Generally, in the majority of applications, the reliability of the joint is dependent upon the bolts ability to clamp the parts together. Adequate clamping prevents relative motion between parts of the joint and leakage from joints containing gaskets. Measuring a bolts clamp force is difficult, especially under production assembly conditions.

The clamp force generated by a bolt can be indirectly controlled by regulating the applied torque. This method, known as torque control, is by far the most popular method of controlling a bolts clamp force. The initial clamp force generated by the bolt is frequently called preload.

There is a link between the torque applied to a bolt and the resulting preload. A problem exists in that friction has a large influence on how much torque is converted into preload. Besides the torque required to stretch the bolt, torque is also required to overcome friction in the threads and under the nut face. Typically, 10% to 15% of the torque is used to stretch the bolt. Of the remaining torque, typically 30% is dissipated in the threads and 50% to 55% under the nut face. Because friction is an important factor in the relationship between torque and preload, variations in friction have a significant influence on the bolts preload. Different bolt surface finishes generally have different friction values.

The torque required for a socket head screw will not be the same as that required the same size standard hexagon bolt. The larger bearing face of the standard bolt will result in an increased torque being required compared to a socket headed screw. This is because more torque is being dissipated between the nut face and the joint surface.

Stresses indicated into a Bolt: When a bolt is tightened the shank and thread sustain a direct (tensile) stress due to it being stretched. In addition, a torsional stress is induced due to the torque acting on the threads. These two stresses are combined into a single equivalent stress to allow a comparison to be made to the bolts yield strength. In order to effectively utilize the strength of the bolt, yet leave some margin for any loading the bolt would sustain in service, an equivalent stress of 90% of yield is commonly used. It is this approach that is used in this guide.

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In the manufacturing industry, one way to report and evaluate the process capability and process performance is through the statistical measurements, like Cpk.

What is Cpk?

Definition: Cpk = Cpk = Process Capability Index. Adjustment of Cp for the effect of non-centered distribution. Cpk measures how close you are to your target and how consistent you are to around your average performance.

Cpk measures two things: 1) how close the mean of the readings are to the center of the lower and upper spec limits (ideally, the mean of the readings must equal the center of the spec limits); and 2) how widely spread the readings are (ideally, the standard deviation of the readings should be zero). The higher the Cpk, the better is the capability of the process to meet its requirements.     

In the industry, a Cpk of less than 1.66 needs a closer look. A Cpk that’s less than 1.33 needs some action to make it higher, and a Cpk of less than 1.0 means that the process is not capable of meeting its requirements.   

A low Cpk means one of three things: the mean is far from the center of the specs, or the standard deviation of the readings is high (i.e., the readings are widely spread), or both conditions exist.      

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WHAT Is TORQUE?
Torque is a “turning” or “twisting” force and differs from tension, which is created by a straight pull. However, we use torque to create a tension.

HOW?
As the nut and bolt are tightened, the two plates are clamped together. The thread angle in the bolt converts the force applied into tension (or stretch) in the bolt shank. The amount of the tension created in the bolt is critical.

WHY?
A bolt tensioned properly works at its optimum efficiency and will resist coming undone. However, if the tension is too low, the nut could vibrate or work loose. If the tension is too high (overstretched), the bolt could break. Every bolt has a correct optimum torque/tension figure for each fastening application. It is important to have these figures available so that the end product will be safe, efficient and economical.

HOW DO WE MEASURE TORQUE?
Torque is the result of multiplying the value of Force applied by the Distance from the point of application.

Comparing the two examples, please note that the same Torque result can be achieved with a lower Force if the Distance from the nut/bolt is increased.

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Mountz, Inc. offers a couple of torque tool options for ensuring the correct torque is applied for applications that involve RF/COAX & SMA Connectors. The correct tightening of COAX and SMA Connectors used in RF cabling applications is essential to ensure optimum performance. This is particularly important in high frequency applications using stainless steel or beryllium-copper bodies.

The general thought is that hand tightening of RF/COAX cables or SMA connectors are acceptable. It is not. Over-torque can crush the insulation, as well as the center contacts and ground mating surfaces. Any change to those can alter the impedance and VSWR signal. Under torque can be just as bad if an air-gap is left in the insulator, or the center pin or ground-mating surface isn’t fully engaged.

If manufacturers and operators want to “do it right,” then they need to find a torque tool that’s designed specifically to prevent over-tightening for RF/COAX and SMA connectors. The torque tool should be designed to limit the amount of torque applied to an assembly or fastening application and minimize the shock to assembly.

Mountz Inc. offers TB Break-Over wrenches and Hand Torque Screwdrivers that can be used for accurately tightening COAX and SMA connectors and ensuring that over tightening doesn’t occur. The TB is commonly used with an open-ended spanner, while the pre-set torque screwdriver with a Crowsfoot Spanner Head enables vertical access in places where conventional torque tools cannot be used. The Spanners are available to suit the standard sizes of all the major connector manufacturers.

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A Pulse tool has the speed of an impact wrench with nearly the repeatability of a clutch based shut-off tool. Containing precision-machined parts the pulse unit is a sealed chamber that is filled with a formulated hydraulic fluid. Pulse blades with a custom designed precision roller push the fluid inside the rotating chamber generating hydraulic pressure that produces torque. Air power is the power source that spins the anvil and resistance against the fastener causes the pulse unit to activate.

Pulse tools are designed for safe, reaction free operation for various industrial assembly applications. Durability, power, speed, low noise decibels, minimal vibration and low torque reaction are key reasons for selecting a safe ergonomic pulse tool. For fastening applications a pulse tool will increase productivity, provide excellent ergonomics, reliability and quality.

Productivity
Rugged Pulse tools increase productivity and enhance product quality through precision torque control and user comfort. A lightweight pulse tool with more torque capability is an ideal tool for an operator. A pulse tool with a good power-to-weight ratio will enhance the cycle time for the operator with a high-speed rundown. The Flex Power Pulse tools, by Mountz Inc., feature a double chamber motor that allows the tool to quickly reach the required torque and increase productivity. The roller inside the pulse unit reduces friction, which increases power and allows the “pulse fluid” to last longer.

The Flex Power pulse tool features a unique “block valve” design to control the flow of oil and keep it on the designated path. The block valve improves the shut-off timing of the tool for better repeatability and reduced wear on the springs.

Other pulse tools have a shut-off mechanism that operates by the movement of steel ball and spring. For these pulse tools, the pressure inside the pulse unit could cause the spring to wear down quickly, which would impact the accuracy of the torque output and increase maintenance costs as well as downtime.

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