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Everything You Need to Know About Fasteners ... and More
Important Technical Information
The commercial fastener industry, without sufficient exposure to aerospace technology, is generally unaware of the materials or processes required to produce high strength/high ductility fasteners of this caliber. The use of carbon steels has been their standard for years. The problem is easy to understand if you know that carbon steels are not very structurally pure materials. When cold form headers came of age, there was a need for cleaner materials. Alloys that would prevent splitting during the forging process. The materials of choice in most commercial bolts and socket screws can be safely heat treated to 160,000-180,000 psi. Anything above this leads to embrittlement or a loss of ductility. Commercial bolt companies would have you believe that when the tensile strength exceeds 200,000 psi the fastener becomes too brittle. That may hold true for their materials but ... ARP fasteners are different. (This article uses information borrowed from published ARP data)
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What You Will Find in This Article:
- Special alloys make a difference
- ARP carefully evaluates available materials
- Fastener design is critically important
- Proper fastener retention (tightening)
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SPECIAL ALLOYS MAKE A DIFFERENCE
Those in the aerospace industry know better, and for years they have used alloys capable of 300,000 psi while retaining excellent ductility. The variety of materials used for threaded fasteners are characterized by their strength. These can range from moderate to ultra high "super alloys". Reciprocating loads, like as in connecting rods are the most significant applications for these broad range of materials. The most cost-effective design is the one in which the bolt strength is great enough to handle the anticipated load, plus a safety margin for occasional overloads. Using a material that provides more strength than is required is generally not cost effective, because as strength ratings increase so does material cost. Although in many instances that increase is quite significant. ARP® engineers typically works with (7) primary alloys.
In the Quench and Temper group they use: 8740 Chrome Moly, ARP® 2000 and L19. They are continually developing new materials and manufacturing processes like the following "super alloys": INCONEL 718, MP-35 & MP-159, Custom Age 625 and Aermet 100. (specs for these materials are found HERE ...  )
MATERIALS ARE CAREFULLY EVALUATED
ARP® features a complete in-house laboratory faculty for evaluating materials as well as testing the performance of threaded fasteners. Tensile and fatigue strength are the primary characteristics studied to be able to develop the best possible products. Actual breaking tests are performed to determine "Exact" fastener capabilities, not just using some arbitrary specification. Such specifications only establish minimum performance and strength values. Accurate knowledge of strength values permits efficient and cost effective fastener design.
Only Part of the Picture
Materials and tensile strength selection is only part of the picture when manufacturing racing quality fasteners. All ARP® products are designed to specifically meet their intended application.
DESIGN IS CRITICALLY IMPORTANT
The occurrence of fastener fatigue is principally due to cyclic loading and off-loading of the fastener. While fatigue resistance can be partially attained by material selection and tensile strength, specific fastener designs are equally important. Undercutting the shank of a bolt or stud increases it's flexibility and spreads clamping loads more uniformly.
Many Factors Contribute to Overall Reliability
Full engagement of threads reduces stress concentrations at the root of the radius of the last thread. Shot-Peening sets up a compressive stress on the surface that helps prevent cracking. Perhaps the most significant process is thread rolling AFTER heat treatment. This is the most significant because it alone can increase the fatigue life of a bolt by more than 100% over a fastener threaded prior to heat treatment. Careful attention to the shape of the thread root radius and it's blending with the flanks is crucial. All ARP® threads are rolled in accordance with MIL-S-8879 (the well known fatigue resistant J-Form thread). This ensures that notch sensitivity of the root section of the thread is reduced to the Mil-Spec minimum. The following paragraph is from this specification and is simply for your information.
"6.1 INTENDED USE. Threads covered by this specification are recommended for high temperature use and for applications requiring very high fatigue life stress levels commensurate with the physical size an weight of the product. Applications are found in aircraft engine and airframe, missile. space vehicle, and similar designs areas where size and weight are critical."
