What's the point to use for calculating torque?

Hey there, I know I'm brand new here, but I've got a question that I'm trying to figure out. I'm not an engineer of any sort btw, just a humble millwright ;) So anyways, I'm working on a bit of a project in my head right now, and I need to know what distance to use when calculating torque. I have a 8 inch nut that needs to be tightened down on a shaft. Would I calculate my torque off of the OD of the nut, or would I measure off of the center of the nut? Any help would be muchly appreciated! Thanks a lot!
Austin

Hi Austin, and welcome to the forum. we are all humble here, well most of us are anyway. :D

It is not quite as simple as that. Do you know the load you want the nut to exert on the bolt when it is as tight as you need it? Using the thread Pitch, you will need to calculate the force required to stretch the bolt or crush a lock washer to provide the desired tightness.

Armed with that, you can work towards the length of a wrench for a human or machine to tighten it to said load. Use the center-line of the bolt (or nut, it is the same) and the point on the outer end of the wrench handle where the turning force (your hands) will be applied.

That might get you started. :)

"centerline of bolt" Yup, that's exactly what I needed. So, no, I have no idea about all the forces and pitches, I just know that the engineering department is supposed to be getting me a torque value in the next few days and I have to figure out a way to get the nut there. Somewhere around 1700 ft/lbs i believe ;) Anyways, I really appreciate the help!

[QUOTE=AustinS;2091]I just know that the engineering department is supposed to be getting me a torque value in the next few days and I have to figure out a way to get the nut there. Somewhere around 1700 ft/lbs i believe ;) Anyways, I really appreciate the help![/QUOTE]


At that torque, you’re going to need a long-strong cheater bar on your industrial torque wrench or apparatus.

If the part you’re torquing is seeing significant loading, particularly fatigue that is positive and negative at the nut, you may need to load it first with a known axial tension load - then torque the nut.

Also, if the time and money to torque is a consideration, consider a torque-multiplier. Just search the internet… Might be less expensive and faster than your special torque setup thingy.

Also, see:

[url]http://www.engineersedge.com/manufacturing_spec/torque_wrench_1.htm[/url]
[url]http://www.engineersedge.com/manufacturing_spec/torque_wrench_2.htm[/url]
&
[url]http://www.engineersedge.com/fastener_thread_menu.shtml[/url]

:grin:

An 8" nut... if we're talking steel or alloy... grade 8 or the like is going to require huge torque to seat it. I don't have a chart that goes that high handy, but just for an example... A 3" fine thread grade eight bolt has a recommended seating torque of 430,000 in-lbs. or 35,833 ft-lbs.

In a pinch you can go the long bar route but be very carefull. I've been part of a fifteen foot heavy wall tube slipped over a five foot pipe wrench tightening affair and it was a little spooky. Tack welded a couple of chunks on the end of the tube to keep the strap from slipping. Held the wrench in position with a long board until some load was on it then stood back and squinted real hard... About a thousand pounds in a basket with a strap on top on the forklift slowly lowered onto the end of the stick until the forks got loose. We call the wrench Big Red and it now has a slight curve to it but it held up!! Ridgid makes some tough tools I tell ya... Now had the wrench let loose... It could've been ugly. :surprise2:

Reminds me of a story...
A fellow at work has a brother that worked at the zoo. Apparently they had to weld some huge nuts used in a new orangutan cage they put together because one of the orangutan's kept loosening them up! So if you can't come up with enough oomph to tighten up your nuts you might consider renting Clyde...? :)

[QUOTE=AustinS;2091]"centerline of bolt" Yup, that's exactly what I needed.[/QUOTE]

Happy to get you started, but a few cautions here. First, 1700-ft-lb is a meek little load for a bolt that size, so the "wrench" probably will not need to be all that stout. A 1700-ft-lb is little more akin to hand tightening a 1/2"-UNC than a non-loosening and mechanically useful tightness for 8".

Second, heed Bob's (RJW..) cautionary tale. The correct tightening torque for an 8" bolt requires a prodigious amount of force and the weight of the "wrench" itself and the means to move it will be important as Bob points out.

If the Engineering department is providing the torque figure, they should also be designing the "wrench," rather than dumping that on you.

Many years back I was in Australia, and they had bolts and nuts about that size securing the base-ends of the Sydney Harbor Bridge to the concrete plinths. I often wondered what they used to tighten them, and what that torque figure might have been. :)