How Hydraulic Pumps Work, Types

How Do Hydraulic Pumps Work

Table of Contents

What Is a Hydraulic Pump?
A hydraulic pump converts mechanical energy from a prime mover into hydraulic energy by producing flow. System pressure arises from downstream resistance; the pump does not create pressure. Tight tolerances enable self‑priming and high‑pressure operation.

How It Works

The rotary element creates low pressure at the inlet, atmospheric pressure pushes fluid from the reservoir into the pump. Rotating chambers carry fluid to the outlet, delivering flow to the circuit. Blocking the outlet causes dangerous pressure spikes that can damage the pump.

Pump Classes

Positive‑displacement vs non‑positive‑displacement. Positive‑displacement pumps displace near‑fixed volume per rotation with minimal slip, ideal for high‑pressure hydraulics. Non‑positive pumps allow slip; flow varies with outlet restriction, suitable for circulation tasks.

Common Types

Positive‑displacement pumps used in industrial hydraulics include gear, vane, and piston types; non‑positive (hydrodynamic) pumps are used for circulation. Displacement can be fixed or variable; variable designs change chamber geometry (e.g., swash plate).

Gear Pumps

External gear pump — two external spur gears create inlet suction and transport fluid around the casing. Fixed displacement; typical pressure 150–300 Bar (≈ 2175–4350 PSI). Simple, robust, cost‑effective; efficiency and noise moderate.

Internal gear pump — an internal‑external gear pair in compact layout improves suction and reduces noise; used widely in industrial/mobile systems. Typical pressure up to ~400 Bar (≈ 5800 PSI) depending on product series.

Options: single, bi‑directional, and double (HI‑LO) gear sets; Group sizes standardized for integration.

Vane Pumps

Single‑action and double‑action designs; sliding vanes in a cam ring create alternating increasing/decreasing chamber volumes. Fixed or variable displacement. Typical pressure 70–200 Bar (≈ 1015–2900 PSI). Low noise, smooth flow; requires cleaner oil and precise setup.

Suitable for machine tools and systems prioritizing acoustic comfort and steady flow.

Piston Pumps

Axial piston (swash plate) — pistons arranged around a cylinder block; swash plate angle sets displacement (fixed or variable). High efficiency; typical pressure 250–350 Bar+ (≈ 3625–5075 PSI). Supports pressure/flow compensation and load‑sense control.

Radial piston — pistons arranged radially; very high pressure at lower flows for heavy‑duty systems. Specialized designs can reach ~650 Bar (≈ 9427 PSI).

Pros/Cons: highest efficiency and pressure capability; higher cost and cleanliness/precision maintenance requirements.

Hand Pumps

Manual positive‑displacement pumps used for emergency/backup power or commissioning; simple and reliable with low flow and operator‑driven pressure.

Non‑positive (Hydrodynamic) Pumps

Flow varies with outlet restriction due to intentional slip; suited to circulation and cooling tasks rather than high‑pressure hydraulics.

How to Size a Pump

  1. Define maximum working pressure at the actuator (Bar/PSI).
  2. Determine required flow for target speed (LPM/GPM), then compute displacement if necessary.
  3. Check fluid type, viscosity, and temperature range; design suction to avoid cavitation.
  4. Match prime mover and duty cycle (IEC 60034‑1, S3/S4) to thermal behavior.
  5. Choose displacement (fixed vs variable), control (pressure/flow/load‑sense), ports/manifold, and filtration class.

Quick Conversions

  • 1 GPM ≈ 3.785 L/min
  • 1 Bar ≈ 14.5038 PSI

Gear vs Vane vs Piston: Engineering Comparison

FeatureGearVanePiston
Typical pressure150–300 Bar70–200 Bar250–350 Bar+
EfficiencyModerateModerate–HighHigh
NoiseModerateLow–ModerateModerate
CostLow–ModerateModerateHigher
MaintenanceLowModerateHigher
Control optionsMostly fixed displacementFixed/variable optionsFixed/variable (swash plate)
Typical useIndustrial/mobile general dutyLow‑noise, smooth flow systemsHigh pressure and variable control

Conclusion: Choose gear for robust general duty, vane for lower noise and smooth flow, piston for high pressure and variable control.

Detailed Comparison Tables

External vs Internal Gear Pumps

AttributeExternal GearInternal Gear
LayoutTwo external spur gearsInternal–external gear pair
Typical pressure150–300 BarUp to ~400 Bar (series‑dependent)
NoiseModerateLower
Suction performanceStandardImproved
DisplacementFixedFixed
CostLowerModerate
ApplicationsGeneral industrial/mobileIndustrial/mobile, compact layouts

Single‑action vs Double‑action Vane Pumps

AttributeSingle‑actionDouble‑action
Cycles per revolutionOne suction + one dischargeTwo suction + two discharge
Flow rippleHigherLower
NoiseLow–ModerateLower
Pressure range~70–200 Bar~70–200 Bar
Control optionsFixed or variableFixed or variable
MaintenanceModerate (vane wear)Moderate (more parts)

Axial vs Radial Piston Pumps

AttributeAxial (Swash plate)Radial
Pressure capability250–350 Bar+Very high (specialized up to ~650 Bar)
Flow characteristicsHigher flows, compactLower flow, very high pressure
Displacement controlFixed or variable (swash angle)Typically fixed; specialized controls
EfficiencyHighHigh
Cost & maintenanceHigher; precision partsHigher; precision parts
Typical applicationsHigh pressure, variable control systemsHeavy‑duty systems needing extreme pressure

Variable vs Fixed Displacement

AttributeVariableFixed
Energy efficiencyHigher under varying loadsGood at constant load
ComplexityHigher (controls, compensation)Lower
Cleanliness requirementTighterModerate
Initial costHigherLower
Control optionsLoad‑sense, pressure/flow compensationOn/off, speed‑based flow
Best use casesFluctuating duty, efficiency prioritySimplicity, cost priority

Variable vs Fixed Displacement

Pros (Variable): Better energy efficiency under varying loads; supports load‑sense and proportional control.

Cons (Variable): Higher complexity and cost; tighter cleanliness and setup requirements.

Application Mapping

  • Mobile equipment (tail lifts, scissor platforms): DC pumps, compact manifolds, vibration‑resistant ports.
  • Industrial machinery (machine tools, presses): higher pressure control, filtration class per component sensitivity.
  • Marine systems: corrosion‑resistant materials, sealed connectors, minimized external piping.
 
 
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