Load and failure of Rolling bearings

I. Load characteristics of Rolling bearings

1. The contact load and stress of the rotating ring and the rolling body passing through each point of the bearing zone are varied, and the contact load and stress at each contact point show the characteristic of pulsating cycle, and the contact load and stress are not loaded in the non-bearing area.

2. For the fixed ring, the load and stress at each point are different, but the contact load and stress at each bearing point show the same characteristics of pulsating cycle, but the amplitude is different.

The highest load value for deep groove ball bearings (six types) at the lowest end: F0=（4.37/Z）Fr.

II. Failure form of Rolling bearing

1. The fatigue pitting of the inner and outer ring and the rolling body for the normal running bearing is 10 r / min < n < nlim.

2. The plastic deformation of the inner ring, outer ring and rolling body for bearing of static or low rotational speed (n ≤ 10 r / min) or intermittent swing.

3. The inevitable friction and wear of inner and outer ring and rolling body.

III. Design criteria for rolling bearings

1. For the normal running bearing-to prevent fatigue pitting corrosion, basing on the calculation of fatigue strength, the life of bearing is calculated.

2. For low-speed bearing, or for bearing continuous load or discontinuous load without rotating bearing, it is necessary to control plastic deformation to calculate static strength.

3. For high-speed running bearings, apart from life calculation, it also needs to check the ultimate speed of bearings.

Bearing failure

We may find the following problems while identifying bearing failure:

1. Cage Damage – During use, the cage damage may occour, such as vibration, excessive speed, wear, & blockage.

2. Contamination – Contamination may be caused by the entrance of foreign objects into the bearing, such as scratches, pits, and lapping of the raceways.

3. Fatigue Failure – It is characterized by flaking or spalling of the raceway.  Fatigue failure is caused by granular weakening of the bearing steel which begins at the sub-surface and spreads to the raceway. 

4. Lubrication Failure – The original color of grease will usually turn to dark. If the grease is stiff or changed in color, it indicates lubrication failure. The grease will have an odor of burnt petroleum oil. If there is lack of oil, the lubricating oil will disappear. 

5. Misalignment – When one side of bearing is misaligned in relation to the other, the ball or roller path will run from one side of the race. This can result in high temperature and destroy of lubricant.

2018 National Bearing Industry Quality Conference

Thrust ball bearings, also known as thrust bearings, consist of a row of steel balls (with cage), a shaft ring (tightly fitted with the shaft) and a race (with a clearance from the shaft and tightly fitted with the housing bore). Steel ball is rotating between the shaft ring and the race. It can only withstand axial loads in one direction and cannot withstand radial loads. Since the axial load is evenly distributed on each steel ball, the load capacity is large; however, when working, the temperature rise is large, and the allowed limit speed is low.

Thrust ball bearings can not limit the radial movement of the shaft or casing, but can limit the axial movement of the shaft and casing in one direction. Therefore, such bearings are often used in conjunction with deep groove ball bearings.

When installing, the axis of the shaft and housing bore must be kept concentric, otherwise the bearing will be prematurely damaged due to stress concentration. In order to eliminate this phenomenon, a radial gap of 0.5 to 1 mm may be left between the outer diameter of the race and the hole of the outer casing. The shaft center line and the bearing support surface should be vertical, and the shaft should not be inclined or deflected. Otherwise, the bearing will be damaged prematurely due to uneven load distribution. In order to eliminate the inclination of the bearing axis, the support surface of the race can be padded with an elastic material such as an oil-resistant rubber, leather, or thrust ball bearing with a spherical seat.

Thrust ball bearings are mainly used in automobiles, machine tools, oil rigs, iron and steel machinery industries.

6 methods to distinguish the false bearings

Bearing is an important part in mechanical equipment. Its main function is to support the mechanical rotator, reduce the friction coefficient during its motion, and ensure its rotation accuracy. In production and processing, once fake bearings are used, the consequences are not only machine maintenance and even downtime, but also millions of losses for the company. Today I will share 6 methods to distinguish the false bearings.

1. Check the logo and characteristic on the package is clear or not. 
2. Check certificate of quality is standard or not. 
3. Check whether the steel print on the bearing is clear or not.
4. Check the overall quality of bearings.
5. Check the metallic luster of bearings.
6. Rotate the bearings to check whether there is noise or not. 

Features of Deep groove ball bearings

Features Of Deep Groove Ball Bearings:

• The cheapest and most commonly used bearing type; produced in many designs and sizes
• The bearings are characterised by simple designs and cannot be disassembled
• The bearings can handle operational conditions and their maintenance is simple
• The bearings show relatively good load rating both in radial and axial directions
• The bearings are suitable for high and very high speeds
• The bearings require good alignment of the journal and bearing body; the permissible tilting angle is approx. 10'
• The bearings are delivered with shields or seals

Selection of Angular contact ball bearing

The selection of angular contact ball bearing should consider working condition and limited space. Working conditions mainly include use environment, precision, speed, force direction and size, and so on. 

High speed, small load generally use non-metal cage; low speed, heavy load use metal cage; High temperature environment use metal cage; medium temperature environment generally use engineering plastic cage; general nylon cage temperature can not exceed 120 degrees.

For ultra-high speed, wear resistance, low temperature rise, long life and high availability environment, hybrid ceramic ball bearings can be used; balls use silicon nitride ceramics. 

If the accuracy of the shaft is high, for example, P4 or P2 bearing is generally used for machine tool spindle; P5 or P4 grade is generally chosen for medium precision, and P0 or P6 grade can be selected for general precision. 

If high rotational speed is required, light series bearings with contact angle of 15 ° should be selected first；2 series or 3 series angular contact ball bearing with 40 °contact angle can be selected if bearing needs to bear big axial load; the angular contact ball bearing with contact angle of 25 °can be selected if the rotational speed and axial load should be taken into account at the same time.

The requirements of oil and gas lubrication for motorized spindle

1. lubrication oil cleanliness grade: 13 / 10 (ISO4406); 

2. Air cleanliness: air filtration accuracy 0.01 micron; 

3. Air dryness: Dew point is +2 ℃; 

4. Inlet gauge pressure in front of bearing nozzle: 2-3 bar; 

5. Oil and gas delivery pipe: inner diameter 2 to 2.5 mm; 

6. The length of oil and gas pipeline: the length should be 1 ~ 4 meters; the helical pipe is favorable for oil storage near the terminal lubricating point. 

When the bearing runs again after stop, the oil and gas can be put in place quickly and the response time will be shortened. Then the selection of oil and gas lubrication products must first meet the above requirements. In addition, it is important to make use of the performance of oil and gas lubricating products in order to transport oil and gas to the motorized spindle and to realize reliable and stable operation.