Insane Hydraulics

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Installation of Axial Piston Pumps Above Oil Level

Today I will be touching on a very controversial topic: installation of variable displacement axial piston pumps above the oil level in a classic open-loop configuration. When I say "classic open-loop configuration" - I mean "zero suction aids", like booster pumps, semi-closed loop shenanigans, or pressurized tanks - just a suction pipe reaching down for the hydraulic fluid.

The issue of the top-mounted pumps has polarized the "hydraulic population" because every sane person knows that drinking a shot by raising the glass to your lips and then pouring it into your mouth is way easier than drinking it through a straw, and yet:

a) sometimes well-established brands build power-packs with top-mounted axial piston pumps, and when a tech sees an axial piston unit above the oil level and then reads "Rexroth" on the inspection cover, he accepts the installation because "surely they know what they are doing".

b) most pump manufacturers will list several "above reservoir installation" recommendations in their catalogs, which means that, at the very least, this practice has the right to exist.

Just have a look at these random quotes:

Duplomatic VPPL:

"...In the case of installation above the oil level, check that the minimal inlet pressure is not lower than -0.2 bars (relative)..."

Rexroth A10VO series 3x:

"...Observe the maximum permissible suction height hS max = 800 mm (31.5 inch)..."

Rexroth A11Vo series 1:

"...Observe the maximum permissible suction height hs max = 800 mm..."

Parker PV:

"...Inlet pressure, even during compensation, never should drop below 0.8 bar (absolute)..." (truth be told, they never specifically mention the above fluid lever installation).

Parker PAVC:

"...Not to exceed .17 bar (5 In. Hg.). Vacuum at 1800 RPM on petroleum base fluids..."

It goes on and on, along with typical recommendations on routing the drain line or installing a check valve in it to prevent draining.

Now, let me tell you what I think about it. For me - installing axial-pisotn pumps above the fluid level is a bad practice, that can be almost directly compared to suction line filtration, i.e. bad for pumps but good for the pump business.

No matter what anyone tells you, lower inlet pressure puts more strain on the axial piston group, especially on the piston shoe retainer plate, which results in a shorter life span. No workarounds here. Oh yes, and that 0.5 bar check valve that you put in the drain line to make sure it doesn't drain dry is actually adding up to the strain!

Another thing - ideal installations do not exist, and any HPU will age. Only the HPUs that employ pumps with flooded inlets age "more gracefully", because a leaking seal or a crack in the suction hose will cause fewer issues in comparison to a top-mounted unit that can drain dry during a prolonged downtime, and then start dry the next time the HPU is turned on (now you know why your HPU screams at you every Monday morning). Of course, leaking seals and deteriorated suction hoses should never "happen", but they do.

So, in order to achieve the maximum life span, axial piston pumps should never be installed above the fluid level.

At this point, I imagine that some will say something like "But we had this power-pack with a top-mounted pump that lasted for #insert amount# years and never gave us trouble"!

Well, this sounds to me like "My #insert a relative# has smoked a pack a day for #insert amount# years and he's feeling great!". I am glad he does, but you don't think smoking's good for you, do you? Smoking kills people. And top mount kills pumps. And both of them do it slowly.


The HAWE does put in their catalog the following phrase: "Avoid installing the pump above the tank". You're absolutely right, Mr. HAWE!