SAE Grade-8 bolt snapped @ 135ft/lbs

By Lawrence (*****)

Date 03-09-2005 02:35

This article and chart may be of interest. I learned a couple things.

http://www.zerofast.com/torque.htm

By Sand Man (**)

Date 03-09-2005 03:07

My book shows a standard dry torque of 119 ftlbs for 1/2 grade 8.
Jeff

By OSUtigger (**)

Date 03-09-2005 03:21

Metalman,

The link Lawrence provided is a great tool, but also try to remember why those numbers are there. For example, a grade 8 can hold more than a grade 5 because it is stiffer. Now, I'm not sure if the alloying elements are changed, but even if they are, I think that this stiffening does not raise tensile by as much as it raises yield, and therefore yield is now considerably closer to tensile. Basically, while a grade 5 would stretch considerably if put under extreme load, an 8 would break at about the same time it started to stretch (albeit at a higher internal stress). Perhaps your torque specs are taking it to this point (I noticed 131 ftlbs for a 1/2 inch on the sheet), and a well lubricated bolt does not read near as much torque as a non lubricated one, and, bam, all the torque goes into stretching the bolt, resulting in a grade 8 to fail while a 5 would have had that sluggish feel to it by that time that would tell you that it was about to break.

These differences would be similar to a rubber strap and a piece of glass. The rubber is going to stretch and you will be able to notice it stretching a little too far, but the glass will hold until it shatters without warning.

Also, remember that those numbers on that page are for tension. If you are cantilevering using bolts as the shear points, you have to factor that into the overall stress, which can get quite large.

I realize this is more than you actually asked for, but I hope it helps anyway...

gls

By MetalMan Date 03-09-2005 03:25

Thank you all for the information. I'll be putting it to good use.

By billvanderhoof (****)

Date 03-09-2005 06:38

It's also important that the surfaces that the bolt head and nut bear on are exactly perpendicular to the bore. Otherwise the bolt is forced to bend and stress is increased.

Bolt stretch is a better measure of stress than torque. Thus torque plus turn might be a better method if this is that critical.
Bill

By thcqci

Date 03-09-2005 13:38

Agreed. Those of us in structural work are used to using Skidmores which simulate the joint and help determine the torque a bolt assembly (bolt, nut & washer[s]) should be loaded to to obtain a given tension. Different lots of bolt assemlies can be significantly different. Lubrication (or lack thereof) makes a big difference.

We don't use torque wrench for tensioning bolts here (don't do many tensioned bolts period). We use turn-of-the-nut or TC bolts when tensioning. But in past lives early in my steel inspection career, I was asked to take a torque wrench out and check tension on bolts. While it may have confirmed that bolts had been tightened, as opposed to being left loose, I now know it told me little else. Further, looking back now I wonder if that torque wrench was even calibrated.

By R. Johnson (**)

Date 03-09-2005 12:59

With the torquing of any bolt, torque does not necessarily equate to clamping force. You can get high torque value if the bolts and nuts have been coated or galvanized, the class of threads, the thread surface roughness and whether the bolts and nuts have been lubricated. I assume this is a structural type of application, do you have washers under the turned element. High strength bolts like a grade 8 can be much harder that the base metal you are connecting and the turned element (nut or bolt) can actuall start milling the surface increasing the torque value you are obtaining without increasing clamping force.
There are devices that can measure clamping force and you can determine a more precise torque value for the actual bolt and nut combination that you are using by test your combination in such a device.

By TOMWELDS Date 03-21-2005 06:50

Being a true American, are these american made bolts? (quality!!)