Abstract
The axial piston pump is one of the most important power components in modern hydraulic transmission systems and a fundamental basis for achieving high pressure, high power density, and variable displacement control. Compared with gear pumps and vane pumps, axial piston pumps are characterized by high pressure, high efficiency, convenient variable displacement control, and high power density. They are therefore widely used in construction machinery, industrial equipment, aerospace, agricultural machinery, marine applications, and hydrostatic transmission systems.
From a technological history perspective, the axial piston pump was not a mature product developed at once. Instead, it evolved gradually based on continuous advancements in the principle of piston reciprocating motion, port plate distribution, swashplate mechanism, bent-axis mechanism, variable control, friction pair technology, materials and processing technology, and electronic control technology. Although modern axial piston pumps still follow the basic principle of "rotary motion—piston reciprocating motion—suction and discharge" in their structure, their design has evolved from simple mechanical-hydraulic components into complex mechatronic-hydraulic products integrating mechanics, hydraulics, materials, control, sensing, and digital technologies. [1]
I. What Exactly is an Axial Piston Pump?
The core characteristic of an axial piston pump is that the reciprocating motion direction of the pistons is essentially parallel to the rotational axis of the cylinder block.
Taking the most typical swashplate-type axial piston pump as an example, its main components include:
- Drive shaft
- Cylinder block
- Pistons
- Slippers
- Swashplate
- Return mechanism
- Valve plate (port plate)
- Pump housing
- Variable displacement mechanism
During operation, the drive shaft rotates the cylinder block, and the pistons, via the slippers, contact the swashplate. Because the swashplate has a certain inclination angle, as the cylinder block rotates, the pistons undergo axial reciprocating motion within the cylinder bores.
When a piston moves outward, it creates a suction process; when it moves inward, it creates a discharge process. During one full rotation of the cylinder block, each piston completes one suction and one discharge cycle.
The theoretical displacement can be simply understood as being related to the following factors:
V_g ∝ zAd
More precisely, displacement is related to the number of pistons, the piston area, and the piston stroke, and the piston stroke is determined by the swashplate angle. Increasing the swashplate angle increases the piston stroke and the pump's displacement; decreasing the angle reduces displacement. Variable pumps exploit this characteristic to achieve flow regulation. Danfoss technical literature also clearly states that the displacement of an axial piston pump is determined by the piston size, number, and stroke, which is dictated by the swashplate angle. [2]
This ultimately determined that axial piston pumps could become the core components for high-pressure variable pumps and hydrostatic transmissions.
II. The Technical Origins of Axial Piston Pumps: From Piston Pumps to Modern Hydraulics
Strictly speaking, axial piston pumps did not suddenly appear in the 20th century.
The concept of the piston pump dates back to very early mechanical pumps. In the 16th century, Ramelli designed a leather-sealed piston pump for mine drainage, which already embodied the basic idea of using piston reciprocation to change the volume of a working chamber. [3]
However:
There remains a huge technological gap between early piston pumps and modern axial piston pumps.
Two key problems truly changed the direction of hydraulic technology development:
First, how to convert rotary mechanical motion into reciprocating motion for multiple pistons?
Second, how to reliably perform suction and discharge distribution under high-speed rotation?
The solution to these two problems truly laid the foundation for the modern axial piston pump.
III. Around 1900: The Modern Axial Piston Pump Begins to Take Shape
1. The Williams-Janney Structure Became a Crucial Turning Point
From the late 19th to the early 20th century, hydraulic transmission entered a new phase of development.
There is some variation in the literature regarding the historical starting point of the axial piston pump. Some studies consider the design by Cooper and Hampton in 1893 as an early starting point, noting subsequent improvements by Williams and Janney. However, engineering history more commonly cites the swashplate hydraulic transmission with port plate distribution designed by Williams and Reynold Janney in 1905 as a significant starting point for the modern axial piston pump. [1]
The greatest technical contribution of this period was not simply the "invention of a piston pump," but the establishment of the basic technical framework for the later axial piston pump:
Rotating cylinder block + Axial pistons + Swashplate + Port plate distribution.
This was the prototype of the later classic swashplate-type axial piston pump. This device was then known as the Janney pump.
2. Why was Port Plate Distribution So Important?
A very critical technology in modern axial piston pumps is the valve plate (or port plate).
The cylinder block rotates, while the valve plate usually remains stationary.
The valve plate has suction and discharge ports. As the cylinder block rotates, each piston chamber periodically connects with the suction and discharge ports on the valve plate.
In other words:
The valve plate essentially solves the problem of "how a rotating component continuously sucks in and discharges oil."
