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Hydraulic Axial Piston Pumps, Theory of Operation by Delta Q

Axial Piston Pumps are highly efficient pumping units capable of high pressures and flows.

An axial piston pump with load sense offers flow on demand and with a closed center control valve, little heat is generated when the pump is destroked because no fluid is moved.

Fixed displacement pumps must continually pump fluid back to tank through an open center valve. The heat generated even when no load is on the pump represents power loss and higher operating costs for fixed displacement pumps.

Because of their superior efficiency, small size and weight, and because of today's increased costs of fuel and energy; axial piston pumps are being more frequently specified in applications once filled by fixed displacement vane or gear pumps.

Once axial piston pumps were found primarily in aerospace applications, today these efficient pumps are found everywhere...in large presses, in hydrostatic drives, steering assemblies
- all sorts of stationary and mobile applications.

Because of the closely fitted parts and finely machined surfaces on an axial piston pump, special care must be taken when service is required. But first, the basics.

In the Axial Piston Pump the pistons reciprocate parallel to the drive shaft.
The pistons are fitted into a round cylinder block,much like bullets in a revolver - and, like a revolver,the cylinder block turns.
Displacement in any piston pump is determined by the size and number of pistons as well as the length of stroke. This variable displacement piston pump changes the length of the stroke continuously to match the changing flow requirements of the hydraulic system.
A “swash plate” or cam is mounted on pivots at the
drive end of the pump.
Each piston is fitted with a ball joint shoe and each shoe has a flange.
The return plate with nine holes in it fits over the shoes and holds them against the wear surface of the cam.
If the cam is tipped, the pistons will be forced into and out of their respective bores as the cylinder block rotates.
If the cam is not tipped, the pistons remain stationary
in their bores as the cylinder block is turned and no
oil is pumped.
To reduce friction between the rotating shoes and the
wear plate, a small hole permits hydraulic oil to flow
from the hollow piston, through the ball joint to the
back of the shoe, creating a hydrostatic bearing.
Because of hydrostatic balance very little of the fluid
escapes the cavity.

The shoes and the ball joint are also separated by an
oil film.
Looking at the cover end of the pump, we see the
valve plate.
There are two kidney shaped holes in the valve plate,
the larger of the two is the intake port.
The smaller kidney shaped opening is the discharge
port.
As the bores in the rotating cylinder block turn past
the valve plate intake port, the pistons are being
pulled from their bores, drawing oil in at the intake.
As the bores in the rotating cylinder block turn past
the discharge port, the pistons are being pushed into
their bores, forcing oil out.
The small round holes at either side of the discharge
port allow oil to be released into the port gradually,
reducing vibration and noise.
The very small hole near the intake port allows any fluid
trapped in the bore to escape into the case as the
piston fully bottoms in the cylinder block.
As you can imagine, firm contact between the valve
plate and the rotating cylinder block is critical for efficient
pump operation.
Also, the return plate, which holds the shoes against
the cam surface must have a firm, constant contact to
do its job.
One large spring provides firm contact for both the
valve plate and the return plate.

This large spring is fitted into the center of the cylinder
block and the pump shaft passes through it.
The spring is seated against a large snap ring (at the
valve plate end of the cylinder block).




Three small steel pins pushed by the spring hold the
return plate tightly against the shoes...
but at the
same time, the spring is pushed against the snap
ring forcing the entire cylinder block toward the valve
plate.
The cam is tipped to change the displacement
according to system demand. The cam has several
forces acting upon it.
The purpose of that spring is to push the cam full tilt
and make sure the pump is on full stroke when first
starting up.
A small piston within the spring is pushed by discharge pressure, providing still more force to keep the pump at maximum displacement after it picks up pressure.
At the bottom of the pump is the control sleeve - a single acting device which can move the cam fully off
stroke to zero pump displacement.

So when oil is directed to the control sleeve, the cam
moves the pump off stroke and compresses the
spring.
Oil to this control sleeve is directed by a pressure compensated control - a spool valve directs oil to the control sleeve when a predetermined pressure is
reached.
All content in this article is courtesy of Delta Q


DeltaQ Plant Location: (Recently Expanded)
10827 Tower Oaks Blvd.
Houston, TX 77070
800-650-3110 > Toll Free
281-807-1840 > Phone
281-807-4457 > Fax
http://www.deltaq.com

 
Threadless connectors make simple, fast, leak-free hydraulic coupling installations

Threadless connectors are an alternative to conventional threaded fittings commonly used in fluid-power systems. The male and female halves simply push together -- without wrenches or special assembly tools -- to form a leak-free connection.

