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Hydraulic systems usually must be designed for installation onto an existing structure. Sometimes designers have no choice but to have components mounted in hard-to-reach locations. However, whenever possible, it’s best to place pumps, valves, and actuators where they are easily accessible for service, maintenance, and replacement. This is especially true for filters, because the tougher they are to access, the less likely their element will be replaced when required.
Hydraulic-system designers should strive to use as few fluid conductors as possible by implementing manifolds to interconnect components. In most cases, this requires external piping. Minimizing potential leakage points and making maintenance as simple as possible should be the goals of any astute designer.
When planning where hydraulic components will be installed on a machine, designers should provide adequate space to route hose and tubing. In addition, they need to coordinate hydraulic-system planning and design with other systems (electrical, fuel, lube, torque converter, etc.) on the machine.
Whenever possible, hydraulic lines should run parallel within the machine envelope and follow its contours. Smooth, parallel routing can be accomplished with a well-planned layout and proper clamping. Parallel routing can often save money by:
• reducing line lengths and the number of adapters;
• minimizing the number of sharp bends;
• making the machine more serviceable; and
• protecting lines from external damage.
To help keep lines parallel, study port positions on components and carefully pre-plan the location of valves, filters, heat exchangers, and the reservoir. Components should also be spaced far enough apart to provide room for proper installation of the adapters and fittings on connecting hoses and/or tubing.
Hose or Tubing?
System designers must first determine whether using hose, tubing, or a combination of the two is best for a particular application. Hose and tubing should not be viewed as separate entities, but rather as companion items, each offering specific and unique benefits. For example, tubing can:
• be bent to smaller radii than hose and installed in tighter spaces;
• be routed through areas of higher ambient heat;
• handle hotter fluids than hose; and
• provide superior heat transfer.
On the other hand, tubing can be flattened or damaged when struck, whereas hose is resilient and more likely to return to its original shape after absorbing a blow. Tubing may also fatigue when connected to high-frequency vibrating components, while hose will absorb the vibration.
If lines are long, tubing may require a series of intricate, close-tolerance bends that may complicate installation and ultimately create service problems. The flexing properties of hose, on the other hand, allow it to follow desired contours and thus simplify hose installation. Hose can also absorb some high transient-pressure shocks, providing more uniform flow patterns and smoother, quieter operation. Hose is not recommended when hydraulic rigidity is required due to its tendency to act as an accumulator.
Good Designs Promote Good Maintenance
To correctly route and properly install fluid-conveying components during a machine prototyping, follow these 10 general rules. These guidelines should be most beneficial during machine prototyping. Follow normal production procedures after eliminating all bugs.
1. Start with large lines—Install the largest diameter lines first because they are the most difficult to bend and maneuver, especially in tight spaces. After that, the job becomes easier. Smaller lines provide greater routing versatility and can be more easily worked into tight spaces, so route each line to conserve maximum space. This not only results in a neater-looking machine, but makes future modifications or additions of accessories easier, more convenient, and more economical.
2. Aim for optimum length—The appearance and efficient operation of a system often depends on using proper-length hoses. Making them too long increases pressure drops and system cost. Hose assemblies are commonly manufactured to specified lengths as well as increments of lengths to minimize the size of the inventory, which must be carried. When computing hose length, remember that hose can elongate as much as 2% or contract as much as 4%.
3. Hose flexing—A hose assembly is designed to bend, not twist. In fact, if a large-diameter, high-pressure hose is twisted only 7 deg., its service life can be significantly reduced—in some cases by as much as 90%.
High-pressure hose must be routed to flex in only one plane (Fig. 1). If routing requires hose to bend in more than one plane, the hose should be divided into two or more sections so that each will flex through only one plane. Better yet, a 45- or 90-deg. elbow could be incorporated to prevent bending the hose in two planes.
A spring guard often is used to keep hose from bending beyond its minimum bend radius at the fitting. However, it will not prevent the hose from twisting.
4. Pivot points—When hose must flex, route it through the pivot point around which the component is moving (Fig. 2). This will produce the most efficient flexing of the hose line, use the least amount of hose, and keep the hose within the confines of the machine. To achieve this, the hose should be positioned to bend like a hinge. Otherwise, the hose may tend to take an S-bend, which is most likely to happen when the hose is pushed rather than bent. An S-bend installation results in excessive movement and shorter service life.
When working a hose through a pivot point, consider the relative positioning of the two end fittings to avoid an S-bend. Swing the moving component to its farthest point, where the hose will experience its widest bend. If the fittings are placed in parallel planes at this point, the hose will tend to flex in a hinge-like manner when the component is swung back to the opposite end of its travel.
5. Reciprocating motion—In addition to flexing, the ends of the hose may have to reciprocate. Several methods are used to accomplish this:
• Hose reels—For use with high-pressure hydraulic hose, these reels are equipped with high-pressure swivel joints and a spring return to help rewind the hose.
• Festooning—Hose is hung in loops from a steel cable. As one point of the loop moves away from the other, the loops open to form an almost straight line.
