Proper Torque and lubrication - nuts and bolts

[Gobi King]

I read that in order to achieve proper torque on a bolt the threads have to lubricated. Is this true?

I had to put a towing hitch ball for my Transit Van and the instructions with the ball said 250 lb/ft and DO NOT lubricate the threads.

Why not lube the threads, won't that help achieve proper torque?

[Tiddlerbasher]

In general if you lube the threads you should lower the applied torque. Sometimes to as much as half the rating - you can dowload (from the net) various toque charts like this one (google is your friend ):
https://www.quora.com/Is-there-a-rule-of-thumb-on-how-much-you-should-torque-a-certain-size-bolt-or-nut

[Keta]

Like Tidder said, you can find both dry and wet torque specs.  I have a 250# torque wrench but usually use a large box end wrench with a cheater on it to tighten my trailer balls.  A guess would be I'm getting them around 150# but I have never had one come loose, I should check one of these days.  It takes all I have to do 200# even with pockets full of rocks.

[Gobi King]

Ah, it is all about the tension,
So with lube a lower torque is required. Makes sense.

What is the conversion formula approximately?

[Tiddlerbasher]

seach for a torque chart that describes your type of screw thread. There may be a formula but I've never seen or used one - I just use the torque charts.

[Gobi King]

Found it, around 30% less for lubed
here is the chart and formula

Lubricant   Torque Reduction
(%)
No lube   0
Graphite   50 - 55
White Grease   35 - 45
SAE 30 oil   35 - 45
SAE 40 oil   30 - 40
Example - Reduction of Torque when Bolt is Lubricated
The maximum tightening torque for a slightly lubricated 1" Grade 5 coarse bolt is 483 lbf ft. Dry bolt torque is approximately 30% higher - or 628 lbf ft.

Tdry = (483 lbf ft) (1 + (30%) / (100%))

   = 628 lbf ft

If the bolt is lubricated with SAE 30 oil - the torque compared to a dry bolt is reduced with approximately 40%.

TSAE30 = (628 lbf ft) (1 - (40%) / (100%))

   = 377 lbf ft

Note that if torque specified for a dry or slightly oiled bolt torque is applied to a lubricated bolt  - the bolt may overload and break.

[Gobi King]

Side note:

Oil pan drain bolt have 2O lb/ft for aluminum ones and
60 lb/ft for steel,
as the threads on the oil pan has lube in it, I am assuming the spec is wet torque.

[Keta]

Do you have or have access to a 250# torque wrench?    I have a Snap-On 50# and 150# and a Proto 250# but have no use for the larger ones any more and should sell them but won't.  I could use a good inch pound one now.

[RowdyW]

I only use a torque wrench for head bolts & crank & rod bolts. All other torques are built into my right arm. For very large bolts & nuts the torque is built into both arms, legs, & back, with a good foot brace.       Rudy

[Keta]

I worked on steam turbines and we needed to use the proper torque.  We had hot and cold specs too.

[jurelometer]

I'm not a mechanical engineer, but I think it works like this:

Torque is the force required to achieve the goals,  not the goal itself.  There are two goals here.   

The first is clamping force - the amount of force holding the parts together as a result of tightening the nut.

The second is static friction  - the amount of force required to start the nut turning to loosen it.  It needs to be high enough to prevent the nut from backing off    - usually due to vibration or cyclical load.

Friction is a function of force (pushing the thread surfaces against each other) and the coefficient of friction (how slippery the two thread surfaces are).   Lubrication  will decrease the coefficient of friction.   This means that for the same amount of torque, you will get more clamping force but less friction keeping the nut from backing off.   Lubricant also tends to help ensure that the clamping force will be more predictable for a given torque, which might be useful for things llike bolting a head onto an engine block and not ending up with some warping causing a gasket leak.

Depending on the application, the ratio of clamping to frictional force might be important, and you don't want to mess with it by not following the recommendations for lubricating/not lubricating and which lubricant to use.

Just guessing, but with a trailer ball,  the thread size is fairly large, so it will probably take a large amount of torque to achieve the desired friction. Plus when the trailer swings when you turn, the receiver will be applying rotational force on the ball,  creating a risk of tightening and loosening the threads.   Getting the exact clamping force is probably less important here than having a reliable amount of friction that will keep the nut from getting loose.  Without lubricant, the static coefficient of friction will be higher relative to the dynamic coefficient of friction, which means it will be harder to break loose for the same amount of  force used to tighten it.   


Or something like that. .     The bottom line is that these engineering folk are not all brilliant, but they have usually been taught the right way to determine the forces involved, and we haven 't.    The odds are probably more in our favor when we follow the instructions.

-J

[RowdyW]

Thats why they make lock washers.

[Gobi King]

Do you have or have access to a 250# torque wrench?    I have a Snap-On 50# and 150# and a Proto 250# but have no use for the larger ones any more and should sell them but won't.  I could use a good inch pound one now.

Lee,

Yes Sir,
I have this https://www.jbtools.com/precision-instruments-c3fr250f-torque-wrench-split-beam-1-2-drive-50-250-ft-lb-flex-ratchet

But this is how I torque'd my trailer ball [no laughs pls]

I weight 207 lbs
The wrench I used was 14 inches long, I turned the hitch sideways and put on the wrench and stepped on it and it tighten a bit with one leg on the ground, then a few more grunts and mini jumps on the wrench, voila!

Rowdy, yes it has a locking washer

J, hmm, no lube so it binds properly (i.e. static ....friction)

Good to know, I will keep an eye on it, I am pulling my 2500 lb boat and trailer with it. I will lube the hitch ball with heavy grease.

I will put some witness marks on the ball and the base, that will tell me if it is looks and rotating

[jurelometer]

Thats why they make lock washers.

I always wondered about that. 

Those helical spring (split) "lock" washers provide for  more rotations to achieve  the same change in clamping force until they bottom out, at which point, they behave the same as a regular washer.  If they do not bottom out, then over time, fatigue will cause the clamping force to change, decreasing the friction and increasing the risk of the nut/bolt coming loose.    There was a theory that the sharp edge of the washer might provide some additional friction, but this would be minor and highly variable/unreliable.

Decided to do a quick web check:  There are a bunch of sites that quote the ASME recommendations (ASME B18.21.1‐1999) stating something to the effect that helical spring washers should not be used to prevent the threaded joint from loosening.   I couldn't get past a paywall to read the original spec to verify this though.   I am curious as to when this change of position on the value of lock washers occurred. 

Anyways,  this is what I have read, and it seems logical.

I would still use a lock washer if that is what was recommended by the manufacturer.  When in doubt, follow the instructions 

-J

[Keta]

A lot of heavy machinery does not use lock washers, Cats use hard flat washers.  All of my trailer balls with the exception of the one on my ATV that comes off often have lock washers.  If I did not want to use a lock washer on a traler ball for some reason I would use a hard washer and red Locktite.