COMPONENT FOCUS Edited by Mary C. Gannon • Editor
What measurements are required for hose assembly sizes?
Figure 1. Measuring male fittings
With hydraulic hose assemblies, one should consider several measurements to ensure their correct size — ID (inner diameter), OD (outer diameter), and length.
Inner diameter is the bore of the hose and the size of the orifice that the fluid has to travel through over a particular length. The OD of the hose could be critical for various things like where the hose is going to fit and how strong the hose is. A very strong high-pressure hose tends to have a larger thickness as a ratio to its ID. And of course, the length will help you determine what kind of flow rates you can handle, factored in with your ID. Obviously a smaller hose will have a higher pressure drop or an overall long length than will a larger diameter hose. A hose must be sized appropriately to handle the flow with as little pressure drop as possible because that pressure drop is wasted just trying to move fluid through a conduit. The total amount of pressure drop should be as minimal as possible, not only for efficiency, but because you want all the energy you’re putting 54
FLUID POWER WORLD
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into your prime mover to eventually do useful work at one of your actuators, whether they are a motor or cylinders. Specifying the wrong length is a common mistake. You absolutely cannot use a hose that it is too short, because it won’t either fit or will break if stretched too far. If it’s too long, it could have too much slack. A hose with slack can rub, fray or cause a catastrophic failure from moving too much around the machine. These longer hoses can also sag, which creates forces on the crimp and hose interface. This can cause wear and leakages over time. In addition, hanging and sagging hoses can get caught on something or be a tripping hazard. If a hose is only mildly too long — an inch, say — you can bolt that down to ensure safety. Overall length is not the tip-to-tip of all the components. What really matters is measuring the interface point between the fittings on the assembly. For example, if you have a swivel JIC, you do not measure to the end of the nut, but instead, will measure to where the seat attaches to the face of the JIC interface. In the example in Figure 1, you’re measuring the male fittings. If you have your hose or tube that has a male JIC, you want to measure to the interface point on that male. This ensures accurate hose length. This procedure is a little more difficult when you have something like a 45° or a bend fitting. With those, you must measure in the center of the fitting because the angled interface crosses various points of the entire hose length, so the middle point is obviously the average length that the hose would expand over that period. When making the hose assembly, you must consider the cutoff length, as seen in Figure 2. If looking at this hose assembly, you see the hose cut length. On the coupling
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