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Avoiding damage caused by vibra­tions

Fans used in venti­la­tion, refrig­er­a­tion and air condi­tioning or other indus­trial appli­ca­tions have to prove them­selves in long endurance tests. But in prac­tice, fail­ures still occur after far fewer hours of oper­a­tion than spec­i­fied in. Ruined motor bear­ings are a typical kind of damage, usually caused by vibra­tions resulting from the way the fans are installed.

The heart beats, wings vibrate, music is played and heard – life without vibra­tions is diffi­cult to imagine. Even though vibra­tions are every­where and have bene­fi­cial uses, they can also be destruc­tive. Poorly balanced car wheels can cause the steering wheel to shake, and vibra­tions can loosen screws or damage ball and roller bear­ings.

Even large struc­tures can collapse when they are subjected to rhythmic vibra­tions at their reso­nance frequency, because then a system capable of vibra­tion can absorb large amounts of energy, which can lead to cata­strophic failure. That was what happened when the Tacoma Narrows Bridge collapsed in 1940 after being forced into oscil­la­tions by unusual wind condi­tions, only a few months after its dedi­ca­tion.

Condi­tions of instal­la­tion matter

Fans are not immune to reso­nance-related damage either, even though their manu­fac­turers use state-of-the-art tech­niques to balance them precisely during produc­tion, as in the RadiPac series from ebm-papst (Fig. 1). A crucial role is played by the condi­tions of a fan’s instal­la­tion, which unfor­tu­nately can neither be predicted nor allowed for.

When a fan is installed in a piece of equip­ment, a new config­u­ra­tion arises that is capable of vibra­tion at a specific struc­tural reso­nance frequency. In addi­tion, the fan’s reso­nance char­ac­ter­is­tics change when it is fixed in posi­tion. With the speed-controlled oper­a­tion that is now wide­spread, the like­li­hood that a fan will at times be oper­ated at reso­nance increases consid­er­ably. Other factors are trans­port and handling. For example, the impeller may not be damaged by impacts, unsuit­able lifting equip­ment and the like, and fans may only be placed on suit­able surfaces.

It is not unusual for vibra­tions to be trans­mitted from external system compo­nents to the fan, for example from a compressor. During oper­a­tion, vibra­tions can result from imbal­ances caused by dirt on the impeller or from flow-related oscil­la­tions, for example if the gap between the impeller and the housing is too small or unfa­vor­able intake condi­tions lead to turbu­lence.

Vibra­tion-absorbing elements and speed range

UVibra­tion-absorbing elements (Fig. 2), such as appro­pri­ately designed springs or rubber elements, help to isolate fans from vibra­tions in their surround­ings. However, certain things need to be consid­ered when selecting them. In addi­tion to the fan system’s natural frequency, the addi­tion of vibra­tion-absorbing elements results in a new spring-mass system with its own reso­nance frequency.

Fig. 3: Basic trace of vibra­tion speed over the speed range of a fan with vibra­tion-absorbing elements: range below the reso­nance frequency (1), range near the reso­nance frequency (2) and range above the reso­nance frequency (3). (source: ebm-papst)

When a fan starts up, it passes through three vibra­tion-rele­vant speed ranges (Fig. 3). In the range below the reso­nance frequency, the vibra­tion severity is below the allowed limit of 3.5 mm/s (according to ISO 14694). Oper­a­tion of the fan is possible in this range, but the vibra­tion-absorbing elements have no effect for phys­ical reasons.

. In the neigh­boring reso­nance speed range, the vibra­tion velocity is some­times well above the allowed limit. There is no imme­diate damage to the device, but lengthy oper­a­tion in this range will shorten the overall service life. Besides that, high noise levels are reached. This speed range should there­fore be passed through as quickly as possible, and sustained oper­a­tion there should be absolutely avoided.

The speed range in which the vibra­tion level is well below the limit starts suffi­ciently far from the reso­nance peak. Only in this range, above the minimum speed, can the vibra­tion-absorbing elements isolate the fan from the vibra­tions of its system or building. To select the right vibra­tion-absorbing elements, the oper­ating speed of the fan in its appli­ca­tion must be known. For every fan, one can find correctly sized vibra­tion-absorbing elements and the asso­ci­ated minimum speed. If others are to be used, the afore­men­tioned prin­ci­ples need to be consid­ered.

Protecting against damage with vibra­tion measure­ments

As explained, there is a host of reasons and effects that can lead to exces­sive vibra­tion levels, but they are not all predictable and are often unavoid­able. Following the instal­la­tion of a fan, vibra­tion measure­ments and/or a search for reso­nance points should there­fore be performed throughout the entire speed control range. This provides an overall impres­sion of the system’s vibra­tion char­ac­ter­is­tics and reveals all unpre­dictable effects and also possible mistakes made during the run-up to commis­sioning.

With a view to achieving longer service life, this is neces­sary as the conse­quences of exces­sive vibra­tion severity due to a struc­tural reso­nance or unsuit­able vibra­tion-absorbing elements can be disas­trous. More­over, incor­rectly dimen­sioned vibra­tion-absorbing elements do not adequately prevent the trans­mis­sion of struc­ture-borne noise.

Fig. 6: Proper setup for multiple fans: Each fan rests on specially designed vibra­tion-absorbing elements (such as springs or rubber elements) on a sturdy frame­work that is firmly fastened to the floor. (source: ebm-papst)

This may cause the entire venti­la­tion system to vibrate, resulting in a high noise level and leading through feed­back effects to bearing damage in the fan. Since measure­ment results are also strongly depen­dent on the posi­tioning of the vibra­tion sensors, the sensors have to be attached as shown in Figs. 4 and 5. Because of possible effects during oper­a­tion, such as the forma­tion of dust deposits, this vibra­tion check needs to be performed repeat­edly, at least within the inter­vals spec­i­fied in the oper­ating instruc­tions.

On the safe side

For vibra­tion analysis of its RadiPac centrifugal fans, ebm-papst recom­mends measuring vibra­tion in all three axes (Fig. 4), or at least in two axes (Fig. 5) radial and axial to the axis of rota­tion, using a typical vibra­tion measuring unit. If measure­ments reveal ranges with exces­sive vibra­tion, it may be possible to rebal­ance the fan in the field. Ideally, the vibra­tion measure­ment unit can assist in such measures. If such measures are insuf­fi­cient, the system can be subjected to design modi­fi­ca­tions such as the addi­tion of rein­force­ment braces.

One can also verify that the vibra­tion-absorbing elements are working correctly and the fan is not oper­ating below its minimum speed. If multiple fans are in use, they should be adequately spaced and kept from influ­encing one another (Fig. 6).

Alter­na­tively, ranges iden­ti­fied by the measure­ments as having exces­sive vibra­tion levels can be avoided using the system’s speed control. When needed, the experts at ebm-papst are avail­able to advise customers, because consid­ering the vibra­tional aspects when installing a fan is always worth­while. Prop­erly installed fans work reli­ably throughout their service lives, unex­pected fail­ures are prevented, and users also benefit from lower noise emis­sions.

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