Gear Inspection and Quality Control: Methods for Gear NVH Testing

In the fields of modern rail transit, aviation, and high-end mechanical equipment, gear transmission not only requires high efficiency and reliability but also excellent NVH performance (Noise, Vibration, Harshness). The level of NVH directly affects the user experience and service life, and also has a profound impact on equipment maintenance costs and brand image. This article will systematically introduce the testing methods, influencing factors, and optimization strategies for gear NVH.

1. The Importance of NVH in Gearboxes

During gear transmission, any tiny geometric errors, assembly deviations, or material defects can be converted into vibration and noise sources during meshing. For rail train gearboxes, high noise not only affects passenger comfort but also exacerbates fatigue damage to components such as bearings and gears, thereby shortening the service life of the entire machine. Without changing the material and transmission scheme, through scientific NVH testing and optimization, we can achieve the dual benefits of noise reduction and service life improvement.

The vibration and noise generated in the gearbox are transmitted to other parts of the vehicle through the housing response. The excitation source is mainly the transmission error, and the transmission paths include gear-shaft-bearing-housing and gear-air-housing.

2. Main Sources of Gear Noise

Tooth Profile and Helix Errors: Uneven meshing caused by these errors leads to meshing impact, resulting in an increase in noise peaks.

Excessive Gear Surface Roughness: It directly affects the meshing contact state and generates high-frequency noise.

Assembly Eccentricity and Radial Runout: These cause uneven force on the meshing points, leading to periodic noise.

Resonance Frequency Superposition: When the gear meshing frequency is close to the resonance frequency of the box, shafting, or external structure, the noise will be significantly amplified.

3. Gear Noise Testing Methods

3.1 Acoustic Measurement

Use free-field microphones to measure the sound pressure level (dB) of the gearbox during operation.

Sound intensity analysis can locate the main noise sources.

The testing should be carried out in an anechoic chamber or semi-anechoic environment to avoid interference from environmental noise.

For example, in the acoustic testing of trams, microphone arrays are used to detect noise sources in components such as the tram body, bogie structure, and wheel-set elements. Acoustic regions involve the gearbox, bogie cover, etc.

3.2 Vibration Analysis

Use triaxial accelerometers to record vibration signals in various directions of the gearbox.

Through FFT (Fast Fourier Transform) analysis, convert vibration signals into spectrograms to determine the presence of abnormal frequency components.

It can be combined with order analysis to distinguish gear meshing frequency from vibrations of other mechanical components.

The frequency spectrum can show the amplitude corresponding to different frequencies, such as 1x Gear, 1x Pinion, 1xGMF (Gear Meshing Frequency), 2xGMF, 3xGMF, etc. For spur gears, radial vibration is more prominent, while for helical gears, axial vibration is more obvious.

3.3 Surface Roughness Testing

Use surface roughness meters (such as Taylor Hobson Talysurf) to measure parameters like Ra and Rz of the tooth surface.

Excessive surface roughness not only increases friction but also amplifies meshing noise.

For high-speed gears, it is recommended that Ra ≤ 0.4 μm to reduce high-frequency noise components.

4. NVH Optimization Strategies

4.1 Tooth Surface Modification Optimization

Tip and Root Relief: Alleviate the impact when the tooth root engages.

Crowning: Reduce the concentration of load along the tooth direction. By optimizing the modification, the meshing impact force can be effectively reduced, suppressing noise from the source.

There are various modification methods, such as double-crowned helical gears with different parabolic profiles (secondary, quartic, and sextic parabolas), contour crowning gears with features like bottom pressure reduction and tip clearance, etc. Different modification methods result in different contact paths during meshing.

4.2 Improvement of Surface Roughness

Use precision grinding, honing, or polishing and rolling technologies to reduce surface roughness.

Through rolling strengthening, not only can the Ra value be reduced, but also the quality of the tooth surface hardened layer can be improved.

Honing is an effective process. The axis of the honing tool is set appropriately, and the honing tool (a precision-machined internal gear made of abrasive ceramics such as alumina with a specific helix angle) processes the workpiece gear. During operation, the processing (contact) direction of the gear tooth surface is almost the same as that during actual gear meshing.

4.3 Dynamic Balance and Assembly Precision

Conduct dynamic balance tests on gears and shafting to reduce vibration sources.

Control radial runout (Fr) and axial runout (Fa) during assembly to avoid uneven loads.

5. Standards and Testing Requirements

International and industry standards have clear requirements for gear NVH performance:

ISO 1328: Specifies gear accuracy grades and error ranges.

ISO 8579: Deals with gear transmission noise measurement.

ISO 10816: Covers vibration monitoring and evaluation standards.

By integrating NVH testing into the quality control of the entire production process, the quietness and stability of the transmission system can be ensured before the product leaves the factory.

Gear NVH testing is not only a part of the factory inspection but should also run through the entire process of gear design, processing, and assembly. Through systematic acoustic measurement, vibration analysis, and surface roughness measurement, combined with modification optimization and precision processing technology, the operational quietness and service life of the gearbox can be significantly improved without increasing costs. This is not only a manifestation of product competitiveness but also an inevitable trend in the high-quality development of modern machinery manufacturing.