bolt thrust

[wmitty]

I'd like to ask a question regarding bolt thrust; but not quite sure I have a firm grip on the concept. I'll assume the idea means the force (pressure X area) generated by the peak chamber pressure within a cartridge upon firing times the cross sectional area of the interior of the brass casing at it's largest internal diameter.

My question is this: does bolt thrust vary in relation to the shape of the interior of the brass casing? In other words, does the force exerted on the bolt lug(s) increase
if the case head wall thickness decreases and the combustion chamber shape at the rear of the case more closely approaches the shape of a right cylinder? When most rifle brass is cross sectioned axially the shape of the combustion chamber resembles a truncated cone just ahead of the flash hole. Does the variation in this shape which occurs between different brands of brass cause a slight variation in bolt thrust? (Assuming chamber pressure stays the same).

Ackley's improved version of the .30-30 was supposed to have reduced bolt thrust to the point that he fired the lever action rifle without the locking lug engaged in the bolt body successfully. Can someone explain how the reduction in force applied to the bolt body would be calculated? If my concept of this idea is wrong, someone please straighten me out.

[Buckshot]

.............wmitty, I can't answer your question except to add that backthrust applied to the bolt is figured using the cubic inch area of the casehead. Also backthrust is only a small proportion of the total pressure the load generates due to the grip of the case to the chamber walls. Or at least that's what I understood the article to say. The auther never once mentioned internal case design in discussing backthrust.

.............Buckshot

[303Guy]

Somewhere in Varmint Al's website is a calculation and description of what happens. I believe Ackley did not actually hold the rifle being fired with locking lugs removed! It's probably best not to try that one at home.:twisted:

If I can find the details I'll post the link.
:drinks:

[Bret4207]

This is a Felix question. It takes an actual rocket scientist to grasp this stuff and he are one!

[high standard 40]

I shoot a lot with Contender pistols and this question comes up a lot. The internal shape of the case is of less importance than the outside shape. A case with more body taper will impart more bolt thrust than a case with a straighter case body. The straighter the case wall is, the more it grabs the chamber walls when it expands during firing. A heavily tapered case grabs the chamber walls less which increases bolt thrust. Note most if not all Ackley cartridges have very little case taper.

[felix]

Once a case moves upon firing, then all bets are off, unless we have a perfectly square case and chamber design. All cases have taper so we can load and unload the gun. Even the pistol cylinders have some taper, and if they don't per individual gun, that can be a problem with loading from the reloading books at max pressure and fast becomes evident with extremely clean cases. Shooting boolits makes a chamber greasy by default and will force the likeness of a taper if any exists or not. Therefore, this taper business is more important in the neck area than anywhere else. The usual load will show targeting variance between cases with and without primer grit remaining from the previous loading session. Personally, I do not thoroughly clean cases, i.e., no tumbling, but use just enough degreaser in a bucket to make the cases un-sticky enough to reload without contamination. Left over primer grit is a cheap friction enhancer for the next shot. ... felix

[joeb33050]

I'd like to ask a question regarding bolt thrust; but not quite sure I have a firm grip on the concept. I'll assume the idea means the force (pressure X area) generated by the peak chamber pressure within a cartridge upon firing times the cross sectional area of the interior of the brass casing at it's largest internal diameter.

My question is this: does bolt thrust vary in relation to the shape of the interior of the brass casing? In other words, does the force exerted on the bolt lug(s) increase
if the case head wall thickness decreases and the combustion chamber shape at the rear of the case more closely approaches the shape of a right cylinder? YES When most rifle brass is cross sectioned axially the shape of the combustion chamber resembles a truncated cone just ahead of the flash hole. Does the variation in this shape which occurs between different brands of brass cause a slight variation in bolt thrust? (Assuming chamber pressure stays the same). NO

The peak force applied to the case head, bolt and locking lugs(98 Mauser), breech block (1885 Win.) or ???? is (partly) described as the peak chamber pressure times the effective interior base area of the cartridge case. Think of the polar cases:
A ctg. case with a .005" needle hole from the neck to the primer won't exert much force on the bolt.
A ctg. case with .0001" thick walls will exert a lot of force on the bolt.
Effective interior base area is = largest interior area of the ctg. case. No matter what shapes before or after the largest, the largest rules.
Other contributors revolve around the case being forced backward by pressure against a tapered side of the case. Coeff. of friction, area and angle together with pressure give the back force. I'm guessing that with reasonably clean and slightly tapered cases, the contribution ain't great.





Ackley's improved version of the .30-30 was supposed to have reduced bolt thrust to the point that he fired the lever action rifle without the locking lug engaged in the bolt body successfully. Can someone explain how the reduction in force applied to the bolt body would be calculated? If my concept of this idea is wrong, someone please straighten me out.
joe b.

