From Valves to Motion: Knowing How a Cylinder Works to Move a Hydraulic System

Hydraulic systems have to be precise as well as powerful, directing power transmission chains to convert mechanical energy into pressure before turning that flow back into mechanical movement.

When the system is working as it should, it encompasses a principle of Pascal’s law, which is realized in a hydraulic system when hydraulic fluid transmits energy from one point to another. Because the hydraulic fluid is nearly incompressible, it can transmit power instantaneously.

Hydraulic fluid creates this power with help from the system’s pumps and motor. A hydraulic pump, which we specialize in here at HPS, converts the mechanical energy from a hydraulic motor into hydraulic power – specifically, flow. This flow creates the necessary pressure to deliver fluid to other parts of the hydraulic system.

The fluid flows to the cylinder through the valve, before the hydraulic energy converts it back to mechanical energy.

Power (motor and pumps) and delivery (valves) are not considerations when looking at the workings of a hydraulic system. Another factor to consider, especially when troubleshooting system issues, is motion.

And to understand motion, it’s good to know a few things about the hydraulic cylinder.

What is a Hydraulic Cylinder?

Cylinders work by applying transfer mechanics to pressurized hydraulic fluid for the purpose of producing motion. Simply put, a hydraulic cylinder is an actuator that creates linear movement in a way that can be compared to a muscle.

These hydraulic “muscles” can be single-acting or double-acting, tie-rod, welded, or ram. Each of these types have different operating specifications, which include:

• Stroke (the length that the piston travels through the cylinder)
• Maximum operating pressure
• Bore diameter
• Rod diameter
• Hydraulic Valves and Cylinders

As pumps and motors are responsible for supplying power and energy within a hydraulic system, valves are the mechanical devices used to control that power by regulating the flow of fluid into cylinders.

Each type of valve (pressure control, flow control, directional control) works differently to direct the flow to cylinders or actuators.

Here is how each valve type controls flow into cylinders:

• Pressure control valves use check valves to maintain hydraulic pressure or load backpressure on a cylinder
• Flow control valves are used to regulate flow rates to cylinders
• Directional control valves have three main functions: to stop fluid flow, to allow fluid flow, or to change the direction of fluid flow between hydraulic cylinders

It’s a vital partnership of flow and power to create motion in a hydraulic system.

How Does a Hydraulic Cylinder Work?

The flow of liquid from the pump develops pressure and force after being directed by the valve before reaching the cylinder.

But then what?

As we know from Pascal’s principle, a pressure change in one part of a closed container is transmitted without loss to every portion of the fluid and to the walls of the container.

Therefore, pressure is equal to the force divided by the area on which it acts (P = F/A).

The cylinder is there to transform that pressure into movement when the force affects its surface. How that is done depends on the type.

Single-acting vs. Double-acting Cylinders

Single-acting and double-acting cylinders are the two most used types of hydraulic cylinders:

• Single-acting cylinders (illustrated at left) only operate in one direction, and the counter motion happens by itself or by a mechanical structure or by an external load.
• Double-acting hydraulic cylinders (illustrated at right) operate in two directions - oil enters the cylinder in the A port and pushes the piston down and diverts to port B, pushing the piston up

Tie Rod Cylinders

Tie rod-style hydraulic cylinders are often used in industrial factory applications and work by using threaded steel rods to hold the two end caps to the barrel. The inside diameter of the barrel is called the bore, which provides space for the rod to move through. The piston is attached to the rod.

Some quick facts about tie rod cylinders:

• Small-bore cylinders usually have four tie rods
• Large-bore cylinders may require up to 16 or 20 tie rods
• Tie rod-style cylinders can be completely disassembled and are not always customizable

Welded Cylinders

Welded cylinders work like tie rod cylinders but have a narrow body and often a shorter overall length. They have a smooth surface and heavy-duty welded cylinder housing to provide stability. They also differ from tie rod cylinders because they are customizable. 

Most Common Hydraulic Cylinder Failure Modes

Worn Piston Seal – The piston seal retains oil on the work side of the piston, within the cylinder. Often reduced force output, continuous running of a hydraulic pump, slower extend and retract speeds, and limited ability to hold a load is present. Tie rod and Ram cylinders can be disassembled, and the piston seal replaced. Worn Rod Seals – This seal is found in the rod gland. Many cylinders have removable rod glads for maintenance replacement. Worn Buffer Seal – This seal absorbs changes in pressure when working in high-load applications. If the Cylinder has a leaky Buffer Seal, it typically is resolved by replacing the seal from a removable rod gland. Worn Wiper Seals – This seal is used to prevent contamination from entering the cylinder via the rod gland. Worn Wear Bands – Wear Bands are located on the piston and prevent the piston metal from contacting the metal I.D. of the Cylinder tube.                                                                                                                                - Reference: Machine Design

Why Do I Need to Know About Cylinders?

A hydraulic cylinder is one of the four main components of a hydraulic system: a mechanical actuator which creates linear movement and works as a machine’s muscle.

And knowing about how to work that muscle can be a vital resource to those who depend on you for quality pumps, motors, and valves.

Understanding fundamentals, such as how Pascal’s Law is applied to actuators, can help with the design, troubleshooting, and maintenance of fluid power systems.