Deep groove ball bearings are the most widely used type of rolling bearings, but early failures caused by incorrect selection are not uncommon at engineering sites. Starting from the structural characteristics, this article sorts out the selection factors such as code reading, load and speed verification, clearance and accuracy selection, sealing form matching, and installation coordination. It also summarizes four common selection misunderstandings for reference by equipment designers and maintenance personnel.
1. First recognize the boundaries of capabilities: what deep groove ball bearings are good at and what they are not good at
The structural feature of deep groove ball bearings is that the inner and outer rings each have a deep groove raceway with a cross-sectional radius slightly larger than the radius of the steel ball. The rolling elements are single row steel balls and are guided by a cage. This structure determines that the boundaries of its capabilities are quite clear.
·Professional working conditions:
Mainly radial load, can bear a small amount of bidirectional axial load at the same time, is one of the few single row bearings that can independently complete the bidirectional axial positioning of the shaft system;
The friction torque is small, the limit speed is high, and the speed adaptability is outstanding among general bearing types;
It has a simple structure, mature mass production, low cost and good interchangeability in the same size.
·Working conditions that you are not good at:
Heavy loads and shock loads. The point-contact load-bearing ceiling is there, and radial heavy-load situations should give way to cylindrical rollers and spherical rollers;
When the proportion of axial load is relatively high. The axial load-bearing capacity of deep groove balls is limited. When the axial force is dominant, angular contact ball bearings, tapered rollers or thrust bearings should be used instead;
Misalignment. Deep groove balls have very low tolerance for the deflection angle of the inner and outer ring axes. In situations where the deflection of the shaft system is large and the concentricity of the bearing seat hole is poor, self-aligning balls or spherical roller bearings should be used instead.
The first step in model selection is not to look through model samples, but to determine whether the working conditions fall within these three boundaries. There are a large number of early failure cases on site. Post-analysis analysis is often not a problem with the quality of the bearings, but the use of deep groove balls in positions where the structure is not good at it.
2. Code reading: selection information in the model
Take 6208-2RS/C3 as an example to disassemble:
The first digit 6: deep groove ball bearing type code;
Diameter series: 18, 19 are ultra-thin series (68, 69), with compact radial dimensions but low load-bearing; 10 series (60) are Light load; 02, 03 series (62, 63) are the general mainstream, and the load capacity increases sequentially; 04 series (64) are used for heavy load;
Suffix ZZ: double-sided iron dust cover, dustproof but not waterproof, with good temperature resistance and speed adaptability;
Suffix 2RS: Double-sided rubber contact seal, high protection level (with double-lip structure, it can reach IP65), but the contact friction of the sealing lip will lower the limit speed and limit the upper limit of temperature rise;
Suffix C3: clearance group, indicating that the clearance is larger than the standard group (C0/CN).
The value of code reading lies in reverse verification: the model number on the drawing or old part is the answer to the existing working conditions. Once the working conditions change (speed increase, load increase, environment change), copying the old model is a hidden danger.
3. Five elements of selection
1. Load check
First make a list of forces: radial force, axial force at each installation position, and whether there are impact and vibration peaks. The dynamic load rating of deep groove ball bearings is checked according to the ISO 281 life formula. The commonly used experience in engineering is:
In general transmission situations with stable loads, the equivalent dynamic load should be controlled within 1/6~1/10 of the rated dynamic load, and the life margin will be reasonable;
When there are impact conditions (stamping, crushing equipment), the load coefficient is 1.5~2.5, and then substituted for verification;
After the axial load is converted into equivalent load, if the proportion of the axial component is too high, it means that the type has been selected incorrectly, and the angular contact or tapered roller solution should be re-evaluated.
2. Speed check
The limit speed table of the control sample (given separately for grease lubrication and oil lubrication). Pay attention to two points: first, for models with 2RS contact seals, the limit speed is usually discounted, and seal lip friction will become a source of temperature rise at high line speeds; second, temperature rise is coupled with speed and load. When high speed and heavy load occur at the same time, a derating margin must be left in the verification conclusion.
3. Clearance selection
The smaller the clearance, the better. Decision logic follows three things:
Fit: The interference fit of the inner ring will eat up part of the clearance. The larger the interference, the original clearance should be enlarged accordingly;
Temperature difference: The temperature of the inner ring (rotating ring) is usually higher than that of the outer ring. The thermal expansion after operation further compresses the working clearance. C3 is commonly selected for high-speed motor conditions;
Multi-station consistency: When dozens of installation positions of the same equipment share clearance groups, the discrete value of the batch clearance has a greater impact on the life consistency than the nominal value of a single set of clearance - large discrepancy means that individual positions fail first, dragging down the entire line. The national standard has upper limit requirements for clearance. During actual acceptance, attention should be paid to the distribution interval of the entire batch of data rather than just the average value.
