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|04-03-2005, 08:32 AM||#1|
Join Date: Nov 2001
Location: Las Vegas, NV, USA
M14 Spring FAQ
Your feedback is appreciated. Let's make it better! I've asked sven to post this at http://www.imageseek.com/m1a
M14 Spring FAQ by Lee Emerson
Note: This FAQ will be included later in the print Second Edition of M14 Rifle History and Development. Many thanks to Bill Ricca and Jeff Shapiro for their assistance with this paper.
The M14 rifle has fourteen springs though some are not readily obvious: bolt lock spring, butt plate hinge spring, butt plate trap door spring, connector assembly spring, elevation knob screw spring, ejector spring, extractor spring, hammer spring, magazine latch spring, operating rod spring, rear sight cover, safety spring, selector shaft spring and spindle valve spring. Additionally, the National Match hooded rear sight aperture has a small coil spring. The magazine, M3 breech shield, M6 bayonet, M12 blank firing attachment, and M76 grenade launcher each have one spring as well. The M2 bipod has four springs, one for each plunger button.
There are several types of coil springs, compression, die, extension, torsion, etc. A coil spring is typically made by coiling round wire or thin rod into a helical shape. With the exceptions of the elevation knob screw spring, rear sight cover and the safety spring, M14 type rifle springs are classified as constant diameter compression coil springs. A compression coil spring will resist a force applied straight into the end of the spring. When a compression coil spring is compressed axially (end-to-end) other forces are induced on the spring as well: lateral (side-to-side), a rotating moment and a tilting moment. The constant diameter design means the width of the spring does not vary.
Compression coil springs are further categorized by how the end of the spring is formed and how the material is wound. The end of a constant diameter compression coil spring can be formed in one three ways, tangential or open, squared or closed, and pigtail. The open end spring is cut anywhere along the helix so that there is no change in the pitch or spacing of the coil. The pigtail end means the spring is curled inward on the last coil to a smaller diameter then cut. The closed end spring is formed so that the spring will stand upright when placed on end. Closed end and open end compression coil springs are often ground to create very flat surfaces at each end. The following M14 rifle springs have closed ground ends: ejector spring, extractor spring, hammer spring, operating rod spring and spindle valve spring. The selector shaft spring and bolt lock spring have open ends and are not ground. Magazine latch springs observed have closed ends but do not appear to be ground.
Grinding the end of the spring may be done to better distribute the applied force, make assembly of the parts easier, or in some cases prevent buckling. The free length of the compression coil spring compared to its mean diameter is known as the slenderness ratio. A compression coil spring with ground closed ends having a slenderness ratio of less than four will not buckle. A compression coil spring with unground closed ends may buckle if the slenderness ratio exceeds 2.63. The slenderness ratios for the bolt lock spring and the extractor spring are 4.05 and 4.03, respectively. It is not known if the slenderness ratio was considered in the designs of the bolt lock spring and extractor spring or if those values were coincidental to any concern for spring buckling.
The direction of the coil spring helix (left-hand or right-hand) is specified in several of the drawings. The bolt lock spring helical direction is optional per USGI drawing 7267074. The USGI drawing 7267078 specifies a left-hand twist for the magazine spring. The USGI drawings for the ejector spring, extractor spring, magazine latch spring and operating rod spring require a right-hand helical direction for each spring. If a spring does not have a helical direction in agreement with the USGI drawing it is likely a commercial reproduction.
The compression and fatigue strength of a coil spring is dependent upon a number of factors: material composition, wire surface finish, wire diameter and length, coil diameter, total number of coils and heat treatment. Spring manufacturers use a wide variety of materials for coil springs: carbon, alloy and stainless steels, phosphor bronze, copper beryllium alloy and brass depending on the application.
The USGI drawings for the M14 rifle compression coil springs always specify the wire material, the wire diameter, the coil outside or inside diameter, the free length, and the total number of coils. The free length of a compression coil spring is the distance from one end of the spring to the other with no force applied. The USGI drawing 7267079 for the operating rod spring is illustrative of the detailed requirements typical of M14 parts production. Some of the drawing 7267079 specifications are described below.
The operating rod spring material was 17-7 precipitation hardening stainless steel wire per military standard Mil-W-46078 and manufactured under military standard Mil-S-13572 Type 1 Grade B. The physical dimension and performance requirements were as follows: wire diameter 0.054 + or 0.001 , coil outside diameter 0.4575 + or 0.0025 , free length = 15.23 , a total of 104 coils, the direction of the helix must be right-hand wound and have closed ends.
