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View Full Version : Bolts, torque, and grease



Nick1911
03-13-2009, 01:00 AM
When doing machinery repair (in particular, automotive ), do you use any lubricant on the threads of fasteners during reassembly?

I've heard a LOT of different information on this. Notably:

"You should always use grease or motor oil on bolts, so they will come out without trouble next time."
"Anything other then dry threads will affect the clamping force at a given torque, and lead to over tightening."
"Only lubricate fasteners on the suspension/undercarriage, where fasteners experience dynamic loads and attack from the elements."
"Clean bolts with wire wheel, the use axle grease before reinstalling."
"Clean bolts with wire wheel, then install dry."

Personally, I clean them up with a wire wheel if corrosion is present, and almost always install dry to the spec'd torque. I was just wondering if there is an official stance on this. It does seem that lubricant on the threads will lead to over tightening due to lowering the coefficient of friction between the mating threads.

Thoughts?

J Tiers
03-13-2009, 01:13 AM
It messes up torquing, although not the "turn of the nut" method.

But for weather-exposed parts, "Never-Seez" or similar can be the difference between getting the thing apart, or going for the torch, later when you need to fix it.

Paul Alciatore
03-13-2009, 01:59 AM
What?

When you torque a bolt is not about the friction between the threads. If it was, why not just add sand or, better yet, weld the threads together to get the max torque possible? It is a way of making sure that there is a certain amount of preload (tension) on the bolt so that it will resist the forces that will be applied to it in use. Thus, head bolts on an engine must have enough preload to prevent the forces of combustion from lifting the head off of the block. So the head bolts must hold it down with MORE force than the burning gas mixture in the cylinder will push it up.

If you are going to get an accurate preload on the bolt, you must ELIMINATE the friction of the threads, not have it add to the torque that is actually due to stressing the bolt. Thus for the most accurate “torqueing”, the threads should be greased with the best possible, high-pressure lube.

By "the forces that will be applied to it in use" I do NOT mean resisting unscrewing from vibration or whatever. Lock washers and/or other locking devices should be used to prevent the bolts from backing out.

I was tempted to add a disclaimer about the real world vs. theory, but I will not. If the design engineer is ignorant to specify torque with dry threads, please do not use anything he/she designed, as it may be quite dangerous.

hardtail
03-13-2009, 02:20 AM
I would say it's application dependant, we use a wire wheel often but it's usually on 3/4" and bigger studs that have corrosion present, then usually apply some thread protectant or antiseize, a wire wheel can be damaging to threads though so is it a heat exchanger or aerospace your working on???? I would say for close tolerance stuff a tap and thread chaser on the offending parts and reassembled dry would be more appropriate but I would really gauge it by a case by case scenario.......Lots of the new auto stuff are stretch bolts also.....I never use grease.

oldtiffie
03-13-2009, 02:46 AM
This might help:
http://www.raskcycle.com/techtip/webdoc14.html

Forrest Addy
03-13-2009, 06:20 AM
I'm going out on a limb here. In the absense of contrary instruction threads to be tighted to a torque value should always be "lightly lubricated". I take this to mean a film of most any petrolem based lubricant should be casually applied to the threads just before engagement. The exception would be high pressure air and oxygen service where non-flammible lubricant is required y applicalbe code.

I've heard this talked to death, pro and con. Unless the threads to be torqued are prepared with some anti-friction dry coating, dry assembly it likely to result is erratic fastener tensioning. If some cases dry troque may result in thread damage via welding and/or glavanic or fret corrosion. In the absense of specific guidance from competent authority all threaded fasteners should be lubricated by some means before tightiening or torquiing.

Specific demanding situations where galling, corrosion, and chloride stress corrosion as in assembly fasteners used in outboard motor lower ends and similar trying applications may be better served by rejecting failed products and going to first principles; re-establishing fastener sealant/lubricant criteria. .

