Quite often I stumble upon the "hole" in understanding of the concept of hydraulic balance among people who closely work with hydraulic components, and have no problems understanding that force equals area multiplied by pressure, but fail to see how this very principle is used to "direct and trim" the forces between pressure exposed parts in hydraulic assemblies. Hopefully, this post will start filling it.
Let us consider the following "hydraulic system", and to simplify things let us imagine that this is an ideal zero-leakage system with no gaps between parts:
The upper plate is creating a 1000N downward force on the rod that has a section area of 1 sq.cm. This pressurizes the hydraulic fluid inside the cylinder to 100 bar. The pressure acts on the bottom area of the cylinder creating a downward force of 1000N, and of course, it acts on the rod itself, creating an equal opposing force of 1000N. A very simple arrangement, that mimics the rotary group on an axial piston pump. As you can see, the normal force (i.e the force that is compressing surfaces together) between the rod and the upper plate, and between the cylinder and the base plate is 1000N.
Now let us change the "design" by drilling a hole through the rod and the cylinder, with the area of 0.9 sq.cm:
The situation becomes much more interesting. Since we are still effectively applying a force of 1000N over an area of one square centimeter, the fluid is still pressurized to 100 bar, however now the normal force between the rod and the upper plate, as well as the normal force between the cylinder and the base plate is ten times smaller because the area that these parts are "exposing" to the pressurized fluid is also ten times as small. In other words - by drilling the hole in our components we transferred 90% of the load to the column of pressurized fluid!
If we reproduced the two contraptions "in steel", and then tried to move our cylinders sideways, one could think that the fact the normal force between the "hard" parts in the second design is ten times smaller would result in the friction force ten times as small as well, however, as it is always the case in real life - its more complicated than that, and there are more "hidden benefits" to the second design.
In the first case, even if the parts were submerged in oil, and theoretically we would be dealing with lubricated friction (which is in itself a super complicated phenomenon), a load increase could "squeeze out" the oil film between the parts, and then the good old dry (or maybe marginal) friction would kick in and cause the parts to seize and destroy themselves. The second design, on the other hand, aside from the much smaller normal force and the fact that sliding a column of pressurized fluid on a surface is much easier than sliding a piece of hard material, would also introduce the huge benefit of forced lubrication, exposing the gap between the sliding parts to the pressurized fluid. And if the load increased, so would the oil pressure, thus keeping the sliding conditions in the so desired wear-free lubricated friction regime.
This crude example serves to demonstrate how easily oil pressure, existing in hydraulic systems, can be used to minimize (or balance out, if you will) compressing forces between pressure exposed components, while still making sure the force is directed the way we want it to be directed.
The most beautiful application of this principle, in my opinion, is the rotary group of an axial piston unit. The cylinder blocks are cleverly balancing out the system pressure-induced compressing and separating forces to keep the cylinder block pressed against the valve plate, and the piston assemblies have shoe cavities designed in a way that makes sure the system pressure keeps their "feet on the ground" (i.e. shoes pressed against the swash-plate - more details about the piston shoes and their design here).
By the way - this is why I always cringe when I hear techs commenting that the rotary group pre-load in an axial piston unit affects its volumetric efficiency. Maybe this myth is not universal - I am not sure about other countries, but around here I've heard it all too often. It starts like - "...oh my god oh my go oh my god!!! - We've lapped the barrel and the valve plate and now we are not sure if we should put a 0,1mm shim or a 0,15mm shim over the bias spring to compensate! We need to compensate, damn you! Get the manual, stat!.."
This is bad. And sad. The fact is - most of the time no "compensation" is required after lapping because a properly balanced (and, obviously, correctly lapped) cylinder block does not depend on a bias spring to function at all. And a 0,1 mm increase in the pre-load length of a coil spring that had compressed 2 centimeters makes no difference.
I once did an interesting experiment (to be honest - it was more of a dumb mistake rather than a purposeful experiment) - when I rebuilt an A4VG closed-loop pump (can't remember the size now, I think it was a 56) - and it was only after I finished all the bench tests (which the pump passed with flying colors) when I found the barrel bias spring on the top of my bench. The pre-load from the cup springs behind the ball guide was more than enough for the pump to function 100%!
Naturally, the lack of pre-load will drastically affect the pump's resistance to the block-lift - but that's something completely different.
Anyhow, the fact that you can balance out pressure-induced forces to your advantage by cleverly sizing exposed areas is pretty cool!
And if you really start looking into it - you will find that most of the moving parts in a hydraulic system that are exposed to the system pressure can be called "hydraulically balanced" - similarly to axial units, gear and vane pumps and motors use balanced pressure plates, the same goes for radial piston unit valve plates, orbital motor valve plates, swashplates with hydro-static bearings (now you see why the cavity on the pressure side is bigger, and why the swashplate stops "swashing" when the lubrication orifice is plugged!), spools, poppets, logic elements, you name it!
A clear understanding of what hydraulic balance is and how it is typically used in hydraulic components is very important, and if you want to be an ace hydraulic technician make sure you have no doubts about it.