Clean design with hydraulic manifolds

Josh Cosford, Contributing Editor

Manifolds simplify hydraulic circuit design in hydraulic machinery, saving space and weight for streamlines machines.

The word manifold is a bit of a blanket term in hydraulics to describe one of many objects used to consolidateplumbing or components for the purpose of fluid transmission. In the strictest sense of the word, a manifold is a single piece of material with many ports used as a junction or header to feed other parts of a circuit or to combine returning flow.

For example, a plumbing header may have a single large port machined in one end, then include two or more smaller, perpendicular ports running down itslength. You may have a 20 SAE portfeeding into the manifold lined with 12SAE outlet ports. The idea is to feed a highvolume into a large port and then provideequal proportions of reduced flow out theauxiliary ports.

A return line manifold provides an equal and opposite function to the header; multiple tank lines provide you with a neat and tidy return location that skips the messy jumble of fittings teed into a single reservoir tank line. By feeding smaller return lines into a single header, you also provide a single point of flow into the reservoir. The consolidated flow may be filtered through a single unit, if necessary, although care should be taken for drain lines. A pump’s case drain line should avoid any significant backpressure related to filtration.

On the other hand, a plumbing header is not likely the image that first pops into your head when you think of a manifold. A more likely image would be the bar stock manifolds used to mount stack valves or the custom-made ported manifolds using cartridge valves for integrated circuits. This article will focus primarily on bar stock manifolds, since custom manifolds constitute an entire new missive.

Standard bar stock manifolds

Manufactured from either 6061 aluminum or ductile iron, bar manifolds come in a series of standard configurations to suit the application’s needs. Aluminum suits the needs of most hydraulic systems operating at 3,000 psi or less and is also relatively inexpensive and easy to machine. Ductile iron is a highly machinable metal with improved characteristics over plain cast iron and is suitable to 5,000 psi.

Bar manifolds come in a range of sizes to accommodate not only the different sizes of ISO (CETOP) industrial stack valves from D02 to D10, but also different flow rates. A D03 (NG6) valve flows up to 20 gpm (75 lpm) in some cases, so the bar stock manifold manufacturers offer a solution to suit. While a standard 14 gpm (36 lpm) manifold comes equipped with SAE 10 and 08 P/T and work ports, respectively, the high flow version employs pressure and tank ports at SAE 16 with work ports expanded to SAE 12 to handle the extra flow. The additional flow capacity enables up to 25 gpm (95 lpm), increasing the flow potential across a multi-station manifold, which has to share that volume.

Although it depends on the individual manufacturer, you can purchase bar manifolds up to twenty stations long. That’s an incredibly long chunk of metal requiring specialized CNC gun-drilling equipment. The manifold’s length doesn’t change its flow rating, so understand that a long manifold must share the same input of flow.

Also, larger valves such as D07s, for example, cannot support quite so many stations as the smaller manifolds. And 8-station D07 manifold approaches three feet long and tips the scales at over 250 lb when specified in ductile iron. Conversely, the sixteen-station D03 manifold weighs about half as much, although it is much longer. Obviously, aluminum, which is half the weight, is an excellent option for any manifold running 3,000 psi or less.

Regardless of the valve size or number of stations, some features are common to most bar manifolds. Most hydraulic valve stacks require porting for pressure, tank and work ports alike. At first glance, bar manifolds look the same, with pressure and tank ports mirrored at the far ends of the manifold and then work ports stacked atop each other and equal to two times the number of stations.

Standard manifold options from Daman Products range from ISO 02 valve patterns through ISO 10.

Parallel and series designs

Of course, we all know that looks can be deceiving, and as you might expect, there is more than one plumbing configuration to bar manifolds. You can purchase them in either series or parallel flow paths, and both require understanding for your hydraulic circuit to function correctly.

The parallel bar manifold is simpler to manufacture; two gun-drilled passages run the manifold’s length, joining the left side pressure port to the right-side pressure port. Similarly, the tank port runs the manifold’s length, allowing flexibility in plumbing from either side of the valve assembly. Both the pressure and tank port may be plumbed from the right side, left side, or both.

Each valve station of a parallel manifold picks up its valve’s ports from downward cross-drillings. These drills are located in four or more places, joining ports P, T, A and B to their respective locations in the valve mounted above. Valves larger than D03 may contain dual tank ports, and others employ separate pilot and drain ports, as required. The parallel ported manifold provides equal flow to each valve station along its length but must deal with one of the all-too- infrequently misunderstood concepts of fluid power — the path of least resistance. You cannot overlook the tendency for fluid to move from a place of high energy to that of lower energy (lowest energy, in fact). Parallel circuits provide that path of least resistance, so take precaution and use flow controls or pressure compensation to prevent an entire circuit’s worth of flow from rivering through only a single valve.

The series circuit will provide fluid power novices with just as much confusion if they’re not paying attention. The series circuit sends fluid through the center of each top mounted valve. Commonly known as tandem center valves, they flow P to T in neutral. But rather than drain to tank, when using these valves on a series manifold, their tank line feeds the next valve’s pressure port.

A series setup is complicated in a couple of ways, but it allows the use of open center hydraulics and readily available fixed displacement pumps. A parallel circuit is susceptible to flow taking the path of least resistance. The series manifold experiences pilfered flow from each valve in the circuit ahead of the trailing valves.

Manifold adapters, like these stack modules from Daman, mount between the existing manifold and a valve. They convert the existing manifold valve pattern to an alternative size valve without disrupting the existing configuration.

If there are four valves in series and no functions operating, fluid passes from one valve to the next until after the last valve; it then returns to tank. Should function one activate, fluid flows out to the actuator where work is done and then returns to flow through the second function. What you must consider is the pressure available to downstream functions while the first function is operating. If the first function uses 1,500 psi of available pressure and the main relief valve set to 3,000 psi, only 1,500 psi is available for all three following functions.

Design choices

You can see how one must intelligently design circuits using either parallel or series type manifolds. A parallel manifold requires a pressure compensated pump or an innovative unloading circuit. A series manifold is exclusively used in fixed flow circuits because it’s a bad idea to unload pressure compensated pumps.

Regardless of the circuit type, there’s more than just the number of stations to consider when selecting a bar manifold. The raw material used in the manifold’s creation comes in slightly more exotic metals, such as stainless steel or chromoly alloy. On top of metal choices, there are standard and special options to ensure your circuit is optimized.

Porting options for bar manifolds come in as many flavors as the general fluid power industry offers. SAE, NPT, BSPP or Metric are not uncommon, but flanged ports are also available. Work ports may come out the front (which could be the back if so mounted) but employ bottom work ports. A bar manifold with bottom work ports makes a vertical mounting method a great option.

Header and junction blocks, also known as “distribution manifolds,” connect several threaded connections into one common block to eliminate leaks and reduce plumbing labor. Image courtesy of Daman Products.

A common option on especially smaller manifolds is the ported cavity to install a relief valve. Cartridge valves are inexpensive, and the extra cost to machine a 2-way cavity into the manifold is quite reasonable. The same goes for a gauge port, which provides an economical and clean method to observe system pressure without the mess of adaptors typically required to add a gauge as an afterthought.

Manifolds are foundational in the operation of hydraulic machinery. They have provided an effective solution to creating an entire hydraulic circuit by merely joining a series of valves on top of a ported chunk of metal.