Three methods exist that can be employed to determine how much tension is exerted upon a fastener.
- Using a torque wrench
- Measuring the amount of stretch
- 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 proper stretch by using quality tools (an accurate torque wrench) and premium 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 effectively RUINED and must be replaced! Replace this fastener or deal with the consequences!
“If the fastener is longer than manufactured, even if it is only 0.001”, 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 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 that is 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 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.
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. This is also true for a steel main cap on an Aluminum block vs. a Cast Iron block. Furthermore, 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 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.
NOTE: It is still possible for even the most expensive, highest quality torque wrenches to lose accuracy. I have personally noticed fluctuations by as much as “Ten (10) Foot Pounds” from wrench to wrench. It is a good idea to have your torque wrench professionally calibrated periodically to maintain its accuracy.
Tightening – The Stretch Gauge
It is highly recommended to use a stretch gauge when installing connecting rod bolts and on other fasteners where it is possible to measure the overall length of the fastener. The stretch gauge is the most accurate way to determine the correct pre-load in connecting rod bolts. 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 that you keep a written chart or table that documents the before and after lengths for every connecting rod bolt—and do not mix bolts and their associated nut! By doing this, upon disassembly, if there is a bolt that has experienced a permanent growth in length beyond the 0.001″ margin, you’ll know that it is time for replacement. Additionally, if any deformation to the threads or any portion of the bolt exists, this should also mandate replacement.
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