Contact Fatigue | Life - Surface Strength

Contact Fatigue | Life - Surface Strength - Hardness Relationships

Рет қаралды 7,150

6 жыл бұрын

Here the phenomenon of surface fatigue resulting from contact stress is presented and explained. The reason why surface fatigue results in surface pitting is shown in terms of the subsurface location of worst-case stress. It is discussed how the contact area increases with increasing force, leading to a non-linear relationship between force and stress. The two alternatives for the application of a design factor (i.e. with respect to force v. with respect to stress) are demonstrated. The meaning and usage of each of the terms in the equation for surface stress are discussed, including the AGMA elastic coefficient Cp. An example problem is completed in which the minimum allowable surface hardness is found for a cylindrical elliptical cam where the desired life, the load (via spring deflection), and the geometry are given. The problem is solved for both the case where the design factor is with respect to force, and where the design factor is with respect to stress.
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This lecture was presented on February 21, 2018. All retainable rights are claimed by Michael Swanbom.
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Пікірлер: 23

@cipaisone 4 ай бұрын

You have a great teaching style.

@TheBomPE 3 ай бұрын

I'm glad you like my material! Thanks for watching!

@bo-bx5hn 4 жыл бұрын

So basically you're 45.62X more helpful than the design teacher I had. Don't ask how I derived that, it's not from shigleys.

@TheBomPE 4 жыл бұрын

I knew a professor once who would write IOCO (intuitively obvious to the casual observer) whenever he didn't have a reference to justify one of his steps. I'm not saying that applies here...ha ha. I'm glad it had an improved helpfulness factor, thanks for watching!

@user-wb3xw8vo1u 3 жыл бұрын

You are a great teacher. That's a huge help for my degree thesis.

@TheBomPE 3 жыл бұрын

I'm glad you find it useful! Thanks for the encouragement!

@raghavendarranganathan102 Жыл бұрын

Nice lecture and illustration. is there a general equation to calculate cycles to failure for different types of contact surfaces? I am looking into a bearing in a raceway of a ball screw and I am interested in identifying the contact fatigue of the screw and nut and not the ball. Particularly for the screw, I want to find the number of cycles to failure for a given contact pressure max.

@sambhavkharel9194 4 жыл бұрын

How we do differentiate (in real-life cases) whether to guard against excessive force or against excessive stress? Required Hardnesses differ significantly from one another. P.S. Extremely thankful for your wonderful lectures, sir. Your lectures have turned very very effective for me.

@TheBomPE 4 жыл бұрын

The difference between the two ways of applying the safety factor has more to do with how you'd like to interpret it. If you use the one focused on load, the safety factor represents how many times (as a multiplier, not a number of cycles) the amount of load you used in your calculation could hypothetically be applied before causing an issue. If you use the one focused on stress, it represents how many times the amount of stress could be applied before causing an issue. In many engineering analyses, these two FS numbers are actually the same, but due to the nonlinearity introduced by the variable contact area, they are different in Hertzian contact. I suspect that it is more common that designers would be interested in the one focused on load more than the one focused on stress in most cases. It seems to me to be more directly useful to most design questions.

@jamescarruthers1967 Жыл бұрын

Do you know why the max contact stress is subsurface? Is it some sort of stress lensing effect? (Not able to watch the whole video right now, but got up to ~10min)

@TheBomPE Жыл бұрын

Basically, max shear stress (like from a Mohr circle perspective) happens beneath the surface because the lateral normal compressive stresses drop off more quickly than the normal compressive stresses as you move from the surface toward the center. This article shows the math and the plots: analysischamp.com/stressbook/BS02-19.HTM

@solrochman1323 2 жыл бұрын

I’m getting quite a bit out of watching your videos. Thanks for posting them. Question: what computer program do you use for your presentations? It appears to be an excellent tool.

@TheBomPE 2 жыл бұрын

I'm glad you are enjoying the videos! I use a Fujitsu T901 with an i7 and 16GB of ram running win7 and onenote 2007. I have newer hardware and software, but it doesn't work as well. Thanks for watching!

@joellasseigne4652 5 жыл бұрын

WHAT IF THE CAM HAS A CONCAVE SHAPE. OR what if the we are looking at a male cone compressing against a female cone creating a line contact with each other?

@TheBomPE 5 жыл бұрын

You might find this article a helpful starting point (look under the heading "Hertz contact stress") en.wikipedia.org/wiki/Bearing_pressure If you want some more depth: iieta.org/sites/default/files/Journals/MMC/MMC_B/86.2_12.pdf You might also want to check this one out :) newrepublic.com/article/117390/netiquette-capitalization-how-caps-became-code-yelling

@CuriousEngineer955 3 жыл бұрын

so we got a low value compared with the hardness of the follower. So I think the cam surface will damage first. what if we have the same hardness values on both of the cam and follower, is it leads to overdesign?

@TheBomPE 3 жыл бұрын

That is right, the cam has to be harder than the follower to achieve the same design factors. This is due to the tighter radius of curvature on the cam. Thanks for watching!

@CuriousEngineer955 3 жыл бұрын

@@TheBomPE Thank you for reaching back and clarifying the doubt.

@enggfundas2937 3 жыл бұрын

Hello Sir, One question - what is the criterion to consider the problem falls under 'contact stresses' analysis and not regular compression stresses? I hope I could put my question in proper words.

@TheBomPE 3 жыл бұрын

There are often high stress gradients right around where two bodies press against each other. The "contact stresses" I discuss in this lecture include an attempt to model those gradients. Generally when thinking of plain compressive stress, you aren't trying to deal with strong gradients.

@enggfundas2937 3 жыл бұрын

@@TheBomPE Thank you for answer. I liked your lecture very much. Lot of solved examples make it better.

@davidkoper6146 3 жыл бұрын

What textbook are you using for this?

@TheBomPE 3 жыл бұрын

I use Shigley's Mechanical Engineering Design, 10th edition for this course, so that is what I use in this video. For this topic, I would say Shigley's does not do a very good job though, unfortunately.