A Variety of Designs for Specific Reasons
Undercutting:
Reducing the shank diameter of fasteners reduces head gasket sealing problems. Short fasteners are very stiff and the amount of bolt stretch available is about the same as the amount of compression in the gasket. Should the gasket lose some of it's compression due to set, it can quickly eliminate the pre-load (torque) in the fastener - which will unload the gasket. This results in a blown head gasket. Providing optimum stretch is absolutely critical. Using an undercut stud helps control head to gasket clamping force better than conventional studs.
Step-Down Studs:
Cylinder head studs that are 7/16"-14 on the block end and 3/8" shank and a 3/8"-24 nut thread is an example of a step-down stud . These are made specifically for cylinder heads that have been rolled over or severely angle milled. This feature is a great benefit to engine builders because it allows us not to start enlarging or changing the direction of the fastener's original passage through the cylinder head. Modifications to this area will usually get a water jack or port opened up (which is not what we want). Another benefit is be able to place cylinder head studs with small diameter bolt holes on cylinder blocks with larger diameter mounting bolt threads with no modifications.
12-Point Head:
Reduced wrench diameter fastener heads give added clearance on racing applications that utilize repositioned valves and large diameter valve springs. This eliminates the need for disassembly of valvetrain components to facilitate re-torquing. Used in many other areas as well for the same basic principal of giving one more access to properly tighten the fastener. Examples are Intake Manifolds, and Headers.
Rocker Arm Studs:
Rocker studs are subjected to severe bending loads. This cyclic bending is the main source of Rocker Stud failure (fatigue). Therefore, a material with good ductility at higher tensile strength is indicated. To ensure proper valve train geometry, concentricity is maintained to a tolerance of .0050" maximum T.I.R. from end to end. Rocker studs from ARP® are offered in two tensile strength values: 170,000 psi for high performance applications, and 190,000 psi for maximum and extreme applications where minimum deflection at extended RPM's required. Also, many of ARP® rocker studs have a special step flange for the use of push rod guide plates.
Accessory Fasteners:
Even though there are few critical loading issues with the mounting of carburetors, water pumps, oil pans, timing covers, etc. ARP® has put a great deal of effort into the manufacturing of only the best fasteners for these tasks. A number of automotive manufacturers use a material called "Leadloy" for these low stress items. It's very cheap and easy to machine, however the material (as easy to see as in the name) carries a high proportion of lead. Some problems that are associated with these "Leadloy" fasteners include: galling of the studs and nuts, inferior clamping ability (which usually means a "leak"). All ARP®'s fasteners are made of heat treated alloys that are at a minimum 170,000 psi, which are 15% stronger than Grade 8 hardware. They are available in both Black Oxide as well as Stainless Steel.
PROPER FASTENER RETENTION
There are essentially three methods that can be employed to determine how much tension is exerted on a fastener; using a torque wrench, measuring the amount of stretch, and turning the fastener a predetermined amount (torque angle). Of these methods, the stretch gauge is most accurate. It is important to note that in order for a fastener to function properly, it must be "Stretched" a specific amount. The material's ability to "rebound" like a spring is what provides the clamping force.
We can control this stretch by using an accurate torque wrench and a good thread lubricant. On the other hand, if a fastener is over-torqued and becomes stretched too much, you have now exceeded the yield strength, and the fastener is RUINED. (replace it or deal with the consequences).
"If the fastener is longer than manufactured, even if it is only .001", the fastener is in a partially failed condition."
Therefore, all ARP® fasteners are designed to stretch a given amount and must be somewhat elastic.
Heat, primarily in aluminum is another problem area. Because of the thermal expansion of the material (aluminum) is much greater than steel it is possible to stretch a fastener beyond yield as the aluminum expands under heat. An effective way of combating or counteracting this material expansion is to manufacture a more flexible fastener when being used to clamp aluminum components.
