Insane Hydraulics

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Three "CETOP 3 Oh-Oh Moments" in the Life of Every Hydraulic Technician

Today I will be talking about the most common sandwich interface size in the world, can you tell which one is it?

There are, of course, bigger and smaller base-plate patterns, but this one is, by far, the most ubiquitous of them all, and, as is always the case with industrial standards, it goes by many names: CETOP 3, NG 6, ISO 3, N.F.P.A.T3.5.1 MR1, N.F.P.A. D03 or ANSI-B93.7 D03 and there's probably more where these came from.

Here, in Portugal, this size is commonly referred to as "NG6" and I guarantee you that if you ask a local tech why the "NG" and the 6, you'll hear something along the lines of "Well... it's like... NG6... because everyone calls it NG6!" (sigh) Industrial folklore, how I love thee.

The "NG" comes from the German "Nenngröße", which means "nominal size", and the "6" stands for 6 mm, which would be the nominal diameter of the sub-plate port hole according to the DIN (Deutsches Institut für Normung) standard 24340. The 6 mm hole is actually a small deviation from the ISO (and the CETOP ) standards, which allow for a bigger 7.5 mm port hole. The sub-plate on the picture above has 7.5 mm holes, so, technically speaking, calling it an "NG6" would be a mistake.

CETOP stands for Comité Européen des Transmissions Oléohydrauliques et Pneumatiques - the European Fluid Power Committee, ISO for International Organization for Standardization, with the standard describing the interface being the ISO 4401:2005. Why the 3? The full numeric code for the ISO standard would be the 4401-03-02-0-05 (for "normal" valves) and 4401-03-03-0-05 (for valves with pilot holes X and Y - something that you will probably never come across), and since the second number of this code is what defines the size of the interface - it is common to reduce this to just ISO 03.

And, of course, all our friends from the US know that N.F.P.A. stands for National Fluid Power Association (even though, apparently, this abbreviation has serious competition coming from The National Fire Protection Association), and the ANSI - for the American National Standards Institute.

By the way, most industrial standards are closed from the free viewing and it's such a shame, in my opinion! I think they should be accessible for free to everyone.

But no matter what you call it - it always boils down to the following baseplate pattern (I didn't draw the outlines of the plate, just the ports and the M5 holes):

So, what can we see here? We can see that the interface is symmetrical along the horizontal center-line crossing the A and B port centers, and asymmetrical along the vertical line crossing the P and T port centers. This is good because you can now identify the ports just by looking at the base plate.

I like memorizing this in the following fashion - "most people are right-handed, which is why the right side is always bigger". And for this interface size, when the plate surface is facing upwards, and the P port is pointing at you (just like on the drawing above) everything is "pulling to the right" - the distance between the threaded holes is bigger on the right side, the ports are "skewed" (or pulled) to the right, and the optional locating bore is also located on the right side. Then all I need to remember is that the A and B follow our natural tendency to place things alphabetically from left to right (in other words - the "A" port is on the left and the "B" port is on the right). Thus - if you can tell which side of the rectangle formed by the four threaded holes is bigger (it's actually a trapezoid rather than a rectangle) - you can tell immediately which hole corresponds to which port.

Naturally, as soon as a tech assembles a couple of NG6 manifolds with simple directional valves for the first time, he experiences his first CETOP 3 "Oh-Oh" moment, when he learns that there's only one way a valve can fit onto such a base-plate because of the asymmetrical mounting holes. Then he thinks "How convenient! I will never will be able to mount one of these the other way around!" Only... he later discovers that this is not true when he comes about a valve stack that uses a sandwich manifold that does not have the o-ring bores on one of the faces but rather uses two plain faces and a thin seal plate:

This is the second CETOP 3 "Oh-Oh" moment in the life of every hydraulic technician - when he realizes that he can flip the valve around the horizontal axis and switch those check-valve throttles from metering-in to metering-out! The Rexroth Z2FS depicted in the cutaway view above is a good example of such a valve.

Now let us go to the third CETOP 3 "Oh-Oh" moment every tech experiences sooner or later - the moment when he comes across a universal ISO 03 sandwich body, which has symmetrical holes (the "whole shebang" - perfectly rectangular mounting hole pattern and the port diamond smack dub in the middle) - and this shatters his hard-learned belief that all CETOP 3 bodies must be asymmetrical to fit on the respective baseplate! Here's an example - the SUN's GBA universal ISO 03 body for a single T-13A cavity:

So, the logic behind the universal bodies is simple. Flipping the modules about the horizontal axis is great because it allows you to swap the metering direction, but what if you have a body with a single throttle valve in, say "A" port? You can flip the metering direction all right (if you have a body with two flat faces) but you still can't pass the valve to the "B" side because of the asymmetrical mounting holes. And this is where the universal bodies "shine" because they not only allow you to flip about the horizontal axis, but also around the vertical, and they can do this because they have much wider (7.5 mm against the usual 5.1-5.4 mm) symmetrically placed holes for the M5 screws, which allow you to, pretty much, mount them in all (four) possible positions!

But... What about the port holes? The baseplate holes are skewed to the right, so how would you make sure that the seals are properly aligned now that the body can, basically, "flap about" the M5 screws with the enormous 7.5 mm holes? I am glad you asked! Usually, it is the seal-carrying plate that does the centering, because it (unlike the "wide-holed" body) still carries normal 5.1 mm mounting holes, and usually uses an emboss (SUN calls it "nib") or a small pin in the middle of the port holes to align the seals with the ports. Here's what it looks like (note the excentric position of the screws in the larger holes of the body):

Pretty cool, don't you agree? Now, you can use the same single cavity body for metering in and out on either "A" or "B"!

So, today's "CETOP 3" takeaways are:

1) You can identify the CETOP 3 base plate ports by finding where the larger base of the trapezoid is (and knowing that it is always on the right).

2) There are: