Selecting Hydraulic Connectors for Leak-free Hydraulic Plumbing

Brendan Casey

Leaks rank No. 1 in the list of most common maintenance issues involving hydraulic equipment. While not a new problem, the real cost of hydraulic oil leaks to industry - which include makeup fluid, cleanup, disposal, contaminant ingression and safety - are only now being fully considered.

Hydraulic connection leaks are commonly considered to be an inherent characteristic of hydraulic machines. While this may have been the case 30 years ago, advances in sealing technology and the development of reliable connection systems mean that today, leak-free hydraulic plumbing can be achieved.

Reliable Connections

Ideally, leak-free reliability begins at the design stage, when the type of hydraulic connection is selected for port, tube-end and hose-end connections.

Ports - Connectors that incorporate an elastomeric seal such as UNO, BSPP and SAE 4-bolt flange offer the highest seal reliability. NPT is the least reliable type of connector for high-pressure hydraulic systems because the thread itself provides a leak path.

The threads are deformed when tightened and as a result, any subsequent loosening or tightening of the connection increases the potential for leaks. Therefore, the use of NPT is not recommended for high-pressure hydraulic systems. In existing systems, consider replacing pipe thread connections with UNO or BSPP to achieve leak-free reliability.

Tube and Hose Ends - Flared connections have gained widespread acceptance due to their simplicity and low cost. The JIC 37-degree flare is the world’s most commonly used hydraulic connection. Its popularity is due to its ease of fabrication, wide size range, imperial to metric adaptability and ready availability.

However, the metal-to-metal seal of the flare means that a permanent, leak-free joint is not always achieved, particularly in the case of tube-end connections. As hydraulic system pressures have steadily increased, the flared connection has become prone to weeping, which results in dirty, sludge-covered systems.

Alternative hydraulic connectors are gaining acceptance, most notably the O-ring face seal (ORFS). ORFS tube- and hose-end connections feature the high seal reliability afforded by an elastomeric seal. However ORFS connectors are larger in size, offer fewer adaptor options, are more difficult to install (alignment must be perfect or O-ring extrusion occurs), have limited availability and are typically double the cost of a flared connection. For these reasons, ORFS is not as widely used as compression fittings and the 37-degree flare.

Leaking flare joints can be eliminated by installing a conical washer between the JIC nose and flare. One type of flare seal, manufactured by Flaretite, is a stainless-steel stamping with concentric ribs that contain preapplied sealant.

When tightened, the ribs crush between the two faces of the joint, eliminating misalignment and surface imperfections. The combination of the crush on the ribs and the sealant ensure that a leak-free joint is achieved and helps protect the sealing faces from fretting, galling and overtightening.

Incorrect Torque

A common cause of leaks from 37-degree flare joints is incorrect torque. Insufficient torque results in inadequate seat contact, while excessive torque can result in damage to the tube and connector through cold working. The following is a simple method to ensure flare joints are correctly tightened:

  1. Finger-tighten the nut until it bottoms on the seat.

  2. Using a permanent marker, draw a line lengthwise across the nut and connector hex.

  3. Wrench-tighten the nut until it has been rotated the number of hex flats listed in Table 1.

Tube Dash Size
Hex Flats
1.5 to 2.0
0.75 to 1.0
0.75 to 1.0
0.5 to 0.75
Table 1


Vibration can stress plumbing, affecting hydraulic connector torque and causing fatigue. Tube is more susceptible than hose. If vibration is excessive, the root cause should be addressed. The propagation of structure-born vibration from the vibrating mass of the power unit (the pump and its prime mover) can be minimized by eliminating bridges between the power unit and tank, and the power unit and valves.

This is normally achieved through the use of flexible connections such as rubber mounting blocks and flexible hoses, but in some situations it is necessary to introduce additional mass, the inertia that reduces the transmission of vibration at bridging points. Always ensure all conductors are adequately supported and if necessary, replace problematic tubes with hose.

Seal Damage

Having outlined the benefits of hydraulic connectors that incorporate an elastomeric seal, it is important to note that their reliability is contingent on fluid temperature being maintained within acceptable limits. Fluid operating temperatures above 82°C damage most seal compounds. A single overtemperature event of sufficient magnitude can damage all the seals in a hydraulic system, resulting in numerous leaks.

Hydraulic systems are often considered perennial consumers of oil and make-up fluid, an inherent cost of operating hydraulic equipment. However, a leak-free hydraulic system should be considered the norm for modern hydraulic machines - not the exception. The proper selection, installation and maintenance of hydraulic plumbing are essential to ensure leak-free reliability.

Read more on hydraulic leakage best practices:

How to Combat Leaking Hydraulic Connections

The Real Cost of Fluid Power Leaks

Hydraulic Plumbing Leaks - Causes and Remedies

Subscribe to Machinery Lubrication

About the Author

Brendan Casey has more than 20 years experience in the maintenance, repair and overhaul of mobile and industrial equipment. For more information on reducing the operating cost and increasing the...