A common complaint about hydraulic and pneumatic systems is that they leak. While it is true that sometimes the component, pumps, valves, and cylinders are where the leak is, most often the leaky items are the fluid conductors. Why is this? It’s usually because the fluid conductors have been poorly chosen, installed, and maintained. Let’s explore each of these areas in an effort to determine the causes of leakage and how to prevent them.

Almost every major hose and fitting manufacturer includes a section in their catalogs that discusses some aspect of the selection, installation and maintenance process. In addition, the Fluid Power Society is almost ready to introduce the new Connector and Conductor Certification which contains a great amount of information related to the selection and assembly of connectors and conductors. Every designer, installer/fabricator, and maintenance person should become familiar with the various publications available in order to educate themselves in the proper selection, installation, and maintenance of fluid connectors and conductors.

An excellent source of information on many of the port and tube end configurations used worldwide may be found in Parker’s “Industrial Tube Fittings” Catalog 4300. This catalog includes a 53 page technical section that covers design criteria, nomenclature, a comparison of various fittings, tube end connection standards, port connection standards, fluid compatibility issues, guidelines on choosing the tube material and type, tube and fitting material compatibility, O-ring material selection, choosing the tube size, plumbing and assembly hints, tube line routing and clamping, how to properly assemble and install fittings, pressure drop, and contact information on governing bodies such as ANSI, DIN, ISO, JIS, and others.

A good source of engineering information on tubing and pipe, as well as many other fluid power topics, is the “Lighting Reference Manual” published by Berendsen Fluid Power. The “Fluid Power Design Handbook,” published by B&T Hydraulics & Pneumatics, Inc. (ISBN 0-9653599-0-5) is similar to, but not identical to, the “Lighting Reference Manual.” While there is somewhat of an overlap of information, each reference contains enough non-duplicated information to justify having both books on hand.
Items to consider when choosing the connectors and conductors to be used are: 1) operating pressure; 2) fluid compatibility; 3) temperatures both of the fluid as well as ambient conditions; 4) environmental concerns, such as corrosive liquids or gasses, ultraviolet exposure, etc, 5) restrictions based on customer specifications; 6) system characteristics, such as shock or frequent pulsing; 7) whether non-conductive hose is required; and, 8) relative motion between machine members. This is not to be taken as an all inclusive list.

Specifying the Fluid Conductors:

Many times the engineer who designs the circuit and specifies the major components neglects to specify the fluid conductors. Typically, line sizes, material and wall thicknesses, or hose specifications (i.e. R100-2) would be specified on the circuit schematic, but the decision of exactly what adapters shall be used is left to the fabricator. The schematic may fail to include a note as to what type of tube end configuration, 37”SAE flare vs. O-ring Face Seal (OFS or ORS) vs. one of the various types of compression designs that are available. Granted, the individual selecting the major components also decides on just what ports each component includes, but a poor decision may be made. For example, threaded pipe ports, rather than SAE O-ring ports, might be specified out of ignorance.

The port end of a connector is determined by the ports of the major components. The type of conductor may be specified by the system designer, or it may be left to the discretion of the machine fabricator. Whoever ends up making the actual decisions needs to ensure that the conductors meet the system’s requirements. Hose should be used only where there is relative motion between two machine members. Hose should not be used to compensate for a lack of skill in producing either bent tube assemblies or welded pipe assemblies. Properly fabricated and installed tubing and pipe has a much longer life span than hose. An excellent booklet on how to bend tubing is “Principles of Tube Line Fabrication” (Bulletin 4306-B5) published by the Tube Fittings Division of Parker-Hannifin.

Installation:
Assuming well chosen connectors and conductors have been specified, the next step in the process is the installation of the various lines. Again, well illustrated source material is readily available that shows the right and wrong ways to run pipe, tubing, and hose. The Gates Rubber Company of Denver, CO publishes “Safe Hydraulics” (Publication # 39595), a guide to preventative maintenance and safety in the use of hydraulic hose and fittings. This publication covers inspection, troubleshooting, hose and coupling selection, coupling identification, routing cleanliness, assembly, and installation. It is geared toward mechanics who work on hydraulic systems.
Aeroquip’s JA316H Industrial Products Catalog, as well as giving some pointers on routing of hose and tubing, also covers the application of swivel unions. Swivel unions, used in conjunction with hose, should be used where the movement between two machine members would over flex a hose. In addition to these two topics, there is also an excellent section on analyzing hose failures. It doesn’t make much sense to replace a failed hose without determining and rectifying the cause of the failure.

Fluid conductors need to be carefully routed to reduce possibility of damage. Sometimes guarding may be needed in order to protect the conductor from impact or heat. Tubing, as well as hose, must be protected from abrasion. Tubes that are routed right against a machine member, and are also subject to vibration, will eventually wear through the tube’s wall. In applications where a hose is subject to abrasion, Kevlar socks should be fitted over the hose. Hose used on mobile machinery must be able to withstand ultraviolet light.
Hoses installed on the boom of aerial platform trucks must be of non-conductive construction. The ends of non-conductive hose must be capped off while the hose is being stored regardless of whether or not the hose has been fitted with the end couplings. Failure to do so will allow moisture to enter the hose, degrading its dielectric properties. There are several industrial applications that also require non-conductive hose, such as hoses used on electric arc furnaces.

Fluid conductors need to be properly supported. There is a variety of clamping systems on the market. Clamps need to be spaced properly and be located at appropriate points. Companies such as Parker, Stauff, Behringer, EPHA, and Hydrocraft publish guidelines on the use of these clamps.

Maintenance:
As with any machine, once it is built and operated, it needs to be maintained. Most companies have discovered the cost benefit of preventative maintenance. Periodic inspection of the fluid conductors should be a part of the PM process. It’s a simple task for a machine repairman, an oiler, or a machine operator to make a quick inspection of the machine. Such an inspection could easily be a task required of the operator at the beginning of each shift. The person inspecting the machine should obviously be on the lookout for signs of leaks, either dripping hydraulic fluid or escaping air. Going beyond that though, one should look for crushed lines and place where a fluid conductor is rubbing against another machine part, causing abrasive wear. Hoses should be inspected for cracking or brittleness of the cover. Immediate steps should be taken to resolve the problem in order to avoid machine downtime.

An example of how poor routing can have a negative impact on the machine is in a situation where lines have been routed in front of an access panel. In order to gain entry to the panel, the hydraulic or pneumatic system must be shut down and locked out so that the lines obstructing entry can be removed. In the case of a hydraulic system, this will also necessitate the messy job of draining the lines. Then, while the lines are open, contamination can enter the system. Finally, in the case of a hydraulic system, air will need to be bled from the system. All of that could have been prevented, and time saved, if the lines were thoughtfully routed around the access panel in the first place. Isn’t it said there’s never time to do it right the first time, but always time to do it over again?

In Summation:
This article touches on but a few of the things designers, machine builders, and maintenance people need to be aware of. Obviously, the amount of information these people need to know is far beyond the scope of this article. The intent here is simply to foster a sense of awareness that specifying, installing, and maintaining fluid conductors and connectors is a complex subject that warrants, and to lend some sense of direction as to where to locate this information.

By: Don DeRose