Abstract

Scraping is an important finishing method used in mechanical manufacturing and maintenance for various surfaces, such as machine-tool guideways, mating surfaces, bearing shells, and spherical fitting surfaces. This paper introduces the scraping methods used for the installation and maintenance of sliding bearings.

Keywords: mechanical equipment; scraping method; sliding bearing

1. Scraping of Bearing Shells

Before scraping a sliding bearing, the journal should be carefully inspected to ensure that its surface is smooth and free from defects such as rust and impact damage. If any defects are found, they should first be eliminated.

The contact condition between the journal and the bearing should then be checked. A thin layer of marking compound, such as red lead powder or red marking oil, is applied to the journal. The journal is then installed into the bearing and rotated manually two or three turns in both directions. After removing the shaft, the distribution of the contact marks on the bearing surface is examined.

Scraping can be carried out when the contact marks are improperly distributed or when the shaft cannot be rotated at all. During scraping, the following principles should be followed: scrape heavily first and lightly later; remove more material from high spots while retaining low spots; remove larger areas first and smaller areas later.

During the initial scraping operations, relatively greater force may be applied to remove more metal and quickly achieve satisfactory contact. When the contact area reaches approximately 50%, lighter scraping should be adopted.

After each scraping operation, the bearing surface should be cleaned. The marking compound is then reapplied to the journal for verification, and further scraping is performed according to the contact condition until the technical requirements are satisfied.

During scraping, not only must the number and distribution of contact points meet the technical requirements, but the side clearance and contact angle must also satisfy the specified requirements.

In general, the contact points are adjusted first while simultaneously considering the contact angle, followed by adjustment of the side clearance. There should be no obvious boundary between the contact and non-contact areas. When the bearing surface is rubbed with a finger, no noticeable step or irregularity should be felt.

This paper discusses and analyzes the scraping methods for bearing shells, providing references for the installation and maintenance of mechanical equipment. 

1.1 Contact Angle Between the Journal and Bearing Shell

The contact angle between the journal and the bearing shell is shown in Table 1.

1.2 Number of Contact Points on the Bearing Shell

The number of contact points on the bearing shell is shown in Table 2.

The requirements for the contact accuracy of a bearing shell are determined by the number of contact spots within an area of 25 × 25 mm².

The scraped contact spots should be small and deep, without sharp edges, and their distribution should be appropriately arranged according to the operating conditions of the bearing in order to achieve good operating performance.

The contact spots can be inspected using the marking method.

1.3 Adjustment of Side Clearance

When scraping a bearing shell, not only must the contact points satisfy the technical requirements, but the side clearance must also fall within the permissible range.

The side clearance is generally one-half of the top clearance. For certain high-speed main shafts operating under relatively low lubricating-oil pressure, the side clearance may be increased slightly, but it should not exceed two-thirds of the top clearance.

The contact angle must also remain within the specified range.

In general, the contact points should be scraped first while taking the contact angle into consideration, followed by adjustment of the side clearance.

Particular attention should be paid to the transition between the contact and non-contact areas. There should be no obvious boundary, and the surface should be scraped to form a smooth transition. When the surface of the bearing shell is rubbed with a finger, it should feel very smooth.

1.4 Bearing Shells with Shims Between the Upper and Lower Halves

For bearing shells with shims installed between the upper and lower halves, the upper and lower halves should be scraped simultaneously.

After scraping is completed, the top clearance of the bearing should be adjusted using the shims.

1.5 Bearing Shells Without Adjustment Shims

If there are no adjustment shims between the upper and lower bearing shells, the upper shell can be removed directly from the bearing housing and scraped against the journal.

The side clearance, contact angle, and contact points of the upper shell should be the same as those of the lower shell.

When scraping the upper shell, the top clearance should be adjusted first. Once the top clearance is approximately correct, the contact angle and contact points should then be adjusted.

