Fastener Technology
Having knowledge on fastener technology that includes fastener materials, specifications, and an understanding torque values and measuring techniques allow our engines to last longer and survive the most extreme use. This article provides a starting point for acquiring that knowledge.
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 the highest caliber. The use of carbon steels has been the fastener manufacturing standard for years. However, the problem is easy to recognize once acknowledging that carbon steels are not structurally pure materials.
When cold form headers came of age, there was a need for cleaner base materials, including alloys that would prevent splitting during the forging process. The materials of choice for most commercial bolts and socket screws can be safely heat treated to approximately 160,000-180,000 psi. Anything above this leads to embrittlement, or a loss of ductility. Commercial fastener manufacturing companies would have us believe that when the tensile strength exceeds 200,000 psi the fastener becomes too brittle. That may hold true for common materials, however, the fasteners manufactured by Automotive Racing Products (ARP) are different. (NOTE: This article uses information borrowed from published ARP data.)
Contents of This Article:
- Special alloys make a difference
- ARP carefully evaluates available materials
- Fastener design is critically important
- Proper fastener retention (tightening)
Special Alloys Make a Difference
Carefully Evaluated Materials
Critical Importance of Design
Many Factors Contribute to Overall Reliability
A Variety of Designs for Specific Uses
Undercutting:
Step-Down Studs:
12-Point Head:
Rocker Arm Studs:
Accessory Fasteners:
Proper Fastener Retention
- Using a torque wrench
- Measuring the amount of stretch
- Turning the fastener a predetermined amount (torque angle).
Tightening – The Torque Angle Method
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 0.001 (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.
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 its highest value when the fastener is first tightened. Each additional time the fastener is torqued and loosened (cycled) 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, (5) cycles are required before final torquing.
- The lubricant used is the main factor in determining friction, and therefore the torque value 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. Therefore, it is important to observe the torque recommendations supplied with each fastener.
Tightening – The Stretch Gauge
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