The maintenance cost of a leaking hydraulic system is so high that many equipment managers have regarded leak-free couplings as one of their most concerned maintenance issues. Easier said than done, but pay attention to matching the replacement parts with those on the machine and install them correctly. You can maintain a relatively dry fleet.

The coupling has two functional ends, the hose end and the port (or "thread") end. Both ends must match the size and type of the port or hose with which it is mated. Since nearly two types of couplings are used, it is not always easy to identify the coupling on the device. Gates’ Safety Hydraulics Manual contains a wealth of information, including drawings that identify the type of coupling. This manual is available to attendees of the Gates Safety Hydraulics seminar.

O-ring face seals are becoming more and more popular, so much so that they may eventually become some kind of standard. But different countries, and even some manufacturers, insist on unique coupling designs. Kevin Dugan, a fluid power system engineer at Parker Hannifin, said that Parker would prefer to standardize SAE-straight threads (SAE-J1926), ISO 6149 (SAE-J2244) metric port threads or Code 61/62 four-bolt split flanges. "But due to the differences between North American and overseas manufacturing, standardization is difficult." The number of old couplings still in use on aging machines compounded the problem.

Fortunately, suppliers provide measurement kits to eliminate the guesswork of identifying accessories. You will find the basic principles of identifying accessories in the illustrations attached to this story.

Correctly matching the coupling is an indispensable start to maintain the integrity of the hydraulic system, but the correct assembly of the joints is equally important. Proper torque is a key. Even if the coupling with poor torque does not leak, other problems may occur. An over-tightened 37-degree coupling may deform, restricting flow and causing turbulence in the hydraulic fluid. Restriction and turbulence generate heat and reduce system performance.

Dennis Kemper, an application engineer at the Gates Hose and Connector Group, says that over-tightening can cause as many problems as under-torque. Excessive torque can cause thread peeling, joint deformation, scratches or cracks on the cone or valve seat. "Either of these situations will prevent proper sealing."

Kemper emphasized the importance of retaining torque specifications. "A twisted hose during the assembly process will want to straighten when system pressure is applied. This can cause hose failure or loose connections. This is sometimes referred to as'de-torque'. Using a spare wrench when tightening a rotary joint will have Help prevent distortion.

"Service personnel should always consider the manufacturer's minimum bend radius for the hose," Kemper said. "And avoid lines that twist the hose or cause it to bend immediately behind the joint."

If you want to prevent leakage, you must also thoroughly inspect the coupling. Caterpillar's Reusable Parts and Recycling Operations Guide (media number SEBF8176-01) discusses which parts can be reused and which defects can be repaired. Generally speaking, couplings with small gaps or burrs can be repaired, but more serious damage-sealing surface or thread, twisted flange surface, cracked or twisted valve stem or rotating nut, or any welded or Parts subject to high temperature-reduce coupling scrap.

Gates' proposal is more stringent. "Any coupling should not be reused," Kemper said. "There is always a danger that some defects may be missed, which increases the possibility of leaks or serious hose assembly failures."

Gary Steen of Caterpillar Hose and Coupling Engineering 6 Sigma Black Belt says that some couplings are more sensitive to defects than others. "With JIC couplings, if you can feel the scratches with your nails, then it can cause problems. One of the great advantages of O-ring seals is that they are less sensitive to scratches, scratches and dents. Use them , The main thing is not to damage the O-ring during the assembly process."

In fact, O-ring seals are so good that they can hide assembly errors-at first. "If you forget the O-ring, if assembled to the correct torque, the accessory can seal metal to metal for a period of time," said Dugan from Parker Hannifin. "Or with the O-ring in place and the connection is only tightened by hand, the O-ring will seal until the vibration loosens the joint and the O-ring is squeezed out."

Dugan also warned technicians not to use connectors with different types of coatings. "When we went from cadmium plating to zinc dichromate plating, some users still use old cad hose ends. The different plating makes it easier for vibrations to loosen the connection and cause leakage."

Kemper added two warnings about O-rings. "It is important that the seal material is compatible with the type of hydraulic fluid used. Remember that even the correct seal will degrade over time due to exposure to circuit heat."

Using only parts from one supplier can minimize compatibility issues. John Thibault, Senior Marketing Consultant for Caterpillar Hydraulics, said: “Unless the seller provides proof that the hoses and fittings have been tested together and certified to meet SAE standards, do not mix brands.” Thibault recommends that technicians treat the system as a whole, not as a single unit. Part, and remember that the entire system must be compatible to maintain performance.

The Gates Safety Hydraulic Manual warns against reuse of old hoses with permanent or field connectable joints, and do not reuse field connectable joints that have been put into use.

Couplings are not as sensitive as hoses to certain conditions that shorten their service life (such as pressure peaks, continuous use at maximum rated pressure or maximum temperature). But they are equally sensitive to other abuses—maybe even more sensitive. The operator should not hang things on the hose, nor should the hose be used as a step. They should take care to prevent impact damage from rocks, limbs and other debris.

Preventing fluid coupling leaks requires technicians to spend time to complete hydraulic maintenance and repairs in the correct way. Although the added time will cost money, it is not as expensive as a leak (an accessory that drips a drop of oil every 10 seconds will waste 40 gallons of oil a year). Your investment in getting the right parts and the time required for the correct installation cannot even compare with the cost of downtime and emergency repairs.

In order to match the accessories to avoid leakage, you need a thread identification kit. Use tools carefully. "Many couplings have very similar threads, but they are not interchangeable," said Fred "Buster" Herman of TICO Industries. "The thread pitch is particularly tricky. The pitch gauge must fit the external thread completely, and the internal thread should engage with the external thread without tilting or binding."

The first step in a leak-free fit is to measure the internal and external dimensions of the threaded joint. Measure the thread at the largest point. Be sure to measure the National Pipe Tapered (NPT) external thread at its bottom and the internal thread at its opening.

The difference between certain types of threads is very small, and you should place the joint and thread pitch gauge in front of a white background (such as a piece of paper) to see how the teeth of the gauge are fixed in the thread. There should be no gap between the meter and the coupling. Connect the internal thread component to an external thread with a known thread pitch to identify the internal thread. The two should be worn together without resistance, and the fit should not be sloppy.

Three angles are the most common: National Pipe thread 30°, JIC 37° bell mouth and SAE 45° bell mouth. If you don't read the angle gauge carefully, it's easy to go wrong. For example, the NP straight thread (NPSM) and the internal thread of the British Standard Parallel Pipe (BSPP) used for mechanical joints both use a 30° valve seat, but the parts are not interchangeable. Both SAE 45° flares and SAE inverted flares use a 45° angle on the male fish, but the inverted horn actually uses a 42° angle on the female fish.

Minimizing leakage at the connector port or hose end requires proper matching and proper assembly of parts. At the port end, the correct assembly partly depends on the torque. There is no similar convenience value at the end of the hose to ensure a leak-free result. This is why many fleet managers use hydraulic crimping machines in the workshop and on-site. These systems are not foolproof, and care must be taken in the selection and assembly of components. But the use of these machines does greatly reduce the risk of leakage and blow-off.