Short thread engagement using oversized bolt
Short thread engagement using oversized bolt
(OP)
Hello,
I am trying to redesign a bolted joint to make it faster to install. Currently we use a properly sized bolt that has a thread engagement of around 1.5 diameters but it is desired to make it faster to install this bolt.
I am thinking what if you use an over sized bolt so you could minimize the thread engagement. Doing this gives you a big tensile stress area and a small shear stress area which is bad practice, because the threads will fail before the bolt. But if you use a big enough bolt, you could have a huge shear stress safety factor to ensure the joint will not fail at all. The length of the wrench used for installation known, so I will assume some super human hand force to size the bolt. If the bolt was big enough could you get away with just one thread holding it together?
This seems wrong to do but it could work in this situation. Is there anything that I am not thinking about that makes this a bad idea?
Thank you!
Kevin
I am trying to redesign a bolted joint to make it faster to install. Currently we use a properly sized bolt that has a thread engagement of around 1.5 diameters but it is desired to make it faster to install this bolt.
I am thinking what if you use an over sized bolt so you could minimize the thread engagement. Doing this gives you a big tensile stress area and a small shear stress area which is bad practice, because the threads will fail before the bolt. But if you use a big enough bolt, you could have a huge shear stress safety factor to ensure the joint will not fail at all. The length of the wrench used for installation known, so I will assume some super human hand force to size the bolt. If the bolt was big enough could you get away with just one thread holding it together?
This seems wrong to do but it could work in this situation. Is there anything that I am not thinking about that makes this a bad idea?
Thank you!
Kevin





RE: Short thread engagement using oversized bolt
RE: Short thread engagement using oversized bolt
RE: Short thread engagement using oversized bolt
another day in paradise, or is paradise one day closer ?
RE: Short thread engagement using oversized bolt
You also need to make up for part thickness tolerance, which may add 1-3 threads depending on pitch.
Given that quarter-turn fasteners don't even have a full thread, you can certainly try, but the tolerance for fabrication problems becomes smaller.
RE: Short thread engagement using oversized bolt
RE: Short thread engagement using oversized bolt
if you use a larger bolt and I assume your tightening into a tapped hole, if that is the case the shear area of the tapped hole is always greater than that of the bolt. So you can get away with a smaller tapping depth on a bigger bolt but you still need to compare the yield strengths of the bolt and tapped hole material to avoid tapping a hole to shallow.
The link below gives female and male stress area's for tensile and shear of various screw thread sizes.
http://www.roymech.co.uk/Useful_Tables/Screws/Thre...
RE: Short thread engagement using oversized bolt
you want to reduce to time to release a bolt, by reducing the number of threads engaged.
is this for a production tool, a small scale production run, a large scale run ? (different selections mean different variability)
how much time are you going to save by reducing the number of threads engaged ?
what size bolt are we talking about ?
do you have access to both sides ?
if you reduce the number of threads engaged, how will you ascertain how many are engaged, given that there's some variability. I'd expect to see a bunch of tests done to demonstrate the load capability with a range of threads engaged.
doing this will change the fatigue performance of the bolt, is that an issue ?
another day in paradise, or is paradise one day closer ?
RE: Short thread engagement using oversized bolt
Similar ideas have been explored many times in the past and generally don't work out well. One thing to consider is that your larger, shorter bolt is going to cost more because the increased forming force will force it to be run on a larger, slower machine. In general, the time saved rotating the fastener is a tiny portion of the assembly time. If you have a 3/4-10 bolt with 1" of thread and you are driving at 500 rpm, the actual rotating time will be 1.2 seconds. Making the bolt larger and going to 5TPI will reduce it by .6 seconds. You need to make sure that every other step in your process is optimized before you worry about the rotating time for screws. Are you using an automatic feed driving system, if not- that is 2-3 seconds per bolt to locate it and put the driver on it.
RE: Short thread engagement using oversized bolt
another day in paradise, or is paradise one day closer ?
RE: Short thread engagement using oversized bolt
RE: Short thread engagement using oversized bolt
Ted
RE: Short thread engagement using oversized bolt
RE: Short thread engagement using oversized bolt
we're reducing the number of threads engaged by
1) shortening thread length, or
2) increasing the thread pitch (same length, lower tpi).
i guess 2) is slightly superior to 1).
another day in paradise, or is paradise one day closer ?
RE: Short thread engagement using oversized bolt
Thanks!
RE: Short thread engagement using oversized bolt
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?