General principles: Choose the standard group (CN/C0) for normal working conditions; choose C3 for high speed, interference fit of the inner ring or poor heat dissipation of the outer ring; lower clearance (C2) is only used in occasions with special requirements for rigidity or accuracy.
4. Accuracy and Vibration Level
The accuracy level (P0, P6, P5, etc.) determines the dimensional tolerance and rotation accuracy; for noise-sensitive applications such as motors, fans, and home appliances, the vibration acceleration level (Z1~Z4) and speed level (V1~V4) are often more worthy of attention than the accuracy level - the Z4V4 level is for low-noise and low-vibration application requirements. Precision selection follows"Just enough": If P5 can satisfy P0, the cost will double but the benefits will be limited.
5. Sealed form
Choose according to the environment and maintenance strategy: clean environment, open or ZZ dust cover that can be re-lubricated regularly; choose 2RS contact seal for wet, dusty, long maintenance-free occasions; semi-closed conditions can use a combination of ZZ on one side and 2RS on the other. The misunderstanding is that the ZZ is omnipotent - the iron cover only blocks coarse particles, and it cannot do anything in the environment where water or emulsion enters. Rust and stuck still occur at such sites. Most of them are caused by the wrong sealing form rather than product quality problems.
4. Cooperation and installation: the last mile of correct selection
Inner ring rotation (most working conditions): The inner ring and the shaft adopt an interference transition fit (commonly used in k5 and m5), and the outer ring and the seat hole adopt a gap transition (H7, J7), so that the outer ring can swim slightly to release thermal elongation;
Outer ring rotation (belt rollers, pulleys): the matching relationship is reversed;
Installation method: For small sizes, use a press to press the inner ring; for medium sizes, give priority to induction heating of the inner ring and then hot-install it. Do not knock the cage or outer ring to transmit force; the heating temperature generally does not exceed 120℃ to avoid tempering effects and damage to the hardness;
Disassembly: Use a puller to act on the inner ring to avoid indentation of the raceway caused by pulling the outer ring alone;
considerable proportion"Bearing quality complaints", and was ultimately determined to be improper installation: hammer damage, crooked installation, overheating, and dirt mixed in. The raceway traces of this type of failure have typical characteristics and can be distinguished by a failure analysis.
Five and four common selection misunderstandings
Take the deep groove ball and carry it hard with heavy load and misalignment. Deep groove balls are used on multi-roller equipment and long-span shaft systems. The stress concentration on the edge of the raceway leads to rapid peeling, which is manifested as periodic abnormal noise and then stuck. The correct solution for this type of working condition is spherical roller bearings, which use their alignment angle to absorb installation deviation and shaft system deflection.
Sealed form fits all. Select all ZZ pictures to save trouble, and batch rust will occur in humid environments; or select all 2RS, and the temperature rise of high-speed bits will exceed the standard.
The clearance only looks at the nominal value and does not look at the batch discreteness. A single set is qualified, but the whole batch is large and scattered. The service life after installation is uneven, and short-lived parts are frequently replaced.
Only recognize the import and not the acceptance data. The acceptance criteria for alternative procurement should fall on the data: clearance test report, hardness (HRC62-64 after quenching and tempering of GCr15 bearing steel is the normal range), measured vibration level, and steel ball accuracy level. Acceptance based on data is more reliable than acceptance based on brand belief.
6. Conclusion
The selection of deep groove ball bearings is ultimately a matter of"Boundary judgment + parameter matching"Engineering question: first use force and rotational speed to determine the type boundary, then check the five elements of load, rotational speed, clearance, accuracy, and sealing in sequence, and finally end with correct coordination and installation, supplemented by the incoming acceptance of batch data. If the type is chosen correctly, the parameters are matched, and the installation is in place, the deep groove ball bearing can run quietly for many years; conversely, if any link is missing, replacing it with a more expensive brand will only delay the failure time.
For working conditions that are uncertain during selection, it is recommended to compile the force list, speed range, installation space, and environmental conditions into text and submit them to the bearing factory’s technical department for review—most selection errors can be eliminated at the drawing stage.
(Source of this article: Guangdong Zhongzheng Bearing Technology Co., Ltd.)