bolt thrust pressure

[brianf]

trying to find or figure out a formula for calculating bolt thrust

i already know the formula for calculating bolt thrust, so i dont need that regurgitated from the same 5 web pages

just because the pure math says its "x", just because its a 50k cartridge does that mean that exactly 50k actually pushes on the bolt face/lugs

that formula doesnt take into account the brass gripping the chamber walls

brass expansion

does the case head aid in distribution of bolt thrust force in other directions

info of that nature, or is there no other way but to sent it out to a cartridge testing center to get actual data

thanks

 

[BurnOut]

I'm by no means an expert in this area, and to be honest, I may not even have enough knowledge to be considered an amateur, but I would think that there are still too many variables in regard to the parameters you've mentioned. Questions like, what is the finish on the chamber? What is the static clearance between the case and the chamber? How hard is the brass? How thick is the brass?

I'm sure there are other considerations as well, but those are just some off the top of my head that could factor in to the calculation that you're after.

 

[brianf]

100%

if there is a study done (that i havnt found yet), that says even in a oil slicked chamber with brass like puddy only 80% of the PSI gets to the face etc

that would be a starting point...everything else is "better"

 

[morganlamprecht]

Rifle Chamber Finish & Friction Effects on Bolt Load and Case Head Thinning Calculations done with LS-DYNA

have u seen this? idk if its exactly what youre looking for...i havent read it all, just remember someone sending me the link a while back when talking about chamber polishes

 

[brianf]

thanks will read again today, havnt read that page in a long time...actually forgot about it

 

[Supersubes]

Probably not answerable without actual testing. What’s your goal?

You may have already seen this.

A Look at Bolt Lug Strength - Lilja

All action designers are concerned primarily with producing a safe and functional product. All other considerations such as weight, finish, eye appeal and other cosmetics are secondary. From a safety standpoint, the strength of the bolt lugs is of prime concern. Bolt lug shear strength depends...

riflebarrels.com

 

[jphil108]

I’m definitely not an expert on what most manufacturers use as far as brass friction with the chamber wall but I suspect they use max chamber pressure X some safety factor in their lug stress calculations and consider chamber friction as an added safety factor. Lots of factors to to consider but the last thing that most designers would want is to calculate bolt lug stress down to a gnats ass and reduce size to optimize weight and then have a bolt fail in fatigue or have someone’s handloads Exceed material UTS. My guess is most of the big manufacturers settled on a size, material and heat treatment that works safely In most conditions even if it’s massively overkill. They probably rely legally on “proven equipment” and don’t modify these parts except under extreme circumstances. I work as a design engineer for heavy brake applications and we almost never get to do many product optimizations even though a lot of our designs are 50 years old or older. We now have FEA tools that show we can make things significantly smaller and lighter but Nobody wants to take the risk, so instead we do incremental improvements and redesign parts a little at a time so that overall everything stays “service proven”. The end result is most times the general product architecture can’t be changed and things end up being heavier and stronger than really needed.

 

[Ledzep]

The ultimate easy and conservative way to do it is take chamber pressure multiplied by the surface area of the rim (even more conservative), or the web. This assumes no friction on the chamber walls.

I'm fairly familiar (bachelor's in ME) with static loading conditions, but I don't know how stress/strain plays with dynamic loading like what you get in a cartridge firing, nor necessarily the plastic/elastic flow of the material in the chamber. It's not as simple a situation as you might initially think, and the links posted above give some indication of that.

 

[brianf]

Currently working on some FEA simulations.

And when using software there is always room for additional variables.

In my opinion there is no reason on earth that there isn’t a spreadsheet (from 1970 lol) that has:

Chamber material
Chamber finish
Brass cartridge /alloy
Friction coefficient of said alloy when manipulated and deformed by internal pressure.
Extraction force
And a few others

Choose the initial performance or any parameter...that would give you your best available options.

A lot of smiths and knowledgeable people have learned through experience but there is no collection or distribution of that data.

Figure I’m a OCD sponge for useless shit..lol

 

[jphil108]

