What Determines the Speed of a Hydraulic Motor?
The speed of a hydraulic motor is determined mainly by the effective flow rate into the motor. The formula is as follows:
n = Q / q × ηᵥ
n is the speed of the motor
Q is the actual flow rate into the motor
q is the displacement of the motor (the amount of oil needed per revolution)
ηᵥ = volumetric efficiency (the effective ratio after taking into account the leakage)
So the higher the flow rate, the higher the speed of the same motor. The higher the displacement, the lower the RPM for the same flow rate.
Many people will mistakenly believe that the speed of a hydraulic motor is related to pressure. In fact, pressure determines torque, not speed.
One more thing to note here. Effective flow refers to the portion of the flow that actually enters the hydraulic motor and participates in doing work, not the theoretical flow of the pump or the nominal flow of the valve.
What Factors Affect the Flow Supplied to a Hydraulic Motor?
Pump Output and Control Method
The displacement of the pump and the speed of the engine determine the flow rate. The higher the engine speed or the greater the pump displacement, the greater the flow rate of the oil supply.
The way the pump is controlled also affects the amount of flow. Variable pumps have a variety of control methods, which include flow control, constant power, constant pressure control, and more. You can use different control strategies to adjust the flow rate as the load on the system changes.
Valves and Throttling
Different types of throttle valves have different flow control capabilities.
When ordinary throttle valves (such as fixed orifice and needle valves) are throttling, once the load changes, the pressure difference between the front and rear of the valve will change, and the flow rate of oil supply to the motor will fluctuate.
Pressure-compensated flow control valves maintain the differential pressure before and after the valve through an internal compensating mechanism when the load changes, thus making the oil supply flow to the motor more stable.
Throttle Locations
Different throttle locations also affect the flow rate into the hydraulic motor.
Throttling the inlet is done by installing the throttle valve in front of the motor inlet, limiting the flow to the motor.
Return throttling is where the throttle valve is mounted after the motor outlet, limiting the flow out of the motor.
Bypass throttling is where the throttle valve is installed next to the main line and restricts the flow to the bypass line.
The table below will show you the difference in stability and energy efficiency between the three types of throttling.
| Throttling Method | Impact of Load Changes on Flow | Stability | Energy Efficiency |
| Throttling of Oil Inlet | Big | Low | Medium |
| Return Throttle | Small | High | Low |
| Bypass Throttle | Medium | Medium | Very Low |
Load and System Pressure
Load variations can cause system pressure fluctuations, altering the pressure difference between the front and rear of the valve and leading to changes in the flow rate through the valve control system.
As the system pressure increases, internal leakage increases, and the effective flow rate to the hydraulic motor decreases.
Therefore, pressure changes are an important indirect factor affecting flow.
Internal Leakage and Volumetric Efficiency
Internal leakage exists in both pumps, valves, and motor bodies. As the pressure and temperature of the system increase, the clearances in the parts become larger and the seals become less effective, resulting in increased internal leakage. Since the leaking fluid is not involved in doing work, there is a difference between the actual effective flow rate into the motor and the theoretical flow rate.
Volumetric efficiency is the ratio of the effective flow rate to the theoretical flow rate. The greater the amount of internal leakage, the less effective the flow rate entering the motor, and the lower the volumetric efficiency.
Viscosity and Temperature
The physical properties of the fluid itself also affect the flow rate.
When the temperature increases, the viscosity of the fluid decreases, and the fluid is more likely to leak. When the temperature decreases, the viscosity of the fluid increases, and the resistance to flow becomes greater.
Contaminants and air bubbles in the fluid can cause localized resistance changes or cavitation, thus indirectly reducing the effective flow rate.
Pipelines and Pressure Drop
Before entering the motor, the oil needs to pass through pipes and fittings.
Pipes that are too small or too long can cause increased friction and pressure drop, thus reducing the effective flow rate into the motor.
An excessive number of elbows and fittings can also cause a pressure drop, which affects the actual flow that ultimately reaches the hydraulic motor.
System Architecture
The overall architecture of the system also affects the effective flow rate of each hydraulic motor.
In complex systems, a single pump will often provide flow to multiple hydraulic motors.
