Finite Element Stress Analysis of a 1903 Springfield Rifle Bolt
by
Daniel J. Flavin
An Engineering Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
Master of Engineering
Major Subject: Mechanical Engineering
Approved:
______
Professor Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, CT
December, 2014
© Copyright 2014
by
Daniel Flavin
All Rights Reserved
CONTENTS
CONTENTS iii
LIST OF TABLES iv
LIST OF FIGURES v
GLOSSARY vi
KEY WORDS vii
ABSTRACT viii
1. BACKGROUND 1
1.1 PROBLEM DESCRIPTION 3
2. METHODOLOGY 5
2.1 GEOMETRY 5
2.2 MATERIALS 6
2.3 FEA 7
3. RESULTS AND DISCUSSION 10
3.1 TWO DIMENSIONAL EXAMPLE 10
3.2 THREE DIMENSIONAL RESULTS 11
3.2.1 Displacement 12
3.2.2 Peak Stress 12
3.2.3 Plastic Deformation 14
4. CONCLUSION 17
REFERENCES 19
Appendix A: Material Properties, SAE 2340 Steel 20
Appendix B: COMSOL Report 21
LIST OF TABLES
Table 1: Material Properties of Heat Treated SAE 2340 6
LIST OF FIGURES
Figure 1: Bolt Action Rifle Nomenclature and Cross Section (Brophy 67) 2
Figure 2: Bare Rifle Bolt 2
Figure 3: Outline Diagram 4
Figure 4: Bolt Model in CAD (exploded view) 5
Figure 5: FEA Geometry 7
Figure 6: Boundary Load 8
Figure 7: Fixed Constraint 8
Figure 8: Model Mesh 9
Figure 9: Two Dimensional FEA Sample (Von Mises Stress) 10
Figure 10: Calculated Values of Displacement 11
Figure 11: Surface Stress Values (Von Mises) 12
Figure 12: Surface Stress Values (Von Mises), Peak 13
Figure 13: Calculated Areas of Failure 14
Figure 14: Calculated Areas of Failure (Cross Section) 15
Figure 15: Percentage of Yield 16
Figure 16: Barrel Rupture Due to Obstructed Barrel (Hatcher 195) 17
GLOSSARY
Action: the mechanical parts of a firearm, which manipulate the cartridge and seal the breech.
Barrel: a metal tube, through which the projectile is propelled by a controlled explosion and the resulting rapid expansion of gas.
Bolt: the part of the firearm action which blocks the rear of the chamber (sealing the breach) during firing, but is moved to allow a new cartridge to be loaded.
Bolt action: a specific form of firearm action in which the bolt is manipulated by hand between each round fired.
Bolt thrust: the amount of rearward force exerted by the cartridge during firing. The force is transmitted into the bolt, which must be strong enough to contain it.
Breech: the end of the barrel closest to the operator, where the chamber is found.
Cartridge: a unit of ammunition consisting of a bullet, gunpowder, casing, and primer assembled into a single piece.
Chamber: The portion of the barrel in which the cartridge is inserted prior to firing. The shape of the chamber will closely match the shape of the cartridge.
Firearm: a portable weapon which launches projectiles through an explosive force
Lug: a projecting portion of the assembly for the purpose of transferring forces between two parts
Muzzle: the end of the barrel from which the projectile exits
Proof Load: a cartridge loaded to a higher than standard pressure, used for safety testing of newly manufactured firearms
Receiver: a portion of the firearm action which houses the operating pieces and connects to the stock and barrel assemblies.
Stock: the portion of the firearm used by the operator for support and aiming by holding against the shoulder
Trigger: the mechanical lever used to actuate the firing mechanism
KEY WORDS
· Bolt action
· COMSOL
· Finite Element Analysis (FEA)
· Locking lug
· M1903 Springfield
· Receiver
· Rifle
· Stress
ABSTRACT
This study analyzed the worst-case scenario loading and resulting stressed of the bolt lugs in a M1903 Springfield bolt action rifle. The load-carrying portion of the bolt head and the corresponding portion of the receiver were modeled in Solidworks, using legacy drawings and surviving examples of spare bolts. The model was then loaded into COMSOL finite element analysis software using material properties of the now obsolete steel. Using the static solid mechanics module with plasticity, the loading and internal stresses of the bolt lugs were calculated. Results showed that the stresses in the bolt lugs approached, but did not exceed, the yield strength of the material.
