AutomotiveDesign Improvement for Increasing Axial Load Capacity of Cylindrical Roller Bearings

# Design Improvement for Increasing Axial Load Capacity of Cylindrical Roller Bearings

For cylindrical roller bearings of NF, NJ, NUP and other structural forms with ribs on the inner and outer rings, the ribs can bear a certain axial load. Under normal circumstances, the load that the bearing can bear is based on the fatigue life during the operation of the bearing, but the axial load carrying capacity of the cylindrical roller bearing does not depend on the fatigue life, but on the strength of the rib and the rollers. The degree of heat generation of sliding friction between the end face and the rib of the ferrule. Especially for high-speed or high-vibration conditions, if the lubrication is poor or the impact is too large, it is more likely to cause wear and scratches on the contact surface between the rib and the roller, resulting in early failure of the bearing. Reasonable contact design can effectively improve the lubrication at the contact point between the roller and the rib, reduce the heating of the bearing and improve the bearing’s axial bearing capacity.

1 The maximum allowable axial load of cylindrical roller bearings with ribs Regardless of any working conditions, the axial load that cylindrical roller bearings with ribs can bear depends first on the strength of the ribs. In order to prevent the rib from breaking, the axial load should not exceed CAmax1 [1]. For 2 diameter series bearings CAmax1 = 0. 004 5D1. 5 ; ( 1) CAmax1 = 0 for other diameter series bearings. 002 3D1. 7. (2) CAmax1 = 0 for 2-diameter series bearings subjected to transient or impact axial loads. 013D1. 5 ; ( 3) CAmax1 = 0 for other diameter series bearings. 007D1. 7, (4) where: CAmax1 is the maximum axial load that can be endured to prevent the rib from breaking, kN; D is the outer diameter of the bearing, mm.

At the same time, the axial load that the bearing can bear also depends on the degree of contact friction and heat generation between the rib of the ring and the end face of the roller. Therefore, the following factors need to be considered for the allowable axial load of the bearing: (1) The contact area, machining accuracy and running-in degree between the roller end face and the rib contact surface; (2) The size and action time of the axial load; (3) ) Lubricant and lubrication method; ( 4) Bearing speed or sliding speed; ( 5) Cooling and heat dissipation conditions. The study found that there is a proportional relationship between the heat generation Q in sliding friction and the contact pressure P, the sliding velocity v and the sliding friction factor μ, that is, Q ∝ μPv . The dynamic axial load capacity is the allowable axial load value that can ensure the stable operation of the bearing under the condition that the lubrication is sufficient (viscosity ratio κ ≥2) and the bearing temperature is 60 °C higher than the ambient temperature. According to the size of the bearing surface heat loss AR ( AR = πB( D + d ) ), the maximum allowable dynamic axial load CAmax2 of the bearing is calculated

The additional cooling system can increase the allowable axial load to a certain extent, at this time, the maximum allowable dynamic axial load is taken as 1 for continuous load. 0, take as intermittent load 2． 0, when the impact load is taken as 3. 0; k1 is the lubrication coefficient, which is taken as 1 for oil lubrication. 5. Take it for grease lubrication 1． 0; C0 is the basic static load rating, kN; n is the rotational speed of the inner ring, r/min; d is the inner diameter of the bearing, mm;

B is the bearing width, mm; k2 is the lubrication coefficient, which is taken as 0 for oil lubrication. 15. Take 0 for grease lubrication. 1; Fr is the actual bearing radial load, kN; ΔTs is the temperature difference between the oil in and out of the axle box, °C; V·s is the oil flow through the bearing, L/min. From equations (5) to (7), it can be seen that the greater the radial load the bearing can bear, the greater the dynamic axial load it can bear. For low speed conditions, the allowable axial load of cylindrical roller bearings with ribs depends only on the strength of the ribs and does not exceed CAmax1. However, for other working conditions such as high speed or large vibration, the axial load carrying capacity of the bearing must also consider the strength of the rib and the contact lubrication between the rib and the roller, that is, compare CAmax1 and CAmax2 ( C’Amax2 ), take the two The minimum value of is the maximum allowable axial load of the bearing. The exact force of the rib can be calculated by software such as finite element analysis. In addition to considering factors such as the shape of the rib, contact method and lubrication, the axial load carrying capacity of the cylindrical roller bearing should also consider the influence of the size of the overrun groove between the rib and the raceway, the thickness of the rib, etc. Normal failure probability.

2 Improvement of the contact method between the roller and the rib In the general structural design of the cylindrical roller bearing, the end face of the roller and the rib are in plane-to-plane contact, as shown in Figure 1a. This structure can well correct the skew of the roller during the movement, but the plane contact sliding between the roller end face and the rib will generate large friction, which restricts the axial bearing capacity of the bearing. Cylindrical roller bearings used in railway vehicles, large vibration and other working conditions, use the formulas (5) to (7) to calculate the axial load capacity in the design.

force is estimated. In order to make the rib well bear the axial load when working at high speed, under the premise of ensuring the strength of the rib, the rib and the end face of the roller are respectively designed as a conical surface and a spherical base surface (Fig. 1b), so that the two The contact is changed from surface contact to point contact, thereby improving the force of the rib and the lubrication of the rib and the end face of the roller. At the same time, it avoids the possibility of arc-shaped rib (Fig. 2) during the processing of the flat rib, and eliminates the edge stress of the rib.

It should be noted that the dimension H1 of the contact position between the roller and the rib must satisfy H ＞ H1 ＞ S (preferably the contact point is in the middle of the rib), S is the radial maximum value of the overrun groove, and H is the rib. Side height (Figure 3, Figure 4). In this way, even if the rollers are slightly inclined, the formation of a lubricating oil film can be promoted, the sliding friction can be reduced, and the stress at the root of the rib can be reduced to a minimum. If the load acts on the edge of the rib or the edge of the overrun groove, there will be more contact stress and the optimal lubrication state of the contact will not be achieved.

At present, the inclined rib design of cylindrical roller bearings takes into account the axial bearing capacity, rotational speed and stability of the bearing. In the actual design, the radius Re of the base surface of the roller ball can be initially taken as Re = [( Dw /2) – H1]/sin θf, where (8): Dw is the diameter of the roller; H1 can be taken initially as the effective rib Half of the contact length, and take the minimum value between the inner and outer ribs. Due to the tolerance of Re and θf, in order to ensure that the contact point is within the required reasonable area, the contact position must be checked H1min = ( Dw /2) – Remax sin θfmax ＞ S, ( 9) H1max = ( Dw /2) – Remin sin θfmin ＜ H. ( 10) If the requirements are not met, Re can be adjusted until equations ( 9) to ( 10) are established.

## Conclusion

For occasions with high speed and a certain axial load, controlling the temperature rise of the bearing and improving the stress condition of the rib has always been a difficult problem to be solved urgently in the cylindrical roller bearing with rib. On the other hand, the rib and the end face of the roller are respectively designed as cone surfaces and spherical base surfaces with a certain angle, and the surface contact between the roller and the rib is changed to point contact, which not only reduces the contact area but also improves bearing lubrication. , reducing the temperature rise of the bearing and improving the axial bearing capacity of the cylindrical roller bearing.