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Note: This is the first post in a regular series we’re calling Smarter Fastening. Designed to be a quick-reference knowledge hub for all things fastening and joining, this series will be published by various members of the Field team, which has over 200 years of combined technical experience in the fastener industry.
While many reference books that provide specifications and information on fasteners and joint design, there are very few published “rules of thumb” to serve as starting points. Field has developed these rules of thumb but recognizes the need to test and validate the specific performance of all joints, especially critical joints.
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Joint Design Rules of Thumb
- Design mode of failure is screw or bolt breaking, not stripping of the internal threads.
- The length of thread engagement in a steel nut member should be equal to or greater than one full diameter of the bolt and be of comparable strength (i.e. if the screw is ¼” diameter, the thread engagement needs to be a minimum of ¼”).
- Either the nut or bolt can be rotated as long as the bearing surface is designed for that purpose (some form of washer face).
Torque/Tension Rules of Thumb
- Most of the applied torque is NOT used to general tension in the joint; it is generally consumed 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.
- Torque to approximately 70% of yield strength (for hard joints).
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.
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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, President
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