Excessive Vibration in an Underground Axial Fan: A Step-by-Step Diagnosis

novembre 7, 2026

Most vibration in underground axial fans comes from four sources: an unbalanced impeller, looseness in the base or bolts, worn motor bearings, and the way the fan is connected to its duct. You can usually tell which one by measuring vibration velocity on the motor housing and noting how it behaves over time and under load. As a reference, ISO 14694 gives 4.5 mm/s rms at start-up, 7.1 mm/s as the alarm level and 9.0 mm/s as the shutdown level for a typical rigidly mounted industrial fan. This guide explains how to measure, how to read the result and how to fix each cause.

Why Vibration on a Mine Fan Cannot Wait

A fan that shakes is wearing itself out. Vibration overloads the motor bearings, cracks welds and brackets, and loosens bolts on the casing and the duct connection. On explosion-proof fans it can also loosen junction box covers and cable entries, which weakens the flameproof protection the fan was certified with. If the fan finally stops, the heading it ventilates has to be cleared. Finding the cause early is far cheaper than a bearing failure or a damaged impeller.

Step 1: Measure Before You Change Anything

Feeling the casing by hand tells you something is wrong, not what. Use a handheld vibration meter and record vibration velocity in mm/s rms:

Measure on each motor housing, as close to the bearings as possible. A counter-rotating fan has two motors, so measure both.
Take readings in three directions: horizontal, vertical and axial (along the shaft).
Measure with the fan running at its normal duty, connected to its duct, and write down the date, location and direction of each reading.
Compare with earlier readings. A steady rise matters as much as the absolute value.

For judging the readings, ISO 14694 (vibration and balance of industrial fans) gives the following commonly used values for category BV-3, which covers most industrial axial and centrifugal fans:

Condition	Rigidly mounted fan, mm/s rms
Factory test	2.8
Start-up on site	4.5
Alarm: find the cause	7.1
Shutdown: stop the fan	9.0

Step 2: Read What the Vibration Is Telling You

Each cause leaves its own pattern. If your meter shows frequency, compare it with the running speed: for example, a motor turning at about 2,950 rpm gives a 1× frequency of about 49 Hz and a 2× frequency of about 98 Hz. Even without a frequency reading, how the vibration started and in which direction it is strongest narrow the search:

What you see	Likely cause	What to do
High at 1×, mainly radial, rising slowly over weeks	Dust, mud or corrosion building up on the impeller	Clean the impeller and check balance (Step 3)
Sudden jump at 1×	Damaged blade, lost balance weight, object hit the impeller	Stop the fan and inspect the impeller
High axial vibration, strong 2×	Motor not sitting flat, impeller not square on the shaft, bent shaft	Check for soft foot and impeller fit (Step 4)
Unsteady, changes when bolts are tightened	Loose foundation, mounting or hub bolts	Tighten and stiffen the base (Step 4)
High-pitched noise, bearing getting hotter	Bearing wear or poor lubrication	Grease or replace bearings (Step 5)
Rubbing or scraping sound	Blade tips touching the casing	Check tip clearance and casing roundness
Pulsing roar, vibration changes with duct condition	Fan working near stall: duct too long, damaged or blocked	Inspect the duct and operating point (Step 6)

On a counter-rotating fan, find out which stage the vibration comes from. Compare the readings on the two motors, and if the fan’s controls allow each stage to run alone, run them one at a time.

Step 3: Fix Impeller Imbalance

Imbalance is the most common cause in underground fans, because mine air carries dust, moisture and sometimes acid. Deposits never build up evenly, so the rotor goes out of balance as it gets dirty.

Isolate and lock out the fan, then clean the impeller and hub with a brush or low-pressure water. Clean every blade the same way; partial cleaning can make imbalance worse.
Check each blade for cracks, bent tips, worn leading edges and corrosion at the root. Replace damaged blades or the whole impeller.
On impellers with adjustable blades, make sure every blade is set to the same angle and locked.
After repair or blade replacement, have the impeller balanced. ISO 14694 gives balance grade G6.3 for BV-3 fans.

Why blades corrode and collect dust, and how to slow it down, is covered in our article on blade corrosion and dust buildup.