This seems simple, but it is extremely difficult under high pressure and high rotational speed conditions.
The port plate pair faces numerous challenges simultaneously:
- High fluid pressure
- High-speed relative motion
- Extremely small clearances
- Hydraulic shocks
- Friction and wear
- Leakage
- Thermal deformation
- Cavitation
- Pressure pulsation
Consequently, many subsequent technological advances in axial piston pumps have essentially revolved around "how to make these friction pairs work stably for a long time under high pressure and high speed."
IV. Around 1907: Hydrostatic Transmission Began to Gain Attention
Around 1907, the Janney structure underwent further improvements, increasing the efficiency of hydraulic transmission.
The significance of this phase was that:
The axial piston pump began to cease being just a standalone "pump" and gradually became part of a hydraulic energy transfer system.
Mechanical Energy:
Engine → Pump → Hydraulic Energy → Motor → Mechanical Load
This formed the basic concept of modern hydrostatic transmission.
Compared to traditional mechanical gear transmissions, hydraulic transmissions can more easily achieve:
- Stepless speed regulation
- Remote power transmission
- High power density
- Load adaptation
- Forward/reverse rotation
- Variable displacement control
This was also a key reason why axial piston pumps could later enter the fields of construction and mobile machinery.
V. Around 1930: The Bent-Axis Axial Piston Pump Emerged
The development of axial piston pumps did not follow a single path.
Alongside the development of the swashplate structure, the bent-axis axial piston pump gradually emerged.
Around 1930, Swiss engineer Hans Thoma designed a representative bent-axis axial piston pump, which led to this structure later often being referred to as the Thoma pump. [3]
The most significant difference between the bent-axis and swashplate types is:
The axis of the cylinder block forms an angle with the axis of the drive shaft.
In a swashplate pump, the cylinder block and drive shaft are typically coaxial, and the swashplate induces the reciprocating motion of the pistons.
These two structures form the two main technological routes of modern axial piston pumps:
Swashplate Type
The drive shaft and cylinder block are coaxial; the swashplate drives the pistons.
Characteristics:
- Compact structure
- Relatively convenient variable displacement mechanism
- Good response speed
- Suitable for through-drive configurations
- Suitable for large-scale industrial production
Bent-Axis Type
The drive shaft and cylinder block axes are angled; pistons are driven via connecting rods or similar mechanisms.
Characteristics:
- Can accommodate larger angles
- Stronger torque capability
- Advantageous mechanical efficiency
- Suitable for certain high-torque, large-displacement applications
Therefore, modern high-end axial piston pumps do not feature the "swashplate type eliminating the bent-axis type." Instead, they exhibit a long-term coexistence of the two structures, each adapting to different application scenarios.
VI. The 1950s: The First Major Technological Leap for Axial Piston Pumps
If the period around 1905 solved the problem of "how an axial piston pump works," then the 1950s addressed the challenge of:
How to make axial piston pumps truly enter the phase of high pressure, high speed, and industrial application?
In the mid-1950s, companies like Denison (USA) and Lucas (UK) began adopting more mature bearing-supported cylinder block structures.
The core idea was:
To let the drive shaft mainly bear torque while reducing unnecessary bending moments and side loads.
This improved the operating conditions between the cylinder block and the valve plate.
At the same time, continuous advances in manufacturing technology allowed for more stable machining precision for the port plate pair, piston/cylinder pair, and slipper/swashplate pair.
Relevant technical literature indicates that the structural improvements during this period increased the working pressure of axial piston pumps to about 35 MPa, while also noticeably improving speed capability. This is considered a significant leap in the development history of axial piston pumps. [3]
This phase was crucial because the axial piston pump then truly possessed the technical advantages of:
High Pressure + High Speed + High Efficiency + Variable Displacement.
VII. The 1960s: The Emergence of Through-Drive Pumps, Axial Piston Pumps Moving Towards System Integration
In the 1960s, construction and mobile machinery developed rapidly.
A new requirement emerged for hydraulic systems:
It was no longer enough for the pump alone to have high performance; the entire hydraulic system needed to become more compact.
This led to the emergence of the through-drive axial piston pump.
A through-drive means the pump's main drive shaft can be extended rearward to mount:
- Auxiliary pumps
- Gear pumps
- Pilot pumps
- Charge pumps
- Other hydraulic components
This enables a single main pump to form a combination of multiple hydraulic units.
For example, in construction machinery, an integrated structure like:
Main Pump + Pilot Pump + Gear Pump
can be formed.