Anything sworn to by shade tree mechanics, auto parts counter yokels, good 'ol boys, or "fellers taught me" may not technically authorative.

I've assembled only about a zillion torqued assemblies at all levels of confidence and never ever have I assmbled any joint with dry threads even in nuclear power applications. Some form of lubrican has always been specified. It pays to look these things up. If there's no info available, the mechanic has to use his wits and work out his best path. That's said, no threaded fastener has failed due to lubricated assembly at full specified torque bit more than a few have failed because dry fasteners siezed and twisted off while being torqued. I recall a flathead Ford....

derekm
03-13-2009, 09:06 AM
I'm going out on a limb here. In the absense of contrary instruction threads to be tighted to a torque value should always be "lightly lubricated". I take this to mean a film of most any petrolem based lubricant should be casually applied to the threads just before engagement. The exception would be high pressure air and oxygen service where non-flammible lubricant is required y applicalbe code.

I've heard this talked to death, pro and con. Unless the threads to be torqued are prepared with some anti-friction dry coating, dry assembly it likely to result is erratic fastener tensioning. If some cases dry troque may result in thread damage via welding and/or glavanic or fret corrosion. In the absense of specific guidance from competent authority all threaded fasteners should be lubricated by some means before tightiening or torquiing.

...
Anything sworn to by shade tree mechanics, auto parts counter yokels, good 'ol boys, or "fellers taught me" may not technically authorative.

I've assembled only about a zillion torqued assemblies at all levels of confidence and never ever have I assmbled any joint with dry threads even in nuclear power applications. Some form of lubrican has always been specified. It pays to look these things up. If there's no info available, the mechanic has to use his wits and work out his best path. That's said, no threaded fastener has failed due to lubricated assembly at full specified torque bit more than a few have failed because dry fasteners siezed and twisted off while being torqued. I recall a flathead Ford....

Short answer, I agree.
Long answer.
The most serious torquing I did was on high pressure (5000psi) multistage feed pumps and even for temporary assembly for pressure testing you applied the anti-sieze mixture (graphite / moly stuff)...
on plated bolts you may find the rubbed off plating on the threads acts a lubricant of sorts.

On the "Good old boys" my father insisted his crews used molykote 321R
when anything serious was being torqued up on large steelwork assemblies.

btw This is the stuff used from cylinder head bolts to the assembly of the solid stage boosters for the shuttle (post-challenger) see this Nasa document (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900004345_1990004345.pdf)
I would understand that the focus on those assemblies post 1986 can be taken confidently as being well looked into :)

J Tiers
03-13-2009, 09:47 AM
Paul, you-all can do whatever you want. it's YOUR problem, not mine.

But if you check the "boltscience" website that was listed here in a recent thread, you will find out that the torque is very substantially "used up" in overcoming frictions inherent in the system, under the bolt head, and in the threads. It may be that 30-40% of the torque value is used in actually closing the joint, the rest in friction.

The link shown above also shows a significant difference, as will ANY reliable reference you can come up with.

Torque values generally DO specify lubed or not lubed. if not, then the assumption is usually made to accept whatever is on them as-shipped, but not to either add more lube, NOR to clean them off. Fasteners are commonly shipped with a light oil on them, if they are black oxided, or unplated. Plated (typically zinc) fasteners need no protective coating, and the plating can also provide some lube effect.

Now, the SPECIFIC case of Never-Seeze or the like is DIFFERENT. There is a medium-heavy body grease-like material, AND particles of graphite, copper, or nickel, typically, depending on the variety/brand.

The lube effect of that material is significant.

If you take a dry torque spec, and apply it to heavily lubricated bolts, you may be spending time extracting busted fasteners, or inadvertently torquing them to the yield point. While that may be a valid tightening method for some applications (high strength bolts in tower cranes) you may not want to tighten to yield with a standard "mush metal" or even a grade 5 bolt.

