A hydraulic system transfers power by moving oil through a controlled circuit. The hydraulic pump supplies flow, valves direct and regulate that flow, and a cylinder or hydraulic motor converts fluid power into movement. Pressure rises only when the moving oil meets resistance from a load, restriction or actuator.
The simplest way to remember the relationship is:
- Pressure determines available force or torque.
- Flow determines cylinder or motor speed.
- The load determines how much pressure the system needs.
How a Hydraulic System Works in Six Steps
- The reservoir stores hydraulic oil and helps release heat and air.
- An engine or electric motor drives the hydraulic pump.
- The pump draws oil from the reservoir and sends a measured volume into the circuit.
- Hydraulic valves control direction, maximum pressure and actuator speed.
- A cylinder converts fluid power into linear force, or a hydraulic motor converts it into rotary torque.
- Oil returns through the valve, filter and cooler before being reused.
Pascal's principle explains force multiplication: an increase in pressure applied to a confined liquid is transmitted through that liquid. A larger output piston can therefore produce a larger force, although it moves a shorter distance for the same displaced oil volume.
A Pump Creates Flow; Resistance Creates Pressure
Many operating and purchasing mistakes start with the claim that a pump "creates pressure". A positive-displacement pump primarily moves a specific volume of oil per shaft revolution. Pressure develops when that flow encounters resistance.
A lightly loaded cylinder may move at modest pressure because the system only needs to overcome seal friction, return-line resistance and the mechanism's weight. When the cylinder lifts a heavy load, resistance increases and pressure rises. If the required pressure exceeds the system limit, the actuator stops and the relief valve opens or the pump control reduces displacement.
This distinction improves troubleshooting:
- Low pressure under load may indicate a low relief setting, pump wear or internal leakage.
- Normal pressure with slow movement usually indicates low flow, restriction or leakage.
- High pressure with no movement may indicate overload, mechanical jamming, a blocked return path or an incorrectly shifted valve.
A pressure gauge alone cannot prove that a hydraulic pump is healthy. A worn pump may still reach the relief setting at low flow. Check pressure and flow under load and at normal oil temperature.
Pressure, Flow, Force and Power
| Quantity | What it means | Mainly affects | Units |
|---|---|---|---|
| Pressure | Resistance the system must overcome | Cylinder force or motor torque | bar, MPa, psi |
| Flow | Oil volume per unit of time | Cylinder or motor speed | L/min, GPM |
| Force | Linear pushing or pulling output | Load a cylinder can move | N, kN, lbf |
| Torque | Rotary turning output | Load a motor can rotate | Nm, lb-ft |
| Power | Rate of doing work | Capacity and speed together | kW, hp |
High pressure with low flow can produce strong but slow movement. High flow at low pressure can create speed but may stall against a heavy load.
For metric calculations: Hydraulic power (kW) = Pressure (bar) × Flow (L/min) ÷ 600
A circuit at 160 bar and 30 L/min transmits 8 kW of theoretical hydraulic power. The required engine or electric-motor input is higher because pumps, valves, hoses and actuators introduce losses.
How Hydraulic Cylinder Force Is Calculated
Force = Pressure × Effective Area
For practical metric calculations:
Force (N) = Pressure (bar) × Area (cm²) × 10
Piston area is: Area = π × Bore² ÷ 4
A single-rod, double-acting cylinder has two effective areas. During extension, pressure normally acts on the full piston area. During retraction, the rod occupies part of the piston face: Annular area = Piston area − Rod area
Therefore, at the same pressure, retraction force is lower. At the same flow, retraction speed is higher.
Worked Example: 80 mm Bore at 160 Bar
Assume an 80 mm bore, 45 mm rod, 30 L/min pump flow and 160 bar measured at the cylinder inlet.
The full piston area is about 50.27 cm²: 160 × 50.27 × 10 = 80,432 N
Theoretical extension force is about 80.4 kN, or 8.2 metric tonnes-force.
The annular area is about 34.36 cm²: 160 × 34.36 × 10 = 54,976 N
Theoretical retraction force is about 55.0 kN, or 5.6 metric tonnes-force.
At 30 L/min, theoretical extension speed is about 99 mm/s, while retraction speed is about 146 mm/s.
Actual values will be lower because of seal friction, internal leakage, mechanical geometry and pressure loss. Return-line backpressure also opposes the working side and should not be ignored.
What the Main Components Control
Hydraulic Pump: Flow
The hydraulic pump's displacement and shaft speed determine theoretical flow: Pump flow (L/min) = Displacement (cm³/rev) × Speed (rpm) ÷ 1,000
Actual flow is lower because of internal leakage. The difference usually grows with wear, pressure and high oil temperature.
Do not select a hydraulic pump from maximum pressure alone. Confirm flow at the actual drive speed, continuous and peak pressure, available input power, circuit type, control, rotation, shaft, flange, ports, fluid and duty cycle.
Hydraulic Valves: Direction, Pressure and Speed
Directional valves route oil. Relief valves limit pressure. Flow-control valves regulate speed. Check valves prevent reverse flow. Counterbalance valves control suspended or overrunning loads. Proportional valves vary pressure or flow from an electrical command.
A valve can have a sufficient pressure rating but inadequate flow capacity. Excessive pressure drop across an undersized valve converts useful power into heat and slows the actuator.
