2022 December the First Week VISTA Technical Knowledge: Solution for asymmetric bearing group with low reverse load

Hot tags: angular contact ball bearing(9)


The contact angle of angular contact ball bearings is usually 40°. SKF has now launched a new 25° contact angle bearing series, greatly expanding the product portfolio of angular contact ball bearings. With the introduction of this new product series, it is the first time to easily realize the asymmetric bearing assembly, effectively solving the well-known minimum load problem, thus extending the service life of bearings in axial load applications such as pumps or motors.

 

Each rolling bearing has a contact angle, which is used to transfer the force inside the bearing. The simplest example is the deep groove ball bearing. The contact angle under pure radial load is 0 °, while the contact angle of angular contact ball bearing is always greater than 0 °.

 

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The larger the contact angle is, the higher the axial load capacity of the rolling bearing is. In extreme cases, when the contact angle is 90 °, it becomes a pure axial bearing. Angular contact ball bearing is between pure radial bearing and pure axial bearing, which can absorb radial load and axial load at the same time.

 

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Smaller contact angle and optimized cage for more options

 

Recently, the product lineup of SKF single row angular contact ball bearing with brass cage has been further expanded, and the contact angle of the bearing is smaller than the common 40°. The following is a brief introduction to the newly introduced 25 ° angular contact ball bearing (suffix AC) series:

 

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A small contact angle will slightly reduce the axial load capacity, but it will bring other advantages. For example, under some operating conditions, the speed may be increased by up to 20% compared with 40 ° contact angle. This is due to the more favorable kinematics conditions in the bearing, which reduces the sliding component as much as possible, thus reducing the generation of heat. In addition, the smaller contact angle ensures a larger radial stiffness, which is a favorable factor in applications mainly bearing radial loads.

 

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SKF also optimizes brass cages for 25° and 40° contact angle bearings. Due to the use of materials with higher strength and improved shape, the cage is not only more robust, but also enables the bearing to achieve higher speed.

 

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The performance of the cage is improved, while the space occupied is reduced, which can make more space for containing the lubricant, thus extending the lubricant replacement interval. In addition, the new brass cage reduces vibration and noise levels by 15%.

 

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Bearing application and layout

 

Angular contact ball bearings are most commonly used in pumps, compressors and motors. With improved SKF angular contact ball bearings, these devices can operate more smoothly and extend their service life.

 

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In these applications, single row angular contact ball bearings are usually (at least) installed in pairs. This is because when the angular contact ball bearing bears pure axial or pure radial load, it will inevitably produce corresponding radial or axial force due to its contact angle. In this design, a bearing can only absorb axial force in one direction. If it is not used in pairs and the axial force comes from the opposite direction, the bearing may be damaged.

 

Since in most applications the axial force will come from both directions, the counter force must be absorbed by the counter bearing. The simplest form of paired bearings is a double row angular contact ball bearing with a fixed "O" layout (following the shape outlined by the contact angle line). Single universal angular contact ball bearings can also be combined in different ways, with both O-type and X-type layouts.

 

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Diagonal contact ball bearing

 

When angular contact ball bearings are installed in pairs, a variety of schemes can be used to set preload or bearing clearance. The most common method is to use so-called "universal mating" bearings (also known as "universal bearings"). The advantage of universal bearings is that they have been matched in the factory, and when they are installed on the bearing pedestal, the prescribed preload/clearance can be achieved. The initial seat clearance between the bearing inner race or outer race is eliminated by clamping the bearing.

 

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In order to obtain accurate preload/bearing clearance, the tolerance of indent and protrusion is only a few microns.

 

 

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Therefore, the assembly of universal bearings is greatly simplified, while other traditional assembly methods require more manpower. For example, the bearing preload can also be adjusted by inserting a special spacer sleeve into the bearing pedestal or shaft, but the bearing must be measured with time and effort, and a separate spacer sleeve must be made for each pair of bearings.

 

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In order to reduce the assembly cost, the new AC angular contact ball bearing with a contact angle of 25 ° has provided a universal matching version that conforms to the SKF Explorer level, and can also provide various preloads and bearing clearance levels as required.

 

Mixed bearing set

The axial force of angular contact ball bearing group mainly comes from one direction, such as the fan or pump rotating in one direction. In such applications, one angular contact ball bearing absorbs the axial force, while the second angular contact ball bearing, namely the reverse support bearing, does not bear the load.

 

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However, rolling bearings require a minimum load to achieve frictionless operation. The load of the unloaded bearing is likely to be lower than this minimum load. This may lead to abnormal rolling behavior of the ball (sliding in rolling contact), which is commonly known as "sticking". "Sticking" will lead to temperature rise, surface damage and/or cage fracture, and eventually lead to premature bearing damage.

 

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Advantages of hybrid bearing sets

 

In the past, this type of application usually installed a bearing group consisting of two identical 40 ° contact angle bearings, with the axial load mainly acting on one side. However, this is not the best configuration, because the larger 40 ° contact angle is more susceptible to the minimum load problem when unloaded.

