Torque and Galling

Methods for Calculating Appropriate Tightening Torque

When you are trying to determine the tightening torque you need for a fastener, there are a couple ways you will want to go about it. It may seem like a fairly easy answer, but that is not always the case.

Use a Torque Calculator

Using a torque calculator to determine the approximate tightening torque is a good place to start. This calculator will request information such as the coefficient of friction, the bolt strength and diameter. A torque calculator is a good way to get started and get an approximate tightening torque, however, it will not be exact in all situations. This is why after using a calculator, it is a good idea to perform some joint testing to verify your results.

Test Your Torque

Once you arrive at the proper torque requirement, you may want to test your torque values to verify the accuracy of your assembly tools. This is important because it helps you ensure the safety, reliability and quality of your project.

You will need a calibrated hand torque wrench to verify the tightening torque of your fasteners. You can select from the First Movement Test, the Loosening Test and the Marking Test as an indication of the applied torque:

  1. The First Movement Test will involve applying force to your torque wrench after the fastener has been tightened. You will do this until the first movement of the fastener is noted.
  2. The Loosening Test requires that you turn the fastener in the opposite direction to loosen it. You will record the torque when the fastener first breaks loose.
  3. For the Marking Test, you will need to mark the fastener and continue to mark onto the surface being clamped. Then you will loosen and retighten until the marks match up.

It is important to use the proper torque on your project to ensure safe, reliable joints. To learn more about torque and methods for measuring it, contact Bossard at ProvenProductivity@bossard.com.

March 17, 2017
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What Fastener Drive Should You Use?

Fastener drive options

What Fastener Drive Should You Use?

Picking a fastener drive may not seem like a big decision to make, but if you choose the wrong one it can have a significant effect on the assembly of your project and the time it takes to complete your project. It is important to make sure you have the right one, and we can help you come to that decision!

The best fastener drive is always the one that is most effective in transferring torque to the fastener to achieve clamp load.

There are many different types of fastener drives and you will want to use the one that best holds your project together and also achieves a good assembly speed. Some of the types of drives are slotted, Phillips, Frearson, combination, hex, one way, square, and six lobe.

The slotted screw and drive were the first to come about because they were very simple. The problem was that fastener drives can more easily slip out of the slot and damage the screw head or product, so cross drives like the Phillips, Frearson, and combination were created. These drives allow for higher torque because they have greater bit engagement. This leads to quicker assembly, so cross drives such as these should be considered if you are trying to speed up your assembly process. Cross drives are also good for screws with low torque because the cross allows the drives to more easily maintain bit engagement to achieve more torque.

Other types of drives include the square, hex, or six lobe recess. These were designed to be used when high torque is required to achieve maximum clamp load. Most high volume production fasteners will use these types of drive recesses. They also require little effort to maintain bit engagement minimizing drive strip out issues.

It can be frustrating trying to fasten something on a project because you may feel like you aren’t making any headway. Picking the correct fastener drive for the job can make your project go a lot more smoothly. Contact Bossard at ProvenProductivity@bossard.com if you have any questions about fastener drives and choosing the best one for your job.

June 17, 2016
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What is Proper Torque?

proper torque

Throughout a typical day, our engineers are asked at least once, “What torque should I use?” for any given size and application. This question is very loaded and requires many details to arrive at a hypothesized torque. The torque is considered hypothesized until testing is performed on an actual application. Until it is tested, it is not confirmed.

The factors that govern any “proper” torque strategy are the strength of the fastener, the surface pressure limits of the mating material, and the friction under the head and in the threads. The ultimate goal of torque is clamp load, so these factors essentially influence total clamp load. Total clamp load, or preload, in most applications is required to keep anything together.

The most common method of finding the proper torque is looking it up on a torque chart. Most companies have some sort of organized torque chart based on size and some refer to the surface finish of the fastener. This is an attempt to consider the friction of common fastener finishes. Our engineers would rate this method very low on an accuracy scale when a specific clamp load is desired. The method we recommend to any design engineer requires a load cell and measuring of the clamp load while tightening. This confirms the design and the engineer’s desired clamp or preload. The torque to get to this clamp load will be the proper torque.

When friction varies and the clamp load needs to be achieved every single time (like a head bolt on an engine) a torque turn method is the proper choice. This method takes a bit more testing to be repeatable but is very accurate. By testing several joints and measuring the clamp load and torque simultaneously, a snug torque can be acquired and then an angle-of-turn added. The snug torque may only be a tenth of the final torque but the angle-of-turn can and will produce accurate and repeatable clamp. For more information on this method, and help determining this angle, contact us any time at ProvenProductivity@bossard.com.

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May 27, 2016
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