Different distribution configurations (parallel/tandem distribution) affect how flow is distributed to multiple actuators.
When there is a preferential or proportional distribution in a system, this can cause the flow to the motors to change as the system action changes.
Suction Conditions
When the filter on the suction side of the hydraulic pump becomes clogged, and the tank level is too low, this can lead to suction emptying and cavitation in the hydraulic pump. At this point, the flow rate fluctuates. This can cause the actual flow rate into the motor to be erratic.
How to Control Hydraulic Motor Speed?
Flow Control Valve
Flow control valves control the speed by restricting the flow into the hydraulic motor.
Control Method
Fixed Orifice Throttle: Flow is controlled by a fixed-size orifice.
Adjustable Orifice Throttle: Controls flow by adjusting the size of the orifice opening.
Pressure Compensated Flow Valve: Maintains a stable flow rate during load variations by an internal pressure-compensating structure.
Applicable Scenarios
Systems with light loads and little pressure variation.
Equipment that does not require a high degree of accuracy in speed regulation.
Equipment with low cost and low stability requirements.
Variable Displacement Pump Control
Variable displacement pumps can control the speed of the hydraulic motor by adjusting the displacement of the variable pump according to the load demand.
Control Method
Mechanical control variable pump: change the displacement of the pump through the mechanical structure, to change the flow rate.
Electrically controlled variable pump: Adjusting the displacement of the pump through the electric control system and pressure sensor, thus controlling the flow rate.
Applicable Scenarios
Systems with heavy loads and large pressure variations.
Systems requiring high energy efficiency.
Equipment that requires high accuracy in speed regulation.
Valve-Pump Combined Control
The speed control can be optimized for different loads and operating conditions by combining valve control and pump control.
Control Method
The flow rate is regulated by controlling the variable pump.
The distribution of the flow is regulated by controlling the throttle valve.
Applicable Scenarios
Systems with large pressure fluctuations.
Systems with frequent dynamic load changes.
Industrial automation systems with high demands on precision.
Closed-Loop Control
The sensor monitors the motor speed in real time and feeds the speed signal back to the controller. The controller then adjusts the flow control valve or pump displacement to precisely control the motor speed.
Control Method
Use an encoder or speed sensor to obtain real-time speed data.
Use PI, PID, and other control algorithms to adjust the input flow rate according to the feedback signal to keep the motor speed stable.
Control the hydraulic components by PLC or a special controller.
Applicable Scenarios
For systems with high requirements for precision and stability.
Systems that require fast tracking of changes in control instructions.
The table below will give you an idea of the advantages and disadvantages of these four types of controls.
| Control Method | Advantages | Disadvantages |
| Flow Control Valve | Simple Structure Low Cost Easy to Install Easy to Maintain | Sensitive to Load Changes High Throttling Losses Low Energy Efficiency Fairly Stable |
| Variable Pump Control | High Energy Efficiency Less Heat Adaptability to Load Stability | Higher Cost Complex Control and Commissioning High System Design Requirements |
| Valve–Pump Combined Control | Fast Dynamic Response Combining Efficiency and Stability Adaptable to Complex Operating Conditions | High System Complexity Higher Cost Difficulty in Commissioning |
| Closed-Loop Control | High Control Accuracy High Anti-interference Capability Good Stability High Adaptability
| High Cost High Maintenance and Commissioning Requirements |
How Do You Choose the Right Speed Control Solution for Your Needs?
Load Characteristics
Whether the load is stable or not determines the method of speed regulation. If the load has small variations, you only need simple control. If the load fluctuates a lot, to make the speed more stable, you need a more stable flow supply and a compensation loop.
Speed Accuracy Requirement
Requirements for speed accuracy vary greatly from application to application. If you don’t need to be exact, a simple valve control strategy will work. If the accuracy requirement is high, you need to use proportional valve control or servo-electronic control.
Dynamic Response
Dynamic response is the speed of the system’s reaction to set point changes or load fluctuations. Highly dynamic systems need to return to a steady state quickly, or they will fluctuate dramatically. This factor determines whether you need closed-loop feedback and high-precision actuators.
