• Torsional ▫ Machine trains prone to torsional problems ▫ Machine trains that do not normally have torsional problems ▫ Synchronous Motor Drives
• Lateral Critical Speed • Axial Natural Frequency • Unbalance
â€˘ Machine breakdowns often appear to be the fault of the part with failed, and are thus attributed to a bad bearing, shaft or gear. However, the real culprit may be TORSIONAL
VIBRATION. â€˘ Good engineering practice calls for a torsional analysis of large rotating equipment at an early stage in the design, so that potential resonance conditions can be identified.
• A damping device may be the only way to reduce vibration-induced stresses, prevent equipment breakdown & enhance machine uptime.
are used extensively in most types of equipment which suffer from torsional vibration.
• A damper coupling will be larger and heavier than a non-damper coupling, such as a gear type. This additional size and weight must be taken into account at the initial design stage.
NATURAL FREQUENCY â€˘ Is a frequency at which a system naturally vibrates once it has been set into motion â€˘ It is the number of times a system will oscillate between its original position and its displaced position, if there is no outside interference.
RESONANCE • It is the build up of large vibration amplitude when a structure or an object is excited at its natural frequency. • DESIRABLE RESONANCE: different musical instruments • UNDESIRABLE MECHANICAL RESONANCE: cause machinery to break or malfunction
POINT OF RESONANCE â€˘ The engine RPM where the amplitude levels from two or more frequencies (one will be a natural frequency) will add together creating a much harsher vibration at that point than at any other point.
TACOMA BRIDE: POINT OF RESONANCE
MOMENT OF INERTIA • It is the rotational analogue of mass. • It is the inertia of a rigid rotating body with respect to its rotation.
THE RADIAN • If the arc length equals the radius length, the angle = 1 radian • 1 radian = 57.2957˚ • 1˚ = 0.0174532 radians Arc
C = 2ΠR = 2Π RADIANS
SELECTING A COUPLING • Depends on whether a system vibration analysis was made. • The design & rubber compound are selected based on the analysis. • Peak, normal & vibratory torque data are used as a basis for selecting the coupling size. • Design engines generally require a torsionally soft coupling which permit 5˚-7˚ angular displacement at peak torque. • For other types of drives, a stiffer rubber coupling is often more effective.
Published on May 5, 2009