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.We Guarantee Cost Savings Through:
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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 a significant effect. A 10% reduction in friction can create a 25% increase in tension.
- Torque to approximately 70% of yield strength (for hard joints).
- Breakaway torque is typically approximately 70% of the assembly torque.
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.
NOTE: Field has developed these rules of thumb but recognizes the need to test and validate the specific performance of all joints, especially critical joints.
-Bill Derry, President
Have a question about fastener materials, grades and strengths? Curious as to how VMI can improve forecasting? Interested in joint design and torque tension?
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