Tech 101: Why Fastener Torque Matters "Güdentight" isn't a torque rating

[bigjohnsd]

Something a lot of weekend wrench-spinners overlook is “torquing” their nuts and bolts. Torque values for fasteners are called out in every shop manual…but does bolt torque matter? The answer may surprise you.

When tightened, fasteners act like a spring that squeeze two surfaces together to keep them from pulling apart (tension) and/or sliding past each other (shear). The key to understanding this concept is fasteners can handle about twice as much force when loaded in tension than in shear! That’s why critical engine fasteners such as connecting rod bolts and head studs are loaded in tension.

However, many fasteners on our cars are tasked with keeping parts from sliding past each other (wheel studs or strut bolts, for example), so how do we keep them from sliding? Making sure the bolts are tight enough! When a properly sized bolt is torqued to specification, the surfaces don’t slide, and the bolt is only loaded in tension (where it’s strongest). If the surfaces slide, the fastener is loaded in shear, and it can fail. As long as the static friction between the parts is greater than the shear force, the fastener is always loaded in tension…but if the fastener isn’t torqued enough and the parts slide, the fastener can break…and bad things happen.

https://www.hemmings.com/stories/why-fa ... e-matters/

[FJRoss]

Güdentight is adequate for most fasteners - coupled with years of experience working on machinery. I drag out the torque wrenches for bearings, pinch bolts, wheel nuts and most stuff on the engine. Not so much for drain plugs, body hardware, or even brake caliper bolts. Haven't broken a fastener, stripped a head, had something I couldn't get apart, or had something back out/fall off in years.

[wheatonFJR]

...
When tightened, fasteners act like a spring that squeeze two surfaces together to keep them from pulling apart (tension) and/or sliding past each other (shear). The key to understanding this concept is fasteners can handle about twice as much force when loaded in tension than in shear! That’s why critical engine fasteners such as connecting rod bolts and head studs are loaded in tension.

The bolded part is a misunderstanding or misstatement. BS.

All I know is that I use a torque wrench to make sure I DON'T overtorque something. Petey was fine with gudenteit. My gudenteit generally is 30-50% over. Except for the rear axle nut.

[natehawk750]

Güdentight is an acceptable torque as long as you know how to fix gûdentight mishaps with out drawing attention from management. As an aircraft mechanic torque values are an important aspect of my job.

Also just for fun one day we put to the test of how close we could get an axil nut (which is 1.75" in size) to the torque spec with just our bare hands. Final torque spec after the torque sequence is 450 in lbs. I came closest to final torque with 425 in lbs. or about 35.5 ft. Lbs.
I

[bigjohnsd]

Fun example of tension via stretch. I worked in a shop where we built steam turbines. As big as they get. The two shells of the turbine case were bolted together with studs that went through the flanges on the outside edge of the case. The studs were about 4 inches in diameter, maybe 3 or 4 feet long, nut on either end. They put a BIG micrometer across the ends, tightened them to a spec. That was my first intro to tension via stretch. Unrelated but equally fun to watch, was the REALLY big micrometers for the main turbine shaft measurements, because it took 2 or 3 men to carry them out to the workpiece.

[FJRoss]

Then you can get into the whole torque-to-angle and torque-to-yield thing...
https://www.felpro.com/technical/tecblo ... bolts.html

[Hppants]

I now use the torque wrench incessively and obsessively. If nothing else, it confirms that I've actually tightened the fasterner.

[raYzerman]

In my automotive career, it seemed to me the engineers set the torques a bit too high, seemingly taking them to some percentage (maybe 80%) of ultimate fastener failure. Of course, variations in torque guns was a problem with those air powered guns. These days they use dedicated DC electric guns and rotate them all through a regular calibration schedule. This solved most problems of stripping and breaking fasteners, but I still always felt some torques were still higher than they needed to be.

Now, we don't have all that, but if I'm using torque wrench, I will stop short of torquing "to spec". There's more than enough clamp load in the vast majority of cases. The torque spec is for a new unlubricated fastener (except where lube is specified and there are other exceptions), and many common fasteners you use are lubricated or contaminated with oil, etc. Prime example, caliper mounting bolts or a drain plug. Here you need to reduce the torque about 25%. Most won't take that into account.

About that FJR rear axle nut for example.... 90 ft. lbs. and can hardly get it off sometimes. All the bikes I've had with similar swingarm applications don't have torques that high. And it will likely be greasy lubricated AND it is a locking nut......

Oil pan drain plug.. 31 ft. lbs. Definitely had to be a dry torque for factory use. It's still too high IMHO because the crush washer crushes around 17. If you buy a Yamaha new, that first drain plug removal is a beatch!

There are situations where you really should leave the torque wrench in the toolbox. Experience helps... If your torque wrench has a tolerance of say 5-10%, then you should never add, but always subtract that 10%.

