Help: Bolt torq, lube, no lube

I always understood that torq values were for lubed threads. But lately I've read that you should use less torq if the threads are lubed (say with anti size compound). Does that make sense?

Torque is usually called out for dry threads. If using any lube/anti-seize/etc, then reduce the torque value.

A bolts job is to provide load. Lube reduces the friction in the threads which increases the load for a given torque. So reduce the torque with lube to get the same load.

Hi Weirdo. Thank for your note. Is there a 'rule of thumb' to use when figuring out the reduced torq?

The rule that I have used is to reduce torque by 25% to 33% with lube.

I saw a chart one time showing different types of lube and how much to reduce the torque for each. Something like anti-seize will be different than locktite because of the difference in viscosity.

Not only viscosity, but lubricity as well.

http://www.permatex.com/documents/tds/Automotive/09975.pdf

It's interesting that #6 under Directions for use specifies normal torque values.

I saw this discussion, interesting stuff.

iwannadie said:

I saw a chart one time showing different types of lube and how much to reduce the torque for each. Something like anti-seize will be different than locktite because of the difference in viscosity.

Click to expand...

It is true that each lubricant will have a different value. It is not based on viscosity because you can have dry coatings that reduce the friction but technically have no viscosity because they are not fluids.

Anyone who wants to research more, the friction factor you are looking for is called the "K-factor" and it is the friction force of surface-on-surface in the equation Applied Torque = K-factor*Force*Bolt Diameter.

Different metal-to-metal combinations and different lubrication will give different K-factors.

For effective retention, a certain amount of tension (tensile preload or tensile stress) is necessary - it's what provides the clamping force on the join, and what provides the enhanced friction that keeps the screw threads from backing out.

Tension is measured directly in rare cases (by measuring the stretch of the bolt directly) or indirectly (interpolation of the tensile stress by measuring the force needed to turn the screw.)

Thus, the torque wrench.

Torque specifications given in the service manual are for "clean, dry" threads unless specifically noted otherwise (for instance, the crankshaft nose screw is meant to be lubricated with clean engine oil then torqued to 80 pound-feet.)

Modifications to the "clean, dry" values are as follows:

- Threadlocker uses full torque - it doesn't change the friction of the threads.
- RTV sealant uses full torque value.
- PTFE paste uses 90% of torque value.
- Graphite uses 85-90% of torque value.
- Engine oil or chassis grease uses 75-80% of torque value
- Never-seez uses 50% of torque value.

The reductions in torque are due to the reduction in friction between the mating thread surfaces - reducing the friction translates to increased tension for a given torque application. Reducing the torque value helps prevent over-tensioning the screw and/or material failure of the mating part.

Interestingly, you also get more consistent application of tensile force - which is why I usually lubricated screw threads for cylinder heads, for instance, which allowed for more consistent clamping force across the deck (variation in preload for "clean, dry" threads runs 20-25% from nominal, while lubricants reduce this potential variation direct proportion to lubricity - the more "slippery" the lubricant used, the more consistent the preload application. Using "torque indicating" washers (like from SPS/Unbrako) or direct measurement of screw length is the most reliable method of measuring preload application in a screw.)