UNC Thread vs. UNF Threaded Bolts

GA for a great link.....

re the linked ASM article - "but as a practical matter today, only 3- coarse (UNC), fine (UNF) and 8 thread (8-UN) are in common use". Surely your imported cars use primarily metric hardware?

LL, the ASM link is excellent, except for one minor exception... metric threads also exist on this planet :)

Metric also has "fine" & "coarse" threads & all the theory within the ASM article applies equally to Imperial (or American, depending on your point of view) & Metric threads (except of course that mertic & Imperial are not interchangeable). Well... you can often substitute a nut/bolt combo with a similsr size, but a metric bolt won't go into an imperial nut & vice versa.

Tony

The ASM Link leaves out a lot of important aspects.

Coarse Pitch :

- Lower Tensile stress area (OD-Pitch= ID is lower) - Preferable as ASM mentioned where the casing is thinner in section.

- Much higher thread strip area (If the usual mode of failure is thread strip then this is preferable - eg soft nut/casing/fasteners)

- Easier to tap.

- More tolerant to pre-drill oversize/ drilled hole shapes (obviously)

- The cross-thread assembly chances are much less, as is the thread damage during assembly and dismantling. And by extension better for fasteners/holes under regular assmebly/dismantling.

- Fatigue endurance limit much lower.

- Loosening under vibration more.

- Assembly faster.

- Number of threads when the threaded member is thin is less, hence not suitable.

- Fine adjstments are more difficult (eg jacking screws)

Some links, again not totally complete are here or check here in "Errors in textbooks"

Also I think machinery handbook gives some specific application guidelines.

Everything important relating to fastening was in the ASM article - except this:

Much higher thread strip area (If the usual mode of failure is thread strip then this is preferable - eg soft nut/casing/fasteners).

And for a fixed OD and penetration this is not universally the case. What is true is that for a fixed OD the fine thread has a stronger core; in addition there is a temptation to use too small a penetration depth.

For what it's worth, being a clumsy old ****, I've sheered of the heads of both coarse and fine screws, and in different circumstances also sheered the threads.

In my practical experience noting the main differences as UNF having less depth of thread allowing a larger minimum diameter equates to greater strength generally.

UNF are more difficult to find then UNC.

Over here, both are more difficult to find than metric.

In simple terms; leverage. You can put a lot more pressure with less torque with a fine thread vs. a coarse thread.

oilcan13