Over torqueing

[Ol'fogasaurus]

This subject came up in another discussion and has been bothering me... a lot!

In their design, fasteners have (maximum) torque reading which is equivalent to the amount of stretch the material/alloy (plus heat treating) can safely handle which is short of, but not that much shorter, of complete stretch causing failure. If you apply to much torque fastener should fail and that failure point is not limited only to the threads area too. Remember that during the making of the fastener the material is manipulated into shape and grain direction. There are basically two ways to form the threads which are cutting or rolling. Both of which changes the grain around the threads. Rolling compresses the grain which cutting (machine of tap and dye) cuts through the grain hence weakening the area somewhat. That is determined by depth of the thread and the thread count.

As fasteners age they often brittle and over torqueing can cause them to fail sooner.

There are also single use fasteners which are usually more common in studs than bolts. You see this more commonly in head studs where the unthreaded part of the studs are smaller in dia. which is usually because the block and heads are of different materials that expand and contract at different temperature and load limits.

The over torqueing is also a possibility in non threaded fasteners also.

The stretching of the threaded area in engine blocks is also a problem. I have seen several engines where the threads were damaged by cross-threading AND/OR the threads pulled out of the block/heads by tightening the fastener(s) to tight.

Also the head gasket part of the discussion can be found by looking it up.

This is nowhere near all there is about the problem but should give a starting idea of what can happen.

Lee

[Ol'fogasaurus]

Many years ago, probably back in the early to mid-60's, I was in a machine shop waiting for something and got to watch them chase the bolt holes in an old engine block (I don't remember the make). One of the things they also did was to check and re-CSK the holes slightly before they chased them. I asked them why and was told that when the bolts/studs were torqued the metal in the block would pull up so if they didn't do it the heads would/could not sit flat and they would get a bad/incomplete torque reading. I think they also said it had to cause limits in the head gasket also. A long time ago but what was said was reiterated later in the mid-70 when I was working and learning about thing in Aircraft.

It wasn't an all the time thing because it depended on what a specific fastener may be being attached into.

Lee

[theKbStockpiler]

Something I recently learned with lug nuts/bolts is that some manufactures, low end GM's and Korean , use a torque spec that is right up to the point that the fastener goes into the plastic phase. That's right! They are using garbage fasteners to hold the wheels on. Unless the fasteners are in good shape, they will stretch or twist being installed or removed. It could be a mistake ,but their lug studs have the strength of a grade 8 and most cars are way higher in the 10.9 range I believe.

Fasteners really should only have a 'lubricated' spec anyways but then someone would have to pay for the lubricant.

[Ol'fogasaurus]

I've been retired now for over 20 years (early retirement/long story) but if I remember correctly (over) lubricating (the material of the lubricant for example may not be proper for the application) can also be a negative in certain situations as it potentially allow the nut/fastener to back off even when properly stretched (torqued) to the proper spec.. Some fasteners come with a proper lubricant already applied.

Without going into names what you are saying about fasteners especially about age, # of cycles, heat in the area or applied directly, over tightening and the other causes of deterioration of fasteners is very close.

The box store stuff is hard to cover as you really don't know the spec or it's origin that the part was made to even if they give the grade (a long story here too). I wish I still had some of the documentation that I was required to work to but that was the company I worked for's stuff so I had to leave it there.

I just bought some fasteners (nuts and washer in this case) 1/4 X 20 but bought the smaller 7/16 washers as the hole was tighter to the bolt than the Box store's 1/4 washer was (a standard way of doing it for most stores) and the outside dia. was smaller (there was a reason for this). For a company to stock all the options of fasteners and related objects the cost would be prohibitive.

Lee

[theKbStockpiler]

The friction on the threads limits clamping force. The friction also limits force that would stretch the fastener. If you look up "clamping torque" , you should find guides that have yield strength, proof load and stuff like that. I don't think heat cycles come into play unless the fastener is going over 260 degrees Fahrenheit. I have bought new expensive speciality bolts and had them break off while still early in a torquing sequence so some of the issues with fasteners failing ,is in the manufacturing process.

[UKLuke72]

I've seen it estimated only around 10% of tightening torque is actually translated into putting tension onto the fastener when dry. Obviously this varies wildly with thread pitches, and fastener finish (bright zinc plated, cadmium plated and raw are the 3 most common in the UK)
Therefore the rest is lost in friction between the head of the bolt or nut-whichever is having the torque applied to it- and friction between the threads. Also, the use of washers, and their hardness all matters too.

For me, unless specifically instructed otherwise, I use a copper based anti seize on everything. It's the tensile elasticity in the fastener that keeps something tight more than it is the friction of the threads. It is easier to reach that level of stretch when lubed than it is when dry. Also, it minimizes any torsional stresses that can build up. If vibration is an issue, I'd rather use a locking compound instead of mechanical means. I don't like aero nuts, binx nuts, nylock nuts, spring washers etc. Nordlocks are good. as is old school locking wire though. Look at conrod bolts. Widely regarded as the most stressed part in an engine and no locking mechanism.

