Optimmum thread design for tensile 2

[joeswoes]

Please help...

I assume that increasing the stress area of a thread increases its tensile force load capacity, so a UNF can withstand more tensile force than a UNC, but are other thread types better yet, such as acme or buttress ???

Thanks in advance to anyone who can help.

Joe

 

[butelja]

Is this a static fastening application or a dynamic lead screw that is used for actuation?

 

[joeswoes]

Static. The bolt/nut is resisting a hydraulic cylinder pushing down trying to seperate them.

 

[CoryPad]

Interesting question. I don't have a lot of experience with alternative thread forms, but here is what is written in Introduction to the Design and Behavior of Bolted Joints by Bickford.

"The ACME thread: this is used for power transmission, for example, to produce traversing motion on machine tools."

"The Buttress thread: used when thrust on the screw is in one direction only, for example, airplane propeller hubs and columns for large presses."

I know Buttress threads are used in the petroleum piping industry to prevent leaks.

I would recommend staying with the standard 60 degree thread form, and you can use fine pitch to increase the stress area.

 

[bobc]

i have spent most of my career working on large presses and associated machinery(4000 to 12000 ton presses) and almost all areas of high stress resistance or reaction, such as columns and pull back rods, rely on a buttress style thread connection.
hope this helps,
bob creely

 

[joeswoes]

Since buttress threaded bolts must be machined from bar stock, what material could be used to make buttress threaded bolts and subsequent heat treatment to attain a 130,000 tensile yield and 12% elongation (ie.. SAE Grade 8)?

Your help is most appreciated.

 

[butelja]

What size range are the bolts to be? The choices are more limited as the size gets larger if you need 130 KSI tensile strength.

 

[Hush]

I haven't plugged through an formulas to confirm it but as I understand it, in general, the smaller your thread angle the better your thread strength. I.E. square thread is stronger than 10 degree butress which is stronger than ACME which is stronger than UN. The trade off is the larger your thread angle the easier it is to machine. If this is wrong please feel free to jump all over it.

As to material if your thread diameter is small enough 4140 Q&T to your required minimums should be readily available.

 

[bobc]

buttress threading systems can range from "0" degree to i believe "13" degree pressure angle, so i wonder about the actual thread strength. buttress threads are single directional, and i believe stronger by far than square in a single direction application.
square threads are dual directional, general purpose and considered to be the strongest in that category.
having machined both types i agree the buttress is easier to machine on a lathe. i cannot testify about thread milling though, as i have no experience in this area.