12
1. BACKGROUND
At its simplest, a firearm can be thought of as a tube strong enough to resist the forces of an explosion, which is plugged at one end and open at the other. In modern cartridge firearms, the plug must be mobile in order to allow loading and removal of cartridges. This mobile plug is called the bolt, and must be capable of withstanding many thousands of pounds of pressure during firing. To do so, some low pressure firearms will use the inertia of the bolt, but most will require the bolt to interlock with the receiver, the main structural component of a rifle. Locking methods, and the methods of operating them, vary widely amongst different types of firearms. One common method is the manually operated bolt, or bolt action, rifle. First developed in the mid-1800s, and used by both military and civilians, it is still a popular method of achieving power and accuracy in sporting and hunting applications.
Many modern bolt action rifles are "Mauser-style" rifles, based off a design developed for the German military in 1898. This action uses two large locking lugs at the front of the bolt to engage with the receiver. The operational portions of a rifle of this style are shown in Figure 1, with important portions of the assembly color-coded. A bare bolt of this style is shown in Figure 2. The chamber, with the loaded cartridge, would be to the right of the bolt face.
While any individual rifle is capable of firing a specific cartridge, a family of rifles may share identical receivers and bolts with only minor modifications required to fire a variety of different cartridges. Each unique caliber of cartridge has a specified peak pressure, which until recently was a "best guess" due to the difficulty in measuring pressures upwards of 50 ksi in a matter of milliseconds. In order to ensure strength, designs were often very conservative. Some designs, such as the Japanese Type 99 Arisaka, have been known to regularly withstand pressure well above the design cartridge values (Hatcher 210). Less conservative designs, particularly for military arms which saw high firing counts, would occasionally have to be recalled and rebuilt in order to ensure the safety of the user. Modern measuring methods have resulted in much more accurate measurements of peak pressure, and these values are regulated and published by two organizations, the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) in the United States, and the Commission Internationale Permanente pour
Figure 1: Bolt Action Rifle Nomenclature and Cross Section (Brophy 67)Red: Bolt Body / Orange: Receiver / Blue: Barrel / Yellow: Cartridge
Figure 2: Bare Rifle Bolt
1: Bolt face 2: Locking lugs 3: Extractor grooves 4: Bolt body 5: Backup lug 6: Bolt handle
l'Epreuve des Armes à Feu Portatives (Permanent International Commission for Firearms Testing, or CIP) in Europe. By knowing the maximum cartridge pressure and the size of the base of the cartridge, the force imparted by the cartridge on the bolt face can be calculated. Called the “bolt thrust”, this value is the maximum loading which the bolt may be expected to withstand during firing. Any given firearm action has a maximum bolt thrust value; as long as each cartridge chosen for use remains below the calculated bolt thrust, the action is safe to operate.
To test the level of safety, manufacturers are required to proof test their firearms, under both SAAMI and CIP regulations. Proof testing usually consists of firing “proof rounds,” cartridges loaded at approximately 130% of the standard peak pressure of the firearm being tested. The firearm is then disassembled and examined for any sign of fracture or plastic deformation. Particular attention must be paid to the locking mechanism, such as the bolt lugs described previously.
In older firearms, with less advanced manufacturing techniques, this testing procedure was particularly important to ensure safety. Despite the testing, issues would occasionally arise. For example, in the period leading up to the First World War, the United States army used the M1903 Springfield rifle, a bolt action built on the Mauser pattern (so closely, in fact, that the Mauser company later sued the US government and won royalties for patent infringement (Brophy 323)). Inconsistent heat treatment of the rifle receivers, built of high-carbon steel, resulted in some rifles in the field suffering catastrophic failures of the receiver during firing, at times severely injuring the individual holding the rifle. To solve this problem, better heat treat procedures were implemented. During the build-up to the Second World War, the raw materials were changes to a nickel-bearing steel known as WD 2340, with the heat treatment altered to reflect the requirements of the new steel.
