Good old gear flow divider - the most perfect solution for synchronizing hydraulic functions!
Only... it is not perfect at all! In fact, it can be pretty bad at synchronizing stuff, and if you are thinking about employing a gear flow divider in one of your projects - you must know that some OEMs set up their catalogs as if they were sales pitches (to be honest, they are kind of supposed to be) and some of their claims should (definitely) be taken with a (large) grain of salt.
This topic is brought up by a random coincidence, by the way. It's been years since I worked on a volumetric flow divider, and then - bam! - I stumble into a "gear flow divider situation", and it costs me dearly, to say the least. I will be detailing on this battle next week, but for now, I just want to say a few words about the gear flow dividers.
I still remember the first time I got to work on one of these. It was a custom-built drum-crushing press at a juice factory. The crushing plate was driven by two hydraulic cylinders synchronized via an external-gear flow divider, and the engineers were wondering why the plate was skewing so badly while crushing drums when the brand-new gear flow divider was supposed to keep the cylinder travels within 2% of one another. I was but a shop hand back then, and my job was doing what I was told to while keeping my mouth shut - but I admit I also found it strange - fixed displacement stuff is supposed to dish out fixed flow, right? So, let us talk about that.
When I say that most gear flow divider catalogs resemble sales pitches, what I want to say is that they are very good at "getting one's hopes up" before providing "pessimistic details in fine print." Or at least that's how it seems to me. In fact, sometimes the "fine print" is omitted altogether - which may lead to situations when a bet on a gear flow divider is a project killer!
Look at what some (random) manufacturers say at the "getting one's hopes up stage":
Casappa HDD:
"...the ratio of the output flow from each section is proportional and constant to its individual displacement capacity, excluding small losses in volumetric efficiency. Rotary flow dividers do not dissipate energy..."
"...Flow dividing gear motors are suited for applications where the flow must be divided equally with maximum actuator synchronization difference of ± 2 %..."
Salmi 2RDE:
"..unlike standard static dividers with variable ports, the flow dividers do not cause dissipation and are much more precise..."
Vivolo RV-2:
"...The portion of flow utilized by each element is solely determined by its nominal flow rate. Therefore, unlike standard static dividers with variable ports, the flow dividers do not cause dissipation and are also much more precise..."
(Wow! It seems that Salami and Vivolo not only went to the same school but also shared a desk. Who copied who, I wonder...)
Galtech SF:
"...External flow gear dividers are non-dissipative hydraulic components consisting of multiple interconnected sections, which divide the inflow into equal or mutually proportional parts, allowing for synchronized movement of multiple actuators independently of the load..."
Bucher QXT:
"...The division ratios are constant and are unaffected by the loads at the actuators. They can be used, for example, to provide synchronized movement of unequally loaded cylinders. Several hydraulic motors can be driven at the same speed, irrespective of their external loads..."
Sounds like a holy grail of everything hydraulic, doesn't it? The ability to divide flow without throttling losses and irrespective of pressure?! Yes, sir! - I'll take a thousand of each, please!
But then comes the "fine print":
Casappa PLD:
"...Flow accuracy equalizers is within ± 2 % if they are rotating in the recommended speed range and the differential pressure between sections is less than 1450 psi (100 bar)..."
Salami 2RDE:
"...For division error less than 4% a maximum level of differential pressure of 50 bar between elements is suggested..."
Vivolo:
"...To obtain errors of division inferior to 3% there must be no difference of pressure between the elements superior to 30 bar..."
"...The flow division error is lower than ± 1.5% with a pressure difference between one element and another until 30 bar. For bigger differences we can approximate an error increase of 1% for each 10 additional bar..."
Galtech:
"...To obtain low division errors, the pressure difference between the stages must be lower than 30 bar..."
And, finally, the only "truly honest" catalog (from Bucher, of course) presents a beautiful performance chart for its QXT 32-016/32-016 divider, that really shows "the whole deal " with gear flow dividers (Q0 is the inlet flow rate, Q1 nad Q2 are the outlet flow rates, and Qmax is the maximum recommended inlet flow rate):
And this, my friends, would be the one chart that everybody who thinks about using a gear flow divider should learn by heart, because it clearly shows the tendency of all gear flow dividers - as soon as you operate them below an optimal max speed and with a pressure difference between the functions higher than some tens of bar - the division (as well as efficiency) becomes way worse than one would expect. And, by the way, the QXT is the internal gear unit (high pressure + high efficiency + low noise) - the "Ferrari of gear pumps", if you will, so for common external-gear flow dividers the situation is actually worse.
Using the graph above you can estimate that a perfectly functional QXT, running at 0.4 of its max speed, synchronizing two cylinders operating with loads that differ by 200 bar, would divide the flow of 1 in portions of 0.57 + 0.43 - about 7% up and down from the middle point, which does not sound that bad, but it would actually result in a 25% travel deviation of the actuators because 0.43/0.57 = 0.75! In other words - when the "fast" cylinder would travel 1 m, the loaded one would only do 0.75 m! And this is not a malfunction, this is a perfectly normal operation!
Another example - imagine a system driven by a diesel engine, with a gear flow divider sized for the flow of the main pump at 2200 rpm to supply, say, two individual track circuits. You may be surprised to discover that when the engine is idling, the efficiency of the divider is much lower than you would expect when you would need to drive a single track, with its speed being some 30% (!) below theoretical.
This doesn't mean that gear flow dividers don't have the right to exist - of course they do! But it is important to know that their volumetric dividing capacity only works "as promised" at their max. speed and with relativity insignificant pressure differential between outlets (and, of course, at optimal oil viscosity as well - let us not forget about that!) As soon as you drop the speed or employ an especially pressure-hungry function (or decide to use one of these as a pressure amplifier) - expect greater losses and speed deviations much higher than the expected 3-4%! And then, you know, "synchronize accordingly."
I suspect that most gear flow divider manufacturers do not include charts similar to the one provided by Bucher not because they didn't test their flow dividers, but because the numbers for sub-optimal regimes would scare their clients off!