Thus, the emergence of the through-drive pump was not just a mechanical structural change; it represented the beginning of the axial piston pump's evolution from:
a "single hydraulic component"
towards
a "hydraulic power platform."
This technology later proved particularly suitable for construction machinery, mobile machinery, and closed-loop hydrostatic transmissions. [3]
VIII. The 1970s–1980s: Variable Displacement Control Became the Core Competitiveness
From the 1970s onwards, axial piston pump technology entered a very important phase:
The competitive focus began shifting from "whether it can achieve high pressure" to "how to control pressure, flow, and power more precisely."
During this period, a large number of variable displacement control technologies emerged.
Typical control methods include:
Constant Pressure Control
When the system pressure reaches a set value, the pump automatically reduces its displacement.
The goal is: Reduce unnecessary flow → Lower power loss → Reduce heat generation.
Constant Power Control
By adjusting the pump displacement, the product:
P=pQ
is kept relatively constant within a certain range.
This is particularly important for construction machinery because diesel engine power is limited. If the pump maintains a large flow at low pressure, the engine is prone to overload.
Constant power control ensures:
The engine power matches the hydraulic pump power more effectively.
Load Sensing Control
The load sensing system automatically adjusts the pump displacement based on the load pressure demand.
When the actuator needs flow: The pump increases displacement.
When the actuator demand decreases: The pump reduces displacement.
This significantly improves the energy efficiency of the hydraulic system.
Therefore, from the 1970s and 1980s onwards:
Variable displacement control technology gradually became an important technical demarcation line between axial piston pumps and ordinary fixed-displacement pumps.
Academic research also began to focus heavily on the response speed, stability, and control characteristics of variable mechanisms. Research from 1979 had already conducted theoretical and experimental analyses of the servo control mechanisms of both swashplate and bent-axis variable pumps. [4]
IX. The 1980s–1990s: High Pressure and High Power Density Became the Main Theme
Entering the 1980s, industries such as construction machinery, plastics machinery, metallurgical equipment, and marine applications imposed higher demands on hydraulic systems.
Users were no longer satisfied with: "The pump works."
Instead, the requirements included:
- Higher pressure
- Higher speed
- Smaller volume
- Lighter weight
- Lower noise
- Higher efficiency
- Longer life
Consequently, axial piston pumps began moving towards high power density.
During this period, structures like tapered pistons in bent-axis pumps began to be studied in depth. An SAE technical paper from 1985 specifically studied bent-axis axial piston units with tapered pistons, pointing out that this structure could reduce volume and weight under the same displacement conditions and increase power density. [5]
This indicates that technological competition had clearly entered a phase of engineering optimization:
Achieving higher pressure × flow within a limited volume.
That is:
P_h=pQ
where hydraulic power is directly related to pressure and flow. Therefore, increasing pressure, increasing speed, and reducing volume all ultimately pointed to a single goal:
Increase Power Density.
X. The 1990s: Axial Piston Pumps Entered a Phase of Maturity and Seriation
Entering the 1990s, the international axial piston pump industry gradually formed a relatively mature technological system.
Companies like Germany's Bosch Rexroth, America's Denison, Vickers, Sundstrand, and the later Sauer-Danfoss, developed extensive product series around different markets.
A noticeable characteristic of this period was:
The competition for axial piston pumps began shifting from "single product competition" to competition involving "complete series + control methods + system solutions."
For example, the same displacement platform could feature:
- Manual displacement control
- Hydraulic displacement control
- Electro-hydraulic displacement control
- Constant pressure control
- Constant power control
- Load sensing control
- Pressure cut-off
- Multiple pump combinations
- Through-drive configuration
This meant that the pump's competitiveness was no longer just about "the pump itself."
It was the overall competition of:
Pump + Control Mechanism + Hydraulic System.
XI. Development of Axial Piston Pumps in China
The development of axial piston pumps in China has a clear chronological correspondence with international technological progress.
In the 1960s, China began independent research and development of axial piston pumps.
Around 1966, China developed the representative CY14-1 axial piston pump/motor. Subsequent improvements were made to the CY series, including standardization, structural optimization, noise reduction, reliability improvement, and through-drive configurations. [3]
This phase holds significant historical importance because:
China began establishing its own system for designing, manufacturing, and applying axial piston pumps.
However, compared with advanced international levels, China's axial piston pump industry lagged for a considerable period.
The most critical issue was not simply "not knowing how to design the structure," but:
Axial piston pumps are products highly dependent on manufacturing capability.