1.1 PROBLEM DESCRIPTION
The most dangerous failure of a bolt action rile is the failure of the locking lugs, as this will both release the high pressures normally contained with the chamber, and propel the bolt rearwards, into the face of the firer. The sensible engineering approach, then, is to design the bolt-to-receiver interface with a large factor of safety, particularly in military firearms which are expected to have a long service life despite harsh environments, careless handling, and potentially inconsistent ammunition. Historically, these design calculations have been done with pencil and paper, but modern technology allows a more advanced and accurate approach. Utilizing computer aided design (CAD) and finite element analysis (FEA), a design can be analyzed electronically to determine failure points and modes. In this report, the M1903 Springfield rifle bolt was measured, modeled, and analyzed for stress loads under firing conditions. A schematic view of the bolt cross section is given in Figure 3, which includes the primary parts and forces involved.
Figure 3: Outline Diagram2. METHODOLOGY
2.1 GEOMETRY
The model was built in SolidWorks computer aided design (CAD) software. Dimensions were pulled from a government drawing of a field guage (Brophy 596), used to establish safe headspace in a rifle. This was compared to a vintage M1903 bolt, to ensure that all critial dimensions on the field guage matched the critcal dimension of the “live” bolt. All dimensions were modeled as least material tolerance limits, in order to take the most conservative case.
Figure 4: Bolt Model in CAD (exploded view)1: Cartridge Base / 2: Bolt Head / 3: Receiver Ring
Only the head of the bolt, the bolt face to the rear of the locking lugs, was considered for the FEA. This was done in order to reduce calculation time, as the remaining portions of the bolt are not stressed to any significant factor. Portions of the bolt forward of the front bolt face were not modeled for similar reasons. All units were done in US customary units (inch/pounds/seconds), rather than metric, to match source material.
Models were also created for the base of the cartridge, using the SAAMI standard sizing, and a receiver ring. By modeling the base of the cartridge, known load pressures could be directly input into the model, reducing the likelihood of a calculation error. The receiver ring is a stand in for the main body of the receiver. Clearance hole sizing on the reaction ring was taken from the machining plan for the receiver (Brophy 549), but other dimensional values may be approximate due to the difficulty in locating an accurate dimensional drawing of the finished receiver.
2.2 MATERIALS
Both the bolt and the receiver are made from a now-obsolete steel known as WD 2340 (Hatcher 224). This is the War Department’s designation for Society of Automotive Engineering (SAE) 2340 steel (Hatcher 231), which was taken off the SAE standards in the early 1950s. The bolt material was hardened to a Rockwell Hardness C of 33-44 HRC (Hatcher 226), equal to approximately 280 – 400 on the Brinell scale of hardness. Using the lower end of the hardness scale, aproximately 300 Brinell, results in the material properties shown in Table 1, taken from the material available in Appendix A.
Property / ValueYoung’s Modulus, elastic (typical to steels) / 29,700 ksi
Young’s Modulus, plastic (estimated) / 300 ksi
Poisson’s Ration (typical to steels) / 0.29
Density / 0.284 lb/in3
Yield Stress / 128 ksi
Terminal Stress / 150 ksi
Table 1: Material Properties of Heat Treated SAE 2340
2.3 FEA
The CAD model was imported into COMSOL Finite Element Analysis (FEA) software, using the stationary (static) solid mechanics module. This was chosen in lieu of a dynamic analysis to reduce the effect of the high strain rate on the strength of the materials. As the strain rate increases, the values for yield and terminal strenegth will also rise. By utilizing a static analysis, the results are more conservative than the values given by the dynamic module.
The orientation of the model is show in Figure 5. The material properties described in section 2.2 were input for the receiver ring and bolt. Material properties for C26000 annealed cartridge brass were used for the base of the receiver (MatWeb, LLC).