What is single/double shear?

I see it on here all the time, but i dont really know what it means. I do know that it is how things are fastened together, but i'm wanting to see if anyone has a link or pics that can illustrate the difference between the two.

I'm not 110% sure of this but I believe that single shear would be using one tab to mount something (such as links for suspension) and double shear would be using two tabs to mount something.
for example single shear would look like this: ]-
while double shear would look like this: ]-[

Hope that helps
Chris

2000xjclassic said:

I'm not 110% sure of this but I believe that single shear would be using one tab to mount something (such as links for suspension) and double shear would be using two tabs to mount something.
for example single shear would look like this: ]-
while double shear would look like this: ]-[

Hope that helps
Chris

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X2, thats correct, Hiem joints should always be double shear when possible, well really anything that is suspension related should be double shear if possible. TRE's contradict this but they should be heavy walled enough to take some abuse.

"Single shear"/"double shear" refers to how a screw/bolt is loaded when the force is perpendicular to the axis of the fastener (when the fastener is loaded parallel to the axis, it is loaded "in tension.")

If a screw/bolt is loaded in shear, it's usually being used as a pivot, and there are other design considerations that should be applied as well (for instance, a threaded fastener should never be loaded in shear on the threads - it should be loaded in shear on the shank, or "grip.")

"Single shear" refers to the fact that there is a single plane of shear force through the fastener - an example would be a tie rod end stud. Even with the taper to locate the parts, there is a single shear plane - it exists between the tie rod end housing and the knuckle surface (typically considered to be even with the lower plane of the knuckle surface.)

"Double shear" would mean that there are two shear planes through the fastener, or that the total shear force is "split" into two locations on the fastener body. It's most common to see hitch pins loaded in double shear - a good example of this would be the cross pin for your trailer hitch. You know how you put the shank of the ball mount into the receiver, and then retain it with a large (~5/8") pin? That pin is loaded in "double shear" - one plane on either side of the ball mount shank.

Loading a fastener in "double shear" allows you to either not quite double the load on it, or to use a smaller fastener (since the load is split.) You'll still want to select for the shear load to be placed on the shank instead of the threads (where the available shear strength is rather lower!) While I like to leave a good safe margin for anything (without testing, shear strength can be estimated as being 75% of tensile strength, and tensile strengths for ISO and SAE fasteners are easy to find,) this still gives guidelines for not wandering too far into overkill.

Note that there is a fairly significant reduction in strength when loading a fastener in shear over the threads (due to reduced cross-sectional area.) I don't have it to hand, but I've been planning a "fastener primer" that will explain just how you can figure the reduction in strength due to threading. There is a similar reduction in strength when loading in tension - same reason. You use the minimum cross-sectional area at the thread root for strength calculations in tension, and you should be able to use the full nominal diameter in shear (unless you screwed up, and loaded it in shear across the threads...)

lawsoncl said:

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Thanks - I didn't have time to draw a picture, and I didn't have one scanned or saved. (Repeated because it's a simple explanation - and saved for later...)

gosh dang that was frinkin the best gosh darn explanation i've ever encountered.... 5-90's words followed up with a perfect illustration. thanks guys i learned something

Thanks 5-90 and lawsoncl, perfect explanation and pic. From the names alone, that's what i pictured in my head, but really wasn't sure because I'm not familiar with the different kinds of joints that people refer to on here. I know what shear forces are, and from the name was able to make a rough guess of what it was, but wasn't totally sure.

thanks

Another downside to single shear is that the bolt/screw is also loaded in bending and not just shear. When analyzing the single shear connection bending moment should be checked to see if that, or shear is the limiting criteria.

Thats simplified but I think pretty clear.

stoneattic said:

Another downside to single shear is that the bolt/screw is also loaded in bending and not just shear. When analyzing the single shear connection bending moment should be checked to see if that, or shear is the limiting criteria.

Thats simplified but I think pretty clear.

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Especially if the bolt gets loose. Think of a loose lugnut.

stoneattic said:

Another downside to single shear is that the bolt/screw is also loaded in bending and not just shear. When analyzing the single shear connection bending moment should be checked to see if that, or shear is the limiting criteria.

Thats simplified but I think pretty clear.

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Only if the bolt is loose. If a properly graded fastener is properly tightened it should still have sufficient strength to resist the bending moment. Again, this relies on the fastener not being overloaded in the first place.

A double-shear design is handy from the point of view that it gives a few more options in terms of mechanical design: ie a smaller fastener can be used to carry a heavier load where a larger fastener cannot be used, etc.

Not dismissing the obvious superiority of a dbl-shear design in our applications since we tend to put mechanical design limits to the test on a regular basis. Just pointing out that there isn't anything inherently wrong with a sngl-shear to begin with.

cdn_xj said:

Only if the bolt is loose. If a properly graded fastener is properly tightened it should still have sufficient strength to resist the bending moment. Again, this relies on the fastener not being overloaded in the first place.

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That should be true if the pieces are loaded perpendicular to the bolt. How often does that happen in the real world?

stoneattic said:

That should be true if the pieces are loaded perpendicular to the bolt. How often does that happen in the real world?

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If they're loaded in shear, never. Shear, by definition, is a stress force perpendicular to the axis of the fastener.

"Tension" - a straight "pulling" action
"Shear" - a straight "cutting" action
"Torsion" - a "twisting" action (usually computed as a combination of tension and shear.)

And, if a fastener is used in shear, it's probably either not properly preloaded, or an excess of material is used (to allow for sufficient preload while allowing for relative motion.) Shear loading is typically used on screws and pins to allow for relative angular motion between parts - think of that tie rod end, for instance. Or of a clevis or shackle (the pin that retains them is loaded - very heavily! - in double shear.) Or, shear loading is used to simplify assembly - think of that receiver hitch pin again.

5-90 did a great job of explaining it.

Here's a good writeup of fasteners also for those interested...

http://www.pirate4x4.com/tech/billavista/NutsandBolts/index.html