A mature axial piston pump requires simultaneously addressing:
- Piston-cylinder bore fit
- Slipper-swashplate friction
- Cylinder block-valve plate friction
- Valve plate clearances
- Surface roughness
- Material heat treatment
- Hydraulic balancing
- Leakage control
- Cleanliness
- Assembly precision
- Dynamic balancing
- Noise control
This is why axial piston pumps are often called "high-tech products" among hydraulic components.
XII. Why Are Axial Piston Pumps So Difficult to Manufacture?
This is a very important point for understanding the history of axial piston pump development.
The core technology of an axial piston pump does not reside in any single part, but in several critical friction pairs.
1. Piston-Cylinder Bore Pair
Requires simultaneously meeting: High-pressure sealing + Low friction + Wear resistance.
Too large a clearance: → Increased leakage.
Too small a clearance: → Increased friction.
Insufficient machining precision: → Susceptibility to abnormal wear.
2. Slipper-Swashplate Pair
The slipper bears significant loads during high-speed motion.
Therefore, the following must be addressed:
- Fluid film formation
- Friction
- Wear
- Temperature rise
- Off-center loading
- Slipper tilt or overturning
The slipper is actually one of the most important friction pairs in an axial piston pump.
3. Cylinder Block-Valve Plate Pair
This is the most typical high-pressure end-face friction pair in an axial piston pump.
It must maintain stable operation under: High Pressure + High-Speed Rotation + Extremely Small Clearances.
Modern research still extensively focuses on surface strengthening, coatings, and surface texturing of the critical friction pairs in piston pumps, as these technologies can improve friction, wear, and lifespan. [6]
Therefore, it can be said:
The development history of the axial piston pump is, to a large extent, the development history of its critical friction pair technologies.
XIII. The 21st Century: Axial Piston Pumps Enter the Phase of "High Efficiency + Intelligence"
Entering the 21st century, axial piston pump technology began to exhibit new changes.
The past focus was on: High pressure, high speed, high reliability.
Now, the focus is further expanding to include: High efficiency, low noise, intelligent control, digitalization, and system-level energy savings.
For example, the Floating Cup axial piston pump proposed by the Dutch company Innas represents one of the important cases of structural innovation in traditional axial piston pumps.
Its design attempts to improve the following through a new piston and cylinder structure:
- Axial force balance
- Friction losses
- Flow pulsation
- Noise
- Power density
Relevant technical literature views this as one of the important directions for structural innovation in 21st-century axial piston pumps. [3]
This indicates a very clear trend:
The development of axial piston pumps has begun shifting from "improving traditional structures" to "rethinking the pump's fundamental structure."
XIV. Electronic Control is Redefining Variable Displacement Piston Pumps
In modern construction machinery, axial piston pumps increasingly rely less on purely mechanical variable mechanisms.
The current trend is:
Mechanical-hydraulic control → Hydraulic servo control → Electro-hydraulic proportional control → Electronic closed-loop control.
For example:
Engine speed sensor
↓
Controller
↓
Pressure/Flow/Load Signals
↓
Electro-hydraulic control valve
↓
Variable mechanism
↓
Swashplate angle
↓
Pump displacement
↓
Actuator
This forms a closed-loop system.
Therefore, the modern axial piston pump has essentially become:
An integrated system of mechanical structure + hydraulic control + electronic control.
This is also why high-end construction machinery today increasingly emphasizes "electro-hydraulic synergy."
XV. From "Hydraulic Pump" to "Energy Manager"
Summarizing the development history of axial piston pumps reveals a rather interesting transformation.
Phase 1: Solving "Whether it can work"
Core issue: How do the pistons reciprocate? Representative: Early piston pumps, Janney structure.
Phase 2: Solving "Whether it can work at high pressure"
Core issue: How to withstand high pressure and high speed? Representative: Bearing-supported cylinder block, improved valve plate pairs, materials, and processing technology.
Phase 3: Solving "Whether it can be variable"
Core issue: How to change displacement? Representative: Swashplate variable mechanism, bent-axis variable mechanism, servo variable mechanism.
Phase 4: Solving "Whether it can save energy"
Core issue: Does the pump's output match the load demand? Representative: Constant pressure, constant power, load sensing control.
Phase 5: Solving "Whether it can be intelligent"
Core issue: Can the pump actively determine what output is needed based on system status? Representative: Electro-hydraulic control, sensors, controllers, digital pumps.
XVI. Today's Axial Piston Pumps Have Become the "Heart" of Construction Machinery
Take an excavator as an example.
After the engine outputs mechanical power, it does not directly drive the hydraulic cylinders.
