Hydraulic Pump Pressure vs. Flow

Pressure and flow are fundamental concepts in hydraulic systems, but they are often confused. It’s a common misconception that if a high-pressure hydraulic pump isn’t producing pressure, it needs replacement. However, this might not be the real issue, as pumps primarily provide flow.

Pressure in a Automotive Lift Repair Florida hydraulic system is created by the resistance the flowing fluid encounters. To illustrate, consider a garden hose:

– If the flow from the hose is restricted, the water builds up back-pressure and can shoot farther.

– If the water flows freely, it may only reach a short distance without any back-pressure.

– When a Tee fitting is present and only one side is restricted, the water won’t spray far. The restriction must be applied to both sides of the Tee to build up pressure for effective spraying.

Similarly, in a Automotive Lift Repair Florida hydraulic system, fluid takes the path of least resistance to return to the reservoir. This could be through a cylinder to perform work, across a relief valve if the load is too high, or through a leak-path if the fluid escapes without doing any work. If a valve allows fluid to return to the tank without building sufficient pressure, it’s not the pump’s fault; the pump’s role is to generate flow, and it’s designed to handle that flow up to a certain pressure limit.

In essence, the resistance of a load generates pressure in a hydraulic circuit. The difference between hydraulic pump pressure and flow is that the pump provides the flow rate, while pressure is a result of resistance encountered by that flow. Pressure is the measure of this resistance as force per unit area. When referring to a pump’s pressure rating, it indicates the maximum pressure the pump can generate and sustain.

Each Automotive Lift Repair Florida pump has two key specifications: pressure and flow. Pressure is usually rated in PSI (Pounds per Square Inch) and flow in GPM (Gallons Per Minute), with metric equivalents being Bar and LPM (Liters Per Minute). One Bar equals 14.5 PSI, and one Liter is approximately 0.26 gallons. The pump’s flow rating must be sufficient to operate all actuators at their designed speed, and the pressure rating must meet the force requirements of the actuators.

Flow and pressure are interrelated. Low pressure might result from insufficient flow, which can occur if the flow is bypassing back to the reservoir, similar to a leaking piston seal. Understanding how flow affects pressure is crucial for assessing hydraulic system performance, selecting suitable power sources and motors, and accounting for internal losses due to flow turbulence, leakage, fluid viscosity, and friction.

Hydraulic Fluid Power and Pump Function

Hydraulic fluid power is generated when the pump pushes fluid through the system. The fluid moves through valves that control the direction of flow and manage pressure as needed. This fluid action drives the cylinder, converting hydraulic energy into mechanical energy.

So, does a Automotive Lift Repair Florida hydraulic pump create pressure or flow?

A hydraulic pump performs two key functions:

1. It generates mechanical action that creates a vacuum at the inlet, allowing atmospheric pressure to push hydraulic fluid from the reservoir into the inlet line.

2. It forces the fluid from the pump outlet into the hydraulic system, causing the liquid to flow.

The pump itself does not generate pressure; it supplies the necessary flow of hydraulic fluid. Pressure is generated by the resistance the fluid encounters as it flows through the system.

It’s important to understand that flow is the rate at which pressure is developed. However, pressure is what drives the movement of the hydraulic components, such as a piston rod pushing or pulling on machinery or extending and retracting aircraft landing gear.

Automotive Lift Repair Florida Hydraulic Flow Control Valves

In a hydraulic system, various flow control valves regulate the flow within different sections of the circuit. 

These valves control the flow rate to cylinders and motors (i.e., hydraulic actuators), thus regulating their speed. They also manage the rate of energy transfer at a given pressure, which translates into work. The actuator force multiplied by the distance traveled equals the work done on the load. Essentially, the energy transferred is equal to the work done, with speed being a function of flow rate.

Understanding flow rate is crucial, as it can be measured in different ways, affecting the selection of hydraulic flow control valves. The two primary measurements are:

1. Mass Flow Rate: The mass of the liquid passing through a system per unit of time, calculated by multiplying the volume flow rate by the fluid’s mass density (ρ).

2. Volumetric Flow Rate: The volume of liquid flowing through a system per unit of time, calculated by multiplying the flow velocity (v) by the cross-sectional area (A) of the hose, tube, pipe, or valve orifice.

Automotive Lift Repair Florida Directional Hydraulic Control Valves

Directional control valves serve a different purpose than flow control valves. They direct energy to the appropriate location at the right time. These valves keep fluid in standby mode to prevent flow until needed for specific tasks. When actuated, they perform the necessary function and return to the neutral position once the task is completed. Some directional control valves can also manage pressure and flow rate by throttling fluid flow.

Hydraulics is about force transmission, not just fluid movement. Just as pushing on a steel rod transmits force, pushing on hydraulic fluid does the same, though the fluid is less compact. 

It’s a common misconception that a hydraulic pump generates pressure. In reality, the pump creates flow, and pressure results from the resistance to that flow. Without flow, there is no pressure. If an operator notes insufficient pressure, the real issue is often the flow rate. Understanding the distinction between hydraulic flow and pressure—and how one affects the other—is crucial for effective pump performance.

To reiterate: a Automotive Lift Repair Florida hydraulic pump doesn’t deliver pressure; it produces a flow rate of fluid over time to perform work.

The hydraulic reservoir holds non-pressurized hydraulic fluid, usually hydraulic oil, which is essential for the hydraulic system. This oil passes through a filter that removes impurities.