Worn rear axle threads

[Pete]

Originally Posted by Joe K

Pete's instruction seem generally spot on - he's welding with material with 100K effective tensile strength (which is probably 80K when the previous 80 percent of yield is considered.) Still, pretty good overlay, probably better than the original.

However, I might not "cut" the thread - the original thread was "rolled." Instructions today for refreshing the thread on the axle generally go to a "thread reformer" of which there are several versions around, and all do not cut thread but rather "mush" it back into dimensional conformance.

Rolled thread tends to be stronger than cut - which is why most grade 8 bolts today incorporate rolled thread. In fact MOST commercial bolts today incorporate rolled thread since that allows selection of a cheaper steel and yet attain the necessary ANSI property as a bolt.

I have "cleaned" up an axle thread using a "little giant" tap & die - a die system which allows adjustment of the die cut. I start out "loose" and cut the minimum amount of thread necessary to get the nut to start - then use the nut to "form" the thread.

NOW I have a clamp-style thread reformer. Much superior and less chance of crossing the thread with the die.

Joe K

I might say ALL STANDARD commercial threads of the type encountered in automotive use, have rolled threads now.
NOT because they are stronger, (they aren't) but because they are FAR cheaper to produce.
Here is a short paragraph from one of the biggest fastener manufacturers in the USA. Make note of the last sentence.

"Strength of Rolled and Cut Threads

In the past, our customers have asked whether rolled thread bolts are weaker products due to their smaller bar diameter. A threaded bar breaks at its weakest point, which is in the root of the thread, and because the thread root diameter is smaller than the bar stock on cut or rolled thread products, the bar diameter is not relevant when considering tensile strength. What is relevant is the work that has been performed in the weakest point on the threaded product. For a cut thread bolt, no work has been done on the material. Metal removal is used to form the thread. For a rolled thread bolt, the threaded section is formed by metal movement during the cold forming process. This process is what makes the rolled threads stronger during a tensile test. However, it is important to note that heat treated cut thread fasteners, such as our 1-1/2” A325 bolts, will perform at the same level as any rolled thread bolt, because the heat treatment will determine the strength of the bolt as opposed to the method of forming them."

[Joe K]

Not to dispute the bolt manufacturer, the failure phenomenon associated with square discontinuities (like the root of a thread) is known as "stress concentration."

For a simple tensile test, what you and the bolt manufacturers say is true. However, bolting is rarely tensile loading alone - generally there is some cyclic function as well - which is where notch sensitivity has its application.

Described well at Wikipedia https://en.wikipedia.org/wiki/Stress_concentration

The feeling generally in engineering (yes I think I can say that) is that rolled threads are preferred design construction. And I can't think of anything more predictably cyclic than the threads on a Model A rear axle.

Rolled threads difficult for most repair shops to produce - at least I haven't seen a thread rolling machine recently.

Look to the failure of those axle threads you have no doubt repaired. The thread fails at it's root - not across the minimum section.

It is possible to buy "rounded" form thread dies. Very pricey. Probably better to upgrade the metallurgy (as you have done) and cut threads with a die or single point tool in a lathe with the sharpness of the point "eased." (Three strokes with a whetstone on each the two corners and its is done.)

Joe K

[Pete]

Originally Posted by Joe K

Not to dispute the bolt manufacturer, the failure phenomenon associated with square discontinuities (like the root of a thread) is known as "stress concentration."

For a simple tensile test, what you and the bolt manufacturers say is true. However, bolting is rarely tensile loading alone - generally there is some cyclic function as well - which is where notch sensitivity has its application.

Described well at Wikipedia https://en.wikipedia.org/wiki/Stress_concentration

The feeling generally in engineering (yes I think I can say that) is that rolled threads are preferred design construction. And I can't think of anything more predictably cyclic than the threads on a Model A rear axle.

Rolled threads difficult for most repair shops to produce - at least I haven't seen a thread rolling machine recently.

Look to the failure of those axle threads you have no doubt repaired. The thread fails at it's root - not across the minimum section.

It is possible to buy "rounded" form thread dies. Very pricey. Probably better to upgrade the metallurgy (as you have done) and cut threads with a die or single point tool in a lathe with the sharpness of the point "eased." (Three strokes with a whetstone on each the two corners and its is done.)

Joe K

Yup.