Richard Wilson
03-13-2009, 12:25 PM
When I was involved with erecting big steel bridges, we used High Strength Friction Grip bolts, which are tightened to yield (1 use only, if you take them out, throw them away), and the instructions with those were to lightly grease them with tallow.
The only threads I put together dry are those on gas bottles

Richard

Carld
03-13-2009, 12:34 PM
The fact is, FOLLOW THE INSTRUCTIONS IN THE FACTORY MANUAL. Sometimes you lube the threads and sometimes you don't. You have to do what the manufacturer says.

Sometimes you lube the head bolt threads and sometimes you don't. Sometimes you use the torque method and sometimes you use the torque/position method.

You can't assume to use dry or lubed threads, you have to know what they want it to be.

As I remember it, the torque is reduced by 10% when lube is used on the threads or you will over torque it.

Doozer
03-13-2009, 02:10 PM
Caroll Smith's book "Nuts, Bolts, Fasteners, and Plumbing" is a good book that covers this topic well.
--Doozer

cuslog
03-13-2009, 03:19 PM
Here's some interesting reading on the subject.
http://www.arpfasteners.com/Tech/TechInstall.html

Jpfalt
03-13-2009, 04:47 PM
This came from a box of machine screws:


"Increase torque until the fastener breaks. Then back off 10%."


It turns out the instruction was intended for use in setting up a clutch controlled screw gun.


In practice, unless you are using a torque wrench it doesn't make much difference. If you are using a torque wrench, then you probably read the instructions.

Jeffw5555
03-13-2009, 06:14 PM
All good responses to this post, but most missed out the OP's original question, which related to "automotive". Unless otherwise stated, automotive fasteners are always lubed with oil for the torque specs. I have seen some special fasteners in automotive applications that have a dry spec, but this is unusual....

Carld
03-13-2009, 11:12 PM
The only thing I can remember using oil on the threads was head bolts, rod bolts and main bolts. The rest of the engine bolts were put in dry. None of the other fastners on cars or trucks called for lubed threads that I can remember.

As I said before, you have to read the shop manual as to lubed or dry threads.

Peter.
03-14-2009, 06:19 AM
The rod cap bolts on by bike were supplied by Carllo with a tube of some kind of moly-sulphide grease to apply before fitting.

chief
03-14-2009, 08:18 AM
Common sense and some research are in order. Blanket statements pro or con are wrong. A few years ago I knew a man who oiled the engine mount bolts on a 900 Kawasaki, he then applied loctite to the nuts (both of these actions were spelled out in his service manual). He rode about three miles before the bolts loosen and vibrated out, he was almost killed when the engine decide it wanted to go sideways. The nuts were nylock titanium.

If you aren't sure,evaluate the following:
The fastener application (bolt ,stud)
Material of the fastener
through or blind hole
type of material threaded into (ferrous, non-ferrous)
temperature range environment the fastener will be subjected to.
Look at the link below:

NSTM (NAVAL SHIPS TECHNICAL MANUAL) 075 FASTENERS

http://www.hnsa.org/doc/nstm/ch075.pdf

J Tiers
03-14-2009, 12:47 PM
It is worth noting that the Navy torque table is 100% based on lubricated threads, per paragraph 075-4.3.2.2.4 b. They don't provide any information for unlubricated threads, which may be specified on some drawings.

The manual does provide a practical way of determining the proper torque in nearly every possible case for lubed threads.

Dawai
03-14-2009, 02:57 PM
Some people building fast engines have gone to relaxed versus stretched bolt lengths. Works if you know the bolts are a proper alloy and the stretch they work at best.

CHALK.. a excellent thread galler that works as good as locktite sometimes.

Wet threads, well I knew one certain hillbilly that built engines in the 70s and 80s.. I, I mean he, assembled a engine using STP on the rod cap bearings.. MOTOR started knocking after about ten laps drying the track. THE normal torque no longer mattered with that thick film taking up clearance. I've tapped them with a brass hammer ever since.