The Science of Hose-Connection Interfaces

Without a doubt, high-quality hose assemblies are essential to the reliability of hydraulic systems. The flexibility of hose enables components to be positioned in the most efficient or convenient places, because the hose has the ability to bend around corners, through tight spaces, or across long distances. Yet, designers may assume they’re getting a high-quality hose assembly by specifying hose from one manufacturer and end connections (couplings) from another. Even if both are of high quality, are there consequences to mixing components from different manufacturers?

SAE standards

Before answering that question, first, a little background. SAE International provides its J517 hydraulic hose standard, which serves as the most popular benchmark in the realm of industrial hydraulics today. More specifically, J517 is a set of guidelines that applies to the current SAE 100R Series of hoses. Currently, 16 such hose styles exist, and they are designated as 100R1 through 100R19. Each of the styles must meet a set of dimensional and performance characteristics as set forth by SAE. However, SAE issues no approval source lists, certification, or letters of approval-conformance to these standards by manufacturers is strictly voluntary. In short, the standards only assure a similarity of products among different manufacturers.

Hydraulic-hose construction

Modern hydraulic hose typically consists of at least three parts: an inner tube that carries the fluid, a reinforcement layer, and a protective outer layer.

The inner tube must have some flexibility and needs to be compatible with the type of fluid it will carry. Commonly used compounds include synthetic rubber, thermoplastics, and Polytetrafluoroethylene (PTFE). The reinforcement layer consists of one or more sheaths of braided wire, spiral-wound wire, or textile yarn. The outer layer is often weather-, oil-, or abrasion-resistant, depending upon the environment the hose is designed for.

Not surprisingly, hydraulic hoses have a finite life. Proper sizing and use of the correct type of hose will certainly extend the life of a hose assembly, but many different factors affect a hose’s life-span. SAE identifies some of the worst offenses as:

• Flexing the hose to less than the specified minimum bend radius
• ŸTwisting, pulling, kinking, crushing, or abrading the hose
• ŸOperating the hydraulic system above maximum or below minimum temperature
• ŸExposing the hose to rapid or transient rises (surges) in pressure above the maximum operating pressure
• ŸIntermixing hose, fittings, or assembly equipment not recommended as compatible by the manufacturer or not following the manufacturer’s instructions for fabricating hose assemblies.

Connecting a 3,000-psi hose with a 3,000-psi coupling doesn’t necessarily mean you have a 3,000-psi hose assembly. The hose-coupling interface is often overlooked when it comes to hose performance. This interface is the science of controlling how the coupling is connected to the hose and the study of their interaction. Unless the hose and coupling are specifically designed for each other, the assembly may have a lower pressure rating, shorter life, or even worse, a catastrophic failure.

Factors affecting the hose-coupling interface include stem and ferrule design, hose reinforcement, hose tube material, and the two most important factors — the outside diameter of the hose (hose OD) and the outside diameter of a crimped coupling (crimp OD) — all variable factors. High-quality manufacturing processes can reduce hose OD variance, but some variation will still occur. Crimp OD will also show deviation from an exact dimension due to the minimal variation in coupling and ferrule manufacturing tolerances, crimper performance, and other external factors.

Additionally, hose impulse life (performance) is variable. Impulse life is a function of the quality of the components that make up a hose assembly and the resulting hose-coupling interface. Figure 1 shows what three important normal distribution curves (hose impulse life, hose OD, and crimp OD) would look like.

Maximum impulse life is achieved when variation is accounted for in:

1. The design of the hose-coupling interface
2. The manufacturing control of all components in an assembly
3. The validation of this system through stringent test requirements.

All factors affecting the interface must be taken into account during the hose’s design and manufacture. Continual validation ensures that the hose, couplings, and ferrule will make a high-performance, risk-free hydraulic hose assembly, Figure 2. A visualization of this is shown in Figure 3.Issues that can affect interface performance include: “What if the hose and couplings are not designed for each other?” “What if the sources of variation in hose-coupling interface are not accounted for in design and testing?” “What if different companies make different components of the hose assembly?”

The answer to these questions is drastically reduced hose performance, even to the point of catastrophic failures. Figure 4 demonstrates what might happen if hose and couplings not designed for each other are used.

Figure 4 shows that, even in the best-case scenario, impulse performance is significantly reduced as compared to an assembly designed as a system and validated through a rigorous test regiment. As Figure 5 shows, things get even worse if all variables are taken into account.

As variation increases even slightly for each component, a significant reduction in hose assembly life is guaranteed, and the risk for catastrophic failure increases.

The next time you grab a hose and coupling, ask yourself:

• “Who manufactured the components?”
• “Were the components designed together? Validated together? And if not, what are the risks?”
• “Am I willing to accept an assembly with a guarantee for reduced impulse life and reduced performance?”
• “Am I willing to risk a catastrophic failure?”

If the hose, couplings, ferrules, and crimp tolerances have been designed for each other and passed exacting standards, these questions need not be asked. But if the components are not designed for each other, you may, instead, be asking for trouble.

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