[high standard 40]

Once a case moves upon firing, then all bets are off, unless we have a perfectly square case and chamber design. All cases have taper so we can load and unload the gun. Even the pistol cylinders have some taper, and if they don't per individual gun, that can be a problem with loading from the reloading books at max pressure and fast becomes evident with extremely clean cases. Shooting boolits makes a chamber greasy by default and will force the likeness of a taper if any exists or not. Therefore, this taper business is more important in the neck area than anywhere else. The usual load will show targeting variance between cases with and without primer grit remaining from the previous loading session. Personally, I do not thoroughly clean cases, i.e., no tumbling, but use just enough degreaser in a bucket to make the cases un-sticky enough to reload without contamination. Left over primer grit is a cheap friction enhancer for the next shot. ... felix

The lube from a cast bullet will indeed reduce the amount that a case can grab the case walls, true. And all cases have some taper, true. But with all factors considered, a case body with less taper WILL give less bolt thrust than a case with more taper. It's the reason the selection of high performance wildcat cartridges are limited on the Contender action to cases with little taper, the JDJ cartridges being a good example where chamber pressures can approach levels of some cartridges with higher case taper that are not safe in the Contender.

[StarMetal]

Actually oil, grease, lube, etc., don't seem to make much of a difference in case friction against the chamber wall. Varmint Al has a test chart with many variations such as rough chamber, smooth chamber, polished chamber, dry brass, smooth brass, polished brass, greased brass, etc., etc., etc.. Read it here:

http://www.varmintal.com/a243zold.htm

Scroll down about 1/2 page to:

Summary Table with Estimated and Measured Coefficients of Friction
See the Friction Test Results (http://www.varmintal.com/afric.htm)

Then see chart. Very interesting.

Joe

[high standard 40]

That's a great article Joe. According to his tests, addition of the lube to the case increased bolt thrust by about 4% and that's not much. Thanks for the link.

[felix]

Yeah, Joe, not even close as the author admitted by: "One wonders if the brass may be forced into the steel “valleys” a good deal more vigorously by the cartridge case internal pressure and the real friction under firing dynamics may well be quite a bit higher. That is a much more difficult experiment to run".

A light bulb burns out when it is turned on, right? This is because of the transient response effect of the "square wave" type step-impulse into the resistive element which did not hold up to the dynamics. Another thing not discussed is the expansion of the chamber along with the cartridge case following. Case will stick if the elastic limit was not reached by the barrel, and was indeed reached by the case. This situation will indicate a false pressure reading when the brass is not kosher, but entirely satisfactory in terms of holding up with that amount of pressure (until the case cracks when either shot or reloaded). The only way to tell is with a pressure metering device between the barrel and the bolt. ... felix

[303Guy]

I keep my cases and chambers lubed at all times. There can be no doubt that the case grips the chamber walls with the presence of lube. The difference is that without lube, the case may bind, ie. fail to slide or creep as the pressure forces the case head to follow the bolt face. When that happens, the case fails by plastic deformation at it's weakest point, that being just above the expansion ring. With a non binding case, the case body progressively creeps rearward, keeping the brass within its elastic limit and springs back as the pressure drops again. Too much pressure can cause binding too, hence the bolt lift test. A lubed case does not 'grow' in length.

Evidence of this creep can be seen by the length of the 'creep' marks on the case - they get progressively shorter toward the shoulder.

It is entirely possible for the case to be held forward by the 'grip'. This shows up as a backed out primer. I've seen a 303 Brit do it. (Mostly, the cases failed in this gun - excess headspace clearance).

[StarMetal]

Yeah, Joe, not even close as the author admitted by: "One wonders if the brass may be forced into the steel “valleys” a good deal more vigorously by the cartridge case internal pressure and the real friction under firing dynamics may well be quite a bit higher. That is a much more difficult experiment to run".

A light bulb burns out when it is turned on, right? This is because of the transient response effect of the "square wave" type step-impulse into the resistive element which did not hold up to the dynamics. Another thing not discussed is the expansion of the chamber along with the cartridge case following. Case will stick if the elastic limit was not reached by the barrel, and was indeed reached by the case. This situation will indicate a false pressure reading when the brass is not kosher, but entirely satisfactory in terms of holding up with that amount of pressure (until the case cracks when either shot or reloaded). The only way to tell is with a pressure metering device between the barrel and the bolt. ... felix


Felix,

Here is a little bit on what you've mentioned. Not the chamber, but at least the receiver and bolt. Take a look:

http://www.varmintal.com/abolt.htm

Joe

[leftiye]

Wmitty, you've got the formula basically right - inside area of case X peak pressure. This gives a number with a little safety margin due to the brass strength when gripping the chamber when under pressure. *

As I remember the Ackley experiment, he unscrewed the barrel of a 94 Win. and fitted the bolt with an extra long firing pin. The case head was not against the bolt face at all. He then fired progressively higher pressure loads until he got case failure. This happened at almost full pressure loads for a 30-30 (35,000 psi). * Quite a little margin - eh?

