When using bearings, particularly SKF imported bearings, in certain instruments, it is always necessary to pay attention to the tolerance fit between the bearing and the shaft.

I. Rolling Bearings

① When the bearing bore tolerance zone forms a fit with the shaft tolerance zone, the tolerance designation originally classified as a transitional fit in the general hole system (e.g., k5, k6, m5, m6, n6) will become an interference fit, though the interference is minimal. When the bearing bore tolerance zone forms a fit with h5, h6, g5, g6, etc., it ceases to be a clearance fit and becomes an interference fit.

② The bearing outer diameter tolerance zone constitutes a special tolerance zone due to differing tolerance values from standard reference shafts. In most cases, the outer ring is fixed within the housing bore. Certain bearing component structures necessitate adjustment, thus the fit should not be excessively tight. Commonly employed fits include H6, H7, J6, J7, Js6, and Js7.

Appendix:

Generally, shafts are typically marked 0 to +0.005. For infrequent disassembly, an interference fit of +0.005 to +0.01 suffices. For frequent assembly and disassembly, a transition fit is appropriate. We must also account for thermal expansion of the shaft material during rotation. Consequently, for larger bearings, a clearance fit of -0.005 to 0 is preferable, with the maximum clearance fit not exceeding 0.01.

1. The fit between the bearing and shaft journal shall be based on the bore system, while the fit between the bearing and housing journal shall be based on the shaft system.

Bearing dimensional tolerances and rotational precision values shall comply with GB307-84 specifications for corrosion-resistant pumps.

2. The bearing-engaged shaft journals and bearing housing bores shall conform to GB1031-83 journal specifications, with shaft journal surface roughness Ra < 1.6 μm and bearing housing bore roughness Ra < 2.5 μm.

3. When bearing rings and rollers are manufactured from GCr15 and ZGCr15 steel, their hardness values shall be 61–65 HRC. When manufactured from GCr15SiMn and ZGCr15SiMn steel, their hardness values shall be 60–64 HRC. Hardness inspection methods and uniformity of hardness within identical components shall comply with the provisions of JB1255.

4. Inspection of radial and axial clearance shall conform to the requirements of GB4604-84.

5. The raceways of the inner and outer rings of rolling bearings shall be free from spalling or severe wear, and neither ring shall exhibit post-cracking. The balls shall show no wear, the cage shall be free from severe deformation, and rotation shall produce no abnormal noise or vibration. When stopped, the bearing shall come to rest gradually.

6. For C-grade tolerance tapered roller bearings, the contact accuracy between rollers and raceway surfaces under pump load shall be verified via colour penetration inspection. Contact traces must be continuous, with contact length not less than 80% of the roller's generatrix.

II. Plain Bearings

1. For Radial Thick-Wall Bushings

① Measure bearing clearance and bushing interference using the lead-injection method, shaft-lifting method, or other techniques. Shaft clearance must meet specified requirements, while bushing interference should be 0–0.02 mm.

② Inspect all components for damage or cracks. Bushings must be free from spalling, porosity, cracks, groove wear, or scorching.

③ Inspect bearing-shaft contact using the dye penetration method. Contact angles should range between 60° and 90° (lower limit for speeds exceeding 1000 r/min; upper limit for speeds below 1000 r/min). Contact within the bearing area must be uniform, with 2–4 contact points per square centimetre. Poor contact requires thorough scraping.

④ Clean the bearing housing to ensure all oil ports are unobstructed, with no cracks or leakage.

⑤ The bearing shell back should be in close, uniform contact with the bearing housing. Inspect using the dye penetration method; the contact area must not be less than 50%.

2. For Radial Thin-Wall Bearings

① The alloy layer of the bearing shell must be firmly and tightly bonded to the shell body, with no delamination or shell separation. The alloy layer surface and the inner base surfaces of both bearing shell halves shall be smooth and flat, with no cracks, pores, laminations, inclusions, or impact damage permitted.

② The bearing shell back and the inner bore surface of the bearing housing shall have a tight and uniform fit. Using the dye penetration method, the contact area shall not be less than 85% for inner diameters less than 180mm, and not less than 70% for inner diameters greater than or equal to 180mm.

③ The clearance and contact condition between the bearing shell and journal shall be ensured by machining accuracy. Contact area generally requires no scraping; if axial contact is uneven, minor adjustment is permissible.

④ After assembly, the inner split surface shall be checked with a 0.02mm feeler gauge; no insertion shall be possible to meet specifications.

3. For thrust bearings

① Bearing shells must be free from wear, deformation, cracks, scratches, delamination, rolling defects, or scorching. Contact marks with the thrust plate should be uniform, with a contact area no less than 70% and evenly distributed across all shells around the circumference. Thickness variation between shells in the same set must not exceed 0.01mm. Babbitt alloy shells should be rounded in the direction of rotation to facilitate oil wedge formation, allowing lubricant ingress. The load-bearing surface on the back shall be flat and smooth.

② Adjustment shims shall be smooth, flat, and free from warping. A single layer of shims with a thickness difference not exceeding 0.01 mm shall be used.

③ After assembly with the bearing cover, repeatedly measure the thrust bearing clearance using the shaft-pushing method. The value shall fall within the specified range. Measurements taken using this method must correspond with those obtained via shaft displacement probe for thrust bearing clearance, with the probe's zero position adjusted as specified.

④ The bearing housing's horizontal mating surfaces must be tightly fitted without misalignment. The oil temperature measurement port must align precisely with the bearing cap's oil hole, free from skewing. The oil hole must be clean and unobstructed.