Hydraulic hose is one of the least considered components of a high-pressure fluid power system; however, its habit of being overlooked does not reduce its importance. Any machine with actuators attached to moving segments requires flexible plumbing, and that plumbing is hydraulic hose. Hydraulic hose has come a long way with regards to pressure limit and flexibility, especially since the proliferation of synthetic materials, such as PTFE and buna nitrile. Also, there are many benefits of hydraulic hose in addition to flexibility, making it an option for complete system plumbing.
Hydraulic hose has many advantages over fixed plumbing, such as tube. Advantages to hose are its ability to absorb vibration and noise, which can be troublesome with pipe or tube, especially if they are resonating or amplifying a sound created elsewhere. Hose assemblies are also easier to manufacturer than hard plumbing, and because hard plumbing can be labor-intensive, hose is often less expensive. Also, hydraulic hose is constructed with a rubber outer carcass, so it has no potential to rust as hard plumbing might, which is often painted to prevent rust when it is not constructed of stainless steel.
The construction technique and material of hydraulic hose differs based on its intended application. The various application parameters required when selecting hose are size, temperature, fluid type and pressure, to name a few. Depending on these criteria, the choice of hose construction will change to suit the conditions. The majority of hydraulic machines are not exotic, however, and typically your standard 100R1 or 100R2 (“1-wire” and “2-wire” respectively) hoses are appropriate. However, when extreme pressures and exotic fluids come into play, the options for suitable hose are reduced.
Tough under pressure
The primary consideration for hydraulic hose—essentially what separates it from other hoses—is its pressure holding capacity, which can be anywhere from 1,000 to 6,000 psi or more. Hydraulic hose uses a three-layer construction: the inner tube, the reinforcement layer and the outer cover (figure 1).
The tube construction material is dictated partly by the fluid being used in the hydraulic system. For example, nitrile or rubber is fantastic for petroleum-based hydraulic oil, but would not be compatible with a synthetic fluid such as phosphate ester, which requires Viton or Teflon. Special attention should also be paid to chemical compatibility of the outer cover as well. There could be ambient conditions containing chemicals incompatible with the cover material, which is commonly nitrile. Tube material varies less than does cover material, which can be made from neoprene, PVC, nitrile or even non-rubber woven textile.
Tube and cover composition also play a role in the temperature range hose is best suited to operate within. Standard hose construction allows for anything between –40 to 202° F, such as with the common combination of nitrile tube and neoprene cover, but special formulations of these polymers are required for colder conditions. Cold weather polymers need to be highly malleable to prevent cracking and subsequent hose failure.
At the other extreme, ambient or operating heat can cause premature hose failure due to a softened tube and cover. Although most hydraulic systems—and the oil itself—are not happy at extreme temperatures, sometimes localized heat must be guarded against, such as in steel mills. For conditions anywhere up to 400° F, Teflon can be used for the tube, and then wrapped with a stainless steel cover. The downside of Teflon hose is that it’s not terribly flexible and has a large bend radius.
Keep it covered for safety’s sake
The cover’s primary job is to protect the tube and reinforcement from environmental conditions, above and beyond the temperature and chemical resistance already discussed. Not all hose is created equally, and because a common failure of hydraulic hose is the result of cover abrasion, special attention should be paid to the cover construction material.
Although abrasion can easily be avoided through proper installation and use of hose clamps, sometimes friction is unavoidable, like with a portable concrete saw tool, which has a remote power unit feeding the saw through 50 ft or more of hose strewn across the job site. In a case like this, special hose construction with an abrasion resistant cover is a solid choice (as is the use of protective hose wrap). I should point out that an old-school textile hose cover—although dubious looking—is actually superior to most neoprene rubbers in regards to abrasion resistance.
The central defining criteria most people associate with hydraulic hose is its pressure holding capacity, and it owes that capacity to the reinforcement layer. To say the reinforcement is sandwiched between the tube and cover would do it injustice and imply the reinforcement is loosely wrapped. More accurately, the reinforcement layer(s) is tightly wound upon the tube with a spiral winding or braiding machine. The cover is then applied, and you have a complete hose.
Steel wire is by and large the most popular choice for the hose reinforcement layer, and is either cross-braided or spiral wound, as seen in Figure 2. Cross-braided wire has alternating rows of perpendicular wire woven in opposite directions around the tube. Braided steel hose comes in one or two layers, and is usually separated by another layer of rubber when two layers are used. As mentioned, the most common types of hydraulic hose are the SAE 100R1 and 100R2 braided hose, affectionately referred to as 1-wire and 2-wire, respectively.
Braided hose is the most popular type of hydraulic hose, and is available in a vast array of sizes and pressure ratings. It should be noted that hose pressure rating is reduced as tube ID increases due to higher internal forces. Also, hose working pressure is rated a quarter of burst pressure, giving the hose a safety factor of four to one, but I digress. Braided hose has excellent flexibility for its strength, and most manufacturers make high-flex braided hose with half the bend radius as its standard cousin.
There are two types of spiral-wound hydraulic hose. The helically-coiled hose is a mild reinforcement used to increase pressure capacity, but is limited to suction lines where little or no pressure exists, or sometimes used on return lines where pressure spikes are of no concern. However, the tightly wound spiral layers used for hydraulic hose are used on ultra-high pressure systems. Four layers of high-tensile steel wire is laid directly on top of each other perpendicular to each subsequent layer.
The four-layer spiral wound hydraulic hose is extremely strong and is common in high-pressure, large-diameter hose assemblies. This 4-wire hose is available up to a 5,000 psi rating at a 2-in. diameter, although sometimes 6-wire hose is used for such extreme requirements. The downside to 4- and 6-wire hose is its flexibility, or lack thereof. Because of rigid, multi-layer construction, 4-wire 100R12 hose is quite stiff; however, advances in materials and construction have allowed the major hose manufacturers to reduce the bend radius significantly.
All hydraulic hose is manufactured to North American or international standards to ensure construction techniques and quality guidelines are met or exceeded, and because of the potential safety hazards in and around hydraulic machinery, this is a good thing. There has never been a wider selection of hydraulic hose available, particularly for specialized applications. Because of the increased demands for high-pressure performance, I imagine the choice of hydraulic hose will only continue to grow.