With the advancement of industrialisation, bearings have become increasingly prevalent in diverse equipment. Amongst these, the adjustment, measurement, and installation methods of bearing clearance constitute a critical aspect of bearing utilisation. This paper focuses on examining certain inspection and adjustment techniques pertaining to bearing clearance.

1. Bearing Clearance

A certain clearance exists between the inner and outer rings and the rolling elements of a rolling bearing, allowing relative displacement between the rings. When no load is applied, the amount by which one ring can move radially and axially from one limit position to another relative to a fixed ring is termed radial clearance and axial clearance respectively.

Clearance is categorised into three types based on the bearing's operational state.

(1) Original clearance. This denotes the clearance in the bearing's free state prior to installation, determined by the manufacturer's machining and assembly processes.

(2) Installation clearance, also termed fitting clearance. This is the clearance present after the bearing has been fitted to the shaft and housing but before it commences operation. Due to interference fitting, either the inner ring expands, the outer ring contracts, or both occur simultaneously, resulting in installation clearance being smaller than the original clearance.

(3) Operating clearance. This is the clearance when the rolling bearing is in operation. During operation, the inner ring experiences the greatest temperature rise and thermal expansion, reducing the bearing clearance. Simultaneously, under load, elastic deformation occurs at the contact points between the rolling elements and raceways, increasing the bearing clearance. Whether the operating clearance is greater or lesser than the installation clearance depends on the combined effect of these two factors.

2. Hazards of Inappropriate Operating Clearance

Operating clearance constitutes a critical quality indicator for rolling bearings and a vital application parameter. In practical use, bearing operating clearance influences load distribution, vibration, noise, friction torque, and service life. Inappropriate clearance poses significant risks to equipment.

(1) Excessively Small Operating Clearance.

Excessively small operating clearance increases the bearing's friction torque, generating substantial heat and predisposing the bearing to overheating and failure. This occurs because insufficient clearance impairs lubrication between the rolling elements and the inner/outer rings, leading to dry friction. This generates excessive heat, causing wear, adhesion, and potential cracking of the inner/outer rings, ultimately resulting in bearing damage.

(2) Excessive bearing operating clearance.

Excessive bearing clearance primarily results from selecting an overly large natural clearance or insufficient bearing preload. For instance, in high-speed gearboxes, a large natural clearance leads to correspondingly large operating clearance. This causes significant vibration during operation, reducing bearing service life.

3. Bearing Clearance Measurement

Primary methods for measuring bearing clearance include specialised instrumentation, simplified techniques, and feeler gauge inspection.

The feeler gauge method is most widely employed on-site for measuring radial clearance in large and extra-large cylindrical roller bearings. Measurements may be taken with the bearing either upright or horizontal; in cases of dispute, the horizontal measurement shall prevail.

Determining maximum and minimum radial clearance values: When measuring with a feeler gauge along the circumference between rollers and raceways, rotate the bearing assembly (inner ring and roller cage) once. The maximum thickness of the feeler gauge that passes between three consecutive rollers constitutes the maximum radial clearance value.

The minimum radial clearance value is the minimum thickness of the feeler gauge that cannot pass between three consecutive rollers. The arithmetic mean of the maximum and minimum radial clearance values shall be taken as the bearing's radial clearance value. Clearance values measured using the feeler gauge method may include tolerances for gauge thickness.

4. Adjustment of Bearing Clearance

(1) Adjustment of axial bearing clearance.

The bearing inner ring is positioned by the shaft shoulder, while the outer ring is preloaded by the bearing thrust washers on both sides. The axial clearance is adjusted via the preload force of these thrust washers. Considering thermal contraction reducing clearance, a certain axial clearance should be maintained. No national standard specifies axial clearance requirements for bearings. In practice, axial clearance is minimally affected by factors such as interference fits and loaded operation. Therefore, during installation, adjustment is typically based on the bearing's original clearance.

Specific adjustment method: With the reducer cover removed, fully assemble the shaft into position and securely tighten the bearing cover bolts on both sides. Then apply axial pressure to one end of the shaft.

The magnitude of this axial force may be referenced from the axial load the shaft will endure during operation. Subsequently, use a feeler gauge to measure Clearance 1 and Clearance 2. After measurement, calculate the sum of Clearance 1 and Clearance 2, and compare this with the bearing's original clearance. Ensure the difference between the two falls within ±40μm. If the requirement cannot be met, adjust by adding shims until the specification is achieved.

(2) Adjustment of Bearing Radial Clearance.

Radial clearance is critical for stable bearing operation. Relevant standards for radial clearance are specified in GB/T 4604-2006. Therefore, in practical applications, the upper and lower limits of radial clearance can be determined by consulting the relevant tables.

Specific adjustment procedure: For ease of measurement, remove both bearing end caps prior to adjustment. Install the bearing in the housing, replace the upper cover, and uniformly tighten the four fixing bolts on both sides using a torque wrench. Bolt preload values should comply with relevant national standards. After securing the bolts, measure the clearance using a feeler gauge and compare the reading with the standard values from the reference table.