Justin Smith and Preston Ingalls
Are torque techniques running wild in your establishment? Does it always seem to be up to one’s own interpretation as to what method is the best? Well, it’s time to grab the reins and gain control, but first, we must understand what exactly we are trying to control.
When using a wrench to apply torque to a fastener something happens beyond the joining of surfaces. Once those two surfaces come in contact and you continue to rotate (torque) the nut, you actually begin to stretch the bolt. This bolt stretching creates tension known as ‘preload,’ which makes the bot like a rigid spring, which creates the needed clamping force.
What most people don’t know is the three things that affect the amount of torque that’s being applied:
- The bolt’s diameter determines the necessary amount of torque and clamping force
- The grade (tensile strength) of the bolt. (Figure 1 shows an example of torque levels for various size and grade of fasteners)
- The amount of friction that’s present.
An example of this third point would be the requirement of greater to torque to stretch the bolt when the mating surface is soft i.e., aluminum or copper vs.a metal which is much stronger like steel or titanium due to its increased hardness. This is because softer metals compress before you reach the desired torque specification, while harder metals withstand the applied force with less compression. This harder metal will also allow you to gain higher loads of torque and reduces the chances of the fastener losing its torque later.
How Metal Hardness Affects Torque
When thinking about metal softness and the amount of torque required for the different levels, we must be cognizant of what torquing the fastener may do to the mating surface. If the mating surface is aluminum, you would want to avoid flathead-type fasteners. These types of fasteners will create unacceptable tensile stress between the parts you are attempting to join and leave indentations in the metal.
This is because the bolt is more of a cone shape than being completely flat. Although this isn’t time for a snack break, thinking of smores illustrates the point. If I were to push my thumb down on the middle of the marshmallow instead of the standard size graham cracker what would happen? Obviously a sticky finger, but more importantly the marshmallow would rise around my finger the harder I pressed. In this example, my thumb is like the conical head fastener.
For this type of application, you’re better off using a pan head or round head bolt or screw because they disperse the force more evenly on the mating surface, which eliminates creation of high tensile stress. You could also use a flat washer with the fastener to increase the surface area in which you are applying force.
Think about a time when you may have replaced a missing front bumper fastener on your vehicle. Manufacturers usually recommend using a fender washer, which are larger and flatter washers than those typically found elsewhere on your vehicle. The soft plastic composition of these washers is ideal, and without them several things can happen:
- The fastener could sink into the surface resulting in a poor hold
- The fastener pushes clear through the surface which would render that fastener useless.
- You cannot expand the area where you are applying torque, which results in gaps between the bumper until you approach your next fastener location.
So now that we’ve determined what torque needs to be applied by identifying the type of fastener and the mating surface to which it will be used with, now we need to know how we regulate the amount of torque to apply.
Properly Applying Torque
There is nothing that worries me more than when I see someone putting lug nuts back on their vehicle, and I notice them using a long handle breaker bar with the jack handle slid over the end to create a cheater bar. You may have also heard someone say, “tighten it until it strips, then back it off a thread.”So, when I say there’s more than one way to skin a cat I mean there are many ways for us to be able to apply the correct amount of torque that’s recommended by the manufacturer and to do it consistently.
If you are using an air impact wrench to tighten fasteners, you can attach air regulators to each hose that allows you to adjust the pressure. The downside to this method is that if the machine bogs down from multiple people using the air compressor or a malfunction occurs, you wouldn’t know unless you double-checked your work with a calibrated torque wrench.
We would hope anyone doing work that required specific torque to be applied would indeed double-check them to ensure accuracy. In reality though, with our fast-paced world and the“I needed it yesterday” attitudes, it doesn’t always happen. Some people will opt-out on the regulator for each hose and instead will let the compressor fill up and shut off, and then back down the pressure.
This will still result in the same inefficiencies of the individual air regulator example. This is because once the air compressor drops down and fires back up you will lose consistent torque due to the pressure drop while in use. If someone else is using the same unit but requires more pressure, they will not be able to get it until you have finished with your work.
There are three common procedures for checking the torque applied to bolts after their installation; namely, taking the reading on a torque gauge when:
- The socket begins to move away from the tightened position in the tightening direction. This method is frequently referred to as the “crack-on” method.
- The socket begins to move away from the tightened position in the un-tightening direction. This method is frequently referred to as the “crack-off” method.
- The fastener is re-tightened up to a marked position. With the “marked fastener” method the socket approaches a marked position in the tightening direction.
In this third method, clear marks are first scribed on the socket and onto the joint surface which will remain stationary when the nut is rotated. (Avoid scribing or marking on the washers since they may rotate with the nut.) The nut is backed off by about 30 degrees, followed by re-tightening so that the scribed lines coincide. But equally important to the right methods for accurate toque is the tools that you use.
Best Tools for Accurate Torque
The best alternative to applying torque with an air impact is to use torque sticks (Figure 2). These can be purchased individually or in complete sets. Most manufacturers standardize their torque specs for reasons such as this. These sticks will start to flex once the torque level reaches their designed limits. This will prevent you from overtightening or completely ringing a bolt off when tightening fasteners with an impact wrench.
Another great tool for ensuring the right amount of applied torque is the trusty old manual style torque wrench. In my experience with servicing equipment, it has always been the most accurate and unsurpassed tool to use. Using this tool gives me the confidence that I have applied the appropriate amount of torque. I not only dial it into the amount of torque I desire but I also get to feel it myself.
So, if you are like me and this is your go-to method, there are still things that you must be mindful of. The first is what type of torque wrench is best suited for your application? There are quite a few options out there such as electronic, hydraulic, beam, and clicker to name a few. Each one has its pros and cons depending on what your need may be.
Proper Care of Torque Tools
Just like any other measurement tool, these require a great amount of care, calibration and preventive maintenance to be performed on a scheduled basis. When I say care, I don’t mean you have to bathe it and tuck it in at night. I mean you always want to make sure that you wipe it down with a dry cloth (no chemicals) after each use and be sure to store it in its case or somewhere the environment won’t affect it, and no other tools will be slamming into it.
Let’s think about a 4-foot level. How confident would you be of its accuracy if you knew that it had been dropped on the floor multiple times or stored where things are constantly sliding and slamming into it? If you are like me, you wouldn’t want that to be your go-to level when framing up your new home. The same would go for the torque wrench. I wouldn’t be comfortable knowing that was the measurement tool that was used when installing lug nuts on the vehicle that carries my kids to and from school.
Another thing while on the care topic is making sure you are not using this precision measurement tool to loosen stubborn bolts. I know this seems to be an ideal breaker bar in a pinch, but if you apply greater force than the tool is designed to out-put you can cause severe damage with the result being inaccurate readings.
Finally, comes the preventive maintenance and calibration of our tools. These tools should be calibrated on a scheduled basis such as the amount of hours it has been in use or how many cycles it goes through. Each manufacturer has their own recommendations, but a good rule of thumb is never to let it go beyond 5,000 uses or 12 months per the (ISO Standard 6789) without being calibrated.
Most professional industries would prefer you calibrate them closer to the 3,000-use range. The exception to this would be if you noticed abnormalities during the visual inspections such as; dried grease, loose components, or any parts of the tool that shows excessive wear or corroded parts. This would require an immediate calibration or even replacement.
Now that you have a better understanding of torque and what affects it. It’s time to break the cycle of non-standardized torque techniques in our facilities. I guess we need to tighten up on how we apply torque procedures.