The operating rod spring formed by coiling cold drawn wire had to be heat treated as follows: age hardened at 900 degrees Fahrenheit for one hour then air cooled. After that, the operating rod spring was heat set by heating at 700 degrees Fahrenheit for twenty minutes with the spring compressed to a length between 5.9 and 6.0 inches. Heat setting a spring improves the stress relaxation during use. Age hardening is used to increase the strength and hardness of manufactured parts made from certain alloys, e.g., copper-beryllium and 17-7 cold drawn stainless steel. Each material responds differently to the age (precipitation) hardening process based on a combination of time, temperature and the amount of cold drawing from the initial rod size into the final wire diameter. Depending on the material, the toughness, corrosion resistance, fatigue strength, electrical conductivity or thermal conductivity can be improved upon by stopping the hardening procedure before or after the time needed to achieve maximum strength. By age hardening and heat setting the operating rod spring, service life was significantly increased.
The operating rod spring, like other compression coil springs, was designed with a given spring rate. The spring rate is defined as the amount of load (force) needed to compress the spring one inch. The lower the spring rate the softer the spring. After the operating rod spring had been heat treated and heat set it was compressed to solid length three times and then load tested at several specific lengths. These tests verified the operating rod spring met the design spring rate. The design spring rate for the M14 operating rod spring was 1.95. When the operating rod spring was compressed to a length of 10.97 the applied load had to equal 8.13 pounds + or 0.81 pounds. When the operating rod spring was compressed to 6.42 the required load was 17.00 pounds + or 1.69 pounds.
When the operating rod spring was compressed to the minimum operating length it measured about 7.19 long with an applied load under 16.00 pounds sufficient to engage the bolt lock. The USGI drawing 7267079 for the operating rod spring specified a maximum length of 5.78 for the solid compressed length. The design and manufacturing process ensured the USGI M14 operating rod spring would function properly for thousands of cycles. The M14 rifle enthusiast or collector should proceed with caution when hearing the term mil-spec in casual conversation with such detailed design, manufacturing and testing requirements for USGI M14 parts.
Spring material is subjected to torsional (twisting) stress. When a wire is twisted the stress is greatest at the surface. Surface defects are commonly the site of where fatigue cracks start in springs. Thus, it is important to have a very smooth surface to prevent fatigue cracking of springs. Consequently, wire surface defects, e.g., pits and seams, are controlled in the cold drawing manufacturing process. For example, Seneca Wire & Manufacturing Company (Fostoria, OH) limits the depth of surface defects in its ASTM A877 chromium silicon alloy steel wire to no more than 1.0 % of the wire diameter. ASTM A877 chromium silicon alloy steel wire is suitable for service applications requiring high fatigue strength at moderately high temperatures such, e.g., M14 operating rod spring.
The question often arises as to whether or not a compression spring will take a permanent set if compressed to minimum operating length and held over time, e.g., full magazine or bolt held open. A permanent set occurs when the compression spring is compressed beyond its elastic limit and does not return to the original length. This results in a shorter free length but more significantly, lower spring force. A permanent set in a compression spring is not formed by compressing it to the minimum operating length and leaving it in that condition.
The majority of the compression coil springs in the M14 rifle are subjected to fatigue stress over a very large number of operating cycles. These springs were designed with this service requirement in mind. If a spring weakens enough the M14 rifle will malfunction in one manner or another. A weak magazine spring could cause cartridge feeding problems. If the extractor spring is weak the spent case may stick in the chamber after firing. A soft ejector spring could result in a spent case being caught between the bolt and receiver. In a combat situation, such malfunctions could prove fatal. Nonetheless, over the course of thousands and thousands of cycles, the free lengths of the operating rod spring and the hammer spring will gradually shorten so that they will warrant replacement.
Compression coil springs can be designed to compress to solid height or length without taking a permanent set. If a permanent set is not desired, the spring material and diameter is chosen so that the torsional stress when compressed solid does not exceed approximately 40 % of the material minimum tensile strength. The minimum tensile strength, or yield strength, will vary with the diameter of the wire, e.g., 231,000 to 399,000 psi for ASTM A228 music wire.
