Fine thread bolts - what advantage?

[Ford850]

What advantage do fine thread bolts have over coarse threads? I notice on most old implements, the bolts are fine thread rather than coarse. I'm working on an old plow now and have to replace quite a few bolts as I go. They are various sizes ranging from 3/8 up to 3/4. I can buy coarse thread grade 5 or 8 at TSC, which is handy. But if I want to use fine thread I have to make a 30 mile trip to a dealer each time I need a few.

 

[Hooked_on_HP]

A fine thread bolt has more clamping power for lack of a better term. If you put a 6" wrench on a fine thread bolt and a corse thread and pull with the same force the fine thread will be tighter because of the pitch of the threads. Also fine thread wont vibrate loose as easy.
Bill

 

[jejeosborne]

A fine threaded nut/bolt will be stronger under tension. In addition, a fine thread will allow you to apply more pressure to the joint in tension since the fine thread requires more turns when mating your joint (acts like a lower gear). If your joint is a shear joint, the fine may be less important but you still won't get the joint pinched as tight using course threads. If threading into aluminum or something soft, a course thread is stronger.

Hooked is a faster typer.

 

[SPYDERLK]

A fine threaded nut/bolt will be stronger under tension. In addition, a fine thread will allow you to apply more pressure to the joint in tension since the fine thread requires more turns when mating your joint (acts like a lower gear). If your joint is a shear joint, the fine may be less important but you still won't get the joint pinched as tight using course threads. If threading into aluminum or something soft, a course thread is stronger.

Hooked is a faster typer.

This is an important point added to what Hooked said. [color added] There is a greater stud diameter in the root of a fine thread as compared to equal sized coarse. The fine bolt is stronger. Fine threads require more care in assembly and disassembly however to avoid damage by physical abuse or included dirt.
larry

 

[ampsucker]

i'll also add that fine threaded bolts can be much more difficult to remove when rusted/dirty due to the threads being more easily contaminated. i usually end up twisting or cutting them off if they have much rust.

modern machining techniques allow for more precision in fasteners. i prefer a corse threaded grade 8 bolt with lock washer for most ground engaging equipment. use a little anti-seize compound when putting it together and you'll have no problem doing maintenance and the lock washers or star washers should keep them from vibrating loose.

if you have a really bad problem with vibration (aside from fixing the source of it if possible) you can double nut any bolts that keep vibrating loose even with lock washers.

amp

 

[Redneck in training]

Superbolt, Inc. Products - Multi-Jackbolt Nut and Bolt style Tensioners
Fine thread impleneted in large diameter bolts and nuts often used on large turbines.

 

[Shmudda]

Fine threaded bolts have one advanage and one only, you can torque them tighter and to a higher preload plain and simple.

Torque is torque, no matter if its course threads or fine threads, and the bolt preload is all the same. Why can a fastener with fine threads take more more torque? Because there's more threads spreading the load out over more thread area. When your torque increases your bolt preload and tension increases and thus the friction between the (2) pieces being bolted increases. Remember, other then when a bolt is used in a shear application, you are relying on the friction of the mating surfaces to keep everything together, the bolt is there just to create the friction and clamping force.

Craig

 

[Redneck in training]

Fine threaded bolts have one advanage and one only, you can torque them tighter and to a higher preload plain and simple.

Torque is torque, no matter if its course threads or fine threads, and the bolt preload is all the same. Why can a fastener with fine threads take more more torque? Because there's more threads spreading the load out over more thread area. When your torque increases your bolt preload and tension increases and thus the friction between the (2) pieces being bolted increases. Remember, other then when a bolt is used in a shear application, you are relying on the friction of the mating surfaces to keep everything together, the bolt is there just to create the friction and clamping force.

Craig

It is like going uphill with your tractor pulling certain load at certain speed and maximum power. If the slope is half you will be able to pull twice the load.
The load is the equivalent of bolt preload.

 

[ultrarunner]

In the machine shop, as a rule, we always used coarse threads for bolts going into castings and fine threads when using nuts...

 

[MJPetersen]

Fine Thread with a nut when very high clamping action is needed and you do not want it to vibrate or work loose. For example the cutting edges, corner bits and track pads of a D8 CAT. Never seize and a torque multiplier for the pad bolts.

Mike