The Basics of Fastening
What is a threaded fastener
A discrete piece of hardware that has internal or external screw threads. The simplest example of this is a bolt and a nut.
Why use a threaded fastener?
Ease of assembly and disassembly and the cost of repairs and maintenance
What is a Bolted Joint?
Two or more components joined together by one or more threaded fasteners
What is torque?
Torque is the measure of a twisting force (Torque= Force x Distance)
How tightening a bolted joint works.
Torque is applied to fastener causing fastener to move on threads, converting the rotary motion of applied torque to a linear motion that's reducing the distance between the bolt and the nut squeezing two or more parts together. This condition is called snug. If we continue to apply torque, the bolt will stretch, creating tension in the fastener. When we stop applying torque, compressive stress in the joint is maintained by tension in the bolt, this is called clamp load. Clamp load is what we're trying to achieve with every tightening.
Where does the torque go?
Only 10% or torque goes to stretching the bolt, the other 90% is lost to friction. That 90% is broken up into 40% thread friction and 50% under head friction. The remain 10% is what's used towards developing clamp load.
How Friction Influences Torque
The easiest way to see how friction influences torque is by looking at the difference between a dry and lubed bolt being tightened. A lubricated bolt will always achieve a higher clamp load due to having a lower coefficient of friction. When comparing a dry bolt that is being tightened with the amount of applied torque, it will have a substantially lower clamp load due to a higher coefficient of friction.
What is Angle?
Angle is the measured degree of fastener rotation.
Ex. 1 full rotation is 360 degrees and a 1/4 rotation is 90 degrees.
Joint Rate
ISO 5393 is a standard held by the International Standard Organization that defines joint rate. For users of threaded fastener this standard offers a method for evaluating and specifying performance of power assembly tool. For manufacturers of power tools it allows manufacturers to offer products under correlated specifications. Joint rate is the degree of fastener rotation required to go from snug to final torque. Joint rate cant be influenced by adding things like rubber gasket or plates to make the gap your are trying to close have a buffer between snug and final torque.
Softs joints
A soft joint means once in the snug position the fastener needs to rotate 720 degree or more before completion of tightening.
Hard Joints
A hard joint means once in the snug position the fastener needs to rotate no more than 30 degrees to completion of tightening. It takes very little rotation to hit final torque making speed and control critical.
Tightening curve
Tightening can be broken down into 2 phases: the elastic phase and the plastic phase. The elastic phase is the range we can tighten the fastener and it maintain its elastic properties allowing it to return to its original size and shape. The point you move from the elastic phase and the plastic phase is what's known as the yield point. This is the point in which the fastener begins to lose its elastic properties and will not fully return to it's original size and shape. If you continue to tighten in the plastic phase the fastener will continue to stretch getting smaller in diameter until it eventually breaks. Most tightening is done in the elastic range. This matters because this is what will be referenced by engineers when determining clamp load and deciding which fastener to use. Typically engineers will choose a fastener that reaches clamp load at 75% of the proof load which is 85-95% of the yield point.
Tightening Strategies
The most commonly used strategy is torque with angle monitoring. This strategy is used to achieve a specific target torque while also measuring angle from a programmed threshold torque value. In many cases angle is measured from snug. With this strategy both the torque and the angle have user defined high and low limits. In order for the tightening to pass both the torque and angle need to be within the prescribed high and low limits.
Another tightening strategy is the "prevailing torque" strategy. This is the optimal strategy to use when special fasteners are being implemented that might unintentionally hit the target torque too early in the tightening process. Some examples of this are Nyloc nut, self tapping screws, or fasteners with thread adhesive patches. If a basic torque strategy were used with a target torque of 5 in-lb the tool would achieve torque too early in the process without fully being seated or the fastener achieving clamp load. This strategy allows you to break the tightening down into 3 controllable zones that'll allow the fastener to fully seat and achieve clamp load. Those 3 zones are cut in, prevailing and final torque. Another way to look at this is angle step, another angle step and then final torque step. This will allow you to push through the high cut in torque and insure you're in the high and low limits in the prevailing zone, then simply seat the fastener to the desired torque to achieve clamp load. When programming this strategy, you would prescribe the degrees of rotation and the maximum allowable torque for the cut in zone. You would then prescribe the degrees of rotation with a high and low torque limit for the prevailing zone, then program the final torque target with high and low limits allowing you to have a fully seated fastener that achieves clamp load.
Error Proofing w/ Tightening Strategies
Why do we care about angle? The reason we care about angle and measure it to make sure it's within the prescribed limits, is called "error proofing". Error proofing is the implementation of fail-safe mechanisms that prevent the process from producing defects. There is a Japanese term used in the industry called "Poka-Yoke" which means to avoid inadvertent errors. Using a torque with an angle monitoring strategy we can detect the absence of a washer, crossed threads, blind holes (too shallow or shavings in the hole), misaligned parts, stripped threads, cracked or split parts and warped parts. Using tightening strategies to error proof the process can save time and money by insuring the the clamp load is achieved and quality is met.
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