Lecture 18 Combined axial and radial loads on bearings
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- Опубликовано: 29 окт 2021
- This video presents an approach for combining axial and radial loads into an equivalent radial load for the selection of rolling contact bearings. It also shows how one can use a spreadsheet to simplify the calculations.
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![Basics and Types of Bearings [Common Types]](http://i.ytimg.com/vi/FBXQ-Hc4g8c/mqdefault.jpg)
What we are going to do if we have only spur gears on our shaft, therefore there is no axial force acting on the bearings? As long as I understand we need to have axial force and calculate Fa/C0 ratio in order to continue with the bearing selection process.
You needn't make a correction if you do not have an axial load.
Can you share the catalogue you are looking into in this video ?
Basil...I am simply using the tables provided in the Shigley text book. I downloaded those tables from an electronic version of the book, then used graphgrabber to convert them into an excel file.
5:19 How they get the value of (Fe)/(V*Fr) though we don't know the equivalent radial load(Fe)? or can you explain process of experiment? from 5:19 to 6:02
Yet more info for you. You start with a guess. I always start with Fe=Fr. Then you select a C10. get the C0, and find an updated e by computing Fa/C0. You then select the X and Y values and update your bearing selection....you keep iterating until you get no change in the selected C10 value.
why we don't use vector addition to get equivalent load?
It would be totally sensible to do as you say if fatigue was not so emperical. But there are threshold loads below which your part will last indefinitely. So, the approach presented here is a 'design' approach to determine an equivalent radial load that would have the same lifetime. It comes about from a large number of experiments, not a first principles calculation. That said, you could do a thorough analysis and determine the contact loads in the bearings, then determine the endurance strength and compare the loads. The approach presented is just a simple, time-tested, approach to make it easier to design the bearing. It incorporates a fairly significant factor of safety, as do nearly all of our simplified design calculations. I encourage you to keep thinking as you do, question everything, and determine which topical areas you would like to pursue. It may be that you advance the field in a meaningful way.
I also forgot to mention that the selection tables are based entirely upon radial loads applied to the bearings. So, to use the bearing selection tables when you have a combined axial and radial load, you need to find an equivalent radial load. Think of this in the same way you use the von Mises criterion to find an equivalent uniaxial load from a multi-axial loading state. But in this case, you are using equations that are based upon experiments and curve fitting, not some meaningful mechanics calculation.
Hello I don’t understand how to get C10 search if the Fe is 2.876 how do you get 21.6622
I would really appreciate it if you explain !
This does seem a bit odd, I admit. But you need to select a dramatically over-rated bearing if you want a high lifetime with a high reliability. The tabulated values are only good for a 50% reliability. So, if I want, for instance, a 98% reliability of the bearing for the given lifetime, I need to select a much 'stronger' bearing. If you watch the lecture on bearing reliability shifting, it will begin to make sense. I hope!
I still dont understand how to get a radial load on this calculation
It is rather complicated. First, find the net radial reaction force acting at each bearing. If the outer ring is rotating, you correct that load using the velocity correction Vr=1.2. This creates an equivalant radial load. If that particular bearing is also asked to carry an axial load, you have to use the equivalent radial load equation where Fe=XVFr+YFa. But in order to know if you have to correct for the axial load, you need to know the static load rating of the bearing, C0, so you can calculate Fa/C0 and find the parameter e. e is a function of Fa/C0. It is best to plot it and find a curve fit. Then you calculate Fa/VFr. If Fa/VFr is greater than e, you have to use the equivalent radial load equation....so you have to find values for X and Y in the equation above. But you can't know C0 until you pick a bearing. So, you first pick a bearing ignoring the axial load...that gives you a C10 rating and a C0 value. You use that value of C0 to find e, you compare your normalized axial load to e, and then do the updated equivalent load calculation. And that requires you to find a new C10 and a new C)...it is an iterative process until the solution converges.
Sir, could you share the excel file that you programmed, thanks.
There is a tutorial video that explains what to do. I don't share the files because I often assign this task to my students!!