Lubrication effect on bolt tension and torque.
When a bolt is lubricated - less torque is required to achieve bolt axial load or tension. Reduction of torques for lubricated vs. dry bolts are indicated in the table below.
|Lubricant||Torque Reduction |
|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.