2023 June the Fourth Week KYOCM Technical Knowledge: Improvement on cage of magneto-generator bearing

Abstract: Magneto-generator bearings were generally fitted with a pressed sheet-copper cage. The fabrication technology of such cage is complicated, which needed several sets of mould and tooling, resulting bearings in high cost and inefficiency with assembly. To counter the problem mentioned above, the PA66-GF25 enforced nylon injection molding cage with simple technology and low cost and meeting user's using requirements was used instead of it. The productivity and properties of bearings were both improved while the cost decreased.

Key words: copper sheet pressing; cage; enforced nylon; magneto-generator bearings

1. Introduction

Magneto bearings are shallow groove ball bearings with outer ring separation. Therefore, when designing, the inner components - cage, rolling element, and inner ring must be integrated to meet the requirements of bearing performance. In the past, the magneto bearing cage was a Quincunx shaped copper plate stamping frame. As shown in Figure 1, the cage process was complex, requiring cutting, forming, shaping, channeling and other processes (cage process route: ring cutting → inside turning → outside turning → punching → pressing with process balls → pocket hole processing → process integration, etc.), and also requiring multiple sets of stamping dies, equipment and tooling, resulting in high cost and low assembly efficiency of bearings. Moreover, the price of metallic copper is several times that of reinforced nylon. Therefore, our company has sought a reinforced nylon PA66-GF25 injection molded cage with a simple process, low cost, and ability to meet user requirements, as shown in Figure 2. Geometric shapes that are conducive to bearing operation can be designed to replace copper plate stamping cages. The change in product structure has enabled bearings to have excellent performance such as low noise, low temperature rise, high rotational speed, and strong impact resistance that the original structure did not possess.

2. Introduction to the performance of reinforced nylon cage bearings

Due to the low friction coefficient, high elasticity, low formation of brittle cracks, and good impact resistance of reinforced nylon (PA66-GF25), the bearing cage made of this material can isolate, guide, and improve the stress state between the rolling element and the cage. The bearing using reinforced nylon cage can also meet the requirements of relative sliding between the cage, rolling element, and ring. This is because a certain amount of glass fiber reinforcement and lubricant are added to nylon PA66-GF25, which is a composite material with good thermal and chemical stability, making it have good wear resistance and self-lubricating characteristics. When the bearing loses lubrication, the temperature rises sharply, causing the metal cage to deform, causing the rolling element to get stuck, causing the bearing to lock and break. The initial temperature of the reinforced nylon cage will also rise rapidly, causing the cage to melt and provide lubrication. The bearing will not lock up quickly and can work continuously for a certain period of time; The reinforced nylon cage can operate normally below 120 ° C; And it has the ability to embed solid foreign objects, allowing impurities to adsorb on the cage without entering the raceway of the bearing, avoiding friction, heat generation, and wear; The specific gravity is small, which is 1/7-1/6 of that of metal, greatly reducing the centrifugal force acting on the cage during high-speed rotation of the bearing, reducing the vibration and noise of the bearing, and improving the high-speed performance of the bearing; At the same time, the nylon cage can be injection molded, and its geometric shape depends on the accuracy of the mold. After injection molding, the size consistency is good, making it easy to mass produce.

Figure 1 Quincunx copper sheet stamping cage

Figure 2 Reinforced Nylon Injection Molding Cage

3. Design of a new reinforced nylon cage

According to the working characteristics of the separation of the outer ring of the magneto bearing, it is first required to combine the inner components (cage, rolling element, and inner ring) into a whole, and lock the steel ball in the cage pocket hole with the self-locking ball in the inner and outer diameter directions of the cage as the core, and ensure that the steel ball does not fall out of the pocket hole and rotates flexibly without clamping the ball. Therefore, the design should ensure that the steel ball can be locked in both the inner and outer diameter directions of the cage pocket hole, and the ball is pressed into the pocket hole through the outer locking point in the outer diameter direction of the cage.

(1) Considering that the shrinkage deformation of reinforced nylon is easily affected by the environment, the diameter size of the pocket hole of the reinforced nylon cage Δ C should be slightly larger than the size of the car made pocket hole to avoid deformation and ball clamping. Due to the non-metallic material and light weight of the reinforced nylon cage, increasing the size of the pocket hole will not affect the noise of the bearing.

Δ _C=1.03 D_w

In the formula: D_W - diameter of the steel ball.

(2) Cage outer diameter circumferential locking amount J₂={ Δ _C - (D_c - P_c)} ^1/2

In the equation: Δ _C - diameter of pocket hole,

D_C - Outer diameter of the cage,

P_C - diameter of the center circle of the rolling element.

