Short thread engagement using oversized bolt

[kevingeneer]

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

 

[Compositepro]

If the thickness of what you are threading is the same, a larger thread will have more strength than a smaller thread. With a short thread engagement, the strength of the engangement will simply be much lower than the strength of the bolt. Is that a problem?

 

[kevingeneer]

I'm thinking its not a problem because as long as the combination of large diameter and short engagement gives me a big enough shear area to support the load, then the threads wont fail and it doesn't matter that the bolt is so strong. Although this means that the bolt will not be preloaded to yield at all. Will that result in a loose joint?

 

[rb1957]

could you use a "quick release" pin ?

another day in paradise, or is paradise one day closer ?

 

[3DDave]

Various analyses I've seen indicate that past 5 threads there is no further increase in strength. Add in 1 thread on each end of the junction to make up for partial/incomplete threads and that gives 7 threads of engagement/7 turns to install.

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.

 

[kevingeneer]

rb1957, it is desired to maintain the compression that the bolt provides. The part is basically a cap that is bolted onto the end of a round post. The cap fits loosely over the post, and currently the tighten bolt holds everything in place.

 

[desertfox]

Hi Kevin

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/Thread_Calcs.html

 

[rb1957]

there are many ways to do anything, and many reasons why you can't use them.

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 ?

 

[Screwman1]

Look at a multi-lead thread; a double helix will only take half as many turns to install.
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.

 

[rb1957]

oh, that's something i missed ... the proposal is to reduce the number of threads by using a coarser pitch (fewer tpi) ... i was thikning along the lines of a shorter threaded length (with standard pitch).

another day in paradise, or is paradise one day closer ?

 

[1gibson]

At face value, a shorter bolt is the easiest solution. A larger and shorter bolt, is slightly better, but I have a hard time believing there is enough time savings to offset the additional costs, of say going from a 3/4"-10TPI bolt to a 5/8"-11TPI bolt.

 

[hydtools]

Screw-on bottle caps have a short thread. Some are double-lead.

Ted

 

[CoryPad]

One missing factor is that larger diameter, shorter length joints are more susceptible to vibration loosening. Beware...

 

[rb1957]

looks like there are a couple of interpretations of what's being proposed. is it ...
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 ?

 

[kevingeneer]

Thank you for all of these ideas, this is very helpful. I was originally thinking of using a shorter thread length, and a slightly longer pitch that matches the larger bolt size. I will have to look into using a special extra long pitch or double lead fastener.

Thanks!

 

[KENAT]

Does it have to be a conventional screw?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​