A hydraulic line failure has probably happened to you at least once; it’s when your equipment shuts down and you’re waiting on replacement hose and a tow truck. Most folks think the hose simply wore out, but it was actualy the wrong size to begin with. Hydraulics are something that would seems straight forward if it weren’t for internal diameter (ID), bend radius and working pressure. These three variables is linked together so when you change one, there needs to be a plan for the others.
The first confusion is dash number system. The number followed by that symbol isn’t a length code or even a model code; it’s a direct reference to internal diameter. To obtain the nominal bore in inches, simply divide the dash number by sixteen. So a dash-twelve hose would be three-quarters of an inch while a dash-eight is half an inch. That makes for no guessing in ordering replacements.
Why Size Matters for Hydraulic Hoses
When you have to fit a hose into a tight spot without sacrificing flow, this mathematical relationship come into play. Note also from the chart’s visual scale how rapidly the bore increases with each number increase. That’s where the engineering of flow capacity start. How much do you want to move and how much flow velocity will cause excessive pressure drop or heat? Oversize your suction line to avoid cavitation. What happens is the pump sucks itself dry because it can’t get fluid quickly enough, it sucks itself dry, causing cavitation. Oversized pressure lines can handles more velocity without worrying about erosion within the hose, as long as you keep it within a safe range.
The chart provides gallons per minute by velocity. That way you have something to reference to see if the hose can hold up with the pump’s output. If you push too much fluid through a small bore, you lose efficiency. Pushing too little through a large bore waste money as well. Find that balance and that’s part of the trade.
The thing with pressure ratings is they’re not fixed. They only relate to type of hose you choose. Cheap, flexible single wire braid hoses does not stand up to high pressure. Two wire braids are the workhorses in this industry. They offer a good combination of flex and strength. Hard jobs call for spiral wire designs. The graphic details them all out side by side so you can compare. You’ll note that regardless of what type of hose you’re looking at, the bigger the dash number, the less max working pressure there is. That’s simply because there’s more surface area for the hose to resist pushing against, so either the hose has more layers of reinforcing material or thicker walls.
Hydraulic systems relies on correct bend radius. You may have the best flow rate and pressure rating possible but bending the hose tighter than the minimum radius will harm the reinforcement layers. This creates a weak point that will eventualy burst under pressure. The table below shows the minimum bend radius for each size. When routing your hoses through machinery frames or tight areas around engines, this information is key. Do not force a bend. Get a longer hose or fitting if the hose won’t reach. Crimping a hose into a corner to save space result in premature hose failure.
And those safety factors in the ratings exist with good reason. For example, the four-to-one standard design factor indicates the hose will break under four times its rated working pressure. That is a margin of safety for physical abuse, heat degradation and spikes in pressure. So don’t think you can pump it up higher than the rating. It is a margin of safety for unexpected events. Chemicals, abrasions and heat will diminishes that safety factor. Checking your hoses periodically for any wet areas, blisters, or cracking is just as important as picking out the correct size to begin with.
When selecting a hose, know its limitations in terms of materials. Select the construction type based off your pressure demands. Select the dash size based on your flow demand. Always allow for an adequate bend radius. Once you’ve done the math once, you won’t make messes later. Five minutes spent looking at the chart will save five hours cleaning up hydraulic fluid. Keep the lines straight, keep the system tight, and let the fluid do what it’s designed to do.
