Sliding bearings are a type of bearing with sliding friction characteristics. They are known for smooth operation, reliability, low noise, and the ability to withstand heavy and significant impact loads. Depending on the structural form, they can be categorized into integral, split, and pad types.
Assembly of Integral Sliding Bearings
Integral sliding bearings, commonly known as bushings, are the simplest type of sliding bearings. They are primarily assembled using pressing and hammering methods. In special cases, a hot mounting method is used. Most bushings are made of copper or cast iron. During assembly, care should be taken; wooden hammers or blocks should be used for striking, and if the interference fit tolerance is large, a press should be used. Whether striking or pressing, it is crucial to avoid tilting. After assembly, oil grooves and oil holes should be in the required position.
If the bearing is deformed after assembly, the inner diameter should be reworked. For smaller sizes, a reamer can be used, while larger sizes should be scraped. Additionally, the fit with the shaft should be controlled within tolerance. To prevent rotation during operation, positioning pins or set screws are used on the contact surfaces between the bushing and the housing. Since the hardness of the housing and the bushing materials are different, drilling may cause the drill bit to deviate toward the softer material. Solutions include using a sample punch on the harder material side before drilling, or using a short drill bit to increase rigidity during drilling.
Assembly of Split Bearings
Split bearings, also known as two-piece bearings, are characterized by their simple structure and ease of adjustment and disassembly. They are equipped with two pieces of bearing shell, with shims used to adjust the gap at the joint.
Assembly of the Bearing Shells
The contact between the upper and lower bearing shells and the inner bore of the bearing body must be good. If not, use the inner bore of the thick-walled bearing shell as a reference to scrape the back of the bearing shell, ensuring that both ends of the bearing shell fit tightly against the ends of the bearing body. For thin-walled shells, ensure that the mid-plane of the bearing shell is about 0.1mm higher than the mid-plane of the bearing body; no further scraping is necessary.
Positioning and Adjustment
The bearing shells must not have any radial or axial displacement in the bearing body. Typically, steps at both ends of the bearing shells are used for stopping or positioning with pins.
Scraping and Fitting
For split bearing shells, use matching scraping points. Generally, first scrape the lower shell, then scrape the upper shell. To improve efficiency, the upper shell and cover can be left off during the scraping of the lower shell. Once the contact points of the lower shell meet the requirements, press the upper shell and cover in place, and further correct the contact points of the lower shell while scraping the upper shell. Adjust the bearing clearance by changing shim thickness. When the bearing cover is tightened, the shaft should rotate smoothly with no noticeable gaps, indicating that the scraping is complete.
Measuring Bearing Clearance
Bearing clearance can be adjusted using shims at the mid-plane or by directly scraping the bearing shells. To measure bearing clearance, use the lead wire method. Place several pieces of lead wire, with a diameter larger than the bearing clearance, on the shaft and mid-plane. Tighten the nuts to compress the mid-plane, then remove the nuts and cover, carefully remove the compressed lead wires, and measure their thickness with a micrometer. The average thickness of the lead wires indicates the bearing clearance. Typically, bearing clearance should be 1.5‰-2.5‰ of the shaft diameter (mm). For larger diameters, use the smaller clearance value. For a shaft diameter of 60mm, the bearing clearance should be between 0.09-0.15mm.
Assembly of Rolling Bearings
Rolling bearings have advantages such as low friction, small axial dimensions, easy replacement, and simple maintenance.
Technical Requirements for Assembly
The end face of the rolling bearing marked with a code should be positioned in a visible direction for easy identification during replacement.
The radius of the circular arc at the step of the shaft or housing bore should be smaller than that at the corresponding part of the bearing.
The bearing should be free from misalignment after being mounted on the shaft and in the housing bore.
In the case of two co-axial bearings, one must accommodate axial movement due to thermal expansion of the shaft.
During assembly, it is crucial to prevent any contaminants from entering the bearing.
The assembled bearing should operate smoothly, produce minimal noise, and the operating temperature should generally not exceed 65°C.
Assembly Methods
The basic requirement for bearing assembly is to ensure that any axial force applied acts directly on the end face of the bearing ring (on the inner ring if mounted on the shaft, or on the outer ring if mounted in a bore).
Avoid affecting the rolling elements. Assembly methods include hammering, press fitting, hot mounting, and cold mounting.
Hammering Method
Use a hammer with a copper bar and some soft material to avoid debris from entering the bearing race. Avoid directly striking the inner and outer rings of the bearing with a hammer or punch, as this may affect the bearing's fit accuracy or cause damage.
Screw Press or Hydraulic Press Method
For bearings with significant interference fit tolerances, use a screw press or hydraulic press. Ensure the shaft and bearing are level and apply a small amount of lubricant. The pressing speed should not be too fast, and remove the pressure quickly once the bearing is in place to prevent damage.
Hot Mounting Method
Heat the bearing in oil to 80°C-100°C to expand the inner bore, then fit it onto the shaft. This prevents damage to the shaft and bearing. This method is not suitable for bearings with dust covers and seals, or those pre-filled with grease.
Adjusting the Clearance of Tapered Roller Bearings
The clearance of tapered roller bearings is adjusted after assembly using methods such as shim adjustment, screw adjustment, or nut adjustment.
Assembly of Thrust Ball Bearings
When assembling thrust ball bearings, first distinguish between the tight ring and the loose ring. The inner diameter of the tight ring is slightly smaller. The tight ring should remain stationary relative to the shaft during operation and should always rest against the shaft's step or the end face of the hole. Otherwise, the bearing will lose its rolling function and accelerate wear.