Hex Head vs. Socket Head Screws

Look at the bolt head design. The bearing surface area is the same. The extra area due to the corners can not be counted on as a bearing surface.

If you have clearance issues or you want it to look expensive & Hi-Tech use socket head. If you don't care use hex head. If you want it to look totally dodgy, use a combination of cup head, hex, socket head & even an old square head if you can find one. It really supports the cause if you have hex heads of different size too! :P

You will find that the socket head is a higher tensile strength than your ordinary high tensile bolts. Imperil bolts are normally grade 8 where your socket head will be around grade12.

I know that this is more of an opinion than a true technical answer, but I have standardized on socket heads for everything I can use them on.

Here is the list of reason I have for doing so;

1. Socket heads can be used in all kinds of tolerance situations. Where as hex heads can have issues in some tight spots.
2. Socket head screws seem to be higher quality in general. I can't count the number of hex heads I have stripped or worn away the head. Socket head bolts/screws have never given me a problem.
3. I have one tool set that handles everything I do instead of searching for one tool or another.
4. I happen to like the look of socket head screws, especially the nice stainless ones. :-)
5. I don't need to keep a supply of several types of the same size bolts. My inventory is smaller.

Standardizing is a nice way to go since it simplifies everything overall. Obviously there are situations that need different solutions, but so far this works for me.

My 2 cents.

-Doug

Thank you all for your answers - each one had a slightly different approach, so now thanks to all of you I have a better understanding.

Machine builders tend to prefer the SHCS over HHCS because they are friendlier to work with. However they do tend to pug with debris.

Always keep a can of compressed air (yes, I know it isn't actually air) on hand. It is great for quickly cleaning out the socket heads. Short of really nasty corrosion I have never seen a socket head that couldn't be cleaned out by compressed air.

Doug

Just to add my opinion, the points made so far are all valid, one other aspect is that it is easier to damage an adjacent painted surface with a spanner than with an Allen key. It is also easier to round off a socket head screw than a hex head.

Socket head screws are usually a better alloy than hex heads. You have to take this with a grain of salt in that, I believe, all inch SHCS are stronger than grade 8, while some metric SHCS are.

So, you can use a SHCS when you have a limited space for wrenching, when you need good appearance, when you need the head recessed, or when you want to really preload the heck out of the screw in a tension load.

The downside is that, generally, SHCS cost more than HHCS, you sometimes have the extra cost of counterboring the head, the threads are almost always longer than you need and in some cases too long for a good joint, the heads are usually hardened (you can't drill them for safety locks), and the bearing area under the head is quite small compared to HHCS.

A lot of people "solve" the small bearing area problem by putting a washer under the head. If you use an ordinary washer, it often makes the problem worse! Washers are notoriously soft, so the small bearing area with the high preload really digs into the washer, relaxing the preload and making a lousy joint.

So, if you're gonna torque a SHCS to a high value, you have to use a hardened washer, perhaps even a hardened and ground one. Now, of course, you were probably already using hardened washers under the head of your grade 8 HHCS, so you can just use the same ones.

Finally, if you're gonna use SHCS, buy a decent high strength, hardened wrench.

Congratulations for your "need to know". Not many designers even care to know. It's one of the differences between a good designer and a "not good" designer.

I asked our fastener expert about the manufacturing of the two fasteners:

Material is determined by process, grade and availability. It will be the same for both. Head shape has little bearing if any. The big advantage to hex heads from a manufacturability stand point is that they can be crash form (trimmed) which can be done off the header and is a cheaper process. Cap head must be done on the header. It is a better process and generally you pay a premium for them because of this.

Another point is finish. The Socket Head Cap Screw is usually blackened where the standard grade 8 hex has no finish.

"rule of thumb" - some thoughts of mine. Other than fastener-by-fastener case influenced by clearances and tool access:

If you are looking at a product design and you want to cut cost use hex head. Lowest grade applicable.

If you are designing a custom machine, automation equipment or similar use socket heads. Typically the extra fastener cost is neglegible.

Material thickness and type of fastened materal. Thinner or weaker materials may require the larger surface of a hex or use of a flange head hex.

Hole sizes. Looser tolerances may force you to have larger holes which in turn may lead you to using hex or flange head hex. Conversly tightly toleranced parts allow you to use the socket heads.

.... now that I've started I could go on for hours. And this is just two (3) fastener types.

One thing to remember is that the application dictates the selection. Socket head screws have a good tensile rating but tend to be more brittle.

I experienced a problem with them when mounting two Cibie rectangular headlights and mounting bracket to the triple tree of a springer front end. The thickness of the material of the triple tree base necessitated the use of a 1/4" fastener maximum. To keep with the theme of the chopper, I used non-chromed, loctited socket heads with washers. Within a week I found myself duct taping the headlights to the forks just to get home. I installed two more, figuring it was just an anomaly, and in two days, one of the screws snapped again. I switched to more ductile grade 5 hex head cap screws, thinking that grade 8's might also be brittle (probably not but tensile strength was not the issue) and never had a problem again. They were ugly but they worked. Apparently, at least for me, socket heads and vibration and shock are not compatible.

Perhapse I should try this idea with my mountain bike. I keep breaking the retaining fastener in my seat. Usually on a trail in the middle of nowhere. I'd rather mash a fastener than finish a ride standing on the pedals.

When those socket head cap screws fail in tension it sounds like a gun going off.

It would be impractical to use a hex head screw were the fastener needing to go flush or sub-surface either in a pocket or in a countersunk hole.

Blimey, your reply is to a post from 2008, are you having a slow day?

That would be telling.