Stay pitch formulae mysteries

[GWRdriver]

In his boiler book(s) Martin Evans gives the formula for pitch of firebox stays as follows:

Pitch = (D² x T x 3) ÷ (P x F x 4)

Where
D = stay root diam (3/16")
T = stay tensile strength (38kps for GM)
P = working pressure (100psi)
F = factor of safety (8)

He then follows this with an example

Pitch = (5 x 5 x 38000 x 3) ÷ (32 x 32 x 100 x 8 x 4)

What I do not understand is where the (5 x 5) value for D came from as it seems to me that should be .1875"² or .0352", . . . or where the extra value (32 x 32) which appears in front of P in the denominator came from. Also, this formula does not take into consideration the thickeness of the plate material which I would think would be necessary for any pitch computation. There must be something I'm missing, or is this one of those MAP typos that's never been fixed?

[Myford Matt]

Hi Harry

Using my editor's brain for once, I think I've got it.

He's saying once you've threaded a bit of 3/16" (ie 6/32") it drops down (conservatively) to 5/32", which when squared gives the equation shown. (I'm looking at p.40 Model Locomotive Boilers.)

Hope that's right (and helps!)

Cheers

MM

[GWRdriver]

Matt,
Thanks. I'll have another look as soon as I have time and perhaps I'll find it's the difference in the way Yo' folks and My folks set up equations, but then an old math teacher of mine, who based upon the abilities I demonstrated in his class was certain I'd end up doing 10 to 20 at hard labor, said it's all the same all over the universe.

[Myford Matt]

Hi Harry

There's no diference in how you/we do equations. It's just that Martin Evans:

a) Made an assumption without stating it explicitly (ie the root diameter of a thread on a bit of 3/16" is 5/32"
b) Fed it into the equation and (slightly) rearranged it without explaining the rearrangment, ie saying that:

D² = (5/32)² = (5 x 5) / (32 x 32)

Cheers

MM

[alanstepney]

Harry, I believe Matt is correct.

I have a rather nice Excel spreadsheet for checking boiler calculations, which originated from one of the US clubs (but I dont recall who or where).
If you dont have it, I can send it to you.
USeful as a double check on whatever you presently use.

[Tel]

Get aholt of the AMBSC copper code for a good, clear stay formula

[GWRdriver]

Alan,
Yes, I downloaded that, but since I don't have (or use Excel) I'm searching for a chum who can open it for me. I'll be interested to see how it wroks.

OK, using Evansonian mathematical conventions I've begun to see something within the realm of believability. Plugging in a WP of 100psi, and 1/4" diam non-threaded phos-brz stays (at T = 60kps), I've come up with a pitch of 1.875". A bit much for my comfort. My preliminary design set the pitch at 1-1/8" so before increasing that (or drilling holes) I'm going to cross-check this against say the AMBSC and other resources.

[havoc]

Yes, I downloaded that, but since I don't have (or use Excel)

You can always download and install OpenOffice if it bothers you to use Excel.

[Ansty]

Hi

I agree with Havoc that OpenOffice is a great alternative if a large download. You may also find this program and derivatives offered as cover mounts on computer magazines to avoid download.

Brian

[chameleonrob]

gwrdriver said:

Also, this formula does not take into consideration the thickeness of the plate material which I would think would be necessary for any pitch computation.

if I remember the method correctly, look up the stay pitch suitable for you plate thickness then work the formula you gave backwards to give the minimum stay bolt thickness.

KN Harris gives these figures for 100 - 120 psi

1/16 plate 1/2 spacing
3/32 " 5/8 - 11/16 "
1/8 " 3/4 - 7/8 "
5/32 " 7/8 - 1 1/8 "
3/16 " 1 1/8 - 1 3/8 "

rob

[bobpendleton]

ansty said:

Hi

I agree with Havoc that OpenOffice is a great alternative if a large download. You may also find this program and derivatives offered as cover mounts on computer magazines to avoid download.

Brian

Agreed, it would be a big download, especially on dialup. I've used Open Office for some years now and bought it on a CD from www.caterhamcomputing.co.uk

As I understand it, they do not (cannot) sell for profit, just to cover expense of copying and distribution. It may be that other distributors exist and it would be worth googling around a bit if this address is inaccessible.

Bob

[alanstepney]

The spreadsheet I mentioned was, indeed is, by Paul Seyfrit.


Harry, I am not sure of that spacing you gave.
Seems too much to me.

I'll contact you be email and get all the figures, run them through that spreadsheet, plus check them against some other figures and see what it all looks like.

[GWRdriver]

I am not sure of that spacing you gave. Seems too much to me. - alan

And me also, that's why I want to find out what I'm doing wrong.

[chris vine]

Hi Harry, you need to be very careful of formulae and the units when they are quoted in books, especially if written by non-engineers-physisists-chemists!!.

What I don't like is that the strength is quoted as 38kps and then he puts in 38000. If he was being consistant, the number to plug into the formula would be 38.

Chris.

[GWRdriver]

Chris,
Yes, a block I've encountered and stumbled ober many times.

[Noddy]

Another "beware"

The roots of a thread and any sharp shoulders on a stay will form stress concentrators.

I don't know the formula you are calculating to, and most designs incorporate a healthy "factor of safety" which is to allow for un knowns in the design process and variation in the materials that are being used.

Alternatively, they are based on a great deal of empirical experience, and so long as what you are doing does not deviate from the situations that the experience was gained in, then you are on pretty safe ground.

It does no harm to be aware that the stress at the end of any cut out increases drastically, and the sharper the end of the cut out is, the more it raises the stresses. In use, these areas become the sites for increased corrosion and are then the prefered sites for cracks to start.

With stress formulae, it is always a good idea to check the correctnes of the formula from several texts and to compare the results with either empirical experience or a different calculation method, and to get a friend to check the calc (it's amazing what a fresh pair of eyes can pick up).

Your local library should have a good supply of "strength of materials" texts

Excellent background reading is: Gordon; "the new science of strong materials (or why you don't fall through the floor)"
and "Structures (or why things don't fall down)"

They're very readable, and Gordon was an old man with a lot of experience of engineering and how to teach it well. (any fool can throw lots of maths at students, giving understanding is an art).

Keith

[Shawki Shlemon]

As Tel says get AMBSC boiler code and it will give you in detail what you need (size & stay pitch in metric and imperial) based on copper metal thickness & boiler working pressure . This I believe is for a safety factor of 8 .

[GWRdriver]

Shawki,
Yes I have the AMBSC and am using it as a comparative guide. I have also just revisited Jim Ewins page and his graphs of stay pitch and material thickness seem to be more liberal than most.

[Shawki Shlemon]

Hi Harry
Safety is more important than liberal , To my knowledge we never had any serious incident in OZ and the code is very comprehensive .

[robmort]

In any case Martin Evans' pitch formula is not correct as it completely ignores the boiler sheet thickness.
Obviously if you have very thick sheet you can have wider spacing, but the thinner the plates the closer the spacing needs to be to avoid buckling.