Which is better, 2 big or 4 small? (Bolts)

Given a fixed amount of overlap of two steel parts that have to be fastened together as strongly as possible, how does one decide on the best number and size of bolts?

I have a long (1.5m) length of 20mm diameter EN1A that will have flats at either end to which cross pieces of 16mm thick steel bar will be fastened (forming a wide 'H' shape). The flats on the round bar are 18mm wide and 50 long to match the width of cross bars. In use the theoretical external load will tend to twist the round steel bar.

Because of access reasons the bolts have to pass through the rectangular bars into tapped holes in the 'D' shaped section of the bar. Given the limited area should I use two 8mm bolts (staggered) or 4 M6 as far apart as possible. I will use HT fixings, maybe metric fine if it helps.

Other than FEA how is one supposed to work out this sort of thing?

Ian

\the strength of sa bolt depends on it's cross section area, so approximately

8 x pi = 26 x 2 =48

6 x pi = 18 x 4 =72

The four bolts have it

The strength of sa bolt depends on it's cross sectional area, so approximately

8 x pi = 24 x 2 =48

6 x pi = 18 x 4 =72

The four bolts have it.

PS includes edits <s>

Sorry, I'll try again with the area not circumference!

2 bolts = 96

4 bolts = 108

Approximately

Looking at the information

LINK

4 x m6 is best ...though if you can fit 3x M8 you will get around 50 % increase in the clamping force

As a rule of thumb the depth of the tapped hole should be at least 1.5 x thread diameter or you will pull out the threads as you tightened the HT bolts.

You could also use some loctite on the joint ...... belt and braces

Some good info here

LINK

Steve

Edited By Katy Purvis on 06/03/2013 09:31:23

I never thought of it like that, but it does make sense. (refering to the combined cross sectional area of all the fixings)

But you have made me ponder more, Its the root diameter that matters but my gut feeling is that the proportion of 'root area to overall diameter area' gets worse as the bolt diameter gets smaller so creating a law of diminishing returns.

All else being equal a thread with finer pitch is better, at least thats a fact.

Ian

A bit OT but this is the first time in in 6 months that I am seeing the 'adverts over text' effect here. I have CHANGED NOTHING today or in the last few days so I'm stumped as to why I should suddenly be aflicted.

Thanks for the links, I don't understand the bit about having tapered bosses (near the end of the first article) to even out the thread loading.

Ian

The clamping force of the bolt is achieved by 'stretching' the bolt and the greater the free length that can be stretched the easier it will be to tighten. The 'taper' hole can be formed by relievng the first couple of threads at the top of the tapped hole.

Steve

Edited By The Novice Engineer on 04/03/2013 22:30:18

Steve

The tapered boss mentioned (Para 7 of the Plymouth document) seems to infer that the bolt thread stretch effect can be correctly countered over the whole length of the engaged thread,

So manufacturers have been selling us less than ideal nuts by the million for hundreds of years, palming us off with substandard products. Maybe nuts would be better if the hexagon was tapered, another big advantage would be that any size spanner would be a perfect fit!

I have read para 7 several times and I still dont understand it.

Ian

Ian,

Look at Steve's post, timed 21:05:21

See the long hyperlink ... I'm pretty sure that's what has done it.

I have reported the problem; so hopefully one of the Moderators will tidy-up

MichaelG.

I think that is what it is trying to say, but I also think it is largely academic. While not always true, in general the core of the bolt is weaker in tension that the threads are in shear. So generally the bolt will break before the thread strips.

I'm not convinced that tapering the hole, and hence the depth of engagement of thread in the hole, will distribute the load over more than a couple of threads. I suspect that what will happen is that the threads with a low engagement depth will distort by crushing, transferring the load onto the threads further down with a bigger depth of engagement; which is where we came in.

Regards,

Andrew

Define "better".

twice the number of bolts, of half the diameter..= same tensile strength

so with m6 vs m8..

