Hydraulic Cylinders and Pumps Guide
Also sometimes called hydraulic motors, hydraulic cylinders are hydraulic actuators used to power mechanical operations. Hydraulic cylinders, in turn, are powered by hydraulic fluid such as oil, which is delivered via a hydraulic hand pump, a hydraulic electric pump or a hydraulic air pump.
Hydraulic actuators are composed of a cylindrical tube and a sliding piston, which is enclosed in a chamber with a cylinder cap on one end and a cylinder head on the other. Single-acting hydraulic cylinders refer to devices where fluid pressure is applied to one end of the piston. This means that the fluid moves the piston in only one direction and a spring or gravity moves it in the other. Double-acting cylinders are hydraulic actuators where fluid pressure is applied to both ends of the piston moving it in both directions. Hydraulic actuators are commonly used in the manufacturing and construction industries.
Hydraulic Cylinder Types
There are two main types of hydraulic cylinders — single acting hydraulic cylinders and double acting hydraulic cylinders.
Single Acting Hydraulic Cylinders
In single acting hydraulic cylinders, the hydraulic fluid only moves one side of the piston, which is then moved in the other direction by an internal spring, gravity or the weight of the load. Single acting pneumatic cylinders perform the same function using compressed air rather than hydraulic fluid.
Double Acting Hydraulic Cylinders
Double acting hydraulic cylinders use hydraulic fluid to move the piston back and forth. Double acting pneumatic cylinders perform the same function using compressed air rather than hydraulic fluid.
Jack cylinders come in a variety of sizes. Unlike other cylinders, jack cylinders are usually used from the floor — they are not mounted. They come with lifting weights of 5, 10, 20 and 50 tones. Differently sized cylinders can typically be used with one pump.
Using a cylinder within a cylinder, telescopic cylinders can create long strokes and as such are often used in different types of machinery. They are either single acting or double acting.
Pancake cylinders are compact hydraulic actuators designed to fit into small spaces. They feature a low profile and a short stroke. Invented in 1968, pancake cylinders initially delivered strokes of under 1 inch. Today, pancake cylinders offer up to 4-inch strokes and pressure ratings of up to 35 bar. In addition, pancake cylinders feature eight bore sizes ranging from 0.5 to 4 inches.
Powered by hydraulic cylinders, hydraulic jacks are used to lift heavy objects and loads. Hydraulic jacks generate pressure by transferring oil between two cylinders with the help of a pump plunger, which opens and sucks oil into a pump chamber. When the pump plunger closes, the oil is moved to the cylinder chamber via an external discharge valve. When the suction valve closes, the pressure moves the piston up and down in the chamber, lifting the object or load.
Also called piston jacks, bottle jacks use hydraulic technology to lift vehicles. More specifically, pumping the handle of a bottle jack pushes oil into its main cylinder, raising the piston and the load or object. Bottle jacks are sometimes used to temporarily hold vehicles up in the air for repair or maintenance purposes. Nevertheless, vehicles are typically placed on axle stands during the maintenance and repair process.
Bottle jacks are used to lift large vehicles with a high clearance such as tractors and SUVs. They cannot be used to lift low-rise vehicles. Bottle jacks are preferable to trolley jacks in a number of ways. They are more lightweight and portable despite being able to lift the same amount of weight. Bottle jacks also tend to be more compact than trolley jacks.
Perfect for situations that require precision, toe jacks are used to lift heavy objects such as machinery or other equipment. Toe jacks are composed of a number of parts including a removable or fixed toe that can be adjusted to a particular starting position, swivel feet that can be tailored to low ground clearance and an overload valve for safety. Toe jacks also feature a slide shoe that stops the load from twisting while it is being transferred. Just like bottle jacks, toe jacks are activated by pumping a handle, an action that increases pressure within the hydraulic system. Toe jacks are usually used in bridge building, heavy construction, machinery installation, and offshore oil and gas.
