Bearing types in case fans

Active cooling of computer components has long been no news to anyone. Users are so keen on air flow and pressure inside the case that they forget that not every fan is fully suitable for its assigned role. And the type of fan bearing plays an important role in this.

A bit of history
Initially, the bearings looked very different from what they do now. As the name suggests, this is what the spike is against.

The simple design, due to the small diameter of the axle, creates a large lever arm ratio and even a high coefficient of friction does not create significant resistance to rotation. And to keep the wear as low as possible, a harder material is used as a bearing. Today this design is found in mechanical watches.

One way or another, progress has taken its toll, and modern designs are already more perfect.

Sleeve bearing
A traditional companion for budget fans. Outwardly, the most simple design, consisting of a brass bushing and a steel shaft, but in its work is not so simple.

Slight difference in shaft and sleeve diameters is filled with oil. When the shaft rotates, the frictional forces between the shaft and the oil inject oil into the place of contact between the shaft and the sleeve, creating pressure from the oil wedge. If this pressure is large enough, it will prevent contact between the shaft and sleeve.

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h is the thickness of the lubricant layer, ω is the angular velocity of rotation of the shaft, d is the diameter of the shaft, P is the magnitude of the load, s is the average clearance, e is the eccentricity

As can be seen from the figure, the weak point of this bearing is that the pressure is applied only from one side of the shaft – this does not contribute to damping vibrations, but on the contrary causes them at a low load value.

As it runs, heating makes the oil more fluid, which reduces the pressure of the oil wedge. Also, heating helps to accelerate the evaporation of oil and, as a result, the shaft with the sleeve begins to contact. When the ambient temperature rises by 20 degrees, the service life of such a bearing is reduced by 3 times. That is, for a fan with a conventional sleeve bearing, a place with a low temperature will be the most favorable. And to reduce micro-vibrations that wear out the bushing and eventually become audible vibrations, a load on the shaft is needed. Such conditions in a tower-type assembly are relevant only on the front panel.

With the development of this type of bearing, self-lubricating variations have appeared, as well as with screw threads. Their feature is the greater amount of oil available for lubrication, as well as some semblance of a pump due to screw structures, which ensures the circulation of oil in any position.

The use of polyoxymethylene (POM) is also beneficial. This material is often used in the gearboxes of cheap power tools. But in this case, this is a replacement for a soft copper alloy sleeve, which would crumble instantly in the gearbox. The polymer material reduces the dry friction coefficient and the appearance of abrasive particles, which in turn accelerate wear.

All these tricks do not completely eliminate the design flaws of the sleeve bearing, although they allow it to work for several years even in an unfortunate position. The most tenacious will be a fan with IP6X protection. It uses a grommet to keep out dust, which also prevents oil from evaporating and leaking out.

Hydrodynamic bearing
It is considered eternal, because as long as there is oil in it, the shaft and bushing cannot touch. This is ensured by a special profile of either the sleeve or the shaft, which provides increased pressure in some areas. Usually these are opposite oblique grooves on the bushing. They are easier to perform in soft metal without disturbing the balance of the shaft. But in practice, you can find anything generously flavored with marketing names.

As can be seen from the simulation results, the increased pressure acts on the shaft from all sides. Due to this, the shaft vibrates less and contact with the sleeve is practically excluded. But the main problem of sleeve bearings is that oil drying is also present here. And another one is added: in the supine position, the oil, as it dries, will either accumulate in the oil chamber (while some designs exclude a sufficient supply of oil due to the capillary effect), or will gradually leave the bearing through an insufficiently tight shaft seal.

And to all this is added a very high sensitivity to work at low revs. The oil pressure depends on the rpm, and if they are insufficient, the hydrodynamic bearing turns into a regular plain bearing. It is not for nothing that manufacturers often limit the lower speed of fans with hydrodynamic bearings at 600 rpm. But even with this limitation, users notice the appearance of extraneous sounds.

Magnetic centering bearings
Most fans use magnetic levitation due to the attraction of the permanent magnet of the rotor and stator poles. It is easy to make sure that there is magnetic levitation – just push the impeller along the axis. She moves freely for some distance and immediately returns. In fans with magnetic centering, another magnet is added, which gives more rigidity, and a shaft axis stop, which can be made of either plastic or a hydrodynamic bearing.

The added stiffness reduces shaft vibration at low revs and allows the hydrodynamic bearing to operate at any revs and in any position.

Friction bearing
As the name implies, the principle of its operation is based on rolling. The harder the material, the less the surface roughness and the more accurate the parts, the longer the bearing will last. The lower the operating speed in the rolling bearing, the longer it will work (even in terms of the total number of revolutions).

Orientation in space does not affect work in any way, so fans based on it can be used in any part of the assembly.

But such a bearing is noisy, which makes it pointless to use it at low speeds, and over time, the noise generated increases gradually. The most durable variety is made of ceramic.

And the quietest modification without a separator, in which the balls do not create noise by tapping against each other, most likely we will never see in computer fans.

Conclusion
Computer fan bearings have their own strengths and weaknesses, taking into account which you can avoid accelerated breakdown and unnecessary waste.

A conventional sleeve bearing is cheap, it quickly fails, but on the front panel it can last quite a long time.

Self-lubricating bearings, especially with the use of plastic (POM) and IP6X protection class, can work in any part of the assembly, not inferior in durability to other types.

A fluid dynamic bearing in its simplest design is even more capricious than a conventional plain bearing. It will be optimal to use at speeds close to maximum, if you avoid the “lying” position.

Magnetic centering allows fluid dynamic bearings to operate in any position and speed.

The rolling bearing is the most reliable, but noisy. Often, it warns in advance of its impending breakdown with increased noise, which avoids a sudden stop.