I agree it shouldn’t be too awful to develop something along those lines, just take some time and a lot of testing to verify. you can calculate most of those things as separate variables and create correction factors to deal with simulation/test result alignment. From a brief study on it without having done a free body diagram I would think you could measure the “ bolt lift force” to initiate primary extraction of a fired case, at the bolt handle and work backward through the extraction cam angles and bolt handle length to determine how much axial force it takes to unstick a case during extraction (it’d be a lot easier to calculate on an ARC Archimedese). Knowing the axial force to extract and the angle of the case taper you could calculate or even look up the force to remove a tapered shaft from a tapered bore and figure out how much effective interference you have between the case and wall after the round is fired. The neck should be a little easier to calculate. As you mentioned, there are a lot of factors involved that may be difficult to determine, hardness of the chamber wall will be difficult without cutting a barrel in half and you’d need a method that didn’t put in much heat (maybe wire EDM) and you’ll have to make some wall thickness assumptions on the case due to the transition the case head but it seems like a solvable set of equations. I suspect most of the big name brands have something of this nature in their engineering data, especially with Remington having been a DuPont company (they calculated and documented everything) but I’d imagine it’s proprietary and not widely used. To be sure most of the smaller machine shops and smiths that are building the awesome actions we have available these days are mostly concerned with safety and just change the extraction cam angles if they’re not getting enough extraction and exhaustively test they’re actions with proof loads for legal protection. Get out your Machinery’s Handbook and a sharp pencil.

 

[brianf]

Didn’t jump too fast..on the last bit.

I personally know of several smiths that don’t test fire the rifle before they send it to you.

Never mind firing a proof load.

I’d bet we ask 20 different people what they consider a proof load and we’d get 20 different answers.

Go no go, physical manipulation and in the box

Yes, absolutely absurd but true.

 

[THEIS]

Hi,

Give me a few days and I will put up some data from FEA in regards to at what cartridge pressures our receivers become not functional but remain operator state and at what cartridge pressures anything actually breaks, ruptures, flys off, etc etc.

We are currently running various analysis on our receiver body, bolt and breech cylinder. Once I publish that we will run the analysis with all the small parts and pieces assembled into the receiver/bolt...down to the pins and screws.

The number of variables is past my knowledge but it is what the Engineer gurus do for a living, lol.

Sincerely,
Theis

 

[jphil108]

Sorry to nerd out but it’s a fun question and a slow Friday at work. I would definitely expect at least one round to be chambered and fired to test function, but I’m sure there are plenty that don’t. as for proof loads that would mostly be for development work but you’re right, proof loads and “burst test” probably have a ton of different meanings to different people. Would be very interested to see what THEIS brings to the discussion. I suspect most legacy rifle actions were designed, prototyped, and tested long before any FEA was done, think about it, Enfields, springfields, and maulers were all designed without any modern design tools, you Can Do a lot without FEA, it just costs more in testing.

 

[THEIS]

Hi,

First....Proof loads damn sure should not be "varied" or misinterpreted by anyone; especially an action or firearm manufacturer.
The information is lined out easier to follow than your DMV test.

https://saami.org/wp-content/uploads/2020/02/SAAMI-Product-Codes-and-Supplier-Information-Public-Revised-2-17-2020.xlsx

We are currently sectioning brass from various manufacturers in order for us to make a proportional example in which we can use as the cartridge itself during the FEA. Holy shit at the internal dimension difference, lolol but that is another subject.

Sincerely,
Theis

 

[jphil108]

They all get their volume differences and case life somewhere haha. I understand there is the SAAMI SPEC but I’m sure companies might choose to push above that for testing purposes. we use a burst test pressure in excess of the FRA/CFR standards for our air brakes because some of our customers want it so we have to test to the most rigorous test, of course that’s 300 PSI air not 80 KSI hot gas. Truthfully though, it’s the rail industry and standards Are not always standard, especially freight vs transit.

 

[brianf]

just bringing it up top again

ill start smaller

does anyone know the bolt face pressure of a generic 308 or 6.5 creed

i know in theory its supposed to be the same as internal cartridge pressure but depending on the chamber finish the brass stretching absorbs some of the initial spike so the actual bolt force is less

that may work brass too much and limit loadings, so im just looking for generic numbers to start

of course this is only for cases that stay intact, if the case ruptures 100% of the cartridge pressure is pushing the bolt face

 

[Mech Eng]

Are you trying to find the force or pressure on the bolt face? And there are a lot of variables to find a really good estimate.

 

[brianf]

One in the same.

Pressure I’ll just change to force/lbs.

Once I have 1 number it’s just math after that

knowing there are a ton of variables I was hoping to get a actual measurement or a generic % of cartridge pressure.

if it’s between 85-95% of cartridge pressure that’s good enough to move the conversation forward.

I know it won’t be 50% etc

 

[buffybuster]

In addition to all the variables mentioned, case taper and case design also factor into bolt thrust. I recall Ackely doing some testing with bolt thrust and case design and he was able to successfully fire a cartridge in an chamber without locking lugs.

 

[Mech Eng]

Well I can only think of two ways to find that number one would be to testing with proper equipment but this is expensive. Your next best way would to find that number is to do what Theis is doing by finding the average case dimensions and then use FEA to find the best estimate.
Are you doing this for action strength or case life?

 

[brianf]

little bit of both...as usual

i know you can use the old school crushers or send it to a place that can set up a rig but i was hoping to get some numbers to start with before i really dive in

thanks