[Hppants]

My oil and rear end drain plugs are tightened to 20 ft. pds, but I use the torque wrench to check it. For Pants, it's cheaper that way (DAMHIK.... sigh)

[Festus]

I've built a lot of machinery in my working life. Extremely high speed machines with a lot of moving parts. For example, machines that were making 12,000 items per minute. It's so fast we had to use high speed cameras to fine tune things.

Only the gearboxes and internal components were ever torqued. I'd guess that 70% of the bolts were not torqued. Our machines ran and we were the industry leaders.

Don't get too worried about it.

[raYzerman]

My oil and rear end drain plugs are tightened to 20 ft. pds, but I use the torque wrench to check it. For Pants, it's cheaper that way (DAMHIK.... sigh)

You are aware the rear drive torque spec is 17 ft. lbs.?

[FJRoss]

My oil and rear end drain plugs are tightened to 20 ft. pds, but I use the torque wrench to check it. For Pants, it's cheaper that way (DAMHIK.... sigh)

You are aware the rear drive torque spec is 17 ft. lbs.?

And yet the same size bolt threaded into the oil drain is spec'd at 31 ft-lb (which we all know to be too high).
I haven't used a torque wrench on either. Maybe I will the next time just to see how close I have been.

[Road Runner]

All good stuff here. The only thing I worry about using a torque wrench on is internal engine, internal transmission, internal differential. Everything else I go by feel. I've been a tech all my life, so I have a good feel.

[raYzerman]

If you use the Yammy crush washers, once it's snug it you're basically done and the crush washer might just have reached the point of being fully crushed. If you crush it, then it's just a flat washer. Test it to 15, check it, then to 17 and check it to see if it's crushed (i.e., remove and look). Given that, I use the Honda aluminum washers and I can feel when they are snug enough. Basically saying you don't need a torque wrench on those fill and drain plugs.

[Hppants]

My oil and rear end drain plugs are tightened to 20 ft. pds, but I use the torque wrench to check it. For Pants, it's cheaper that way (DAMHIK.... sigh)

You are aware the rear drive torque spec is 17 ft. lbs.?

Just verified with the book. Huh - I guess I assumed wrongly that it would be the same as the oil pan. The silver lining is that I happen to have a spare rear end housing, should I strip the drain bolt. The far reaches of my dumbass are never ending.

Thank you, Ray. 17 it is for the rear end going forward. 20 for the oil pan. And torque wrench gets used, no exceptions.

[dcarver]

The bolded part is a misunderstanding or misstatement. BS.

Huh? My understanding is that torque is one method of creating tension in the bolt by stretching it. Not compressing it?

I can tell you that Westinghouse 4 loop nuclear reactor heads are tensioned by applying a hydraulic test rig on the stud then stretching it. The amount of stretch correlates to a torque value. The stretch is measured with a special digital depth mic. The stud has a machined hole drilled in the center that the depth mic measures. Multiple studs are simultaneously stretched, measured, then the nut is run down, essentially hand tight. Remove the hydraulic pressure, voila. Here's a good article for those truly interested. https://www.valvemagazine.com/articles/ ... nd-tension

In the Metrology lab, we calibrate force instrumentation (both compression and tension) using Morehouse Proving Rings. These are essentially a hollow circular blog of metal with an analog micrometer and reed valve in the middle. Newer ones use digital measurements.

As you compress the ring, it deflects. The amount of deflection is measured when the reed quits vibrating. The micrometer then measures the deflection. A quadratic equation is then used to determine the correlation between deflection and resultant force.

The highest force instrumentation calibrated in my lab was 100,000 pounds.

Here's a pic of Morehouse proving ring stand

And one of the proving ring

[dcarver]

Oh, BTW, the correct term is "Lb/Ft" NOT "Ft/Lbs"!

[FJRoss]

Oh, BTW, the correct term is "Lb/Ft" NOT "Ft/Lbs"!

Shouldn't it be Lb-Ft as opposed to Lb/Ft????

[dcarver]

Oh, BTW, the correct term is "Lb/Ft" NOT "Ft/Lbs"!

Shouldn't it be Lb-Ft as opposed to Lb/Ft????

Why YES, it should, and you PASS the TEST!

[wheatonFJR]

The bolded part is a misunderstanding or misstatement. BS.

Huh? My understanding is that torque is one method of creating tension in the bolt by stretching it. Not compressing it?

I agree of course that bolts are stretched when placed in tension up to the yield point. The bolt head and nut place a clamping force on the two or three plates there part of the connection. Simple stuff that a beginning engineer understands. That's not what I am calling out. Of course tension bolted connections can have a higher strength than a bolt in shear. However, the bolt doesnt get to decide whether its in shear or not. A shear connection is decided by the designer based on the structural needs. The crux of my critique is that in a shear connection, it can be resisted by either bolts in direct bolt shear or by bolts tensioned so as to clamp the plates together to resist slip between the plates involved in the connection. Typical structural bolts in direct shear will always have a higher capacity than the clamping force created by the tensioned bolts...as opposed to the general statement given in the quoted article.

But what do I care, I'm retired.