Hello Folks

I'm playing at understanding action design, and have just done some analysis based on Dan Lilja's excellent article "Calculating Bolt Lug Strenght".

Now that I have a grasp of this, and understand how much shear area I need, another question arises "how much lug bearing area is required, and why"

Thanks - Foster

Dan's article is excellent. I think bolt thrust calculations are based on the working pressure of the cartridge plus the accepted safety factor. Now to get to the real deal. Have you ever seen a primer backed out of a fired case? What do you think happened inside the action? The short answer is nothing. The action didn't let the primmer back out.
the case was sized pushing the shoulder back too far and when fired the gas pressure sealed the case to the chamber and stopped the case from moving to the rear and making contact with the bolt face. What does that mean, well it means that the bolt face in this case don't hold the case in the chamber when its fired and therefore there was no bolt thrust.
there was an article written by PO Ackley about bolt thrust in his handbook for shooters. He ran several tests and more or less proved there is no bolt thrust in normal shooting. Anyway it is a good question.

[/quote]
there was an article written by PO Ackley about bolt thrust in his handbook for shooters. He ran several tests and more or less proved there is no bolt thrust in normal shooting. Anyway it is a good question.[/quote]


How would you then explain bolt lug set back? The test's P.O. Ackley performed was specifically to prove the holding capability and reduced bolt face thrust of his blown out case design. He would fire a round, back the action off 1 turn and repeat the test progressively lengthening the firing pin. Increase the taper and you increase bolt face thrust. Polish a chamber and you increase bolt face thrust. Bolt face thrust is very real.

Yes, the thrust (pardon the pun) of Ackley's chapter in his book was the reduced bolt thrust when straight or nearly straight cases were fired in dry, clean chambers. If I recall the acid test was a Winchester 30-30 AI lever gun with the vertical locking bar removed.

Please note that as sure as P.O. was about the outcome, he did not stand behind the gun when he fired it!

[/quote]How would you then explain bolt lug set back[/quote]
I have never seen any set back in normal shooting. I feel sure with a load that is grossly over the normal working pressure, set back might occure, but at that point the barrel has been ejected from the action, so why worry about the lug setback. As long as the lugs are stronger than the front ring of the action we are all safe from our mistakes, more or less.

[quote/]I have never seen any set back in normal shooting. I feel sure with a load that is grossly over the normal working pressure, set back might occure, but at that point the barrel has been ejected from the action, so why worry about the lug setback. As long as the lugs are stronger than the front ring of the action we are all safe from our mistakes, more or less.[quote/]

Why do I get the feeling you are not serious?

Tentman-- A good rule of thumb I use is--
A total lug surface of 1 sq inch
is good for about 150,000 lbs of thrust.
Example an enfield has two lugs .45 inch
long and .130 in high for a total bearing
surface of .125 sg inches, good for 18,000
lbs operating thrust without causing setback on properly hardened actions and bolts, with proper headspace.That same enfield has a shear rating on the lugs of about 35,000 lbs thrust.
On my enfields and ruger used for testing I
added exra bearing thrust surface on bolt handles for safety in my load testing..Gave me about 54,000 lbs of shear rating.Ed

Ackley's results were misleading. The case is pushed forward by the primer firing. If the chamber is unlubricated it will stay forward. At around 40 kpsi the rear of the case will move back. When the case comes back it comes with full force. That is why primers pop back with reduced loads. With higher pressure the primer will bulge a bit before the case comes back. That is why some primers look like rivets. Both poped back and riveted primers are indicators of headspace. If you want mathematical verification of this Check "The Okie Shooter" (Sam Watson). The reason Ackley could shoot a 94 Win without a breech lock was that his pressure was low. An improved case had nothing to do with it as all the action is at the rear of the casen not the shoulder.
Good Luck!

Another thought; The shear calculations made are suspect in that surfaces involved are cantelevered. Area has nothing to do with it. It is depth and width that determine stress.
Good luck!

quote:

more or less proved there is no bolt thrust in normal shooting.

If a rifles bolt had no lugs guess what would happen when the gun was fired. A bolt does have thrust.

quote:

there was an article written by PO Ackley about bolt thrust in his handbook for shooters. He ran several tests and more or less proved there is no bolt thrust in normal shooting

...until pressures capable of stretching the brass case are reached.

Wanna guess at how low those pressures are!?!?!?

Every heard of casehead seperation?

Many worship at the altar of Ackley, but he was misleading or wrong on many things...dangerously so in this!

Ackley was firing factory ammo in that 30-30AI. He proved an already low pressure round fired in an improved chamber, which lowers the pressure further, doesn't generate bolt thrust. I have no idea what that has to do with high pressure cartridges!

30-30 has about 3500 lbs bolt thrust, brass will
hold 4000..that is why his experments
showed what they did.An 06 has about 7000
lbs thrust its brass holds about 5000, so
you can see difference...

The more I read about P O Ackley the more amazed I am.

This one man single handedly accounted for more BS than and false beliefs than the next dozen experimenters combined.

quote:

Originally posted by Tentman:
Hello Folks

I'm playing at understanding action design, and have just done some analysis based on Dan Lilja's excellent article "Calculating Bolt Lug Strenght".

