Determination of preload in bolt pretension

- July 5, 2019 at 7:17 pm
  
  shantashreejena97
  Subscriber
  
  Hello everyone I have used M5 Allen bolts made up of 316L stainless steel in my set up which are engaged to certain beams made up of 6063 T5 aluminium alloy. Can anyone tell me how much preload should I give for applying the bolt pretension? Length of the bolts are 3.5cm(head+shank).
- July 5, 2019 at 8:00 pm
  
  Nima
  Subscriber
  
  Dear Shantashree18,
  
  According to Shigley's Mechanical Engineering Design Handbook, 9th Edition, p. 442, you have to consider values for preload as follows:
  
  Fi = 0.75*Fp (For nonpermanent connections, reused fasteners)
  
  Fi = 0.9*Fp (For permanent connections)
  
  Fi is preload and Fp is proof load. You can calculate the proof load for different bolts as follows:
  
  Fp=At*Sp;
  
  Sp is approximately 85% of the yield stress of the bolt material, while At is tensile stress area, which is calculated by (pi/4)*d^2. "d" is the nominal major diameter of the bolt, which is 5 millimeters for an M5 bolt. 316L stainless steel has a yield stress of approximately 170 MPa.
  
  Note: This information is for conventional uses. For specific purposes, you have to follow the instructions provided by the supplier.
- July 5, 2019 at 8:10 pm
  
  peteroznewman
  Subscriber
  
  The desired nominal preload force in a fastener is typically between 70% and 90% of the Proof Strength of the fastener.
  
  When I look up 316 SS, I find it comes in Class 50 and Class 70, which is more common. I didn't look up 316L, so check on that.
  
  The preload force that is 70% of the Proof Strength for an M5 in 316 SS is 653 lbs or 2.9 kN.
  
  To convert that desired force F into the torque T to tighten the fastener, an engineer can use the simple Nut Factor calculation.
  
  T = K x D x F where T = torque, K = nut factor, D = bolt diameter, F = bolt tension.
  
  Using this formula, the Torque is 3.1 N-m. The attachment explains Nut Factor.
  
  The last piece is to decide how long the thread engagement must be to prevent stripping the threads of the aluminum. That is based on the shear strength of the nut material. Given the shear strength, one can simply calculate the length of the thread engagement so that the shear stress in that length of a cylinder of the diameter of the fastener is equal to the shear strength of the material.