90 percent of a large population of these fan types will continue to work. On average, a fan that operates for 50,000 hours will run continuously for over five years.

Temperature Ball-Bearing Fan Sleeve-Bearing Fan % Difference
25°C 95K Hours 80K Hours 18%
40°C 75K Hours 52K Hours 44%
50°C 63K Hours 40K Hours 58%
60°C 54K Hours 30K Hours 80%
70°C 45K Hours N/A -

"Fan Mounting: Fan orientation does not affect the longevity of ball-bearing fans because of preloading, the procedure by which manufacturers build an initial side load, such as a spring or a wave washer against the balls of the bearing. When mounted in vertical positions, sleeve-bearing fans can maintain life spans comparable to their ball-bearing counterparts. However, when sleeve-bearing fans are mounted in any position other than vertical, the fan's life span decreases.

"Noise: Sleeve-bearing fans run more quietly than ball-bearing fans. This is true for applications that have very low fan speeds. The noise advantage of sleeve bearings is much reduced in applications that require faster fan velocities.

"Parts: Typically, sleeve bearings deteriorate under high temperatures because they are made from porous, powered metals from a sintering process. Also, sleeve-bearing fans can develop a high micro-hardness that makes secondary machining difficult. Steel parts make ball-bearing fans more exact. They are precision-ground and super-finished.

"Lubrication: Sealed-for-life ball-bearing systems use thicker lubricants that have more additives, and are less subject to evaporation. While the lubricants within sleeve-bearing fans have a greater concentration of oil, the sleeve-bearings' bushings can only hold a fixed amount of lubricant. Since there is no periodic recharging of the oil, the lubrication within a sleeve-bearing system is more likely to evaporate.

"Point of Contact: Sleeve bearings are line-contact bearings; there is broad contact between the shaft and bearing that generates a good deal of friction. Ball bearings are point-contact bearings, which generate minimal friction. Previously, opponents of ball-bearing fans argued that the ball-bearing system could lead to brinelling or indentations in the raceway. Yet, defenders of the ball-bearing system believe that if a ball-bearing system is assembled correctly, each component will fit perfectly, eliminating the potential for parts damage."

Comair-Rotron, another fan manufacturer, has a somewhat different perspective on the issue of ball versus sleeve:

"Most ball bearing fans are noisier by 1 to 3 dBA over their counterpart sleeve-bearing fan. Also the additional noise is somewhat pure tone in nature. Therefore, the annoyance level is considerably higher than with the sleeve-bearing fan. This higher noise level is also in the higher frequency ranges, which makes it even more annoying.

"Sleeve bearing fans, generally speaking, can easily sustain multiple shocks of 80 g's with duration of 11 msec without impacting noise at all. This is not true for ball bearing fans. Figure 3 shows what can happen to ball bearing fan noise if the fan is subjected to 40 g's (11 msec duration). This is a very important factor since the equipment manufacturer has no control over how this equipment is treated after the fan is installed, particularly in shipment. It is quite common for a ball bearing fan to be noisy before it is even used just from the handling of the equipment it is installed in.

"Typically, sleeve bearing fan noise does not increase due to life. This remains true up until the system begins to fail due to loss of oil. However, ball bearing fans can begin to get noisy in a very short time. This increase in noise is due to many facts, such as grease channeling, loss of grease, damaged bearing camouflaged by the grease, etc. Also, as time goes on, the grease may begin to dry out which allows for a very noisy fan, but it will continue to run for a long time. This brings up an interesting point: the reason for the use of ball bearing fans is to extend the fan life past sleeve bearings. However, if usable life were defined to end when the fan became noisy, it is quite possible the sleeve bearing fan would out live the ball bearing one."

SPCR's Take on Sleeve vs. Ball

The advantages of ball bearings for general application in computer equipment are clearly spelled out in the NMB comparison. For silent computing, the advantages of ball bearing fans are only relevant if the acoustics at low and very low speed are very good. The advantages of sleeve bearings are certainly relevant for silent computing. The Comair-Rotron article shows why PC silencers might prefer sleeve bearing fans. They are...

  • Quieter than ball bearings, especially at low speed, and stay quieter throughout their life — the most compelling aspect for SPCR
  • Good for >3 years continuous use in up to 60°C; longer in cooler conditions — good enough for most well-designed PCs used in quiet home environments. It should be noted that concerns about >5yr lifespans for fans become moot; most PCs do not have much longer usable lifespan.
  • Less prone to damage during shipping and handling. This is a conjecture I made in the Fan Testing Methodology article (Editor's Note at the bottom of page 2); Comair-Rotron gives support to my suspicion.

The caveats against sleeve bearing fans are...

  • Don't use them in any orientation other than vertical. Not only do they not last as long, they quite often make more noise in positions other than vertical.
  • Don't expect them to last as long in very hot conditions. Ball bearing fans in the same hot conditions will last longer though they get noisier, which is why they are almost universally used in rarely-attended server machines that are locked away in air-conditioned rooms. Ball bearings screech as they wear out so there is some warning of pending failure. In contrast, sleeve bearings usually stop working without warning. This does not necessarily spell disaster, as most PCs have some features that warn of high temperatures. Also, overheating of a component causes instability in the PC which will usually be investigated by the user. (If the PC is left on while the user is absent, then the fan failure could turn into a more expensive problem; all the more reason to turn it off when you're not there.) For the average personal PC user, it may be wise to routinely replace a sleeve bearing fan used on a hot heatsink after a few years of service.


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