Tutorial 8 - Selection of Ball Bearings Goals: • Calculate rated loads of bearings • Select a bearing pack from the SKF catalog • Incorporate manufacturing concepts into bearing design.
Problem Statement Given: A gear is mounted on a 1.5 inch diameter shaft and will be supported by two deep groove roller bearings to ensure it meshes properly with a pinion. The bearings must support loads of 400 pounds each and a combined thrust force of 50 lbs. The shaft rotates at a maximum speed of 300 rev/min and must have an L10 life of 50 x 103 hours with a static safety factor of 1.5. Find: Select a bearing for this application.
General Comments A large number of bearing manufacturers are in existence which can provide a bearing for this application. While one could obtain additional bearing catalogs, this analysis is restricted to selecting a bearing from Table 13.6. The design approach is identical to selecting a bearing from any other manufacturer’s catalog, and is therefore representative of a designer’s task.
Calculation of Equivalent Load The equivalent load is P= 265 pounds See the next slide for details of the analysis!
Analysis Details The equivalent load is given by Equation (13.86) as P=XPr+YPa. X and Y are found from Table 13.8 to be 0.6 and 0.5, respectively, for a radial deep-groove ball bearing. The radial loads on each bearing are given as 400 pounds, and the total thrust load is 50 pounds. There is no indication that the thrust load is evenly shared or is entirely borne by one bearing. A worst case analysis would suggest that the situation where one bearing pack supports the entire thrust load should be analyzed. The equivalent load is then P=(0.6)(400lbs)+(0.5)(50lbs)=265 lbs
Bearing Selection A 6208 series bearing is best for this application. See the next slide for details of the analysis!
Analysis Details The safety factor forces us to consider only the bearing packs with basic load ratings of 1.5 x 265 lbs ~ 400 lbs. A number of bearing packs from the SKF catalog can support the basic load rating required in this application. Indeed, the loads are not very large for the size of shaft which is used. Often times shafts are designed so that the angular deflection is small under the torques experienced in application, and long shafts are quite large in diameter. Note that a 6404 series bearing, for example, has a sufficient load rating. But to fit this bearing onto the shaft would require machining the shaft to a 0.78 in diameter and then compromise the stiffness or strength of the shaft. Even if the bearing were located at the edge of the shaft, this is not a good design practice, as a scale drawing of the shaft will readily indicate. To fit onto the existing shaft, a bearing bore (db) of 1.5748 is selected. Of the bearings with this bore, the 6208 series bearing is the smallest bearing with a high enough basic load rating. In theory, there is nothing wrong with selecting a 1.7717 inch bore (or larger) bearing as long as a proper sleeve is provided for mounting the bearing onto the shaft. However, there is an added expense to this approach. Good design practice would now suggest that the shaft design be reviewed and modifications made, if practical, to allow rapid mounting and exposure of the bearing packs.
Concluding Remarks A common problem was encountered here, where small bore bearing packs would have sufficient capacity to support the design loads, but would have compromised the shaft on which they are mounted. This problem illustrated the notion that the mounting of bearings onto shafts must play an integral role in their selection.