Use of class #3 male and class #5 female thread together question

[RickBall]

I work for a company that assembles wheel hubs (cast iron) and the design center specifies a class #5 male and class #3 female threaded hole together using a 5/8-11 thread, where the stud is roll threaded and the tapped hole is just that, tapped. Is this a normal combination where a tight fit is required. They also apply green Loctite to the stud before they screw it in. We are breaking studs off. Is this typical in industry to mix thread classes? Is there really a need for this match? Any suggestions to ease assembly? Thanks! Rick

[Kelly_Bramble]

Use of class #3 male and class #5 female thread together question

Absolutely not!

Thread types nor tolerance classes should ever be mixed. Class 3 with a class three and class 5 with a class 5 no exceptions if you want the threads to work as intended.

See:


Internal Thread Dimensions for Class 5 Interference-Fit Threads


External Threads Class 5 Interference-Fit Threads ANSI/ASME B1.12

[RickBall]

Hello Kelly. Thank you very much for your quick response. I didn't think so either, but Design is going to need something that says per batum that application. I did mess up and the material is cast iron, not steel, which I'm sure doesn't matter much. Also, I had the bolt calles switched around; it's class 5 for the bolt and class 3 for the hole. I went to your referenced websites and didn't see anything that Design will buy off on. I Googled it yesterday before I joined this forum, but I wasn't able to find anything to state what you and I both agree upon; don't mismatch the thread classes. Are you able to perhaps assist me further. it's much appreciated.
Have a great day! Rick

[Kelly_Bramble]

don't mismatch the thread classes.

I am always uncomfortable with statements such as "Where is the industry standard stating Thou Shalt Not" do whatever. With all due respect that is not the purpose of a specification or standard.

At this moment I'm going to assume that the designer knows something you and I don't and that they have some special application and fit in mind. Therefore, being open minded let me change parts of my previous forum post.

From:

"Thread types nor tolerance classes should ever be mixed."

To:

"If you want an external class 5 interference thread to function as intended by the controlling industry standard, mate it with an internal class 5 interference thread". And the same goes for a a class 3 V-thread.

Moreover, since this is a special mix of threads and you are breaking studs on installation I would suggest the design activity define on the assembly drawing a special installation procedure for these studs to prevent breakage.