Bohang hydraulic valves are selected by function, rated pressure, flow capacity and mounting pattern. The available range includes directional, relief, check, throttle-check and proportional valve configurations for industrial and mobile circuits.
Cylinder and Hydraulic Motor: Output
A cylinder produces linear movement. Pressure and area determine force; flow and area determine speed.
A hydraulic motor produces rotary movement. Pressure difference, displacement and mechanical efficiency determine torque. Flow, displacement and volumetric efficiency determine speed.
Higher motor inlet pressure does not automatically mean higher speed. A motor can show high pressure and low speed when flow is restricted, the load is excessive or hot-oil leakage has increased.
Bohang supplies fixed- and variable-displacement hydraulic motors for open and closed circuits, including models rated up to 400 bar nominal and 450 bar peak. The correct choice still depends on torque, speed, case-drain arrangement, return pressure, mounting and duty cycle.
Why Actual Output Is Lower Than the Formula
Operators experience the complete circuit, not an ideal formula. Oil loses pressure through hoses, fittings, filters, valves and coolers. Internal leakage reduces available flow, while friction reduces force and torque.
A pump outlet gauge may show 180 bar while only 160 bar reaches the cylinder. If the opposite chamber has 15 bar backpressure, usable force must be calculated from the pressure acting on both sides of the piston.
When equipment is weak or slow, compare:
- Pump outlet and actuator inlet pressure
- Opposite-port or return pressure
- Pump flow under load
- Oil temperature
- Pump or motor case-drain flow
Measurements near the actuator often reveal losses that a pump-mounted gauge misses.
Common Symptoms Explained
| Symptom | Likely meaning | First checks |
| Slow but still powerful | Pressure is available; flow is low | Pump speed, flow, restriction, hot-oil leakage |
| Fast but weak | Flow is available; usable pressure is low | Relief setting, pump, actuator leakage |
| High pressure, no movement | Overload or blocked flow path | Mechanical jam, valve position, return line |
| System overheats | Hydraulic power is becoming heat | Relief bypass, throttling, undersized valve |
| Motor slows when hot | Internal leakage has increased | Case drain, pump flow, oil temperature |
| Cylinder drifts | Oil crosses a seal or valve | Piston seal, directional or holding valve |
Do not replace the pump solely because a machine is slow. One slow function often points to its valve section or actuator. If every function is slow, check pump speed, inlet condition, main flow and the relief circuit.
Four Common Misconceptions
1. Higher pressure makes the machine faster. Flow primarily controls speed.
2. A larger pump always produces more force. Greater displacement usually produces more flow. Force still depends on pressure and actuator area.
3. The relief setting is normal operating pressure. It is a limit; working pressure should change with the load.
4. Hydraulic oil is completely incompressible. It is nearly incompressible for basic calculations, but oil compresses slightly, hoses expand and trapped air compresses considerably.
Safety: Never Check a Leak by Hand
Official accident records document hydraulic fluid released at 3,000 psi penetrating a worker's skin and requiring hospitalization.
Before service:
- Lower or mechanically support suspended loads.
- Isolate the engine, electric motor or other power source.
- Release stored pressure, including accumulator pressure.
- Confirm the circuit is depressurized.
- Use cardboard, wood or an approved tool to locate leaks.
- Treat a suspected fluid-injection injury as a medical emergency.
Information Needed for Component Selection
Before requesting a replacement pump, motor or valve, collect:
- Machine and application
- Existing manufacturer and complete model code
- Nameplate and installation photographs
- Required flow, speed, force or torque
- Working and peak pressure
- Displacement and circuit type
- Pump control or valve function
- Rotation, shaft, flange and ports
- Case-drain requirements
- Hydraulic fluid, temperature and duty cycle
Two components with the same displacement are not automatically interchangeable. Differences in shafts, ports, controls, mounting dimensions or case-pressure limits can cause installation failure or early damage.
Bohang manufactures and customizes hydraulic pumps, hydraulic motors and hydraulic valves for replacement, OEM and system-integration projects. Send the complete model code, pressure, flow and mounting information so compatibility can be checked before quotation.
Conclusion
The pump supplies flow. The load creates the need for pressure. Valves control where oil goes and how the circuit responds. Pressure acting on cylinder area produces force, while pressure difference acting through motor displacement produces torque. Flow determines how quickly that output creates movement.
For reliable operation, start with the work the machine must perform. Calculate force or torque and speed, then match the pump, motor, valves, plumbing, filtration and cooling to the full duty cycle.
FAQ
Q1. Does a hydraulic pump create pressure or flow?
A: It creates flow. Pressure rises when the flow meets resistance. The pump must deliver the required flow while safely operating at the resulting pressure.
Q2. Why does a cylinder retract faster than it extends?
A: The rod reduces the effective retract-side area. The same flow fills the smaller volume faster, but retracting force is lower.
Q3. What determines hydraulic motor torque?
A: Torque depends mainly on pressure difference across the motor, displacement and mechanical efficiency. Outlet and case pressure must also be considered.
Q4. Why can a system show high pressure but no movement?
A: The actuator may be overloaded or jammed, the valve may not open correctly, the return path may be restricted, or pump flow may be too low.