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How to Create Secure Fastened Joints

What is the key to getting your fastened joints tight and keeping them tight? This is one of the biggest headaches that engineers face today. But with a little education and understanding of the bolted joint, the problem becomes easier to tackle.

Bolts Act Like a Spring

Believe it or not, we want bolts to stretch when we tighten them. By stretching bolts up to, but not beyond their yield strength, they act like a spring. This creates the desired tension in the joint to prevent clamped members from slipping and putting a shear load on the bolt. Bolts are only about 60% as strong in shear as they are in the axial direction, so avoiding this sideloading is key to designing good joints.

How long should your spring be for optimal joint retention? A clamping range of five times the diameter of the bolt is ideal if tightened. For example, an M10 bolt should have fifty millimeters of distance between the head and the nut when tightened to perform at its very best. Junkers vibration testing has proven this to be the best combination to avoid rotational loosening.

What about joint settling? Knowing the surface pressure of the material you are bolting together, compared to the surface pressure of the fastener is key. With the wrong combination, you can get joint settling and loss of clamp load which can lead to fatigue failure.

For more information on how to create secure joints, Bossard offers Expert Education seminars, both as webinars or in person at your own facility, tailored to your specific questions and needs. Contact us at ProvenProductivity@bossard.com for more information.

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May 03, 2019

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