 

Now, the new 25° contact angle bearing series realizes the asymmetric assembly of 40° and 25° contact angle bearings. This bearing set has great advantages. In the asymmetric bearing group, the main axial force is absorbed by the bearing with a large contact angle (B=40°), while the no-load bearing with a small contact angle (AC=25°) reduces the risk of fouling, because it increases the threshold of lifting force. The lifting force is an external axial force. Under the action of this axial force, the reverse support bearing in the preloaded bearing group will be completely unloaded, and the required minimum load cannot be guaranteed.

 

Specifically, this means that under the same external load, the no-load degree of the reverse support bearing with a contact angle of 25 ° is low. This greatly reduces the risk of premature failure of the bearing, thereby improving the operational reliability.

 

The advantage of an asymmetric bearing group in terms of internal load distribution can be illustrated by an example of a preloaded bearing group. The bearing group consists of two angular contact ball bearings with an O-ring layout, which bear a pure axial force F (red arrow). The 40 ° contact angle bearing absorbs the axial force, while the 25 ° contact angle bearing is no-load under the axial force.

 

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In the coordinate system, the x-axis represents the deflection and the y-axis represents the force. The green curve represents 25 ° contact angle bearing, and the blue curve represents 40 ° contact angle bearing. Position 1 represents the load ratio without external loads. Position 2 represents the load ratio with the external axial force F1 at the lifting force level of the 40 ° bearing. Position 3 represents the load ratio with the external axial force F2 at the lifting force level of the 25 ° bearing.

 

Position ①: load ratio without external load after assembly

 

At the intersection between the blue line and the green line (on the y-axis), no external force is applied. Both bearings bear only the set preload. In this example, for simplicity, the preload force F preload=1 is assumed.

 

Positions ② and ③: load ratio when external force is applied in addition to preload

 

When the external axial force F is applied in addition to the preload, the line of sight must be moved to the right from the intermediate intersection in the figure. The 40 ° contact angle bearing (blue) must bear the load brought by force F in addition to the preset preload, so the blue curve rises. At the same time, force F makes the 25 ° contact angle bearing (green) no-load, so the green curve drops. When the green curve touches the x-axis, the preload is exhausted and the bearing is unloaded. This situation must be avoided.

 

The grey dotted line is used for comparison with the reverse support bearing with a contact angle of 40 °. It shows the deflection in a conventional bearing set with the same contact angle (40 °+40 °).

 

Detailed description of position ②: force F1 (40 °) and force F1 (25 °)

 

At this point, the difference between conventional bearing groups (40 °+40 °) and asymmetric bearing groups (40 °+25 °) becomes particularly obvious. In both cases, the external force is exactly the same (F1 (40 °)=F1 (25 °)=2.8 x F preload), while the deflection of the bearing group is different.

 

The force F1 (40 °) is represented by a dotted line, and the force F1 (25 °) is represented by a solid line. The conventional bearing set with two 40 ° contact angle bearings has reached the lifting force point. This can be seen from the grey dotted line intersecting the x-axis representing the deflection of the reverse support bearing at a 40 ° contact angle. At this time, the preload has been completely used up (yellow? At the intersection of dotted lines). This is about 2.8 times of the preload.

 

However, for an asymmetric bearing group, when the same force is applied, there is still some residual preload in the bearing. In this case, the green curve is still above the x-axis (yellow box at the intersection of solid lines).

 

Detailed description of position ③: force F2

 

The lifting force of the reverse support bearing with a 25 ° contact angle is shown here. It is worth noting that the force F2 is significantly greater than the force F1. Specifically, this is about 5.2 times of the preload. This means that compared with the bearing group composed of the same bearing, the asymmetric bearing group can absorb nearly twice the axial force without idling the reverse support bearing.

Therefore, the 25 ° contact angle bearing is more suitable as a reverse support bearing.

 

Conclusion

SKF new generation angular contact ball bearing with contact angle of 25 ° is an ideal solution for reverse support bearing. Especially in the application where the axial force mainly comes from one direction, it is recommended to use an asymmetric bearing group in the design to prevent the problem that the minimum load cannot be achieved and avoid premature failure of the bearing.

 

In addition, 25 ° contact angle bearings can also be used in applications requiring high speed and/or high radial stiffness.

 

More about VISTA Angular Contact Ball Bearing:

Vista angular contact ball bearing High-quality brand have excellent reputation and popularity, good after-sales service system, fast, and guaranteed product quality!

 

As a leading global angular contact ball bearing manufacturer and supplier in China, we’re featured by perfect solution for many kinds of bearing supplying, competitive price and fast delivery for each customer. Please rest assured to buy angular contact ball bearing at the best price from Vista. For any special requirement, contact us now.

 

As angular contact ball bearing manufacturer, Vista supplies single row angular contact ball bearing, double row angular contact ball bearing and four point angular contact ball bearing for customers.

 

Applications: Machine tool, precise instrument, Automobile, Fluid conveying machinery, Light industry machinery, etc.

 

Angular Contact Ball Bearing 

 


2022-12-03

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