ISO 21250-4:2020

Overview

ISO 21250-4:2020 - "Rolling bearings - Noise testing of rolling bearing greases - Part 4: Test and evaluation method NQ" specifies the NQ measurement and evaluation procedure for assessing noise generated by greased rolling bearings. The standard defines how to prepare test bearings, record vibration pulses from grease-related shock events, count noise pulses across fixed intervals, and classify grease noise performance. It is intended to support grease developers, bearing manufacturers and test laboratories in quantifying grease-related bearing noise and improving low‑noise lubrication.

Key topics and technical requirements

• Scope and purpose: Defines the test and evaluation method NQ for rolling bearing grease noise, to be used with ISO 21250‑1 general principles and test machine requirements.
• Test specimen and preparation: Clean bearings with organic solvent filtered through a 0.45 µm membrane, inject specified grease quantity and mount without rotating after injection.
• Test conditions: Specified axial load (default cited 29.4 N in procedure; machine supports 15–100 N) and rotational speed (nominal 1 800 min‑1 with allowable deviation shown).
• Signal processing and frequency band: Use velocity‑proportional transducer and filter to analyse the medium & high band (M&H: 300 Hz – 10 000 Hz). Transducer/filter characteristics reference ISO 15242‑1.
• Peak detection and counting: Rectified band‑pass signal is thresholded and pulse counts are recorded for four time intervals (3 s, 10 s, 30 s, 120 s). Threshold is based on the effective vibration level (normal running) so that shock peaks from solid contaminants are detected.
• Result aggregation: Ten test bearings per grease type; average the noise counts (n) from one side of each bearing to obtain the grease’s noise count.
• Classification: Grease noise is assigned to classes A–D based on the measured counts for each time interval (Class A = low noise, D = high noise).
• Test machine & calibration: Annex A illustrates a machine (air bearing spindle, moving‑coil transducer, peak counter, printer). Annex B describes amplifier and sensor calibration procedures.

Applications and who uses it

• Grease manufacturers use ISO 21250‑4 to develop and validate low‑noise greases and to compare formulations for damping and contamination control.
• Bearing manufacturers apply the method to select greases that minimize running noise in precision bearings and to specify grease performance for customers.
• Independent testing labs and R&D centers implement the NQ method to produce repeatable, standardized noise data for product claims and quality control.
• End‑users (automotive, electric motors, precision machinery) use classification results to choose greases that reduce audible and vibratory bearing noise.

Related standards

• ISO 21250‑1:2020 - Basic principles, testing assembly and test machine
• ISO 21250‑2:2020 - Test and evaluation method BQ+
• ISO 21250‑3:2020 - Test and evaluation method MQ
• ISO 15242 series - Bearing vibration measurement methods

Keywords: ISO 21250-4, rolling bearing grease noise, grease noise testing, NQ test method, bearing vibration measurement, grease classification, low-noise grease, test machine calibration.