Push-to-connect hydraulic fittings: are they suitable for high-pressure systems?

The Push-to-Connect (PTC) fittings are now a standard in production lines, pneumatic circuits, and instrumentation panels since they allow technicians to connect within a matter of seconds, without the need for having to use a wrench. This convenience brings up a concern for those designing hydraulic systems: could similar fittings that connect to compressed air perform the same thing in a hydraulic circuit operating at 3,000, 5,500, or more than 6,000 pounds per square inch? The answer is yes. Fittings that connect via push-to-connect have a significant but a small role to play in hydraulics. Their purpose is stopped well short of the power systems that run at high pressure. Understanding the reason behind this is by looking at the ways these fittings create and maintain pressure.

How do push-to-connect fittings function?

A push-to-connect fitting is based on two parts within to perform what threads and a nut on their own do in a conventional hydraulic fitting consisting of a collet (or grab ring) that pricks the tube's outside whenever someone attempts to remove it and an O-ring that secures the tube's outer diameter. Put the tube in, and then the teeth of the collet stretch upwards, allowing it to go and then re-emerge to secure the tube when it's pulled past them. The pressure inside the line is a result of this design, not against it. The more pressure the system puts in to push the tube away, the more the collet's teeth flex. This is an effective mechanism, which is the reason why these fittings are extremely popular in pneumatics as well as low-pressure fluid transfer, where speed of assembly and the ability to unhook and reconnect with ease with no tools are more important than the capacity of the pressure.

The problem is that the strength of the collet's grip is limited by the size and hardness of the wall that it's gnawing into, and the O-ring seal is a static one made for low pressure differences and not the pulsation and pressure spikes that are characteristic of hydraulic power circuits.

What is the exact location where the pressure ceiling is?

The majority of the commercially available fittings for push-to-connect are evaluated in low-pressure or pneumatic hydraulic terms, usually within the range of 150-250 psi for normal polyurethane or nylon tubing. This can increase to 1,000-1500 psi for fittings made specifically for thermoplastic or thicker-walled nylon hydraulic tubing. There are a few metal-bodied push-to-connect fittings that pair with steel tubes and push even higher; however, even at the top portion of the range, they're not anywhere near the 3,000 to 6,000 PSI working pressures used in industrial power units or high-pressure pumps.

Compare that to the fitting families actually built for hydraulic power transmission: JIC 37-degree flare fittings, ORFS (O-ring face seal), and DIN 24-degree cone fittings are all rated well into the thousands of psi, with many ORFS and DIN configurations comfortably exceeding 6,000 psi and remaining the standard choice for high-pressure circuits precisely because their sealing geometry and metal-to-metal or controlled-compression contact can handle both static pressure and dynamic pressure spikes without relying on a friction-grip collet.

Why is it that the design of the collet and O-ring struggles when pressure is high?

There are three areas in which fittings that connect via push-to-connect can cause problems as pressure rises.

The first is a pure pull-out force. Hydraulic systems create an axial force on a tube proportional to the pressure and cross-sectional size of the bore. As pressure increases, the force grows with it, but at some point it surpasses the force that teeth on the collet are able to withstand, prior to the teeth sliding or the tube wall expanding or shrinking at the point of grip. Hydraulic systems don't simply keep pressure steady; they also cycle, increase when valves shift, and can experience pressure spikes when actuators stop abruptly. Every one of these transients are a time when the axial force of the fitting is significantly higher than the normal system pressure and a fitting that's not working at constant pressure may fail completely in the transient.

The third is seal integrity as time passes. The O-ring inside a push-to-connect fitting performs double duty by sealing against both the tube's surface finish as well as any slight misalignment that may have occurred by the tube being inserted. The hydraulic fluid can be harder on elastomers than compressed air in a variety of applications, especially when using mineral oils at high temperatures, which means that the same O-ring that works effectively in a pneumatic setting could degrade more quickly under conditions of hydraulic service in particular due to the heat generated by an ongoing power circuit.

The third one is vibration and cyclic load. Mobile equipment, in particular, is subject to a constant vibration caused by the engine and the hydraulic pump as well as the environment in which they operate. JIC and ORFS fittings resist vibration loosening because they're torqued to a metal-to-metal or controlled-compression interface that doesn't rely on a spring-loaded mechanism. The grip of a push-to-connect fitting can become looser over time under continuous vibration, in a manner that an appropriately torqued flare or an O-ring face seal does not.

Push-to-connect fittings can work in applications that require hydraulics.

All of this doesn't mean that push-to-connect fittings are not suitable for a hydraulic system. They are a legitimate option in a handful of specific situations. Low-pressure lubrication lines, case drain lines, as well as fill or tank vent lines in hydraulic equipment typically are operated at pressures that are low enough that fitting a PTC fitting that is designed for the specific application is suitable, and the quick assembly advantage is extremely useful for maintenance access that is routine. Sensor tap lines and instrumentation, when the pressure gauge or transducer is sampling the pressure of a line using small-bore tubes rather than being used to carry the flow of the system, are also a typical and accepted use in the event that the fitting's ratings cover the pressure of the line being sampled, with some margin. The pilot lines of certain control circuits, which typically are operated at a fraction of the main system pressure, could also be a good option, based on the pressure that is involved and the rating of the fitting manufacturer for the tube and the fluid combination.

The common thread throughout all of them is that the pressure applied remains within the fitting's specified rating and has a sufficient safety margin and that the line that is in question doesn't carry the primary energy flow of the system.

What can be used instead of high-pressure circuits?

For the main power lines of industrial or mobile hydraulics, the choice of fitting must be based on the same elements that have guided hydraulic fitting selection for decades, including working pressure, pressure rating that has a sufficient safety factor (commonly a 4:1 design factor for burst pressure), as well as vibration exposure, thermal cycling, and serviceability. JIC 37-degree fittings are the most reliable choice for general hydraulic assembly and are widely available, even though they're more sensitive to over-torquing as well as reuse than O-ring seals. ORFS fittings are now the most popular choice for numerous leak-sensitive and high-pressure applications due to the fact that the O-ring face seal isn't dependent upon the perfect flare-to-metal contact and can withstand minor misalignments better while securing reliably when pressures are higher. DIN BSP and DIN BSP fittings remain the norm in all equipment manufactured according to European specifications and come with their own established pressure ratings.

Whatever family is selected, the fitting or hose or tube, as well as the adapters, should share a similar pressure rating. The weakest part of the chain, not necessarily the most highly rated one, in order to determine whether the system can be relied upon to manage it.

Push-to-connect fittings can be a valid engineering solution for a particular issue: quick and tool-free assembly to facilitate lower-pressure transfer fluid. They are but a replacement in place of JIC, ORFS, or similar fittings in any circuit that uses main hydraulic power at pressures that are typical of industrial power units and press circuits. The mechanism for sealing that collets and O-rings allows them to be used is the reason why their pressure limit is far below the high-pressure hydraulic services required, especially when vibration or pressure spikes as well as thermal cycling are taken into consideration. The ideal way to go about it should be to align the appropriate type to the working pressure as well as the operating cycle for every particular line in the system, leaving the push-to-connect fittings for low-pressure lines and using fittings that are rated for pressure for all things that are used to maintain the system's working pressure.