As the number of connections in hydraulic systems increases, so does the likelihood of leaky fittings and of tortuous flow paths that stretch out response time. As a result, hydraulic manifolds are finding greater use in both mobile and industrial systems.
Manifolding drastically reduces the number of external connections required, slashing assembly time and reducing the chance of leakage. But there are other, almost equally important advantages:
- Space requirements are reduced because many valves are combined in a single package.
- Actuator response time can be reduced -- sometimes as much as 50% -- because flow paths are shortened and straightened.
- System cost can be reduced because of decreased requirements for tube, hose, and fittings, and because of decreased assembly time.
- Maintenance costs are reduced because valves can be replaced as units.
With these advantages come some limitations. Probably the most important is that more design time and testing time are required. Whereas a simple list specifying fittings, tubing, and hose for a system can usually be generated in-house within a day, the design of a manifold block is considerably more complex, usually calling for the services of outside experts. Manifold manufacturers typically require one to three weeks to generate a technical proposal. Fabrication of a prototype may take as little as one week if only standard components are used, but may take considerably longer for special components. And the vital step of prototype testing adds another few weeks.
Manifold design and testing are costly, but the cost must be balanced against reduced system assembly costs. For simple manifolds of three to five valves, the break-even point may be as few as 20 units; few manifolds are so costly that the break-even point is more than 1,000 units.
Other factors may limit manifold use in some systems:
- Unit-block manifolds, which combine many valves and flow paths in one manifold block, are sensitive to large contaminant particles such as metal chips, pieces of rag, and sealing tape. In some systems, a coarse screen is used at the manifold inlet to filter out these contaminants, but in most cases the pump filter provides adequate protection. In systems without a pump filter, a manifold filter rated at 20 to 25 ∝m absolute is recommended.
- Because manifolds and valve packages enclose flow paths and place valves close to each other, they tend to produce fluid temperatures somewhat higher than those encountered in conventionally plumbed systems. The higher temperature may affect seal and gasket selection, and may require a larger oil cooler.
- Although valve replacement is easy in both unit-block manifolds and valve packages, fault location is typically more difficult with manifolds than with valve packages or conventional systems. The difficulty arises because flow passages in manifolds are not easily blocked off to isolate different portions of the circuit.
One way to retain many of the benefits of manifolding while avoiding much of the design time and cost associated with unit-block manifolds is to use valve packages, or "modular manifolds." These packages consist essentially of valves that bolt directly to each other, without intermediate plumbing. One set of lines connects all valves in the package.
In addition to directional-control valves, components such as flow dividers, flow regulators, check valves, lock valves, and relief valves are available in modular form. These attachments can be added to a bank of directional-control valves to form packages tailored to specific applications. Because all components are standard units, design effort is minimal. The valve packages are typically furnished without extra assembly charges.
The questions that follow can serve as a quick guide for evaluating the potential benefits of manifolding valves. If half or more of the answers are "yes," chances are good that the use of manifolds will prove cost effective.
- Are valve fittings placed in inaccessible locations?
- Is fluid leakage likely to pose a hazard to personnel or the environment?
- Is space at a premium?
- Is actuator response time significant?
- Is easy valve maintenance important to the system user.
- Are locking or crossover valves needed in association with directional-control valves?
- Is a long production run expected?