Tightening - The Torque Angle Method
Since the amount a bolt or nut advances per degree of rotation is determined by the thread pitch, it would appear that the amount of stretch in a given bolt or not can be accurately predicted by measuring the degrees of turn from the point where the underside of the bolt head or nut face contacts the work surface. Termed the "Torque Angle" method, this procedure has long been the standard of civil engineering. It has been suggested that torque angle is a relatively simple and valid procedure to use in our "blind" installations where it is not possible to physically measure the actual bolt stretch.
Simple calculation of bolt stretch based upon thread pitch is not accurate. No material is incompressible! When a bolt or stud is pre-loaded or stretched, the components being clamped will compress to some extent. When we are looking for bolt stretch in mere .001 (thousandths) of an inch, the amount of clamped material compression becomes a real factor. Investigation has proven that installed stretch is dependent upon not only the pitch of the thread and degree of rotation, but also on the amount of compression of the clamped components, the length of the male fastener, the amount of engaged thread the type of lubrication and the number of times the fastener has been cycled.
For Example: Given the same degree of rotation, the actual amount of bolt stretch will be critically different between an Aluminum cylinder head vs. a Cast Iron one. Or even a steel main cap on an Aluminum block vs. a Cast Iron block. Further, there is a significant difference between long and short head bolts or studs on the same head. The torque angle method can be accurate, but only if each individual installation has been previously calibrated by direct measurement of bolt stretch.
When using the torque angle method, it is best to begin rotation from a small measured torque (no more than 10 lb./ft), rather than the first point of contact with the work face. To achieve accuracy it is also best to cycle the fastener five times before either calibrating or installing.
Tightening - Using a Torque Wrench
If the stretch method cannot be used in a particular installation so that the fasteners must be installed by torque alone, there are certain factors that must be taken into account. ARP® research has verified the following "rules" pertaining to the use of a torque wrench:
- The friction factor changes from one application to the next. That is, the friction is at it's highest value when the fastener is first tightened. Each additional time the fastener is torqued and loosened this value gets smaller. Eventually the friction levels out and becomes constant for all the following repetitions. Therefore, new fasteners should be tightened and loosened several times before applying final torque. The number of times to complete these tighten/loosen repetitions depends upon the lubricant used. For all applications where ARP® lubricants are used, they are designed for (5) cycles before final torquing.
- The lubricant used is the main factor in determining friction, and therefore the torque in that particular application. Motor Oil is a commonly used lubricant because it is readily available. If less friction is desired in order to install the fasteners with less torque, special low friction lubricants are available. With special lubricants, the actual torque values can be lowered 20-30%. It is important to keep in mind that the reverse can also be true. If the torque has been decided on a particular fastener on the basis of a special low-friction lubricant, then installing it with motor oil will result in insufficient pre-load. The torque will need to be upped to compensate for the extra friction caused by the motor oil. Click HERE For Fastener Torque Specs ...
- Surface finish is also important. For example, a black oxide fastener behaves differently than a polished fastener. This makes it important to observe the torque recommendations supplied with each fastener.
NOTE: It is possible in even the most expensive torque wrenches to lose accuracy. I have seen fluctuations as much as "Ten (10) Foot Pounds" from wrench to wrench. It is a good idea to have your torque wrench calibrated periodically to maintain it's accuracy.
Tightening - The Stretch Gauge
It is highly recommended using a stretch gauge when installing rod bolts and other fasteners where it is possible to measure the length of the fastener. The stretch gauge is the most accurate way to determine the correct pre-load in the rod bolt. Simply follow the manufacturer's instructions. Measure the fastener prior to starting and then monitor the overall length during your installation. When the bolt has stretched to the specified amount, the correct pre-load or torque has now been applied. We recommend you keep a chart as to the all the rod bolts before and after lengths. By doing this, upon disassembly, if there is a bolt that has made a permanent growth in length beyond the .001" margin, it is time to replace it. Also if there has been any deformation to the threads or any portion of the bolt it should be replaced as well.
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