2. Application of Curved-Surface Scraping Techniques

2.1 Equipment Using Sliding Bearings as Supports

Sliding bearings are characterized by high load-carrying capacity, high rotational accuracy, reliable and stable operation, low noise, vibration absorption provided by the lubricating-oil film, high resistance to impact loads, and long service life.

Therefore, sliding bearings are generally used to support the main shafts of high-speed, heavy-duty, and precision machinery, as well as large electric motors, gearboxes and reversing gearboxes in rolling-mill systems, and other similar equipment.

2.2 Scraping Methods and Relevant Parameters for sliding Bearings in Centrifugal Blower Maintenance

(1) Installation diagram of the D900-2.5 centrifugal blower

The installation configuration of the D900-2.5 centrifugal blower is shown in the corresponding figure.

(2) Inspection and recording before maintenance

The following inspections should be carried out before maintenance:

a. Inspect the contact condition of the upper bearing shell.

b. Use a 200 mm × 200 mm frame level with an accuracy of 0.02 mm/m to separately measure the overall levelness of the low-speed gear shaft and high-speed gear shaft of the motor speed increaser, as well as the blower shaft.

c. Use the lead-wire method and feeler-gauge method to inspect the top clearance and side clearance of each bearing, as well as the axial clearance of the thrust bearing shell.

d. Inspect the meshing condition of the herringbone gears in the speed increaser and the center distance between the shafts.

e. Use a dial indicator to check the coaxiality between the motor and speed increaser and between the speed increaser and blower.

f. Inspect the operating condition of the lower bearing shell.

(3) Scraping principles for the bearing shells

The following principles should be followed:

a. For high-speed, light-load operation, the top clearance should be 0.0017d–0.0025d, where d is the journal diameter. The side clearance should be one-half of the top clearance.

b. For low-speed, heavy-load operation, the top clearance should be 0.0015d, and the side clearance should be two-thirds of the top clearance.

The scraping parameters are given in Table 3.

c. The interference of the bearing-shell back should be between 0.05 and 0.08 mm and must not exceed 0.10 mm.

The maintenance and scraping methods for other types of blowers are basically similar. The parameters in Table 3 can be used as a reference according to the rotational speed and load of each blower model.

2.3 Scraping Methods and Relevant Parameters for Sliding Bearings in D2000-11 Exhaust Fan Maintenance

(1)

The maintenance method is the same as that used for the D900-2.5 centrifugal blower. However, particular attention should be paid to whether the contact between the spherical back surface of the bearing shell and its mating surface meets the technical requirements.

(2)

The scraping parameters are given in Table 4.

2.4 Maintenance Method for the Reversing Gearbox of a 650 Rolling Mill

The reversing gearbox of the 650 rolling mill is a low-speed, heavy-load piece of equipment subjected to relatively large impact loads. Its maintenance and scraping procedures differ considerably from the two examples described above.

The maintenance procedure is as follows:

a. During maintenance, ensure that the center distance between the shafts and the gear meshing condition meet the technical requirements.

b. Since the gear shaft of the reversing gearbox drives the rolls, relatively large impact loads are generated when steel passes through the rolling mill. Therefore, the upper bearing shell is the primary load-bearing surface.

Consequently, when scraping the bearing shell, particular attention should be paid to the upper shell, and the contact spots should be large and firmly established.

3. Conclusions

Based on the above discussion, combined with the teaching work associated with current production internships and with reference to technical books, textbooks, and technical materials, as well as exchanges with colleagues, the relevant methods and applications of curved-surface scraping techniques in mechanical equipment maintenance have been summarized.

2026 September 2nd Week Marginal Product Recommendation

MG-090 Bronze Wrapped Bushings:
MG-090 bronze self-lubricating bearings use a kind of high density bronze alloy of special compositions as backing, the surface of alloy is rolled with diamond pattern of the oil indents or half ball oil indents, this kind of bearing has a higher density, a higher load capacity.

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