[odoh]

Hmm-m-m, sounds complicated ~ I always thot the pressure in the combustion area would be the same at any location in thet area? Brass ~ slipping/slidding? All brass that I'm aware of tends to be softish ~ like lay a case on the bench, don't even need a hammer, just whack w/a screwdrive handle and heap big dent. Can't see anything it offers in the way of strengthening anything in a combustion chamber scenerio of >30,000psi ~ sealing off of gas? Yes, no doubt.

As a kid back in the '50s, conventional wisdom deducted from actual blowups that lube/oil on the case/in the chamber dictated that it allowed the case to cling less to the chamber walls increasing bolt thrust and hence the failure. Apparently that still holds in the minds of many/most. I really don't know but something isn't right.

Perhaps, just perhaps, the lube/oil in the failure scenario in itself really did not increase only the bolt thrust ~ perhaps like oil in the wrong place of an airgun or like in the presence of high pressure like LOX line, the oil combusted and created an over pressure situation ~ equivalent of adding several grains of powder charge increasing pressure in all directions? If so, (and again, I'm not sure myself), then bolt thrust may need to be re examined.

[StarMetal]

Oil on the case doesn't increase bolt thrust by that much. The bolt, and barreled receiver, are such constructed of proper materials to handle beyond even a little above normal bolt thrust. Bolt thrust is no ways nears what combustion chamber pressures are. Take a round operating at say 50,000 psi. Bolt thrust would be in an approximate area of 4650 foot pounds.

Oil isn't going to combust in a rifle chamber. In a rifle bore maybe, but not the chamber.

In WWII some Japanese machine gun ammo was oiled to ensure proper functioning. I don't recall ever hearing anything about the machine guns blowing up or catching fire because of this.

If you think of a brass rifle case as a gasket then you can understand it's function better. Think we went from a completely closed breech end in a muzzle loader to where we are at today. In the first firearms the breech has to be totally sealed. The same today. To complete today's seal the brass case is the key element. Just as a head gasket in an automobile engines must be properly fitted and supported, so must a brass case.

Joe

[felix]

That link is much better, Joe. It would have been interesting for him to include the impulse through the bolt in a rear locking gun, like 788, winnie/marlin lever gun. Break open and drop actions guns should be considered as well? The Miller and Hall drop actions are made extremely strong for their style. ... felix

[303Guy]

It would have been interesting for him to include the impulse through the bolt Suddenly applied loads can double the effect of that load! They can treble the effect if there is a gap!!! I cannot help but wonder how a case that is gripping the chamber walls and then suddenly gives way and impacts the head against the bolt face, affects the load effect? I get the impression that when there is a headspace problem, the problem grows exponencially! (Maybe only noticeable in Lee Enfields - lots of those around that have taken a hammering!)


My question is this: does bolt thrust vary in relation to the shape of the interior of the brass casing?I would suggest that the it would. An ideal case design would possibly allow grip and strength in a way that reduces bolt fase thrust considerably. It would of course reduce case capacity somewhat but may then allow a higher operating pressure. A bit tricky when the friction factor varies, maybe?

I personally think a lubed cartridge will give a more consistant bolt face thrust, if that is important.

[felix]

Yes, it does. Gas expands in all directions uniformly until it meets resistance, and then changes direction into the location where the pressure opposing the expansion pressure is least. If gas did not expand this way, then our cars would be riding on a bumpy road (because of out-of-round tires) without regard to the irregularities in the road. BR actions are heavily built for two purposes. One to hold that pipe straight, and two to allow the case to be ground into battery without any remaining slop whatsoever. If and when the case moves, that expanding energy is wasted and not imparted to the projectile. Also, if and when the case moves there is no telling in which direction the case is going to move because we do not know where the slop (space) is. So, if you have a head space problem, that will "intentionally" force the slop along the major axis primarily, and, if that slop is greater than the lateral slop, the bolt is going to get hit out of proportion with the total thrust imparted to the chamber WHICH INCLUDES THE INTERNAL SPACE PROVIDED BY THE MOVING PROJECTILE AT ALL TIMES. ... felix

[303Guy]

... WHICH INCLUDES THE INTERNAL SPACE PROVIDED BY THE MOVING PROJECTILE AT ALL TIMES. ... felixWould you mind explaining this, please?