Fastener Retention Overview
There are 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 pre-determined amount (torque angle). Of these methods, use of a stretch gauge is the 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. You should know that different materials react differently to these conditions, and ARP engineers have designed each fastener to operate within specific ranges.
On the other hand, if a fastener is over torqued and becomes stretched too much – you have exceeded the yield strength and it’s ruined. If the fastener is longer than manufactured – even if it is only .001,? it is in a partially failed condition. Therefore, ARP has engineered its fasteners with the ductility to stretch a given amount and rebound for proper clamping.
Heat, primarily in aluminum, is another problem area. Because the thermal expansion rate of aluminum is far greater than that of steel it is possible to stretch a fastener beyond yield as the aluminum expands under heat. An effective way of counteracting material expansion is through producing a more flexible bolt.
The Torque Angle Method
Since the amount that 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 stud 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.
ARP has conducted extensive evaluations of the torque angle method. We have concluded that, for our purposes, it is suitable only when individually calibrated for each installation.
Simple calculation of bolt stretch based on thread pitch is not accurate. No material is incompressible. When a bolt or a stud is preloaded or stretched, the components being clamped compress to some small extent. When we are looking for bolt stretch of only a few thousandths of an inch, the amount of clamped material compression becomes a very real factor.
Our investigation has proven that installed stretch is dependent, not only on the pitch of the thread and the 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 that the fastener has been cycled.
For example, for the same degree of rotation, the actual amount of bolt stretch will be critically different between an aluminum cylinder head and a cast iron cylinder head – or a steel main cap on an aluminum block and a steel main cap on a cast iron block.
Further, there is a significant difference between the long and short cylinder 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 some small measured torque – no more than ten lb./ft. – rather than the first point of contact with the work face. To achieve accuracy it is also best to cycle the fasteners five times before either calibrating or installing.
Using a Torque Wrench
If the stretch method cannot be used in a particular installation, and the fasteners must be installed by torque alone, there are certain factors that should be taken into account. ARP research has verified the following “rules” pertaining to use of a torque wrench:
1. 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, this value gets smaller. Eventually the friction levels out and becomes constant for all following repetitions. Therefore, new fasteners should be tightened and loosened through several cycles before applying final torque. The number of times depends on the lubricant. For all situations where ARP lubricants are used, five cycles are required before final torquing.
2. The lubricant used is the main factor in determining friction, and therefore, the torque for a particular installation. Motor oil is a commonly used lubricant because of it’s ready availability. If less friction is desired in order to install the fasteners with less torque, special low friction lubricants are available. With special lubes, the required torque can be reduced as much as 20 to 30 percent. It is important to keep in mind that the reverse is also true. If the torque value has been specified for a particular fastener on the basis of low friction lube, installing the fastener with motor oil will result in insufficient preload; the torque has to be increased to compensate for the extra friction caused by the motor oil.
3. Surface finish is also important. For example, black oxide behaves differently than a polished fastener. It is therefore important to observe the torque recommendations supplied with each fastener.
NOTE: It is possible for even the most expensive of torque wrenches to lose accuracy. We have seen fluctuations of as much as ten (10) foot pounds of torque from wrench to wrench. Please have your torque wrench checked periodically for accuracy.
Using a Stretch Gauge
We highly recommend using a stretch gauge when installing rod bolts and other fasteners where it is possible to measure the length of the fastener. It is the most accurate way to determine the correct pre-load in the rod bolt.
Simply follow manufacturer’s instructions, or use the chart on page 25 of the ARP catalog for ARP fasteners.
Measure the fastener prior to starting, and monitor overall length during installation. When the bolt has stretched the specified amount, the correct preload, or clamping load, has been applied.
We recommend you maintain a chart of all rod bolts, and copy down the length of the fastener prior to and after installation. If there is a permanent increase of .001? in length, or if there is deformation, the bolt should be replaced.