Why Hydraulic Motor Speed Fluctuates and How to Stabilize It
Fluctuations due to Load Changes
Changes in load can directly cause fluctuations in the speed of the motor. If the load suddenly changes a lot, it can affect the hydraulic motor, which can result in speeds that are not steady. If the load gets bigger, the flow rate and speed get lower. When the load goes down, there is extra flow, and the speed goes up.
You can minimize speed fluctuations caused by load changes by using a pressure-compensated flow valve.
Leakage and Friction
Damage to the seals and bearings inside the hydraulic motor can lead to leakage of hydraulic fluid. The system loses some flow, which affects speed stability. Internal leaks can vary with pressure and temperature, increasing the degree of speed fluctuation.
When the motor is running slowly, there is a lot of friction inside it, which can also make the speed go up and down.
You can reduce the effect of friction and leakage on stability by optimizing seal design and increasing system lubrication.
Unstable Oil Condition
The viscosity of hydraulic fluid is affected by temperature. When the oil gets hotter, it gets less thick and flows faster, which makes the speed go up and down.
If the liquid has impurities or air bubbles in it, it will not flow easily through the system. This will change motor speed.
You need to keep the temperature of the fluid within normal limits and clean and change the hydraulic fluid regularly. You can stop this with an oil filtration system.
Hydraulic Piping and Circuit Dynamics
Motors and loads may not be able to instantly adapt to changes in flow when they change direction or pressure, so the system can’t always adapt instantly to changes in flow, which can cause speed fluctuations.
You need to make the piping design better to make sure the pressure in the circuit stays the same. To reduce changes in pressure, you can use valves or buffers.
The Effect of Control Systems and Actuators
Proportional and servo valves in hydraulic systems are slow to respond to changes in setpoints. This can cause the flow or pressure to go up and down, which then changes the speed.
If the feedback and rules of the control system are not accurate enough, it may cause problems when adjusting it.
You should use high-precision proportional or servo valves and improve the control algorithm and regulation parameters to make the system faster and more stable.
Cavitation and Airborne Effects
When air gets into the hydraulic system, it forms bubbles in the liquid. The swings in air bubbles cause changes in fluid flow, which cause fluctuations in motor speed.
When the pressure in the system is too low, air bubbles form in the liquid and burst. This can make the motor speed go wrong.
Proper venting and cavitation protection will keep the fluid clean and make sure there is enough pressure in the system.
FAQ
Meter-in vs meter-out: which is better for motor speed control?
The inlet throttle control is used to adjust the oil flow into the motor. It has a simple structure and is cheap, so it is good for less accurate systems.
The outgoing throttle control regulates the speed by controlling the return oil flow. It makes the speed more consistent. It is suitable for high-precision systems.
How can I reduce heat during hydraulic speed control?
You can choose variable pumps for speed regulation to reduce throttling losses.
You can use valves that control the flow to make it more stable as the load changes, which saves energy.
The oil temperature can be kept within a suitable range with an efficient cooling system.
Is it possible to control the speed precisely without using servo valves?
Yes, using a proportional valve or closed-loop control system allows precise control of the motor’s speed, and you do not need to use a servo valve.
Why is my hydraulic motor noisy during speed control?
Noise is usually caused by the following:
Cavitation: expansion and compression of air bubbles in the hydraulic fluid.
Internal leakage: Poor or worn seals inside the motor.
Flow fluctuation: Unsteady flow due to an unstable control system or contaminated oil.
What is the difference between speed control and torque control?
Speed control mainly controls the speed of the motor. It is achieved by adjusting the flow rate.
Torque control is how you adjust the motor’s torque. You can adjust the flow and pressure to get the best results.
You can use them together, depending on what you need.
Final Thoughts
Speed control of hydraulic motors is important for the efficient operation of hydraulic systems. Several things can cause this, like how well the water flows, changes in pressure, leaking pipes, and temperature. If you pick the right control method, the system will become more stable and use less energy. It will also generate less heat.
With over 10 years of experience in hydraulic motor speed regulation, Target Hydraulics is dedicated to offering you precise and reliable hydraulic speed control solutions. Whether it’s design optimization or system commissioning, we can provide you with expert support. Contact us today for a customized hydraulic solution.
