So, a bolt you tightened down completely failed and broke into two pieces. Why did it happen? Did you tighten it and use a torque wrench to insure the proper tightness?
The first things you need to determine include what size bolt, how many threads per inch and what grade of bolt. The grade of bolt is marked on the head of the bolt. The chart shown is for Grade, 2, 5 or 8.
The higher the number or grade, the stronger the bolt is. All bolt supply companies have torque value charts. Most bolts are zinc coated, some are chromed or black anodized. You need to look past the coating or finish for the grade markings. Did you know that the torque value shown is for a dry bolt? Will oil, assembly lube, Never Cease or a thread-locking compounds make a difference?
Some bolt companies now show a value for an oil lube on the threads, but what about the thread-locking compounds?
The torque value actually will allow the bolt to slightly stretch, which is a consideration in keeping any bolt in place and also tight. Bolts have a yield strength, which I like to refer to as its working strength. As long as you stay below the assigned torque number, the bolt will not deform; it will stretch slightly, but when loosened will return to its original length.
Go over the yield or torque and the bolt will stretch, retaining a longer length than it originally had. This is why it is so important to put the tightening load on the bolt for what it is designed.
For this test I am using three 3/8-16 Grade 5 bolts from the same make and same box. I first measured each bolt for original length. One bolt will be assembled together dry, the next with 30w oil on the threads and the last one with thread locking compound on the threads. I used as a spacer some 4130 tubing ¾-inch in outside diameter with a hole of .395-inch.
The bolt supply company gave me a spec of 33 foot-pounds of torque for dry bolts. They did not have a spec for oil lube on the threads. Most companies do have these torque specs, but for this article test I wanted to prove there is a difference.
After torqueing the dry bolt, the length stretched by .0012-inch. After disassembly, the bolt did go back to its original length.
Next, I assembled the oil-on-thread bolt and torqued it down to 33 foot-pounds with the torque wrench. This bolt stretched .00195 longer, and after disassembly it was still .00055-inch longer, which proves it actually went over the yield strength of the bolt, which now means it is damaged and will not be as strong as designed.
But what about the bolt with the thread locking compound? After assembly and torqueing to 33 foot-pounds, it stretched .00190 longer and after disassembly it maintained a .00050-inch longer length than it originally was. This meant this bolt suffered damage also.
Not always is this failure caused by improper assembly. Note: There are no-name cheap-shit, no mark of grade bolts out there for you to purchase—beware.
Here’s an example: On a small build of a race engine, the cam chain came with a new cam sprocket along with new cam-sprocket bolts. Their spec was 50 inch pounds of torque. First I torqued them at 30 inch pounds, then 40 inch pounds, before I was going to push the 50 inch pounds.
At 40 inch pounds one bolt just didn’t snug up and instead of continuing to tighten it, I decided to disassemble and inspect the bolt. In the picture you can clearly see the top bolt has been stretched and was close to breaking.
Instead of using any of the supplied fasteners with this kit, I decided to purchase new bolts from a reputable bolt company I used in the past. With the new bolts tightened up, I had peace of mind, while racing this engine.
Spend the extra few dollars, buy from a reputable bolt supply company and use the torques specs they recommend.
Often the owner’s manual has a spec of grade and torque, this is preferred because it is engineered for these numbers. Be sure in any assembly if it does NOT state to use lube, this means a DRY assembly.
Hopefully this will clear up why there are failures of bolts. Be sure to use a calibrated torque wrench, and the proper length and grade bolts have been used.
Here’s a source for testing your torque wrench:
http://www.wikihow.com/Calibrate-a-Torque-Wrench
Bikernet Tech Sources:
Brew Dude
http://www.brewracingframes.com/
2016 is our 42nd anniversary!
Let’s start with the English lesson: brew (broo). It’s a full fabrication and machine shop. We build quality handmade bicycle frames and frames available in road, track, ss, mtb, cx, and touring. We won numerous motorcycle shows, and we have full powder-coating facilities in house. We can build the bike of your dreams using quality products made in the USA. He does ceramic coatings, any color, and writes tech articles for The Horse, American Iron Garage, Ol Skool Rods and of course the badass Bikernet.com
Car Kulture Magazine
http://www.ckdelux.com
