Bearings, Balancing and Vibration: ISO 14694, ISO 20816-3 and Condition Monitoring

Vibration is the early warning system of a blower. A rising trend appears weeks or months before a bearing fails or a blade cracks, and most causes, such as deposits, wear, misalignment, or loose supports, are cheap to fix if found early. Two standards set the framework: ISO 14694 for the balance quality and vibration of industrial fans up to a certain size, and ISO 20816-3 for larger machines. An 800 HP blower falls under the second, which is worth knowing when you write an acceptance specification.

Which Standard Applies

ISO 14694 specifies balance quality and vibration levels for industrial fans, for all applications except those designed solely for air circulation. Its scope covers fans with a power below 300 kW, or with a standard electric motor of up to 355 kW. For larger fans, the applicable vibration criteria are in ISO 10816-3, which was replaced in 2022 by ISO 20816-3. ISO 20816-3 applies to industrial machines with a rated power above 15 kW, and its scope includes fans with a power above 300 kW, or fans that are not flexibly supported.

Blower power	Standard
Below 300 kW, or with a motor up to 355 kW	ISO 14694
Above 300 kW (includes an 800 HP unit, about 597 kW)	ISO 20816-3

A new edition of the vibration standard is in preparation, so check for the current version when you write a specification.

What the Standards Do and Do Not Cover

ISO 14694 covers the fan as delivered. It does not cover foundations or installation, and it does not take into account the effect of vibration on people, equipment, or processes. Site vibration depends on the mass and stiffness of the supporting structure, the alignment of the drive, the quality of connections, and the airflow conditions. For that reason, in-situ vibration is not solely the manufacturer’s responsibility unless the contract says so. Specify the acceptance basis, the measurement position, and the mounting condition.

ISO 20816-3 evaluates vibration in zones, from A to D, which describe the condition of the machine: A for newly commissioned machines, B for machines acceptable for long-term operation, C for machines restricted to limited operation, and D for vibration severe enough to cause damage. The limits depend on the machine group, its support flexibility, and its speed, so consult the standard for the values that apply to your machine.

Balance Quality

Wheels are balanced to a stated grade. For fans under ISO 14694, a common practice is a balance grade of G6.3 for smaller machines and G2.5 for larger, with the category chosen for the application and power. The grade limits the residual unbalance allowed, which falls as speed rises. A better grade costs more, and it lowers vibration and bearing loads. Balancing is best done on the assembled wheel and shaft, and it should be rechecked after repairs, deposits, or blade replacement.

Bearings

Large blowers use rolling-element or sleeve bearings, chosen for the load, speed, and life required.

• Rolling bearings are compact and widely used, with life calculated from load and speed.
• Sleeve bearings suit large, high-speed rotors and offer damping, and they need an oil system.
• Lubrication must suit temperature and speed, and a high-temperature blower needs high-temperature lubricant and cooling.
• Temperature and vibration sensors at the bearings give early warning.

Bearing life should be stated and checked against the running hours, not just assumed. See high-temperature centrifugal blowers.

Common Causes of Vibration

Cause	What happens	Typical clue
Deposit build-up	Uneven mass on the wheel, causing unbalance	Vibration rising gradually at running speed
Blade wear or corrosion	Uneven mass loss, changed profile	Rising vibration, changed performance
Lost balance weight	Sudden unbalance	Step change at running speed
Misalignment	Motor and blower shafts not in line	Vibration at twice running speed, axial component
Bearing wear	Race or rolling element defects	High-frequency signals, temperature rise
Loose bolts or foundation	The machine moves	Erratic, multiple frequencies
Resonance	Running speed near a structural natural frequency	Large vibration at a specific speed
Belt problems	Wear or wrong tension	Vibration at belt frequency
Thermal distortion	Uneven heating of the shaft or casing	Vibration that changes with temperature

Blowers on variable-speed drives can pass through a resonance as the speed changes, so identify critical speeds during commissioning and program the drive to avoid dwelling on them. See flow control for blowers.

Condition Monitoring

Permanently installed sensors on critical blowers give trend data and early warning. Common measurements are vibration velocity and acceleration at each bearing, bearing temperature, and motor current. Set alarms from the standard’s zones and from your own baseline, since a blower’s normal vibration differs from the generic limit. Record a baseline at commissioning, because every later change can be compared with it. A change in motor current at the same flow can indicate deposits, damage, or a change in the system.

Commissioning and Acceptance

At commissioning:

1. Check rotation, alignment, and the tightness of all fixings.
2. Run the blower and record vibration at each bearing, in each direction.
3. Record bearing temperatures and motor current.
4. Check the operating point against the blower curve.
5. Store the results as the baseline.

Writing the Acceptance Clause

Disputes over vibration usually come from vague wording. Write the acceptance clause so it states the standard and its edition, the measurement points and directions, the mounting condition, the running speed and load at which the measurement is taken, and the value that counts as acceptable. Agree who pays for a re-test, and whether the factory test or the site test governs. A clear clause prevents an argument later, and it makes the commissioning baseline more valuable, because everyone is comparing against the same numbers.

Where Our Range Fits

NextAir Systems manufactures centrifugal blowers up to 800 HP (about 597 kW) and up to 500 mm WC (about 4.9 kPa) static pressure. For blowers above 300 kW, ISO 20816-3 applies. Tell us the balance grade, vibration acceptance criteria, and monitoring you need, and we will confirm what we can supply.

Ask About Balance Grade and Monitoring

Send your duty and acceptance criteria, and we’ll confirm balance grade, vibration limits, and monitoring options. Contact us for a free quote.

Frequently Asked Questions