Step 4: Tighten Bolts and Stiffen the Base

Looseness turns a small imbalance into heavy shaking. Work from the ground up:

Base and foundation.The fan must stand on a level, rigid base or a solid skid. A base that flexes or rocks amplifies vibration. Check anchor bolts and any shims.
Soft foot.If one mounting foot does not sit flat, tightening its bolt bends the casing and the motor. Loosen one foot at a time with a dial gauge on the casing; if it moves, shim that foot.
Motor and impeller.In local fans the impeller sits directly on the motor shaft. Check the motor mounting bolts, the impeller hub fixing and the shaft for run-out. Fans with a coupling also need the coupling alignment checked.
Casing, flanges and duct clamps.Tighten all flange bolts and the clamps holding the first duct section.
Explosion-proof parts.After a period of heavy vibration, check junction box covers, cable glands and flameproof joint bolts. Tighten them to the torque in the manual; never add gaskets or change flameproof joints.

Correct installation prevents most of these problems in the first place; see our step-by-step installation guide and safe installation of explosion-proof fans.

Step 5: Check the Bearings and the Motor

Bearing problems usually show up as rising temperature and a harsher, higher-pitched noise before vibration becomes severe. Track the bearing temperature with an infrared thermometer alongside the vibration readings, and follow the greasing interval and grease type given in the manual. Too much grease overheats a bearing just as too little wears it.

Electrical problems can also make a motor vibrate. Unbalanced supply voltage or a winding fault produces vibration that disappears the moment the power is switched off, while mechanical vibration fades gradually as the fan coasts down. That simple test tells you whether to call a mechanic or an electrician. Motor build quality matters here too; see why motor quality matters in mine ventilation fans.

Step 6: Check the Duct and the Operating Point

A healthy fan can still vibrate if the system it works against is wrong:

Duct connection.The first duct section must line up with the fan outlet without pulling on it. A misaligned or rigid connection transfers force into the casing.
Damaged or blocked duct.Kinks, tears, collapsed sections or water lying in the duct raise resistance and push the fan toward stall, where airflow becomes unstable and the fan pulses and vibrates.
Duct longer than the fan was chosen for.As headings advance, the duct gets longer and the required pressure rises. A fan chosen for a short duct may end up working outside its stable range. For long headings, choose a fan sized for the full distance, such as our long-distance tunnel ventilation fan.

When to Call the Manufacturer

Contact us if vibration stays above the alarm level after cleaning, tightening and checking the bearings, if an impeller or blade is damaged, or if the fan vibrates from the first start. For heavy work such as replacing an impeller or rewinding a motor, our guide to major repair of long-distance axial fans explains the process. We can also advise on replacement parts for our FBDY counter-rotating and FBY press-in local fans, and for main fans such as the FBCZ extraction fan and FBCDZ withdrawable fan.

Common mistakes we see:

Changing parts before measuring and recording the vibration
Cleaning only the blades that are easy to reach
Tightening bolts on a base that is not level, which creates soft foot
Ignoring a slow rise in vibration until it reaches the shutdown level
Blaming the fan when the duct is damaged or much longer than planned

FAQ

What vibration level is acceptable for a mine axial fan?

For a rigidly mounted fan in ISO 14694 category BV-3, around 4.5 mm/s rms is acceptable at start-up, 7.1 mm/s is the alarm level and 9.0 mm/s is the shutdown level. Use the manufacturer’s values where they are given.

Can I keep a vibrating fan running until the next planned stop?

Below the alarm level, yes, but measure more often and plan the repair. Above the shutdown level, stop the fan, and make sure the heading has ventilation from a standby fan or is cleared.

Why did vibration get worse after cleaning?

Usually because the impeller was cleaned unevenly, water was left inside the hub, or a blade angle was disturbed. Clean every blade the same way, let it dry and check the blade settings.

How often should vibration be checked?

Record readings at least once a week and after any event such as a duct move, a blast nearby or a repair. Main fans benefit from continuous monitoring; see how vibration monitoring improves reliability.

Do counter-rotating fans vibrate more than single-stage fans?

Not when they are built and maintained well, but each of the two impellers must be balanced and each motor checked. Always measure both stages.

Send Us Your Readings

Tell us the underground axial fan model and power, the vibration readings with their position and direction, the bearing temperatures, the duct length and diameter, and when the vibration started. Photos or a short video of the fan running help us a great deal. Our engineers will help you find the cause and supply the parts you need.