4 bolts will allow at yield proportionaly higher tension ..

smaller bolts of same thread series will have finer pitch...more mechanical advantage to achieving max tenstion.in bolt.

finer pitch..will have a consequence for "secuirity" of assembley...nylocks, threadlock or wire fixes that

finer pitch....more thread leads available in tapped part...."stronger"

Cost ? well more holes/ tapping ops. and 4 m6 prob costs more than 2 m8...

Best of all....... failure modes...3/4 of holding force (lost one fastner) is better than 1/2..

More is ,

stronger
more secure
easier to reach required torque..
probably distriutes loads/clamping force better

all positives..only downside...cost in place

Edited By Steven Vine on 05/03/2013 01:35:40

Edited By Steven Vine on 05/03/2013 01:37:44

Slightly off topic but I also had the adverts over the posts for the first time - switched to the latest version of FireFox (I normally use Chrome) and the problem vanished.

I have the web developers toolbar (a utility to check that web pages are 'valid' and there was a box sizing statement that was not valid, and hence got ignored, which I would guess solved the problem - probably not being ignored in Chrome and hence causing a problem!

I really wish browsers could obey the documented standards - bit like going back to when nuts and bolts were hand made to whatever the toolmaker felt like sometimes!

Perhaps if the site designers read this they could have a look?

Chris

Michael

Thanks for your advice. The material and sizes of the two parts, and the positional relationship between then is already defined, The only thing I can vary is how they are bolted together and the main requirement is to resist torsional movement of the 20mm diameter 'axle'.

The bolts (12.9) have to go into tapped holes in the round bar, I like to have about 2xdia thread length which is easy in 20mm thick material, in fact I could put the bolts past the width of the flat if I spotface where I have to drill.

The width of the flats is about 18mm so the furthest away from the centreline would put M6 bolts 11mm apart. I could use 6 M5 bolts and get those even further apart. Its only a one-off job so cost is not an issue, 4 or 6 takes longer to do than just 2 but time is not too critical.

I wonder whether two small fixings in conjunction with a 'large as possible' tight fitting dowel might be a better approach?

Ian

Hi Ian

I Have posted a spreadsheet given to me by an engineering mate of metric bolt yield strengths over various tensile strength series..

As I am not a structural engineer I am not able to verify the tables however I have found them to be useful. At least for non critical non life endangering use.

**LINK**

Cheers

John

My understanding is as others have mentioned - strength is determined by core diameter as long as more than about 2 threads are engaged, as the cross section of the thread in shear then exceeds the core area. Incidently this is why it is still worth using high-tensile bolts even if they are fitted into a low-carbon steel like EN1A - the threaded depth can be arbitarily deep and strong, the fixing's strength remains the same regardless of length.

This means the difference between 2xM8 and 4xM6 is not huge.

I would have thought that 4xM6 would be the best choice as such an arranegment will better resist twisting and be less likely to work loose under load. Dowels might help stop rotation of the part, but would do little to help reduce 'peeling' forces.

Joey

Given the size of the materials - I'd weld or braze it!

John

Michael

Welding is not an option as these parts are going into a piece of bespoke furniture and need assembling in stages. The structure is to keep two separate ends of a table surface the right distance apart, and level with each other. Most of the time the whole structure will be stagnant and only come under any strain when in in transit, or being moved around a room.

My aim when I started the thread was to establish how I could make the best use of what I had. My first design (after I eliminated splines etc) did have two bolts on the centreline but I then thought that the cross drilling for a M10 or M12 bolt would remove too much of the shafts cross section.

If I use M6 bolts 6mm each side of the centreline I get nearly 10mm of thread engagement on each bolt, which should allow the bolts to be pretty tight.

Ian

.

Agreed, Michael

In fact, I would probably make the saddle piece quite substantial [like the cap for a big-end bearing] and let it drive the "D shaped" end of the shaft.

In this application, I think further bolting is unnecessary.

A coat of Loctite, all over the "D" prior to assembly, should do the job nicely.

MichaelG.