In front of you is an NPTF tapered thread assembly. The recommendation is to ‘turn,’ not ‘torque,’ to assemble the connection. But all you have is a torque wrench. Or maybe you have an SAE straight thread port assembly, which has a suggested assembly torque, and your torque wrench is nowhere in sight.
Time is wasting. The inevitable question pops up: Can’t you just crank that fitting down and be done with it? Tight is tight, right?
In the case of torque, we use a calculation to determine how much torque hydraulic threads can handle without being damaged. Going over or under the recommended torque can lead to leaks. That likely makes sense to you if you’ve gone below recommendations – a loose fitting can lead to leaks – but might make less sense if you’ve gone over.
When you tighten a fitting, in some circumstances you’re actually deforming the metal, which can damage threads and sometimes the fitting, making it unfit for re-use. In other circumstances, an O-ring could be pushed down into the female threads, compromising the seal of the connection. Additionally, if that O-ring is old or dry, it can crack under too much pressure, leaving you without a proper seal and again the possibility of leaks. The same holds true for the methods requiring turning the fitting.
So even if you think you’re a torque wrench or assembly master, when you tighten based merely on your sense of what feels right, you have most likely over-torqued/tightened your fitting.
When a turning method is recommended, you will see a very low number in the tightening recommendations. Generally, that number will be no higher than 3. These numbers are linked to two types of turning methods that do not require a torque wrench:
TFFT (Turns From Finger Tight) is the number of full rotations of the fitting recommended after you have made the connection finger tight.
FFWR (Flats From Wrench Resistance), or sometimes called the “Flats method,” is the recommended number of flats on the fitting’s nut that should pass a certain point after resistance is felt.
Torque is a measurement of force. In the case of hydraulic fittings, it’s the rotational force used when tightening, generally using a torque wrench. The recommended torque value is a measurement of the maximum amount of rotational force necessary to ensure your fitting is tightened properly. Torque can be measured in units, such as pound-foot, or newton meter.
Torque is important because it helps you know how tight your fitting connection needs to be when your system is running at higher pressures.
So when do you use these different methods? Here are some guidelines.
Due to the shape of tapered thread fittings, it’s particularly important that they never be over-tightened. More than any other fitting, they are prone to damage due to over-tightening: damage to the threads or even a cracked port end, which can lead to leaks and make re-use impossible. That’s why TFFT, rather than torque, is recommended for tapered thread assembly.
Some special considerations when making an assembly with a shaped fitting that has a tube end connection: Make sure you achieve proper alignment. Never back-off (loosen) tapered thread connectors to achieve alignment.
See our detailed port end assembly information for step-by-step instructions. You can also view our blog post, How Many Times Can I Reassemble a Hydraulic Fitting.
Assembling this fitting results in a metal-to-metal seal made by the cone on the female adapter or tube flare seating onto the 37° flare on a male adapter. This metal-on-metal contact can deform the fitting’s nose over time and threaten reusability. If over-tightened, the problem can be compounded. That’s why FFWR is recommended in this assembly.
An important tip for this method: When you’ve reached the first point of resistance to your wrench, use permanent ink marker to make a longitudinal mark on one of the flats of the nut and continue it on to the body hex. Then, at the properly tightened position, mark the body hex opposite the previous mark on the nut hex. These marks will serve two important functions:
The displaced marks serve as a quick quality assurance check that the joint has been tightened.
The second mark on the body helps indicate if the fitting has loosened. Upon reassembly, the fitting should be tightened past this mark to create a seal lower on the nose.
See our detailed 37° flare fitting assembly information for step-by-step instructions. You can also view our blog post, How Many Times Can I Reassemble a Hydraulic Fitting.
FFWR is the recommended method for all bite-type fittings. That’s because rebuilds can become a factor if the fittings are over-torqued.
See our detailed bite type fitting assembly information for step-by-step instructions.
O-ring face seal fittings can, and should, be torqued to the recommended specifications. This is because the connection seal is made by an elastomeric seal against metal. The lack of metal-on-metal contact means that only the O-ring is being deformed in assembly and reassembly. This sealing method is very forgiving and, when assembled to the recommended torque specifications, will create a leak-free seal every time.
All straight thread port types should be torqued to the recommended specifications. Much like O-ring face seal ports, the O-ring used with straight thread ports is what is making the seal. Over-tightening can damage the fitting or the seal.
That said, you should take care to tighten only to the recommended torque. Over-tightening can lead to damaged fittings and/or O-rings, cracked ports and most certainly a compromised connection seal.
See our detailed port end assembly information for step by step instructions.
Understanding the difference between a torque and a turn means you’ll never be in doubt when assembling or reassembling hydraulic fitting connections in your system, keeping it leak-free for years to come.
Have any torque wrench tips or horror stories? Tell us below. We’d love to hear from you.
Did you find this post helpful? Subscribe to TFD techConnect posts by email. TFD techConnect is a technically-focused monthly blog written for engineers specifically around motion and control engineering challenges.
If you have any additional questions or comments, please post them and I’ll respond if warranted. If you want to talk to me directly, I can be reached at Parker Tube Fittings Division, 614.279.7070 or via email.
Do you want to receive new product announcements and technology updates from Parker Tube Fittings Division? Subscribe today and stay informed.
Rob King, engineering supervisor, Parker Tube Fittings Division
Related resource articles: