GarandOwner

June 5, 2008, 02:48 PM

Like many others I have a love for the M1 Garand, It is pretty much common knowledge (at least among Garand shooters) that the operating rod is sensitive to buckling due to its length. I have always tried to find what pressure this occurs at but have never found it.

So the engineer in me took over and I decided to calculate it myself. After some research through the multiple books I have on the m1 Garand I have found that the operating rod is made out of 1050 steel:

According to the MIL-HDBK (version 5j) 1050 steel has the following properties:

Elastic modulus: 2.75572x10^7 PSI

Using Euler's theory, a rod under compression will buckle when its axial load reaches:

(pi^2EI)/L^2

where E is the elastic modulus, I is the area moment of inertia of the rod's cross section, and L is the length of the member

This is then divided by the area to get the normal stress required to buckle the rod. The area used to compute is (2*pi*r*t) where r is the outer radius of the rod, and t is the thickness of the cylinder wall. So we get:

(pi^2*2.75572x10^7*(pi/64*(.506^4-.401^4)))/(15.83"^2)

(2*pi*.253"*((.506"-.401")/2))

This gives a value of: 25342.3 PSI

The diameter of end of the op rod where the gas piston is, is .5255"

using Sigma = P/A where sigma is the normal stress (pressure on the end of the rod) P is the axial force and A is the area of the end of the op rod, we get:

(pi^2*2.75572x10^7*(pi/64*(.506^4-.401^4)))/(15.83"^2)

(pi*(.5255"/2)^2)

= 9751.46 PSI

So the max port pressure the Garand can handle before op rod to buckle is about 9752 PSI

(of course you would want to use less than this to be safe)

Things that can cause this number to fluctuate are cyclic fatigue from multiple cycling through the rifles life, variations in the strength of the steel used in each op rod due to imperfections, and the fact that since the op rod is bent (for design purposes) the strength is lower since the rod has already yielded.

Some may find this information interesting or useful, others may find it dry and boring.

So where am I going with this?

Well I came across the difference in gas cylinder locks between the early garands and the late type. And I noticed that the Late type has a valve stem in it to act as a pressure relief valve, so I am curious, how many people have either had or heard of an operating rod being bent by a Garand that had the late type gas cylinder lock on it?

So the engineer in me took over and I decided to calculate it myself. After some research through the multiple books I have on the m1 Garand I have found that the operating rod is made out of 1050 steel:

According to the MIL-HDBK (version 5j) 1050 steel has the following properties:

Elastic modulus: 2.75572x10^7 PSI

Using Euler's theory, a rod under compression will buckle when its axial load reaches:

(pi^2EI)/L^2

where E is the elastic modulus, I is the area moment of inertia of the rod's cross section, and L is the length of the member

This is then divided by the area to get the normal stress required to buckle the rod. The area used to compute is (2*pi*r*t) where r is the outer radius of the rod, and t is the thickness of the cylinder wall. So we get:

(pi^2*2.75572x10^7*(pi/64*(.506^4-.401^4)))/(15.83"^2)

(2*pi*.253"*((.506"-.401")/2))

This gives a value of: 25342.3 PSI

The diameter of end of the op rod where the gas piston is, is .5255"

using Sigma = P/A where sigma is the normal stress (pressure on the end of the rod) P is the axial force and A is the area of the end of the op rod, we get:

(pi^2*2.75572x10^7*(pi/64*(.506^4-.401^4)))/(15.83"^2)

(pi*(.5255"/2)^2)

= 9751.46 PSI

So the max port pressure the Garand can handle before op rod to buckle is about 9752 PSI

(of course you would want to use less than this to be safe)

Things that can cause this number to fluctuate are cyclic fatigue from multiple cycling through the rifles life, variations in the strength of the steel used in each op rod due to imperfections, and the fact that since the op rod is bent (for design purposes) the strength is lower since the rod has already yielded.

Some may find this information interesting or useful, others may find it dry and boring.

So where am I going with this?

Well I came across the difference in gas cylinder locks between the early garands and the late type. And I noticed that the Late type has a valve stem in it to act as a pressure relief valve, so I am curious, how many people have either had or heard of an operating rod being bent by a Garand that had the late type gas cylinder lock on it?