TechArchive

This article was originally written in the period 1995-2000

Every piece of equipment or machinery which incorporates rotating elements has a number of frequencies at which it tends to vibrate. These are known as eigen or natural frequencies. Designers have to try and avoid running machinery close to these points as this can result in a high level of mechanical vibration, noise and excessive wear.

To eliminate these major resonances or to avoid operating the equipment at such critical speeds, SKF engineers have developed a series of practical and theoretical methods of analysing all the various bearing components and the contribution they make to vibration.

The rotor speed at which the shaft/bearing system is in resonance with a periodic excitation force is defined as the critical speed. The most common excitation force is due to a mass unbalance which occurs at the rotor speed. At most speeds the vibration amplitudes will be small except when the rotational speed (and therefore the frequency of unbalance excitation) is near an eigen frequency of the system. In such cases, excessive vibration can cause major problems.

Vibrations are excited by dynamic forces and mechanisms which occur in any rotating shaft/bearing arrangement. However, measures which decrease the vibration amplitudes by converting this energy into energy which is not relevant for the vibrating system (eg heat) are called damping mechanisms. Based on its detailed knowledge of bearings combined with a wide range of practical, experimental and theoretical R&D work; SKF has established a number of major dampening sources within a roller bearing arrangement.

These include the dampening of: the elasto-hydrodynamic lubrication film between the rolling elements and raceways, the interface between the bearing rings and housings (or shaft), by squeezing lubricant within the so-called entry region and, by deformation of the rolling elements and raceways. To separate the effects of the different damping mechanisms, SKF carried out various tests involving: rotor speeds, bearing preloads, vibration excitation forces and signals, conditions of bearing lubrication and the condition of the housing to ring interface.

Engineers at SKF also have access to a series of sophisticated computer programs which enable them to calculate the performance of rolling bearings in arbitrary applications. Static properties such as bearing deflections, contact stresses, load distributions or bearing stiffness values for any operating condition can be calculated. Finite element analysis and rotor dynamics calculations can also be carried out. The rotor dynamics program allows a comprehensive study of the dynamic behaviour of shaft/bearing system and it is connected to a data base including all SKF standard bearings.

One customer was experiencing failures of SKF angular contact ball bearings mounted in air blowers after a relatively short operating period. Bearing defects were excluded following a detailed investigation of the material, the heat treatment and the bearing dimensions. While static overloading of the bearings was also ruled out by way of fairly simple calculations. Dynamic problems were expected, however, when the operating speed of the blower was increased.

Subsequent calculations with SKF’s rotor dynamics program revealed that the lowest eigen frequency (the bending mode of the shaft) was close to that of the operating speed. And this was verified by the customer following a series of practical experiments with the blower.

Applying the results of the rotor dynamic program, the effect of an increased shaft diameter and the influence of the blower location on dynamic behaviour of the entire system could be accurately estimated. Such analysis formed the basis for an improved design of air blower with exhibited none of the previous problems of premature bearing failure.

The use of such computer analysis programs enables engineers to design rotor bearing systems with critical speeds located away from the operational speed by a safe margin. The calculation of undamped critical speeds and mode shapes is an excellent tool for preliminary evaluation of rotor bearing systems.

However, these values can only give relative displacements and a complete unbalance response analysis is needed to determine absolute displacements. This is particularly important for equipment start-up and shut down phases when critical speeds are often passed.

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