Most engineering efforts focus on ways to design and manufacture products that satisfy application requirements as inexpensively as possible. But you can make a satisfactory design better by making it more reliable and easier to maintain - and without spending a lot of money. How? Simply by following simple tips and recommendations offered by manufacturers. These fundamentals concern bending, alignment, motion, and similar basic guidelines.

Application basics

Unlike metal tubing, hose is flexible, so it is used primarily to allow relative motion between components at either end of the hose assembly and to simplify routing and installation. It is much easier to route a hose assembly over, under, around, or through a series of obstacles than it is to bend and install a rigid tubing assembly. Furthermore, replacing a hydraulic line by fabricating a rigid tube assembly often is more costly and time consuming than making a hose assembly.

Most manufacturers offer hose that can be bent to a tighter radius than that published in industry standards. Still, bending hose to a smaller radius than recommended should be avoided to avoid shortening service life. Therefore, route hose in a manners that provides ample bend radius, Figure 1.

Because hose is flexible, you must allow for contraction and expansion when cutting the hose to length. Manufacturers state that, depending on its type, hose can elongate up to 2% when pressurized, but, more importantly, can contract as much as 4%. This length differential can strain hose reinforcement wires and eventually lead to failure, especially at the hose-to-coupling interface. Therefore, cut hoses slightly longer than needed to compensate for contraction, Figure 2.

Bend hose in one plane only to avoid twisting its wire reinforcement, which would reduce the hose's pressure capability. Manufacturers state that twisting a high-pressure hose only 5° can reduce service life by 70%, and 7° of twist can reduce service life up to 90%. Unfortunately, hose routing usually occurs late in the design process, so it may be difficult to find an ideal path. Multi-plane bending often can be avoided by reorienting the hose, Figure 3. If this is not possible, install a hose clamp between bends, Figure 4, and provide enough length on both sides of the clamp to relieve strain on the hose's reinforcement wires. This length depends on the hose ID, degree of bending, and helix angle of the particular hose's reinforcing wire, so manufacturers prefer to evaluate each application individually.

Another alternative is to use a single section of hose for each bend and install a hose-to-hose coupling and hose clamp between bends. This technique is less preferred because it not only is more costly and time consuming to perform, but increases the number of potential leak points in the hose assembly. Also, to help ensure that technicians replace and secure hose assemblies properly, include detailed instructions on hose length, use of hose clamps, and special considerations in service manuals.

Providing protection

Hose manufacturers now offer a variety of products with abrasion-resistant covers. No wonder: manufacturers state that about 80% of hose failures are attributable to external physical damage, with abrasion cited as the major culprit. Abrasion is generated primarily by hoses repeatedly rubbing against equipment surfaces or each other.

To help prevent abrasion, use clamps to secure hose in place and keep it from rubbing against adjacent surfaces, Figure 5. The clamp should have a snug fit around the hose to prevent movement, but not be tight enough to damage the hose by squeezing too tightly. Be sure the hose is slack on both sides of the clamp to compensate for contraction and expansion.

Additional protection can be provided by sleeves. Metal sleeves resemble springs that protect the hose from being crushed. Fabric sleeves help keep abrasive particles away from hoses, and both types can serve the added function of nestling multiple hoses into a compact bundle.

Some types of sleeves must be installed from one unconnected end of the hose and slid along its length. Others have a longitudinal slit to enable installing the sleeve without having to disconnect either end of the hose assembly.

Accommodating movement

In addition to causing twisting and abrasive wear, motion can also quickly spell doom for hoses that do not properly accommodate equipment dynamics. For example, hoses connected to a cylinder that undergoes pivoting motion, Figure 6, must be of proper length and routed to avoid becoming kinked or bent beyond their minimum bend radius.

An item that can make a good design better is a swivel joint, sometimes called a live swivel. Unlike standard swivel fittings, which connect hydraulic lines at any fixed angular position, swivel joints accommodate relative motion between the hose and the component to which it is connected. As Figure 7 shows, swivel joints permit pivoting motion that reduces the bending transmitted to the hose assembly and can reduce the length of hose required.

When multiple lengths of hose lie close to each other, and substantial linear motion will occur, hose carriers keep hoses neatly nestled to prevent tangling, twisting, and rubbing against each other. Depending on which particular type is specified, carriers can also isolate the hoses inside from potentially hostile conditions outside - impact from falling objects, abrasive particles, chemicals, or intermittent high temperatures.