Re: Bolt queries
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Hi Peter, Some good ¡°rules of thumb¡± for designing with bolts.? These were bestowed on me over 35 years of mechanical design and product development from gifted and generous mentors.? 1. For steel bolts into mild or stainless steel, 1.5 diameters of thread will engage the full strength of the bolt threads. For example 1/4-20 ideally should have 3/8 min full thread.? 2. For blind holes, if possible you want an extra 1 diameter of tap drill depth to have room for chips to fall during tapping.? 3. For good steel you can go as little as 1 diameter for the thread strength but 1.5 allows for drilling pilot oversized. Research into thread strength has indicated that as little as 2-4 threads truly carry the full load of a bolt, but redundancy of 4-5 threads is a safe absolute minimum.? It takes into account thread yield and settling in of both bolt and nut threads.? 4. Aluminum bolts should not be considered structural bolts unless highly engineered , and anodized or hard coat anodized. Over time aluminum bolts oxidize and lose ductility. Aluminum bolts used anywhere need grease like lube. And yes over time an aluminum bolt and nut or thread joint will oxidize.? 5. For structural bolts into aluminum it takes 3 diameters of full threads to engage the full strength of a steel or stainless steel bolt. I have used both stainless and alloy steel in aluminum successfully. Best practice is to use grease on both. If you have ever seen aluminum car rims on a car after a few winters with road salt, the mating surfaces are always corroded and pitted and usually seized together from galvanic affect. Grease does help.? Btw 18-8 or 304 stainless bolts are approx 40ksi yield and 80ksi ultimate strength. Socket head screws from unbrako or cam car or metric with stamped heads are usually approx 2x the strength of stainless.? 6. The failure in a bolted joint is always the bolt if it is pure tension. This is due to the small cross section of stress area. For this reason for steel in steel a fine threaded screw is always stronger than a course thread but assumes perfectly formed tapped hole. If the pilot drill drills over size you can quickly go from 75% engagement to 60 or 50 with just a couple number drill sizes. That is why most mechanical designers avoid fine threads in structural applications.? Also this allows you to pair high strength bolts with medium strength nuts because the nuts have properly formed threads.? 7. Important rule of thumb¡.if the screw looks too small, bump it up a size or 2.? Combined stresses in bolted joints can be considerable.? 8.? Minimum distance for a tapped hole or tight clearance hole for a bolt from the edge of a plate should be about 1 diameter. That is from center line of hole to edge. This engages full bolt strength and full clamping assuming the plate is approximately 1 diameter thick minimum. This usually eliminates plate hole tear out.? 9. It is easy to calculate tensile force or shear force in a joint and is tempting to use 1 bolt to do the job. Don¡¯t fall for this temptation in critical joints. Minimum of 2 bolts is better and 3 bolts is best as a minimum. This helps mitigate unplanned bending. There is an old saying that any bracket is a step. Don¡¯t want anyone getting hurt. Assume human body weight.? 10. Factors of safety. Any non-critical joint 1.5x.? Critical joints with shock loading 5x-8x factor of safety.? Overhead lifting 10x-12x FOS.? Disclaimer: ?All above info is for reference only. Any critical fastener joint requires formal engineering. Just being conservative. ;) Enjoy.? On Mon, Aug 14, 2023 at 12:45 PM Dave Seiter <d.seiter@...> wrote:
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