Fan speed, Bearings and Noise

Fan Speed

Some applications demand a high airflow while others require minimal noise. To meet these various requirements, fans are available in different speeds. Increases in the fan speed are positively correlated with higher airflow and noise. Note that this positive correlation applies to all brands of fans.

HIGH SPEED [HS]

High speed fans have a higher airflow but are noisier. By designing the impeller to spin faster, the fan delivers higher levels of air relative to other fans with the same case size. Consequently, increasing the fans speed generates more friction in the bearings which creates more noise. Fans categorized as HS or high speed are designed for applications that demand a high airflow; and the increase in noise level is acceptable.

LOW SPEED [LS]

Low speed fans are quieter but deliver less airflow. By designing the impeller to spin slower, the fan becomes relatively quiet to other fans with the same case size. Consequently, decreasing the fans speed also causes less air to be moved by the slower spinning blades. Fans categorized as LS or low speed are designed for applications that require minimal noise; and the lower airflow is acceptable.

Bearings

The noise a fan produces is also dependent on the bearing system it uses. Initially, the noise level from bearings is overshadowed by the fans rotor blade turbulences. As the fan ages, the bearings get louder and may become the primary source of noise. Sleeve bearing fans start off quieter but become louder at a faster rate due to its lubricant design. On the other hand, dual ball bearings start off slightly louder but will remain more consistent throughout its life.

An axial fan consists of two main parts, the spinning impeller and its non-moving housing. They meet at the center using bearings that rotate on the shaft of the housing. Bearings are responsible for minimizing friction between the two parts. Because the bearing is such a critical component of the fan, the type chosen will result in very different characteristics for the fan. The five key distinctions between the bearings are the noise levels, life expectancy, operating conditions, available mounting positions, and the price. Please keep in mind that many of these parameters mentioned are also influenced by other factors than just bearings.

NOISE LEVEL

The noise level emitted by a fan depends on various factors including the fans speed, age, and bearing system. Regardless of the bearing it uses, fans with higher speeds are louder than lower speed ones. At low speeds, comparable fans produce similar noise levels. But at higher fan speeds, sleeve bearing fans are noticeably quieter and one-ball one-sleeve bearings noticeably louder. Fans are also noisier as they age regardless of speed or bearings. Because sleeve bearing fans have the shortest life expectancy, they will become louder at a faster pace than all other bearing types.

LIFE EXPECTANCY

The life expectancy of a fan depends on numerous factors including operating temperature and bearing system. A fans lifespan becomes shorter as its surrounding temperature increases, no matter what bearings it uses. At lower temperatures, fans will have a similar lifespan. As the temperature increases, dual ball bearing fans will live considerably longer than sleeve bearing fans. The noise level which is partially dependant on the fans age will reflect this relation. Also, the improper mounting of sleeve bearing fans will decrease its life expectancy.

OPERATING ENVIRONMENT

Different types of bearings are suitable for operating for different environments. A fans life will decrease as the temperature of its environment increases no matter what bearing it uses. Although all fans start off with a similar lifespan at very low temperatures, sleeve bearings will have a much shorter life span than ball bearings at higher temperatures. Another point is that ball bearing fans are not suitable in environments where there is a lot of movement. They are very sensitive to shocks and are likely to sleeve bearing fan.

MOUNTING

Due to its lubricant dependent construction, sleeve bearing fans should only be mounted and stored horizontally. Meaning it should always stand upright with the direction of the airflow parallel to the ground. If mounted improperly, its life will shorten and the noise level will also increase. On the other hand, Ball bearing can be mounted various positions without endangering its life span.

COST

The sleeve bearing consisting of a simple sealed cylinder of lubricant is the easiest to manufacture and therefore is the cheapest. As you move towards ball bearings which are constructed with a complex ring of spherical balls with springs, the fan becomes more expensive. Sleeve bearings are very cost effective for applications that have a low temperature environment while dual ball bearings are superior if cost is not a major factor.

Noise

The noise a fan produces can come from various sources such as bearing friction or the vortex flow. The loudest of these noises is stated on the fans specification sheet in decibel units. A higher “A-weighted sound level” or dBA number means a noisier fan. However, this rating is just the starting point and should only be used as a reference. There are many components which can cause the fan to become louder over time. The primary factors of a fans noise level is the fans speed, bearing system, mounting direction, and its structural design.

A faster spinning impeller delivers more air but causes a higher friction to be produced between the bearing and shaft. As a result, there is a positive correlation between airflow and noise. Keeping size and bearing constant, higher airflow fans tend to be noisier. The fans speed is determined by its RPM rating or rotations per minute number. A goal of many engineers is to reduce the fan’s RPM as much as possible without compromising its heat dissipation performance.

IMPROPER MOUNTING AND OIL LOSS

The sleeve bearing system consists of a sealed cylinder of lubricant. This oil based lubricant will eventually evaporate as the fan ages even if it wasn’t used. A lack of lubrication will increase the amount of friction between the bearing and shaft. As a result, the fan will become noisier if not refilled with oil. Improper mounting of sleeve fans will also accelerate the oils evaporation. For this reason, sleeve bearing fans are noisier if not mounted or stored horizontally.

FRAME AND IMPELLER DESIGN

The design of the fan is largely responsible for how much air it delivers and the level of noise it will produce. However, a loud fan does not necessarily mean an inferior design. Many fans are used in applications that have strict high airflow or high static pressure requirements. Others are used in environments that are already very noisy. In these scenarios, the noise level of the fan can be ignored in order to maximizing heat dissipation.