Several coupling manufacturers have introduced specially designed threadless fittings that yield the performance of O-ring seals, but eliminate the cost and O-ring assembly problems for fluid power systems with working pressures up to 6,000 psi. These couplings also provide a metal-to-metal seal − like JIC fittings − but they don’t have to be torqued-on to make the connection.

These fittings are called quick connect couplings.
They are used exclusively at the hydraulic port or manifold on steering units, valve blocks, pumps and motors. For OEMs, they offer several key benefits:
- Leak-proof connections that reduce the cost of warranty claims and fluid leaks from equipment used in the environmentally sensitive forestry and agriculture industries,
- Easier, quicker connections that result in manufacturing cost savings in the assembly process,
- A tactile positive feedback and visual confirmation that the connection has been made,
- Assurance that connections cannot be accidentally made with other threadless couplings,
- Easy serviceability, and
- Easy replacement in the field with traditional and widely available threaded couplings

The biggest advantage of threadless couplings is the reduced time and expense needed to install hydraulic hose and tubing assemblies, and the improved accessibility to the hydraulic system.

Certain kinds of equipment, such as skid steers, are compact in design and have little room for the plumbing and routing of hydraulic sub-systems, which must be designed into tighter spaces where they are subject to heat build-up and other stresses.
Installations with conventional threaded fittings can take several minutes or even hours, and they are subject to leaks because of torquing inconsistencies and other variables. On the other hand, threadless couplings allow even confined, difficult-to-reach connections be completed in minutes.

Instead of screwing on the threaded connector and tightening it with a wrench, threadless connecting halves -- the male end and the adapter -- simply push together to form a leak-free connection. A snap ring positively engages the male and female halves, without any assembly tools. Some threadless coupling designs do require a special tool for removal, however.

Threadless couplings should be considered for truck/bus, agriculture, construction, material handling, and turf care applications. They are especially suited for quickly making difficult-to-reach, confined-area connections.

For additional information about Gates Quick-Lok threadless couplings, contact your Gates Fluid Power division distributor or sales representative. On the Web, go to www.gates.com/quicklok


 

 
Match hydraulic system components to ensure leak-free service life.

Gates Corporation engineers recommend against making a hydraulic assembly with a hose and couplings from different manufacturers.

Here’s why.

Although most American-made hydraulic hoses, and many imported hoses, are built to confirm to SAE (Society of Automotive Engineers) specifications, the SAE allows a whole range of materials to be used.

Hoses from various manufacturers may have comparable dimensions and constructions (SAE), but different rubber compounds and tolerance dimensions (non-SAE).

Summarily, each manufacturer designs a coupling to properly fit its own hose manufacturing tolerances. Also, the proliferation of thread ends from around the world in recent years has dramatically increased the possibility of mismatching threads and seats on various couplings.

An improperly coupled hose will likely fail (blow off; leakage) causing downtime and possible personal injury.

Finally, it's important to: 1) use the crimping equipment of the hose/coupling manufacturer; and 2) always follow the crimp and assembly recommendations of the hose/coupling manufacturer.

Each component of a Gates hydraulic system -- application-specific hoses, a full range of couplings and versatile crimpers -- is designed to work together to give you a proven, reliable and safe product.

Contact your authorized Gates hydraulic distributor or go to www.gates.com/fluidpower

 

 
Use proper torque values when tightening couplings

An over-tightened hydraulic coupling may be just as apt to leak as an under-tightened coupling. Why? Over-tightening may result in overstressing and/or cracking of the seat or the staked nut.

When attaching a coupling, it’s best to use the torque values supplied by the coupling manufacturer.

The minimum value will create a leak-proof seal under most conditions. Applying torque values greater than the maximum recommendation will distort or crack the fitting.

When tightening couplings, make sure that the hose does not twist on the adapter. Twisting will shorten hose life and scar the sealing surfaces of swivel-type couplings (JIC, 45°, etc.), which can create leaks.

For straight couplings, use a torque wrench on the hex-swivel nut and a standard box-wrench on the stem hex.

When a crowfoot wrench is used with a torque wrench, adjustments to the torque readings must be made, otherwise over-tightening will occur.

By replacing staked- and tube-nut JIC style couplings with Gates Full-Torque Nut™ hydraulic couplings, design engineers can dramatically reduce hydraulic leaks due to over-torquing.

For more tips on proper coupling torquing techniques, visit www.gates.com/fulltorque.