Instead, the path is:
Engine
↓
Main Hydraulic Pump
↓
High-Pressure Hydraulic Oil
↓
Main Control Valve
↓
Boom/Arm/Bucket Cylinders
Swing Motor
Travel Motor
Thus, the axial piston pump determines the entire hydraulic system's:
- Flow rate
- Pressure
- Response speed
- Energy efficiency
- Heat generation
- Noise
- Operational performance
Especially in large excavators, multi-pump systems often need to simultaneously meet the combined operational demands of travel, swing, boom, arm, and bucket actuators.
This is precisely why construction machinery demands much more from axial piston pumps than ordinary industrial pumps.
XVII. Future Development Directions for Axial Piston Pumps
In the future, axial piston pumps will not simply follow a single trajectory of "increasing pressure." Instead, they will present several clearer directions.
1. Higher Power Density
Achieved through:
- Increasing speed
- Increasing working pressure
- Optimizing flow paths
- Reducing volume
- New materials
- Novel friction pair designs
Achieving higher power in a smaller volume.
2. Higher Efficiency
Evaluating a pump in the future will not be limited to its rated pressure.
More importantly, the focus will be on: The overall efficiency across the entire operating range.
Especially in construction machinery, a significant amount of time is not spent at rated conditions.
Therefore:
Partial load efficiency will become increasingly important.
3. Lower Noise
The noise of axial piston pumps mainly originates from:
- Flow pulsation
- Pressure pulsation
- Valve plate impact
- Mechanical vibration
- Cavitation
- Structural resonance
Future noise reduction will be achieved through: Valve plate optimization + Flow path optimization + Structural vibration damping + Active control.
4. Greater Intelligence
Future pumps may no longer simply be:
"Adjusting displacement based on pressure signals."
Instead, they will evolve further into:
Intelligent hydraulic power units capable of sensing, judging, predicting, and actively adjusting their working status.
For example, by using data on pressure, temperature, speed, vibration, displacement, etc., the pump could determine:
- If it is wearing out
- If the valve plate pair is abnormal
- If cavitation is occurring
- If there is a decrease in efficiency
- If maintenance is needed
This essentially is predictive maintenance.
XVIII. Conclusion: The Core Logic Behind Over a Century of Technological Evolution
Reviewing the development of the axial piston pump reveals that it did not simply follow a linear path of: Old Product → New Product → Higher Pressure → Higher Speed.
Its true development logic is:
Mechanical Kinematics Innovation → Port Plate Technology → Friction Pair Technology → High-Pressure Capability → Variable Displacement Control → System Integration → Energy-Saving Control → Electro-Hydraulic Control → Intelligence.
In the early 1900s, axial piston pumps addressed the problem of "how to convert rotary motion into piston reciprocating motion." The 1950s tackled "how to make the pump work reliably under high pressure and high speed." The 1960s–1980s focused on "how to achieve variable displacement and system integration." Entering the 21st century, the focus has further shifted to "how to improve efficiency, reduce noise, and achieve intelligent control."
Therefore, it is no coincidence that the axial piston pump has become the core component of modern hydraulic systems. It perfectly meets several of the most important needs of modern hydraulic systems:
High Pressure, High Flow Rate, High Efficiency, High Power Density, Stepless Variable Displacement, and Precise Control.
In this sense, the development history of the axial piston pump is essentially also a microcosm of the evolution of modern hydraulic technology from mechanization to high performance, system integration, and intelligence.
And although the basic working principle of today's axial piston pumps can still be traced back over a century, their internal friction pairs, valve plate designs, materials, machining precision, variable mechanisms, and electronic control technologies have undergone fundamental changes. Modern research continues to advance around port plate mechanisms, friction and wear, efficiency, noise, and new structural concepts. [7]
So, to understand the history of the axial piston pump, the important thing is not remembering a few specific years, but understanding a core principle:
Every major technological advance in axial piston pumps has been essentially aimed at overcoming the contradictions between pressure, speed, efficiency, life, control, and volume.
This is precisely the fundamental reason why, after more than a century, it remains the core power component of high-end hydraulic systems.
References
- Influence of Fluid Compressibility and Movements of the Swash Plate Axis of Rotation on the Volumetric Efficiency of Axial Piston Pumps
- Principles of Operation
- Development History of Axial Piston Pump / Motor Technology >>Motor Technology >> Air Motor Information >> China Air Motor Network
- Characteristics of Displacement Control Mechanisms in Axial Piston Pumps
- 851508: Design Theory of Axial Piston Units with Tapered Pistons - Technical Paper
- Surface Modification and Performance Enhancement of Key Friction Pairs in Aerospace Piston Pumps
- Challenges and Research Progress in the Flow Distribution Mechanism of Piston Pumps: A Review