At Vine Engineering, we constantly recommend the question “How much torque should I apply?” be replaced by “How much Clam Load my Bolted Joint requires?” Additionally, the Joint Materials, Environment, Fasteners Grade and functionality must be carefully examined before determining the initial Target Tightening Torque.
Engineered joints require an optimum Torque value to provide the correct Preload. Applying the Torque to fasteners is commonly achieved using a calibrated torque wrench. The required Torque for a particular fastener application may be calculated, or documented in a published standard.
Torque values depend on several variables such as:
- The Bolt Size (Nominal Diameter)
- The Bolt Grade or Property Class
- The Torque Factor (total Frictional conditions along the Thread Flank, and at the Bearing Surfaces.)
A common relationship used to calculate the torque for a desired preload takes into account the Thread Geometry, Size, and the Friction in the threads and under the bolt head or nut.
The following assumes standard ISO or National Standard bolts and threads are used:
T = K D W
T = the required Torque (in-lb, ft-lb, or N.m)
D = the bolt Nominal Diameter (in, or mm)
W = the Screw or Bolt Tension (Ibf, or kN)
K = Torque Factor, Nut Factor
The Tension of the screw or bolt is calculated by multiplying 75% of Proof Load by the Tensile-Stress Core Area. Proof Load values and Tensile-Stress Core Areas are well documented in several Standards such as SAE J429, SAE J1199, ISO 898-1 etc. On the other hand, the Torque Factor “K” is 0.15/0.20 when Bolts, Nuts, and Washers of the Bolted Joint are clean and coated with a thin film of oil. “K” is 0.10/0.15 when additional lubricating of oil or wax is applied. “K” can also be 0.25/0.40 when dirt, rust, and other defects of field storage and environmental exposure are present.
The following Torque values are presented as a guideline only. Each Bolted Joint has its own characteristics, and the torque must be determined accordingly.
At Vine Engineering, we have the expertise to assist you achieve the optimum torque values based on your Bolted Joint configuration, Materials, Surface Condition, and Functionality.