In my previous blog Torque: Why, How, and Where, we talked about the why. Now we need to achieve a correct clamping force, and so we will discuss how to get there.
Here are the normal ways of torquing hardware, in order from the least accurate to the most accurate.
- Hand tightening
- Using a torque wrench
- Torque plus turn
- Hydraulic rotation
- Hydraulic tension
We are going to break down each of these techniques and see what the results can be. Some are good results; others are not so good.
- Impacting – This is very common because it is easy to do and fast. The amount of force derived from an impact, whether it is battery powered or air driven, is almost impossible to know. How fresh is the battery? How much air pressure and volume is supplied by the hose? How loose are the internals of the impact wrench itself? How loose is the socket on the drive? How loose is the socket on the fastener? Is there an extension between the impact and the socket? More about that extension will be discussed later, but you can tell from all of those questions that there are so many variables that accuracy will always be questionable. The next step up on the ladder of accuracy is hand tightening.
- Hand tightening – Some people have it, some don’t. A lot of what is done in the professional world is done with experience. Certain industries have a level of experience that instills some great “Feel” for the tasks. Aircraft Mechanics win this category, hands-down (if you will excuse the pun), because everything they do is critical and sometimes even exotic. Like a titanium bolt going into an aluminum, or even magnesium, receiver. This requires a level of attention and feel that a lot of technicians just do not possess. They have an ingrained muscle memory from years of this type of work and can get very close to specified torque in a lot of cases. I, personally, do not have that kind of feel. Even with that knowledge and tactile experience, they will always verify with a torque wrench because they must! It’s part of the procedures, and no A&P mechanic wishes to be the one who just let an aircraft fall out of the sky. That leads us to the next rung on the ladder.
- Torque wrench – Here we are using a calibrated (hopefully) device to accurately measure how much rotational force we are inputting into a fastener. This too has some rules that must be observed to perform the job correctly.
a. No double-click: As in, bouncing off the end of the handle like it’s your own shop Pogo stick. The thought behind the double click is to ensure that the fastener is tight, and boy is it! Since the torque wrench did not return to a relaxed position, it just clicked again, but went way past the desired torque. In a case where something like 100 lb./ft. is desired, that second click made it go to perhaps 170 lb./ft. The proper way to check is to pull slowly and smoothly to the “click”, anticipating when it should happen and stopping at that point. Then, let the tool totally relax and pull again the same way. If the tool clicks with the fastener not turning anymore, then you will know it is torqued correctly.
b. The extension we mentioned earlier: A common misconception is that if you add an extension, you lose torque. While this is true with an impact wrench (because you are adding mechanical looseness), this is not the case with an extension on a torque wrench. We have a load cell that we use to prove this concept, and with about 8 feet of extensions, we asked the technician to apply 100 lb./ft. of torque. There was definitely more flex in the tool, but once all of that flex was taken up, our load cell indicated that he had put 102.3 lb./ft. of force into the sensor. So, there is no significant loss of torque from the use of extensions provided that the tool is used correctly.
c. Torque wrench storage – When I was growing up working in the family shop, my grandfather would lose his cool if he found a torque wrench put away at anything other than Zero. He would declare the tool off limits until it could be calibrated again. Unfortunately, this is another misconception about storage. Every tool manufacturer publishes what setting to set that tool to for storage, and it is normally never zero. The spring in that handle actually wants a set amount of force kept on it so it cannot drift with environmental changes like temperature or vibration. Some are specified to be set at 10% of max, or maybe at the lowest available setting, others have a set number. These specs are normally in the manual that came with the tool, but often that has been discarded or misplaced. But a few minutes on Google will surely provide the answer.
- Torque plus Turn – This picked up a lot of adherents in the 1960’s and 70’s. It is way more accurate than the torque wrench, because in a lot of cases, the torque specified was higher than some torque wrenches could register. The idea is to put a lower torque into the joint using the torque wrench, and then adding the degrees to achieve the much higher clamping force. This works great, especially if the manufacturer supplies a gauge to measure that angle. There is very little guesswork, just turn to the indicated degrees. If you need more than that, we need to get more complex equipment involved, such as hydraulics.
- Hydraulic rotation – This is for the big boys. Anyone who has had to torque something big can attest to how hard that can be with hand tools, even with a torque multiplier. Hydraulic rotation takes that task much easier, albeit more costly. This can take a large fastener, rotate it to 1000 lb./ft. or more with no trouble at all. But by the same token it is more critical to know what that force needs to be. Without much effort, it becomes very easy to over-tighten something, and that could be just as bad as under tightening. If you need more accuracy than that, then you need to apply hydraulics in a different way.
- Hydraulic Tension – this is the most accurate way to tighten a large joint. The engineers that design systems that require this know exactly how much clamping force is needed and specify the amount of force to use. In this case, a collar is threaded onto a stud and the specified pressure is applied to that collar. When it stretches the stud by the correct amount, the nut is simply run down by hand and the pressure is released. This allows the “elastic” properties of that stud to clamp the joint together. The only real issue here is the fatigue that is put into that fastener. Some manufacturers specify how many times a stud can be stretched, while others will have the tech measure the stud each time to insure it has not gone into a yield or plasticized condition.
All of these methods have their place. Anything that makes the job more accurate and more repeatable creates the opportunity for better running conditions. We haven’t yet touched on the topics of torque specs, or lubrication. These will be interesting topics for another day.
Visit our Knowledge Center for resources and tools to help you succeed when implementing and using our maintenance technologies.
Maintenance Tips by Chris Greene CRL