Come Together: Hose-Coupling Interfaces Hydraulic hose assemblies are an integral part of a hydraulic system, withstanding thousands of psi of fluid pressure, powering the articulating arms of mobile construction equipment, the hydraulic ram of a 100-ton hydraulic press, and many other heavy-duty applications. The construction of a typical hydraulic hose assembly is relatively simple – a section of rubber hose with metal couplings crimped onto both ends. What is often overlooked is the interaction between the hose and the coupling after being crimped together, referred to as the hose-coupling interface. This carefully balanced combination of a metal ferrule, rubber hose, and coupling stem (also known as a hose insert) gives a hose assembly its overall pressure rating. Both the hose and the couplings are designed for use at a certain working pressure, so it might be assumed that when combined, the assembly’s pressure rating would match the lesser rating of the two components. But the hose-coupling interface, a critical portion of a hose assembly, can often be the limiting factor when determining a complete assembly’s working pressure. To develop a dependable assembly working pressure and potential working life, there are several steps to the process. Choosing the correct ferrule, building a crimping profile and range, testing complete assemblies, including specific crimper settings, and recognizing sources of failure are all involved in the process of designing a reliable hose assembly. To design couplings with a robust hose-coupling interface, there are variables such as the specific hose cover material, tube material, reinforcement layer thickness, and reinforcement ductility. When the ferrule component of a coupling is designed, these factors are taken into consideration and often affect the number and depth of the serrations lining the internal circumference of the ferrule. Many manufacturers of couplings intend for these serrations to penetrate the hose’s cover material and embed themselves in its reinforcement layer, creating a stronger bond with the hose when crimped. Gates calls this a “bite-the-wire” coupling. Also, in a proper crimp, the rubber of the hose flows between the serrations as it is being crimped, further improving the connection’s strength while minimizing intrusion from external elements. This is why ferrule design is closely tied to the type of hose, with some coupling families offering multiple types of ferrules, depending on the hose or application. The first step in qualifying a coupling with a hose is identifying the correct ferrule to pair with the hose. For example, even if the hose inner diameter (ID) is known, it’s possible a different ferrule will be required for an SAE 100R1 hose versus a 100R2 hose because of the differences in hose outside diameter (OD). One-piece couplings come with a ferrule already attached, so the ferrule sizing is considered when choosing the complete coupling. Once the correct coupling components are chosen to be qualified with a specific hose, the next step in qualification is to develop a manufacturer-provided crimp specification. This specification often refers to the final crimped OD of the coupling ferrule after it has been attached to a specific hose, measured using calipers on the flat portions across the ferrule diameter. To develop these crimp specifications, coupling development engineers estimate a target crimp OD using historical data and material property calculations. Engineers perform initial crimps using a hydraulic crimper, with a little trial and error to get to the target crimp OD. During crimping, due to the pressure exerted by the compression of the ferrule and rubber hose, engineers expect that the coupling stem has a certain amount of bore collapse. Using pin gauges or bore gauges (depending on the inner diameter), they take measurements to determine if there is too little collapse, resulting in a weaker clamping force between the ferrule and stem, potentially leading to coupling blowoff when pressurized. Too much collapse constricts the fluid pathway and decreases overall hydraulic system efficiency, in addition to potentially weakening the strength of the stem and leading to premature failure. Using the gauges, engineers compare the measured bore to the minimum bore allowed by SAE J516. They check the concentricity of the crimps and look for uniform crimping results around the circumference. They repeat subsequent crimping trials until they achieve an acceptable bore collapse, recording the crimp OD of the successful crimp result, which will then become the starting crimp OD for performance testing of that coupling-and-hose combination. Testing After determining a starting crimp OD, the real fun begins. The next step in the qualification process is testing the hose assembly under pressure, running it through several tests specified by industry standards like SAE J517 and J343 to determine the assembly’s integrity. Some of these tests are to ensure customer safety, using test parameters that allow for a sufficient safety factor to accommodate spikes in system pressure when in use in the field. Other tests in SAE J517 check the assembly’s performance in simulated use, testing for things like hose dimension changes, hose degradation from external sources, and potential life of assemblies. A few tests stipulated by SAE J517 are pertinent to the hose-coupling interface: the proof test, burst test, leakage test, and finally, the impulse test. Proof testing a hose assembly is a simple test that ensures the assembly is capable of holding pressure, requiring a hydrostatic pressure of twice the working pressure, held for 30-60 seconds, and checked for leaks at the coupling and along the hose length. After passing proof testing, select assemblies are tested for burst and leakage, putting the assemblies into extreme pressure conditions to see how they perform. Burst testing requires taking the assembly to a minimum of four times the working pressure, often higher, until the assembly has catastrophic failure. Leakage testing similarly pushes the assembly to its performance limits, bringing the pressure to 70% of the minimum burst pressure, held for five minutes, removing pressure, then repressurizing to 70% of minimum burst. After advancing through this pressure cycle, engineers check the assembly for leaks or signs of failure. All three tests have binary results, either pass or
