Leak-Free DesignLeak-Free Design

Designing effective, leak-free hydraulic systems requires special attention to torque levels and proper sealing techniques. Recent testing also shows the full torque nut coupling, where the nut is crimped onto the stem as the coupling is assembled during manufacturing, is more leak-free than its staked nut counterpart.

Properly matching hydraulic hoses and couplings from the same manufacturer is the first step to maintaining hydraulic system integrity. Connecting the assembly correctly to a port is just as important to prevent system leaks. Proper torque is one key. An over-tightened coupling may be just as apt to leak as an under-tightened coupling. Even if a poorly torqued coupling doesn’t leak, restriction and turbulence generate heat, which may reduce system performance.

Dennis Kemper, an application engineer with Gates Fluid Power, says over-tightening can cause as many problems as too little torque. “Too much torque can strip threads, distort and overstress the fitting and scratch or crack the cone or seat. Any of these conditions will prevent proper sealing.”

Kemper says values listed in SAE J514 are for qualification testing only, not for setting up torque values for a production environment and engineers should follow the coupling manufacturer’s recommendations. An often-overlooked factor for torquing is that all values are based on the wrench being square with the nut. If the torque wrench is at an angle, a lower torque value is achieved even if the wrench gives the “click” indicating proper torque.

Kemper says a recent improvement to the JIC-style connection is the full-torque nut coupling which makes the connection of wire-braid hydraulic hose assemblies much less prone to being over-torqued. This coupling has been in use for more than a decade but because it was costly to manufacture, its use was restricted to more expensive, high-pressure, spiral wire hydraulic hose, where the coupling is a fraction of the assembly cost.

Testing by Gates and OEM customers indicates the full torque nut coupling is more leak-free than its staked nut counterpart. A staked nut coupling has two machined components: the nut and the stem with integral collar. The nut is staked (crimped) onto the stem as the coupling is assembled. Staking deforms the surface of the metal, forming a thin cross section at the collar. On the assembly line, a worker using a torque wrench can crack the nut at its weak point, even when proper torque is applied.

The full-torque nut coupling has three machined components: the nut, the stem and the collar.The nut is taller and thicker than a staked nut. The full-torque nut coupling design allows the nut to slide over the stem during manufacture. The collar is staked on afterward.

When tightening a staked nut, forces go in all directions. With the full-torque nut, force is applied directly downward because of the right-angle shoulder on the nut. This downward directional force lessens the stresses on the coupling components. The clamp load, force exerted to form a seal at the interface between the adapter and the coupling, is applied directly to the sealing surface.

From a practical standpoint, Kemper says the full-torque nut is less prone to being over-torqued because when the nut is tightened with a box-end wrench, the coupling will reach its clamp load easily. On staked nut designs, applying too much force can crack the seat or the nut. In the field, especially, operators use a box-end wrench to tighten leaking staked nut couplings and make the leak problem worse.