Insane Hydraulics

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How Much of a Difference Can Three Hundredths of a mm Make?

This here is a clone of a Walvoil SD11 monoblock DCV.

Clone of a Walvoil SD11 valve

It's a good clone, by the way. Decent quality, a great price, and probably the best budget DCV I could get (at least at the moment of this writing). I've been using them for years for simple stuff, like front loaders, log splitters, etc.., and I've had zero problems. But this is not a sales pitch, this is a technical article about the radial clearance of spools in spool-type DCVs, and I will be using this particular clone to illustrate how even a seemingly insignificant increase in radial clearance leads to a surprising increase in leakage.

So, I needed to find an economical two-section DCV solution the other day, and one of the sections had to be single-acting. We stock tons of Walvoil valves and the respective parts, but the "economical" part of the client's request was imperative, and so I thought to myself - "I wonder if I can fit the Walvoil's single-acting spool into one of these clones?"

I could also machine the spool, of course, and we do have all the necessary tools in the shop to do it right (although even a simple girding stone would do the job if I were "in a pinch"), but why waste time machining spools when you have parts lying on the shelf?

Here are the two spools one next to another - the original Walvoil at the top (single-acting on A, B plugged, three positions), and the cloned-version at the bottom (closed center, double-acting, 3 positions):

The original Walvoil spool at the top (single-acting on A, B plugged, three positions), and the cloned-version at the bottom (closed center, double-acting, 3 positions)

They look "swappable" to the naked eye, but when I measured the spools, I saw that there was a 0.03mm difference in their diameters. Walvoil's spools measured exactly 20.00 mm in diameter (I grabbed other Walvoil spools from the shelf and measured them as well - and they all had 20.00), and the "cloned" version measured 20.03 mm:

The Walvoil spool measures 20.00 in diameter
The cloned-version spool measures 20.03 in diameter

When I tried the Walvoil spool out in the cloned body, it didn't feel "too bad", but it did feel "a bit too light" for a normal fit. I knew that I wouldn't be able to use it, but I wondered how much of a difference would an increased radial clearance of a mere 0.03mm make for the leakage. And since I didn't have the answer to this question, I decided to find out "empirically". Here's the setup:

Testing spool leakage

The "cloned" spool had the leakage of 18 drops per minute at 200 bar. To give a better perspective of what 18 drops per minute look like, here's what you get after 10 minutes:

18 drops per minute after 10 minutes

Outstanding! If this valve was used in a front loader " not spoiled" by counterbalance valves, this leakage would result in a load dropping at a rate of millimeters per hour. As I said - have been using these valves for years, never had any problems or complaints.

Now, here's the leakage that I got when I installed the spool that had minus 0.03mm in diameter:

This is a lot of leakage for a spool valve (in the left cup)

But wait - there's more!

Don't look at the pictures below this paragraph yet! Knowing that the oil in the cup on the right took 10 minutes at 200 bar to gather, can you guess how much time it took to fill the cup on the left (yes, it took less than 10 minutes)?

Taken a guess already?

Are you sure?

I'll give you the answer now.


It took 34 seconds:

This is a lot of leakage for a spool valve!
It only took 34 seconds at 200 bar to fill this cup

Maybe someday I'll explore the math behind the leakage flow rate in a radial gap, which is very interesting (there are great articles on this already, btw), but for now, I want you to know the following: the leakage flow rate in a spool/housing assembly is directly proportional to the cube of the radial clearance! And this is so for concentric leakage, if your spool/bore combination is eccentric (spool pushed to the side), the leakage flow rate is further increased by 2.5 times!

Normal spool clearances are in the order of one-hundredth of a mm or less. Each time you double the clearance - the leakage increases by 8 times. Say the "good" spool had a 4-micron clearance, and we "loosened it up" by another thirty - in other words, increased it 8-fold. Now, how much is 8 cubed again?

So yes, my friends, a measly three hundredths of a milliliter is a huge deal when you talk about radial clearance of a spool valve!