Learn how three manifold designs — the common hydraulic integrated circuit, stackable bar designs, and simple passageways for pressure or return — are manufactured for hydraulic designs.
By Josh Cosford, Contributing Editor
When discussing manifolds, your mind will go to one of two places — bar manifolds with sandwich valves mounted atop or integrated circuits with a series of cartridge valves constituting a hydraulic circuit. The truest form of manifold contains no valves at all – consider the exhaust manifold on a vehicle, which is just a series of passageways that combine exhaust gas flow into a single outlet.
Such a manifold also serves a purpose in hydraulics and may exist on either the pressure or return sides of a hydraulic circuit. Running the primary line into a manifold provides a tidy method to split downstream functions without the mess of tees and adaptors for a single hydraulic source that supplies many downstream functions. More commonly, the return line manifold joins each separate function’s tank flow into one large chamber, where it may flow into a filter before returning to the reservoir.
Hydraulic integrated circuits are an effective and economical method to organize the valves of your hydraulic machine. Instead of plumbing together separate components throughout a machine, all the passageways and ports are inside a single piece of aluminum or steel. Special cavities are drilled into the block where the cartridge valve is inserted. The cavity looks like a mildly conical shape with cylindrical steps, and at each step is a cross drilling corresponding to each valve port (see Figure 2).
Although the complex shape you see can be achieved with myriad individual CNC drills and tools, specialized roughing and finishing tools achieve the finished shape in two operations in a CNC mill. Although a standard drill may create the pilot hole for the roughing tool to more easily achieve its shape, you’d be surprised at how quickly the basic cavity shape can be hogged out of a billet of aluminum with the appropriate tool. You can see from Figure 1 how many holes may be drilled into a manifold.
Integrated circuits aren’t limited to screw-in cartridge valves. The slip-in cartridge valve offers a similar benefit of clean installation while offering extraordinary flow capacity. Also known as logic elements, these valves may pass flow in the thousands of gallons per minute. They also find themselves installed into cavities, but the shape is more straightforward than the multi-port screw-in cartridge valves. Without going into many details, logic elements are large 2-way, 2-position valves which use various pilot control functions, allowing them to mimic directional, pressure, and flow valves. There are downsides, however, such as the requirement for four valves to replicate standard 4/3 directional valves.
Why should you use HICs?
If manifolds are so difficult to manufacture, why should you choose them over separately plumbed components? Well, there is space efficiency — integrated hydraulic circuits take up less space than traditional hydraulic circuits because the cartridge valves are compact and installed directly into the machine block, reducing the need for external piping and reducing the overall size of the system.
Although the design and manufacture are bespoke, installing cartridge valves makes for a simple and quick process once the blocks are machined. The cartridge valves are inserted directly into the machine block and connected to the hydraulic system with minimal external piping. The time saved through installation on the machine requires fewer hoses and connections for the technician to plumb. Also, consider the reduced potential for leaks. With so few fittings and hoses, leaks are either internal or via the few externally ported connections.
Cartridge valves offer unmatched speed of maintenance, as well. Pressure and flow control valves require only a wrench to remove and replace in less than a minute. Solenoid valves may take only a minute longer to remove and replace the coil(s). In addition, no electrical connections need messing with as the coil can remain wired, requiring only gentle placement out of the way until the valve is changed.
Because there are hundreds of available cartridge valves, integrated circuits offer the designer near-infinite combinations of sophisticated control options. From post-compensation to proportional control, there isn’t any machine outside the complexity of a manifold to handle. With the combination of advanced (and expensive) subplate-mounted valves, there is nothing you can imagine that can’t be created.
Stackable valve designs in bar manifolds
The mention of subplate-mounted valves brings to mind the stackable series very popular in the industrial hydraulic market (Figure 3). They are sometimes called ISO or CETOP valves because of the standards they’re based around, but what’s more important to mention for this discussion is their tendency to employ a different form of the manifold.
Bar manifolds are machined billets of aluminum or ductile iron that offer a readily available system of mounting valves to create simple circuits suitable for most circuits and machines. Machined from lengths of square bar raw material, with dimensions suitable for D03 right up to D10, they are made from 7 x 9 in. raw material.
With generally standard dimensions, valve interfaces and mounting holes, and with designated port locations, the manufacturing process for bar manifolds makes it quite economical compared to custom designs. Even with up to twelve sections or more run abreast, the difficulty scale changes little. Because most of the ports and drilling are perpendicular to the manifold surface, the only challenge comes from the diagonal drillings for the valve interface ports.
Creative fixturing in a CNC mill will allow the machinist to achieve the diagonal porting. Of course, care must be taken in the CNC operation to prevent side load on the drill, which could easily break, but any machinist worth their salt has the experience to select the correct speeds and feeds for such an operation.
Of course, the more sophisticated manufacturers do not waste time with multiple machine setups and instead use 5-axis machines that can complete an entire manifold in just two operations or less. If the threaded holes for the mounting feed are tapped only into the side edges, there’s no reason to remove the manifold to flip it to access the bottom. A 5-axis machine will be many times faster than a traditional mill but also costs many times as much.
Using a combination of stackable sandwich valves, 90% of the circuits any customer may need could be created in short work. Each stack starts with the valve closest to the actuators and builds to the directional valve on top. Locating pins sometimes ensure the valves are facing the correct direction, which is especially important for D03 valves, which have a symmetrical diamond-shaped port design.
The assemblies are held tight using four or six bolts. For smaller stacks, socket-head cap screws are preferred, but the hydraulic shop must stock various lengths. Not every valve in a stack shares the same thickness, so one set of four valves, for example, may have a different installed height from another set. For tall stacks, technicians cut the high-tensile threaded rod to the appropriate length and then top them off with stud nuts. Pro-tip: install a long-threaded rod into one hole of each valve section to act as a guide to help locate each valve as it’s installed, and then remove the rod to install the last bolt.
Bar manifolds may not offer the same space efficiency as custom-designed integrated circuits; they’re certainly a better use of space and plumbing than separately located valves. Each valve in the stack eliminates what would have been previously two to six separately plumbed tubes, pipes, or hoses. Generally, sandwich valves eliminate leak points, but occasionally after decades of use, the O-rings that seal the ports between each valve may become brittle and leak. If leakage does occur, they’re effortless to replace.
Getting creative with your designs
Although circuit creativity offers fewer unique possibilities compared to integrated circuits, designs may still employ creative tactics outside the norm. For example, using an extra stack location in a bar manifold to create a mini circuit of pressure valves to offer creative control over the entire circuit is a distinct possibility. The system relief valve, an unloading valve, or even pressure compensators could be stacked together to control the pressure port of the entire manifold. Although most bar manifolds come with a port to install a screw-in cartridge valve, the performance of cartridge valves rarely matches a high-quality stack valve, especially regarding flow rate.
Bar manifolds come in either parallel or series circuits. Parallel circuits use a single pressure and tank drilling, extending the length of the manifold and providing each valve with the same access to pressure and tank flow. Unless an unloading valve is used, only closed-circuit systems with pressure compensated pumps are suitable for parallel manifolds. For series circuits using fixed pumps, the flow path runs from the first valve station’s tank port into the next station’s pressure port. Care should be taken with series circuits because the P-to-T flow path occurs through the valve spool itself, limiting flow.
Although different in their own ways, each of the three manifold styles has much in common. This fact is not lost on manufacturers because the prominent players in the game make all three styles. As a result, manifolds are popular in hydraulics and have been a cornerstone of most hydraulic systems for the past couple of decades.
Filed Under: Components Oil Coolers, Engineering Basics, Valves & Manifolds