InsaneHydraulics - Sergiy Sydorenko © 2009-2010 All Ridghts Reserved
Several days ago I read an interesting article about
hydraulic balance of mechanical seals (the original article is
from here, I am also putting a copy of it in the Library)
. Very interesting material, definitely worth looking into. As I
was finishing reading the article, an idea of applying the same
principles to piston assembly of an axial piston pump was already
buzzing in my head. Many thanks to mr. Mc Nally for the idea!
Under the Shoe.
Earlier I talked about hydraulic balance and its use
to reduce load forces of pump components. This article is a
follow up, focused on a piston assembly of an axial piston pump
(swashplate type). The pics show a classic piston shoe
design, although the hydraulic balance principle is equally applied to
the "inverted" piston design, where the "ball" is part of the shoe
(Linde, Komatsu,..), a design which allows a bigger swashing plate
angle, thus resulting in a more compact pump design.
Anyway, let us take a closer look at the
forces acting on a piston inside a working pump. Obviously we have
closing force, resulting from system pressure acting on the piston's
end and opening force acting on the shoes cavity. But besides
these two obvious forces there's more. Although the clearance between
the contact surfaces of the shoe and the swashplate is very small,
there's still a radial oil flow and, consequently, a pressure drop in
this area (pic8).
Due to the very small clearance the flow is most likely laminar, and
due to the fact that, as the radius increases, the flow cross section
also increases, the pressure drop is not linear. This means that
the shoe balancing area is, in fact, a little bigger than the cavity
There are two basic designs of a piston shoe, a simple design (pic6, pic7) and a complex (or labyrinth) design (pic4, pic5,
just one example as there are many similar variations). The
"high" areas inside the balancing cavity of a complex piston design
also contribute to the hydrostatic lift as the film of oil under them
I would really like to know why so many
different shoe designs exist. The simple, single cavity no labyrinth
design is the cheapest and the fastest to produce, and its efficiency
and reliability is time proven on both open and closed circuit pumps by
many, so if I were to design a pump I would stick to it. But,
apparently, there are still plenty factories out there, who invent a
new revolutionary shoe cavity design each time a new pump series is
launched. Which can also be explained by the fact that creation of
pump/motor components to a great extend is (A) an empirical process,
and (B) a process greatly influenced by company tradition, which is no
shame at all. So if someone wants to go nuts machining away all sorts
of cavities and cavity-connecting labyrinths in a piston shoe, I say
let them! In fact, nothing can explain better this point than this
abstract from (1):
"The general lack of basic principles for
designing reliable components for hydraulic pumps and motors requires a
strategy of applying experience and full scale testing. Hydraulic pump
and motor components are very sparsely lubricated though they are
immersed in fluid. They therefore slide in "boundary
lubrication", a regime for which there are no methods for predicting,
or even estimating product life or frictional performance. The testing
of sub-components in bench tests or in accelerated tests produces new
uncertainties since the role of the many variables that control wear
and scuffing are not well known. The best design procedure involves full scale testing of components with all of the recirculating contaminants, vibrations and misalignments included..." (highlighted by me)
Now, to dress this theory up in numbers and to find
out the balance ratio of modern piston designs (out of pure scientific
curiosity, of course), I swept shelves in the warehouse and took
measurements of some random pistons (various brands, both closed and
open circuit), then made some calculations and combined the results
into this nice table. For a complex design piston the measurements were
taken in the following manner.
All measurements are in millimeters. To calculate the contact
area lift, an estimated value of 30% of the total area of the
outer ring surface was used. If you ask me where did I get the
30% figure I'll tell you it's a hunch, based on the McNally's article.
In the "balancing area" column you have the area of
the shoe cavity directly exposed to the system pressure (inner rings'
surface of the complex design shoe was not taken into consideration, as
the fluid film under the rings is pressurized thus equally contributing
to the lifting force) plus 30% of the total area of the outer ring.
Balance ratio is the shoe balancing area divided by piston head area .
|Short Pump Reference
||Piston Head Area, sq.cm
||Balancing Area, sq. cm
As you can see, different brands and designs all wound up with the balance
ratio of around 0.9, (the only exception being the A10VO028, with a
smaller ratio of 0.8, which I am guessing can be explained by the small
You may also see that complex shoe design prevails.
Another advantage for the design I might think of is, probably, the
fact that the narrower outer ring of the complex design, when scored by
contaminant particles, affects the balance ratio to a smaller extent
in comparison to the simple design with its relatively wide ring surface.
But hey, check out Sauer Danfoss series 90 closed circuit pumps. Extremely
reliable and more than time proven units that have used single cavity
design pistons for a decade already, and seem to have done just fine.
It is also worth mentioning that aside direct
acting forces caused by the system pressure (and the case pressure)
there are other forces contributing to the piston shoe lift. There's
the hydrodynamic lift, caused by the fluid trying to get under the shoe
as it moves along the swashing plate. There are all sorts of vibrations
and misalignments. There's the centrifugal force, friction forces
between the cylinder block and the piston itself, and probably an
assload of other forces I can't think of at the moment but pretty sure
they exist. All of them being compensated for by the hydraulic balance.
I can only imagine how cool must have been all the
empirical work and failed attempts to design a reliable piston.
Pump designers, I salute you! Keep up the good
work for us, hydraulics-junkies, to have something to back-engineer...
1. Hydraulic Failure Analysis
fluid, components and system effects
George E. Totten, David. K. Wills, Dierk G. Feldmann.