Now that I have a grasp of this, and understand how much shear area I need, another question arises "how much lug bearing area is required, and why"

Thanks - Foster

To answer your question.....you need two factors.
1. shear
2. compression

In the first equation you need the area of the lugs where they attach to the bolt to not shear off. This is a matter of the tensile (or better yet yield) strength of the bolt material and hardness.

In the second equation the compressive strength of the action is the area in square inches of the rear of the bolts lugs where they sit against the action during firing.

Here the action tensile strength (use yield strength) times the number of square inches of contact of the lugs must be greater than the area of the cartridge times the max pressure generated by the cartridge.

Obviously the area of the action required against the bolt lugs for a .223 is smaller than for a .300 magnum under similar pressure.

IMO the margin needs to be at least 2-1.....or twice the holding power required.

To be a bit more correct, use the material allowable applicable to the calculation. Use shear strength for the shear calculation, bearing strength for the bearing calculation....

FWIW, ultimate shear strength of a typical 4140 heat treated to ~125 ksi (tensile) is around 75 ksi, a bit over 60 ksi yeild.

The bearing strength is around 250 ksi ultimate, 150 ksi yeild.

Jon is correct. If you exceed the yield stress you get permanent deformation of the steel. That's something you definately don't want in a rifle action. In addition make sure you get the correct numbers for yield stress for the specific alloy of steel your dealing with. They can vary quite a bit.

"The more taper, the more backward thrust"?? are they sure?

Does that mean that the 300H&H has more thrust than the 300WSM?, or does it mean it has less cause:
1/ the case is longer(more pressure is ditrubuted lateraly)

A case with a little more taper will have a little more thrust, but not in the ranges
of 2 or 3 or 4 times as much...Like example
of 300 H&H, 300 Wea,,the H&h would only give about 10-15 % more thrust than Wea at same pressures.Ed

Case length should be essentially ignored for the discussion at hand. As long as the case is long enough that enough surface grabs...the casehead will stretch before the case slides out.

As for tapered cases...you'll likely get varied opinions, but IF the case/chamber mating surfaces are unlubricated there should be little difference. Any lubricant would allow either to slide backwards...but which does so faster is probably not going to matter in the larger scheme of things (at the level of taper we are dealing with). Both have enough grip that in modern high pressure cartridges the brass will yield first.

The difference between the WSM (and RUM, and other rebated cases in general) and the H-H cases is that the TRUE rearward thrust area is the cross sectional area of the combustion chamber, not the casehead. The rebated case allows a larger combustion diameter for the same casehead. It is popular to approximate the rear thrust by the casehead area, but it is not entirely accurate...look at a cross sectional view and think a little geometry and trig and you can see what I mean.

quote:

Originally posted by hubel458:
A case with a little more taper will have a little more thrust, but not in the ranges
of 2 or 3 or 4 times as much...Like example
of 300 H&H, 300 Wea,,the H&h would only give about 10-15 % more thrust than Wea at same pressures.Ed

With all due respect I'd like to disagree with this.....bolt thrust will be the same with a .300 H&H and a .300 Weatherby loaded to the same pressures.

Taper in the case has nothing to do with the force exerted on the bolt.

I've seen a thou or two of setback in a Rem 700 receiver. The cartridge was a 300 Win Mag and the load developed about 106,000 psi per the NECO program. The chamber had swelled to about .516 just ahead of the belt. The barrel and bolt held just fine. I removed the barrel and that allowed the bolt to be removed. A "gunsmith" installed a new take-off .300 Win mag barrel and the customer went back to shooting the rifle.
The cause of the overload was "I grabbed the .223 powder and used it for the Win Mag, the cans were very close in color."

Stuart Otteson has calculated the shear and bearing area of a lot of actions in his books, "The Bolt Action" a two volume set. Out of print, though.

Hubel458; How can a slight taper alter the thrust?. It can't be friction look at a trig table. Where did you get your information?.
Good Luck!

quote:

Originally posted by hawkins:
Hubel458; How can a slight taper alter the thrust?. It can't be friction look at a trig table. Where did you get your information?.
Good Luck!

Hawkins.....I believe this is from P O Ackley's teachings. Just another in a long series of myth and misinformation he distributed.

It is my belief that P O Ackley did a lot of good in his experimentations. Few folks have done as much as he has. Unfortunately his conclusions are not necessarily as responsible as many would have us believe.

Let see--I'm sorry it didn't get explained right.If The coeffient of friction between sides of case and chamber is the same on a straight case and a slanted, sides case of same
side length, the the slanted surface will grip
the sides of the chamber slightly less than straight one. Due to the slant.May only be real small percentage, or fraction og a percentage.As far as
what the total the case will grip chamber.
In real world practice and in either case the friction is usually more than what the tensile strength of the brass is. That is what I meant to get across.Most guys making improved cases say that all bolt thrust disappears, which
is wrong except where thrust is less than
what brass holds like Ackley's 30-30.Ed