Rules of Thumb for Fastening and Joint Design

Joint Design Rules of Thumb

  • Preferred failure mode is screw or bolt Fracture rather than thread strip.
  • Bolts function like springs and need to be stretched to produce clamp force in the joint.
  • The minimum recommended thread engagement of a bolt threaded into a nut member that is at least as strong as the bolt is one bolt diameter (i.e. if the screw is ¼” diameter, the thread engagement needs to be a minimum of ¼”). Engagement in weaker nut members must be greater in proportion to the relative shear strength of the bolt and nut member.
  • Either the nut or bolt can be rotated as long as the bearing surface is designed for that purpose (some form of washer face). When there is a choice, it generally suggested that the nut is turned as it eliminates the possibility of the bolt rubbing against the edge of the hole during tightening.  This would reduce the amount of clamp load generated for a given installation torque.
  • Lower grade bolts are softer and more ductile while higher grade bolts have higher strength and higher hardness.
  • Fastening is a process and the best fastening process is the one with the least amount of variation in the fastener, assembly components and installation methods.
  • Don’t assume that a bigger bolt is always better and more reliable. Using a larger diameter bolt than needed will tend to have greater variation in initial clamp load and will lose a greater percentage of that clampload.

Torque/Tension Rules of Thumb

  • Most of the applied torque is NOT used  to general tension in the joint; it is generally consumed as follows:
    • 35% overcomes thread friction
    • 55% overcomes under head friction
    • 10% created clamp or tension
  • Friction plays a significant role in the torque/tension relationship.  Changes, which appear to be small (adding wax to the bolt), can have significant effect. A 10% reduction in friction can create a 25% increase in tension.
  • While the coefficient of friction between the threads and under the head have the greatest influence of the torque-tension relationship, it is less widely understood that fastener geometry plays a significant role. For example, all else being equal, a flange head screw would generate less a clamp load than a hex cap screw because the under head friction acts on a larger diameter.  A larger clearance hole would have the same effect for the same reason.
  • The RPM’s of the fastener installation usually affects the torque/ tension relationship, though to varying degree depending on the mating materials and finishes.
  • Because of these variables, and other not mentioned, it is not recommended that installation torque for clamp load critical joints be determined by torque tables based on an assumed nut factor K. The actual clamp load can easily be 50% more or less than the estimate on which these table are based. Various test methods can be employed to ensure more accurate results.
  • The most common range of target clamp load for hard joints is 70% to 80% of the fastener’s proof strength (a tensile load which produces no permanent elongation). Proof strength is about 7-10% lower than yield strength.

Thread-Forming Screw Rules of Thumb

  • Length of engagement is typically the same as guidelines in “Joint design.”  Do not include lead threads in “full thread engagement.”
  • When there is adequate length of engagement, start with a hole size that provide 65% thread engagement.
  • Thread-forming fasteners typically provide superior drive to fail ratios when compared to thread cutting fasteners.
  • Thread-cutting fasteners typically provide lower driving torque than thread forming fasteners.
  • A minimum drive to fail ratio of 1/3 is typically required.
  • Forming threads in materials ranging in hardness from Rb75 to Rb90 is not a problem.
  • Hardness is in excessive Rb90 need to be reviewed on an individual basis.
  • Design criteria should be primarily focused on joint strength requirements and maximizing the drive to fail ratio.

Bonus information!

Impress your friends by knowing the difference between a screw and a bolt! Most would suggest it has something to do with size or strength; not so. A screw is designed to be rotated into the joint. A bolt is designed to be stationary while the nut is rotated onto the bolt.

-Bill Derry, Chairman