Vibration Forcing Frequencies (1x, Blade Pass, Gear Mesh)

The spectrum says WHAT, but only if you know which line belongs to which part -- and those lines are simple multiples of shaft speed that a technician should not have to look up on a phone in a plant aisle. Every rotating part announces itself at a frequency tied to shaft speed: a pump impeller with seven vanes puts energy at seven times running speed, a gear with thirty-one teeth at thirty-one times, a rotor with N bars at N times. Identify the peak's frequency, divide by running speed, and the integer that comes out names the component. The classic confusions are worth naming, because they are what a technician actually gets wrong. UNBALANCE is 1x and dominantly radial. MISALIGNMENT is usually 2x with significant AXIAL energy, and it is that axial content rather than the frequency that separates it from unbalance in the field. LOOSENESS throws a picket fence of harmonics rather than one line. And a peak at twice line frequency is ELECTRICAL, not mechanical, no matter how much it looks like a shaft harmonic -- which becomes a genuine trap on a two-pole motor near 3,540 rpm, where 2x running speed is about 118 Hz and sits almost on top of the 120 Hz of a 60 Hz supply. There, a spectrum alone cannot separate them and the only reliable test is to cut power and watch whether the peak vanishes instantly, because an electrical line disappears with the field while a mechanical one coasts down. A 1,780 rpm pump with a 7-vane impeller runs at 29.67 Hz, puts blade pass at 207.67 Hz, and a peak there at high amplitude means a tight impeller-to-cutwater clearance or a starved suction rather than a bearing. Gear mesh sidebands spaced at shaft speed are reported because their PRESENCE is the finding: a healthy mesh shows the mesh line without them. This computes where to LOOK. It does not analyse a spectrum, identify a fault, or evaluate amplitude, and amplitude is what separates a normal forcing frequency from a problem -- every one of these lines is present on a healthy machine. It does not address resonance, which amplifies whatever excites it and can make a small forcing function dominate, or natural frequencies, critical speeds, and the phase measurements that distinguish several of these faults from one another. The machine and component manufacturers' data and a qualified vibration analyst govern any diagnosis.

running speed 1x = rpm / 60 Hz; blade or vane pass = 1x x the blade count; gear mesh = 1x x the tooth count with sidebands spaced at 1x; belt frequency = 1x x pi x sheave diameter / belt length; twice line frequency = 2 x the supply frequency; rotor bar pass = 1x x the bar count.

The standard rotating-machinery forcing frequencies as condition monitoring practice, by name. Where to LOOK, not a diagnosis: amplitude, phase, axial content and resonance decide what a peak means. The machine and component manufacturers' data and a qualified vibration analyst govern any diagnosis.

Multiplication of shaft speed by part counts the user supplies; no spectrum or manufacturer table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: rpm, blade_count, gear_tooth_count, belt_length_in, sheave_diameter_in, line_frequency_hz, rotor_bar_count, one_x_hz, two_x_hz, blade_pass_hz, gear_mesh_hz, twice_line_hz, two_x_to_line_gap_hz

• Where to look, not what is wrong every one of these lines is present on a healthy machinecondition monitoring practice
• Amplitude is not evaluated it is what separates a normal forcing frequency from a problemcondition monitoring practice
• Resonance is not addressed it amplifies whatever excites it and can make a small forcing function dominatecondition monitoring practice