The spring design must also consider the temperature to which it will be subjected. The gas piston, and to a lesser degree, the operating rod guide, transfer heat to the operating rod and the operating rod spring. Temperature at the forward end of the operating rod has been measured above 500 degrees Fahrenheit. Chromium silicon alloy steel operating rod springs are rated for service to at least 700 degrees Fahrenheit (see Commercial and Chinese Parts).
The spindle valve spring is heated from burnt gun powder gas flowing from the barrel into the gas cylinder. The spindle valve spring is made from stainless steel to resist the high temperature gas. Stainless steel has a higher maximum service temperature than carbon steel. The specific AISI alloy stainless steel for the spindle valve spring was not available but the following serves to illustrate the point. 17-7 precipitation hardening stainless steel wire is rated for a maximum service temperature of 600 degrees Fahrenheit as compared to 250 and 300 degrees Fahrenheit for hard drawn carbon steel and oil tempered carbon steel, respectively.
The hammer spring is made from American Society of Testing and Materials (ASTM) 228 specification music wire according to USGI drawing 6008887. ASTM 228 music wire is a high carbon steel with hardness ranging from 41 to 60 HRC. It is composed of a minimum of 98.4 % iron and a range of 0.2 to 0.6 % for manganese and 0.7 to 1.0 % for carbon content. ASTM A228 music wire is rated for a maximum service temperature of 250 degrees Fahrenheit. This rating is acceptable since the hammer spring is not intended to operate at that high a temperature. ASTM A877 chromium silicon alloy steel wire and ASTM 228 steel music wire are popular choices for applications requiring excellent fatigue strength such as the M14 rifle hammer spring.
|04-03-2005, 10:00 PM||#2|
Join Date: Oct 2004
Different, thanks for the technical info on springs. Knowing what our tools are made of leads to a better understanding of the nuts and bolts of what we are holing in our hands. But really, why do we need to know spring reaction tables? If the darn thing doesn't work, replace it!
I've been a professional automotive mechanic for 28 years, and I couldn't tell you what the spring rate of a standard solenoid is. I DO know that if 12 volts does not provide enough electromagnitism to overcome the spring rate. the device doesn't work.
You have provided us with more information on the M-14 weapons platform than anyone else has ever been able to. But I have to ask....what does this info provide?
I know this will probably ignite some bad feelings, and I truly hope it is not so. I've always looked forward to your postings, KNOWING that I will learn something valuable. But sometimes, 'ya just gotta say...HUH?
|04-03-2005, 11:55 PM||#3|
Join Date: Nov 2001
Location: Las Vegas, NV, USA
Remember that this FAQ will be a small part of Part 3 of the Second Edition of M14 Rifle History and Development. Part 3 of the book concerns USGI and commercial manufacture M14 parts. There are a lot of folks paying a lot for USGI M14 parts. It's no secret that those parts will get more expensive. There have been and will continue to be folks who make, package and sell fake parts or misrepresent commercial parts as USGI. There are fake USGI magazines for sale wrapped in packaging that looks to be genuine. The hump artists are using this packaging to rip you off. How do you tell it's fake if the packaging is genuine?
Example, in the post above you're informed that a USGI M14 operating rod spring has to have 104 coils. Well, I have one in my parts stash that is 101 coils. I now know that is not a USGI part. I have an extractor spring in my parts stash that has a left-hand twist. I know now it is a commercial reproduction spring and not USGI. Also, I've seen hammer springs that are quite a bit shorter than the USGI dimension. Worn spring? Want to put it in your new LRB Arms M14? Hopefully, I have saved some of you from getting ripped off. HTH
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|04-04-2005, 06:43 AM||#5|
Join Date: Jun 2002
Location: New Tripoli, Pa
Different's in depth detail of springs is good to know. It shows, for instance, that the ejector spring was an improvement over the Garand. Knowing the development, specifications, and history of a spring can tell a lot.
In order to limit the size of his chapter, he has left out the requirement for measuring tools and multiple testing done at different points of compression. If he included everything required to a GI spring, it would be overkill.
Different's FAQ is well done and demonstrates the silliness of those who claim their new production is "mil-spec".
BTW just a trade secret here. In the early 1990's I got a batch of M14 Operating Springs on bid at Mechanicsburg. The springs were from a production in 1983. They compress properly and were accepted. However afterwards they were released surplus maybe due to the wrong direction of twist??
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