Experience, amassed from working in heavy industry, with steam, power generation, chemicals and heavy engineering. Oh and old VWs

[Ol'fogasaurus]

I worked in aircraft engineering as a draftsman ending up rated as a tech designer 6 (not enough schooling to go to a TD7 where I could sign in the engineering spot (and yes I was involved when CAD and CAM came into the forefront). I also was rated to sign the last spot on drawings after the engineer and supervision had signed looking for missed requirements or missed design requirements (such as an "Orthographic" design needing "Descriptive Geometry" to get things like dimensions right.

I looked up Nordlocks as I seem to remember hearing about them from the UK engineers and Techs I worked with. I don't think they would be allowed in Aircraft and for any of several reasons but in other applications they might be OK.

Anti-seize I have tried and not used again. I looked it up and "However, anti-seize is not suitable for fast-moving applications, as its high solids content can cause parts to jam. Yet unlike grease, the solid lubricant in anti-seize can withstand high temperatures and protect parts from galling and seizure even under extremely stressful environments."

Washers have their use and the large amount of styles can make a difference. They are usually designed either for general applications or for certain specific applications; the number of different designs for certain applications is shockingly great.

The stack limit to washers is 3. After that loss of contact stress coming from the head of the bolt and/or nut can happen especially during tightening when they want to move around a bit. After the 3 washer limit then spacers are usually required assuming you can't get the right sized and and length for either shanked fasteners (used for shear and tension/clamping applications) or fully threaded fasteners (designed primarily for clamping only applications). On studs where the threads at both ends are larger in dia. than the solid center area is usually means that this is either a single use application or there is a lot of movement between the bodies being clamped together (like the block and the head(s).

And that is only touching on the subject.

Lee

[theKbStockpiler]

On my own vehicles, I use something on every thread but I hand torque most of it. When fasteners are selected by a manufacturer, from what I have read, they want to be at 85 to 95 percent of the max torque before the fastener gets a small degree into the plastic phase. So 95 percent could actually be stretching the fastener and is not a good enough fastener for something I own. If a lube: like anti seize is used, it could put the fastener to over more than 150 percent. Well within the plastic range.

[Ol'fogasaurus]

On my own vehicles, I use something on every thread but I hand torque most of it. When fasteners are selected by a manufacturer, from what I have read, they want to be at 85 to 95 percent of the max torque before the fastener gets a small degree into the plastic phase. So 95 percent could actually be stretching the fastener and is not a good enough fastener for something I own. If a lube: like anti seize is used, it could put the fastener to over more than 150 percent. Well within the plastic range.

KB, it has been a long time since I checked drawing then checked or rechecked drawings (different types of skills used. I had to check for ANSME, MIL-standards and ISO (which is one of the international specs.) design specs. What you call the plastic stage is when the bolt (and nut) are stretched putting the loading on both the material which holds the clamping of the fasteners and what they are connected into and how well the grain of the fastener is correct to the spec.

There are two (as I remember) ways of doing the threads; one is cutting the threads by a tap or dye. The draw back here is the grain of the fastener is compromised as some of the grain is removed. The stronger way is to roll the threads which compacts the grain of the fastener. There are videos on the making of bolts on the web. Not sure about nut types of objects.

As soon as the fastener(s) start to firm up from tightening the load starts to stretch the fastener. The length of the fastener can determine some of the stretch (what I think you are calling the plastic phase). The torque rating on the fastener is somewhat less than the point of damage to the fastener but over-torqueing does affect the life of it hence reducing the strength/flexibility of the fastener. Age, heat et al also affects the life of the fastener or what the fastener is joined to/by.

Just had company show up so for now...

Lee

When I use the term drawings I am talking about drawing that will become blueprints.

Lee

[theKbStockpiler]

The main point is to keep the fastener in it's elastic range. If not it is permanently deformed"stretched".

[Ol'fogasaurus]

The main point is to keep the fastener in it's elastic range. If not it is permanently deformed"stretched".

Yes! And not to reuse them because of old age and/or being stretched. Fasteners are much cheaper to replace than having to replace expensive things like engines, trans and the like.

Lee

[Ol'fogasaurus]

One thing that was forgotten to be stated is that when torqueing a fastener using a torque wrench, you finish all of them in the series, let them sit for a short time then recheck the torque as sometimes you will see a change in your reading. Sometimes it is normal and sometimes it is not normal and maybe you have fasteners that are either aged or have been torqued too much too many times and they could be starting to fail.

Over torqueing does cause stretching and aging so that sometime, when torqueing the bolt fails and at a range usually way before the torque required.

Also remember the three thread showing rule which gets the "easy on taper" at the end of the bolt (not full threads) out of the torque range.

Lee

[david58]

Great post thanks for sharing this information