• Rolling—Hose is arranged in an unbalanced U-shape with hinged tracks carrying the hose. One leg is left stationary and longer than the second, which is free to reciprocate parallel to the first.
6. Rotary motion—Swivel or rotary joints are commonly used to provide rotating motion. Where continuous rotary movement is used, specify a rotating joint. If movement pivots and reverses, a swivel joint would be the better choice. When used with hose, a swivel joint will prevent hose twisting or bending at the fitting.
7. Control oil spray—Fire protection must be incorporated when hydraulic lines are routed near hot, potentially hazardous areas. This prevents oil in a broken line from spraying onto any potential source of ignition. There are several ways to build in such protection:
• Reroute the line through a tunnel made from steel tubing, channel, or angle iron.
• Install a sheet-metal baffle between the lines and potential ignition source.
• Route the lines through a large, open-ended hose or sleeve so that the oil will flow out of the ends in case of line failure.
• Use fire sleeves either to fit over the hose or built into the hose cover.
• To guard against a failed hose that might whip and spray hydraulic oil over an ignition source, anchor the hose to the component to which it is hydraulically connected.
8. Minimum bend radii—The hose must be routed to accommodate the minimum bend radius of that hose (Fig. 3). Minimum bend radii for different hose diameters and pressure ratings are provided in SAE and hose manufacturers’ literature. These figures usually refer to the minimum bend radius at maximum operating pressure for a static line. Bending a hose to less than the minimum radius can result in kinking of the hose and excessive stress at the hose or fitting interface. As a result, the cover can crack more easily or the internal wire reinforcement can fatigue quicker, both of which will reduce service life.
9. Avoid abrasion—Hydraulic hose typically features a tough outer cover to protect the hose reinforcement from abrasion or moisture damage. However, constant abrasion at one point will eventually puncture the outer cover and weaken the reinforcement. This is the number one cause of hydraulic hose failures in the field. To minimize abrasion, either properly route and clamp the hose, or use a protective cover (Fig. 3, again). A variety of protective coverings are available, including coiled springs, coiled strap steel, spiraled plastic, and nylon sleeves.
10. Clamping—A piping installation is not complete until properly clamped. Clamp choice is very important, and often it can be critical to the installation (Fig. 4). Common sheet-metal clamps will not hold a large, high-pressure hose.
Good clamps can be inexpensive, yet highly effective for high-pressure surge lines. Anticipate and plan for a possible length change ranging from an increase of 2% to a decrease of 4% for high-pressure lines. Proper routing and clamping should be planned to avoid areas of vibration. Also, never clamp the hose on a bend.
Properly sized clamps should grip the hose securely. To keep the clamp from abrading the hose, the ID of the clamp should be about 1/32 in. smaller than the OD of the hose.
Good installation techniques are essential to efficient operation and maximizing the life of a hydraulic system. However, hydraulic hose and tubing are fatigue items with a finite service life—eventually these wear items will fail. Typically, basic maintenance techniques, such as visual inspection or time-based preventive schedules, are not enough to sufficiently prevent failures.
The mean time to failure (MTTF) of a component can vary widely based on the duty cycle, installation practices, environment, and robustness of the product. Many users replace hydraulic hoses at predetermined intervals—say, every three years. However, this is an average value, so replacing hoses at three-year intervals does not guarantee that they will last three years. Alternately, hoses in less-demanding applications may last six years. If so, replacing them after only three years of service means they have only achieved half their useful life.
Figure 5 is an example of how the MTTF can vary for a hydraulic hose. This range of performance makes it difficult to determine the exact interval at which each hydraulic hose should be replaced without discarding a good hose or preventing occurrence of a failure.
Eaton’s LifeSense Hose is a condition-based hose-monitoring system designed to detect failure-related events both inside and outside a hydraulic hose. It provides an electronic signal to indicate that it should be replaced before actual failure. The LifeSense feature monitors both internal fatigue and external abrasion—the primary causes of hose failure in the field. Two options are available: a wired system and a wireless system.
The wired system consists of a sensor on the hose assembly connected to a wire that is routed to a hose diagnostic unit. The hose diagnostic unit then interprets the data and alerts the operator if a hose needs replacement.
The wireless system includes a sensor connected on the hose assembly that communicates via a high-frequency radio signal to a hose diagnostic unit. The hose diagnostic unit also doubles as a gateway where it can communicate this information to smart devices via Ethernet or Wi-Fi. With the information in the cloud, it can be sent via text message or e-mail. The wireless system also provides a customer portal that can serve as a user interface for remote monitoring. Now, the true condition of that hydraulic hose can be monitored anywhere, anytime.
By combining good hose-routing practices and advanced condition-monitoring technology—such as Eaton’s LifeSense Hose—the frequency and impact of hydraulic hose failures can be significantly, if not completely, eliminated. The end result is maximized equipment efficiency and reduced safety concerns.
Mike Beining is Engineering Manager, Product Application at Eaton’s hydraulic hose operation in Maumee, Ohio. For more information, call (800) 386-1911 or visit www.eaton.com/hose.