One of the scariest phases of product design is testing & validation. That’s when any unknown flaws or manufacturing defects will see the light of day. As you’ll see, you can eliminate the unknowns by performing something as simple as measuring and recording fastener torque.

Ensuring that your product meets manufacturing standards, operates consistently and does not fail prematurely is no easy task. Customers, regulatory bodies and internal quality control each have traceability requirements that must be satisfied. How do you satisfy the needs of these various groups? The answer lies in providing data that your product meets specifications. Although there are many design aspects to focus on, for this article we’ll concentrate on the literal “nuts and bolts” of your system by addressing the effects of fastener preload and how to solve the problems that may arise from incorrect preload.

Why focus on the fasteners? Your product’s structure relies on its fasteners to distribute loads. An incorrect preload applied to a fastener can lead to the premature failure. For instance, a bolt that has too little preload applied will transmit less stress than its neighbors, overloading them and leading to premature failure. Bolts with too much preload, on the other hand, can damage structures or overly elongate. The latter effect results in an eventual loss in bolt preload over time. An improperly torqued fastener also affects more than just load distribution; it can also affect seal integrity.

Gaskets and seals require sufficient bolt preloading on the fasteners holding them in place. This ensures the gasket meets the desired service life of your product; otherwise, failures can occur. For instance, too much preload will result in premature gasket collapse, while too little preload results in a weak or nonexistent seal.

How do bolts receive the incorrect preload? This issue can be traced back to unexpected friction in the assembly process and the incorrect application of torque. Solving this problem requires you to measure the applied torque and fastener preload and use that to determine the friction in your system and calculate the correct amount of applied torque.

1. Capture bolt preload.

A complete measurement solution requires two sensors, one to measure bolt loading and another to measure applied torque. Measuring bolt loading requires a sensor that allows the bolt shaft to pass freely through the sensor while measuring the force the bolt head applies as the fastener is torqued down. These sensors go by different names, such as through-hole load cells, load washers or donut load cells. The sensors aren’t limited to bolt load auditing; they can also be permanently installed to monitor gaskets and seals. This enables the monitoring of the changes in preload due to gasket fatigue and hardening, allowing you to anticipate and replace the gasket before failure.  With this in mind, the next step is to measure applied torque.

2. Measure applied torque.

Torque measurement can be as simple as using a torque wrench with a digital display or a strain gauge based reaction or rotary torque sensor. The latter can be connected to a digital display and data-logging solution to automatically capture the torque the operator is applying to each fastener, or integrated into automated assembly systems for live torque feedback and monitoring. The sensor that will work best for you will depend on the accuracy you need and demands of your application. Once you have a torque and bolt-loading sensor selected, you can derive the friction in your system to make an accurate calculation of the torque you need to apply.

3. Derive fastener friction and determine appropriate torque values.

Friction derivation is quite simple. You install the bolt-loading sensor between your structure and the fastener. The shaft of the fastener then passes freely through the sensor with the head of the fastener resting against the active sensor area of the transducer. You then torque the sensor down measuring both the applied torque and resulting bolt loading. These tests are repeated until you receive consistent torque and bolt loading values. To calculate friction for your system, you then input your torque and bolt loading values and use a torque equation such as the Farr Screwjack Equation or Motosh Equation.

Once you have friction calculated, you can then determine the necessary torque required to achieve your requisite bolt loading. You then combine this precision torque value with torque sensors during manufacturing to verify that each fastener has received the correct amount of torque and logging it. The captured data is used to ensure the quality and consistency of your manufacturing process to your customers, regulatory agencies and any standards bodies you may encounter. With the data in hand, validation and traceability will no longer be a nightmare.

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