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Hydraulic Hose Assemblies

     However, having to braze a fitting onto a tube is labor-intensive and costly, and bending a tube assembly to fit an application, and trying to install it into an existing machine can be a real nightmare. Hose assemblies, on the other hand, are flexible, can easily be routed around obstacles, and can be fabricated more easily using any of several popular crimping techniques and machines. In the replacement aftermarket, hose is considerably easier to obtain and more cost-effective than bent tubing, which can require custom tube benders, special fittings, and considerable skill to install. Given the availability and routing advantages of hose, it should not be surprising, then, that maintenance workers usually replace a hard-to-reach failed bent-tubing assembly with a hose assembly.
     Hose is also less susceptible to vibration than tubing and is preferred in applications involving port to port movement or odd routing configurations. These advantages often out-weigh those of tubing. In fact, hose assemblies frequently are used for convenience where a tube assembly would be the better choice — a long, straight rigid run, for example. In this case, the hose assembly costs more, but may be easier to fabricate. The tubing assembly may cost less and offer better overall performance, but the convenience of making a hose assembly using standard tools wins out.
     A hose assembly can be more economical than tubing for a straight, rigid run if the tube connects to hose at both ends. In this instance, the total cost (parts, labor, etc.) of making a hose assembly, connecting it to a tube assembly, and ending in another hose assembly can far exceed the cost of a single hose assembly covering the entire length of the branch.

Allowing for flexibility

     In critical routing situations, hose replacement is much easier than tube replacement because the hose is flexible enough to bridge port to port connections. With bent tubing assem-blies, the connecting section must be exactly the same size as the original section to avoid placing stress on the fittings, which are often brazed or welded to the tube. If stress on the joint is high enough, the joint becomes especially susceptible to vibration damage and cracking.
     In many cases, the choice between hose or tubing has been clear: in environments subjected to high temperatures or damage from sharp or heavy objects, a metal tubing assembly was specified. If the fluid line needed to be flexible, a hose assembly became the choice. The development of better tubes and covers for hydraulic hose — combined with more-convenient meth-ods of fabricating tube assemblies — makes the decision between using a hose assembly or tubing assembly in an application tougher than ever.
On one hand, tubing has several distinct advantages. Tubing:
  • can handle higher pressures (exceeding 6000 psi)
  • dissipates heat more readily
  • can cost less (due to high-volume purchasing)
  • has a tighter bend radius
  • weighs less (which is important in long boom arms)
  • can be used in higher ambient temperatures, and
  • it potentially has a longer life than hose.
     Increasingly, designers of hydraulic equipment are integrating bent tube assemblies and hose assemblies into hybrid bent tube/hose assemblies. These assemblies provide the weight and bend advantages of bent tube with the vibration-damping characteristics of hose.
Combination hose/tube assemblies prevent problems caused by vibration, kinking, and abrasive failure.
     These bent-tube/hose assemblies are particularly effective on mobile applications where vibration is often severe. A feasible alternative to port-to-port bent tube assemblies is to connect a port to a location with tubing, then attach a hose.

A weighty problem

     An illustration of how bent tube/hose assemblies can work together can be seen on the mobile agricultural applications that include 60-ft wide boom attachments. In the past, manufacturers preferred to use bent-tube assemblies on these attachments to minimize weight and prolong life of the tool bar. At the same time, equipment manufacturers recognize that these attachments can be a source of vibration — a major factor leading to system failure. Their solution to this problem has been to use primarily hose assemblies where the tool bar will stand up to the weight and a bent tube/hose combination where light weight is important.
     Today’s hydraulic hoses have been designed to be much lighter and provide smaller bend radii than earlier products. With the introduction of these new hoses, the weight advantage of bent tubing has been eliminated, and the small bend radius advantage has been cut in half (see Tables 1 and 2).

Avoid overheating

     The thinner walls in today’s hoses also reduce the insulation properties that can contribute to heat buildup under certain situations. This insulation effect is an advantage in cold weather applications where designers prefer the insulation properties of hose over tubing in order to retain heat in hydraulic oil.
     On the other hand, tubing is considered a better heat dissipater than hose and is preferred for applications that are marginal in terms of sensitivity to heat. This preference extends to the replacement aftermarket, where conventional wisdom has held that substituting hose for bent tubing may result in an overheated system.
     In reality, engineers have been successful in designing cool-running hydraulic systems, so overheating is not a major problem. In certain applicaions, using hose can actually result in less heat buildup because of improved laminar flow through the more gradual bends created between hose connections.
     Even in situations where overheating is a problem, replacement of tube assemblies with hose will have a negligible effect, especially when newer model hoses are used.

High-pressure applications

     One area in which bent tube maintains an advantage over hose is in high-pressure applications, such as injection molders that operate at working pressures up to 10,000 psi. Hose manufacturers are making major strides toward developing new products that will equal or surpass these ultra-high pressure requirements.
     When considering hose for replacement of a bent-tube assembly, it is important to check the manufacturer’s specifications for both pressure and temperature ratings, and determine the correct inside and outside hose diameters using a precision-engineered caliper. Hose OD is especially important when hose routing clamps are used, or when hoses are routed through bulkheads. Check individual hose specification tables for ODs in suppliers’ catalogs.
     The ID must be capable of handling the required fluid flow without generating backpressure. It is not uncommon for pumps to deliver more than 200 gpm to hydraulic cylinders and motors in various types of equipment, which is why it is important to know the type of hose and the working pressure in a system when making replacements. Flow and pressure intensification should also be considered. For example, double-acting telescopic cylinders can generate return flows and pressures during retraction that exceed source flows and pressures by far.
     In situations where hydraulic equipment has been modified to perform special operations, it is not uncommon to see pressure spikes that manufacturers of the equipment and the hose and fittings did not anticipate. As a general rule, when choosing hose as a replacement for tubing, it’s best to allow a generous margin of safety.
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