Crushing plant vibration analysis as the base of predictive maintenance

Why an overall reading is not enough

Crushing plant vibration analysis is the most direct way to learn what is happening inside rotating equipment without opening it. The most common field mistake is taking one overall value with a handheld meter and declaring it high or normal. An overall value warns, but it does not explain: an unbalanced rotor, a loose anchor bolt and an advanced bearing defect can all produce a similar number. What separates them is breaking the signal into its frequency components.

A crushing circuit is a hostile environment for this work. Crusher, screen and feeder run together, each generating vibration at its own speed and transmitting it through the frame. The screen vibrates by design. A measurement plan therefore has to answer three questions in advance: which point, which direction, at which speed.

The measurement plan: point, direction, reference speed

A usable programme fixes three things. Measurement points are marked permanently; horizontal, vertical and axial directions are defined for each point; shaft speed and load condition are recorded at the time of reading. A measurement without recorded load and speed cannot be compared with the next one. A crusher running empty and the same crusher under load behave like two different machines.

Frequency signatures: what each peak says

Peaks in the spectrum are read as multiples of running speed. Running speed is written as 1x, one cycle per revolution. The interpretation rules are well established.

Bearing defect frequencies derive from bearing geometry, meaning the number of rolling elements together with ball and pitch diameters. They are not integer multiples of shaft speed, and that is precisely what identifies them. In the earliest stage the peaks are too small for the standard spectrum and are caught only by envelope analysis or ultrasound. By the time clear peaks and many harmonics appear in a normal spectrum, damage has usually progressed.

The screen needs its own criteria

A vibrating screen runs at high amplitude by design, so general severity criteria written for machine classes do not transfer to it. The quantities to track are different: stroke amplitude, the difference in amplitude measured at the four corners, the angle of the motion ellipse and spring free height. A growing corner-to-corner difference is often the first sign of a cracked body or fatigued springs.

Reading overall severity

For rotating machinery, international standard series evaluate vibration severity from broadband RMS velocity, sorted into zones A through D. Zone A describes a newly commissioned machine, B a level acceptable for unrestricted long-term operation, C a condition tolerable only for a limited period, and D a level capable of causing damage. Those boundaries shift with power class, with rigid or flexible foundations and with machine type, which is why a figure in mm/s carries no meaning until the class is stated. Readings are taken on non-rotating surfaces such as bearing housings.

In practice the strongest yardstick is the machine's own history. A baseline taken after commissioning becomes the reference, and later readings repeat the same point, direction, speed and comparable load. Trend beats absolute value: a level that doubles within a fortnight is far more serious than a higher level that has been flat for a year.

Isolating a loose anchor bolt

Mechanical looseness is the most frequent and the cheapest fault to fix in a crushing circuit. When the bolts of a crusher on its footings slacken, a long series of running speed multiples appears in the spectrum and horizontal amplitude rises noticeably. Taking readings separately on the machine foot and on the concrete beside it settles the question: a large difference between the two means the joint is not carrying. Settled isolation pads, cracked footing concrete and a slack tension bolt all produce the same signature, so the diagnosis is completed by visual inspection.

Putting the programme into service

Three steps build a working programme. List critical equipment and mark measurement points on each item. Take a baseline and record the machine class and reference speed with it. Set the interval; weekly for critical equipment without a spare, monthly where a spare exists, is a common starting point. Programmes die within months when readings are stored apart from maintenance records. Wear part replacements and trend curves have to be read side by side before a decision is possible; further examples of that pairing sit under maintenance and wear.

Common questions

How often should readings be taken?

Weekly on critical equipment without a spare and monthly elsewhere suits most plants. Shorten the interval as soon as the trend breaks.

Is a handheld meter enough, or are permanent sensors required?

Regular handheld readings catch most faults. Permanent sensors earn their place on fast-developing failures and on points that are hard to reach safely.

Does high vibration always mean a bearing?

No. Looseness and imbalance are the more common causes in a crushing circuit, and bearing damage carries a different frequency signature.

Does uneven feed distort the reading?

Yes. Irregular feed makes the overall value fluctuate. Repeat measurements under steady load and, where possible, with the same material.