(Stay tuned next week for Part 2 – How to Measure Tension)
Everyone is always concerned about getting the right torque. Regularly, customers contact us requesting material torque charts. Everyone is so concerned about what they are setting their torque wrench to, that they are missing what matters most: getting the right tension.Getting the right tension cannot be achieved by looking at a torque chart or purchasing a top-of-the-line wrench. Even a brand new bolt will require +/-40% torque to achieve the same tension as another new bolt out of the same box.
The Difference Between Torque & Tension
Torque is simply the effort (force) you apply to turn the bolt in place. When you apply the torque, it stretches the threads of the bolt and nut together. This “stretch” or elongation of the threads is called tension. It is tension that holds everything in place. Tension is critical in securing a bolt properly. If the tension is too week then the fastener could loosen; if it’s too tight the threads may be stripped or the bolt break.
Recently, we had a customer who called with a pile of broken bolts. He sent them back for analysis. In studying the material and the bolt structure, no flaws could be found. After further discussions with the customer we realized that the bolts were over torqued due to a change in lubricant and the tension was too tight, causing the bolts to break.
The Real Truth: Everyone is always concerned about applying the right torque. The Internet is littered with articles on checking torque. The crack-on/off test, the marking test. But the truth is. The same torque, applied to the same bolts, out of the same box will result in varying tensions. Measuring torque doesn’t mean a nickel – it’s measuring tension that counts.
Why Does the Same Torque Result in Different Tension
One word: Friction. The best way to help ensure the tension is correct is to take into account friction variables when applying torque and adjust the torque to allow for these variables. This can help to ensure that the resulting tension is accurate.
Lubrication
If too much lubricant is applied, less friction will cause the wrench to move easier and the nut to torque beyond the required tension (over torqueing)
Debris & Deformity
Think about shoveling a snowy driveway in winter. When there are a few flakes it’s easy to push the shovel, yet when the snow is a foot deep, if you use the same force, you won’t go very far. A bolt with deformed or rough threads, as well as a dirty bolt can act the same as a snowy driveway in a blizzard. If you apply the same torque to a rusty bolt as you did to a smooth new bolt, the toque slows down as it “plows” through the rust and the bolt doesn’t move as far as it would without rust. This means for the same toque the bolt turns a shorted distance and hence the tension is less.
Thread Pitch
Course verses fine threads also affect the torque which you will need to apply. Torque has to overcome friction in order to move the nut around the bolt. Just like the snow in the driveway, the more to move, the harder you have to push or torque for the requirement movement to occur. Hence, finer threaded fasters have more threads in a set length, which means more friction (snow), so greater torque is required.
Want to learn more? Click here to read Torque Part 2: How to REALLY Measure Tension
So Now What Should You Do?
Yes, torque is not the most accurate method of getting the right tension, but it is a starting point. Here is a link to our torque specs. Look out for next week’s sequel we will introduce ways you can accurately measure tension – for when precision is critical.