To ensure that the pocket hole can lock the ball and smoothly fit the steel ball, based on the elastic deformation of the reinforced nylon material and the diameter of the rolling element, the range of pocket hole locking values has been verified through practice: D_w -J₂=0.15~0.4. If this value is too large, the rolling element cannot fit into the pocket hole. From formula (2), it can be seen that after selecting the main parameters D_w and P_c of the bearing, the diameter of the pocket hole Δ _C is also a fixed value, and only by reducing the outer diameter D_c of the cage can the pocket locking value be within a reasonable range. However, if the outer diameter Dc of the cage is reduced, it will affect the strength of the cage. Therefore, the design diameter in the outer diameter direction of the pocket hole is Δ B round fossa（ Δ _B ＞ Δ _c) Adjust the depth h_B of the circular groove to ensure that there is a suitable locking amount J₂ at the locking point of the pocket hole, ensuring that the steel ball is smoothly installed into the cage pocket hole.

(3) Holder inner diameter locking amount

Inner diameter circumferential locking amount:

Inner diameter axial locking amount: J={Δ_c ² -(P_c-d_c) 2 }^1/2

In the formula: d_c - Inner diameter of the cage.

Requirement: J ＞ J₁ meets this equation to ensure that J₁ does not interfere with the steel ball during installation, and to ensure that the steel ball is smoothly installed into the cage pocket hole; If J<J₁ steel ball will be difficult to fit into the pocket, the design parameters can be adjusted appropriately according to the formula.

(4) To ensure the flexibility of bearing rotation and prevent the rolling element from getting stuck or blocked in the pocket, the finished cage must be inspected with a control steel ball. During inspection, the outer diameter of the steel ball group should not be less than D_k, and the inner diameter of the steel ball group should not be greater than d_K.

The size of the steel ball moving in the outer diameter direction of the cage pocket hole: D_k=P_c+D_w+（ Δ _C ²-J₂ ²) ^1/2+(D_w ²-J₂ ²) ^1/2 

D_k1=D_2max-2R_wmax+2 (R_wmax D_wmax/2) cos α _min+D_wmax+ ε ₂

In the formula: D_2max - maximum diameter of the outer ring raceway,

R_wmax - maximum groove curvature radius of the outer ring raceway,

α _min - minimum contact angle of the bearing,

ε ₂- Factor of safety,

ε ₂=0.3~0.8.

Considering various factors such as excessive underwear of the ring or deformation of the cage, it is required that D_k1<D_k. Only by meeting this equation can the designed cage outer diameter direction not get stuck, so that the steel ball can come into contact with the outer race under the maximum limit state of the outer race diameter, and the bearing can work normally.

Size of steel ball movement in the inner diameter direction of the cage pocket hole:

In the equation:

D_2min - minimum diameter of inner race raceway,

R_Nmax - maximum groove curvature radius of the inner race raceway,

ε ₁- Factor of safety,

ε ₁=0.2~0.6.

Considering various factors such as excessive underwear of the ring or deformation of the cage, it is required that d_k1>d_k. Only by meeting this equation can the designed cage inner diameter direction not get stuck, so that the steel ball can come into contact with the inner race under the minimum limit state of the inner race diameter, and the bearing can work normally.

(5) To ensure good elasticity at the locking points of the pocket hole, thin the upper and lower locking points of the pocket hole and design them at angles of α、β And a 25 ° angle.

(6) 

Outer angle of pocket hole

Inner angle of pocket hole

(6) To ensure uniform cooling of the cage after injection molding, eliminate plastic pits, reduce deformation, and reduce the weight of the cage, increase the limit speed of the bearing, save raw materials, and reduce costs. Therefore, while ensuring its strength, excavate the two end faces of the cage into groove shapes, with the following dimensions:

Outer diameter size of excavation groove: D_c1=P_c+0.25D_w,

Inner diameter size of excavation groove: d_c1=P_c-0.25D_w,

Excavation depth dimension: h=0.16D_w.

4 Conclusion

The main discussion was on the structure of the magneto bearing cage and the impact of different structures and materials on the performance of this type of bearing, and an improved design scheme for reinforced nylon cage was provided.

The improved reinforced nylon PA66-GF25 injection molded cage replaces the complex copper plate stamping cage. Reduced costs, improved production efficiency, and improved bearing performance. The change in product structure has enabled bearings to have excellent performance such as low noise, low temperature rise, high rotational speed, and strong impact resistance that the original structure did not possess.

More about KYOCM Self-aligning Ball Bearing:

Self-aligning ball bearings have two rows of balls, a common sphered raceway in the outer ring and two deep uninterrupted raceway grooves in the inner ring. They are available open or sealed. The bearings are insensitive to angular misalignment of the shaft relative to the housing, which can be caused, for example, by shaft deflection.

For dimensions and specifications, contact us now:

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