According to Pascal’s law, force equals pressure multiplied by area. Hydraulic systems source pressure from hydraulic pumping units. Meanwhile, the area of hydraulic cylinders refers to the size of the piston or cylinder rod. As such, the force of a hydraulic cylinder, or the weight that it can push or lift, is determined by the pressure of the hydraulic pumping unit and the size of the piston. The pump also determines the speed of the cylinder. Most pumps range between 3000 and 10,000 psi (210 and 690 Bar). Increasing pump pressure may require adjustments to the cylinder’s valves and seals.
Hydraulic cylinders use pressurised hydraulic fluid to create linear force. Each hydraulic cylinder comes with a range of specifications including cylinder type, operating pressure, stroke, bore diameter and rod diameter.
There are single acting hydraulic cylinders where the fluid only acts on the piston rod from one side and double acting hydraulic cylinders where the piston can move horizontally, vertically and along any other path. Cylinder types include welded, ram and tie-rod. Welded cylinders utilize welded cylinder housing. Ram cylinders feature a piston rod area that is greater than half of the area of their moving components. Usually used in high-pressure applications, tie-rod cylinders feature tie rods on the outside of the cylinder housing for stability.
Hydraulic actuators are composed of a number of parts including a cylinder barrel, piston, piston rod and seals.
- Barrel: The chamber that contains hydraulic pressure.
- Cylinder cap: The cylinder cap closes the barrel at one end to keep the pressure in the chamber. It is attached using a tie rod or by threading, bolting or welding.
- Cylinder head: The cylinder head sits opposite the cylinder cap, sealing the pressure in the chamber. Just like the cylinder cap, it is attached using a tie rod or by threading, bolting or welding.
- Piston and piston rod: The piston separates the low and high pressure that builds up in the barrel or chamber. The piston is attached to a piston rod with nuts, threads or bolts to create the linear movement that lifts objects and loads.
- Seals: Seals prevent the pressurised oil from leaking at different connection points in hydraulic actuators. Hydraulic cylinders typically come with the following seal glands: primary seal, secondary seal, static seals and bearing elements. The type of seals that are required is usually determined by cylinder speed, as well as the working pressure of the hydraulic actuator. Hydraulic cylinder seals are usually made from rubber or metals such as cast iron.
Hydraulic cylinders are either single-acting or double-acting. Single-acting hydraulic cylinders feature pressurised oil that only moves in one direction (they only have one port) while the pressurised oil in double-acting hydraulic cylinders moves in both directions (they have two ports). In single-acting hydraulic cylinders, the piston is moved back with the help of the load, gravity or a retracting spring. In addition, single-acting cylinders require one hydraulic hose while their double-acting counterparts require two hydraulic hoses.
Single-acting hydraulic cylinders feature fewer components than double-acting hydraulic cylinders. As such, they are easier to maintain and perfect for simple jobs where precise and fast retraction is not of the essence. The additional port in double-acting hydraulic cylinders allows for more control and precision, which makes them ideal for applications that require accuracy since they are faster and more predictable.
There are certain things that need to be considered before investing in a hydraulic cylinder.
- Cylinder type: Is the cylinder a tie rod cylinder, a welded rod cylinder or a telescopic cylinder? Do you need a double-acting or a single-acting hydraulic cylinder?
- Bore size: What is the diameter of the bore? This determines the force that can be generated at a certain oil pressure.
- Piston size: What is the diameter of the piston used in the hydraulic cylinder?
- Operating pressure: What is the maximum operating pressure of the hydraulic cylinder?
- Stroke: How long is the stroke of the piston in the hydraulic cylinder? This can be anywhere between under an inch and a few feet.
- Accessories: Do you need to purchase any accessories for the hydraulic cylinder such as hydraulic hoses, hydraulic fittings or hydraulic couplings.
Used to lift heavy objects and loads, hydraulic jacks are commonly used in the automotive industry. Here are the most common types of hydraulic jacks.
- Bottle jacks: Highly portable, bottle jacks feature a lever that can be activated to raise a built-in piston. This, in turn, generates the force to lift an object or a load.
- Trolley jacks: Also called floor jacks, trolley jacks operate horizontally and feature a lifting pad. This makes them more suited than bottle jacks to lifting objects with lower ground clearance.
- Toe jacks: Toe jacks are used in applications that require precision and speed. Toe jacks can be used in situations where the object that requires lifting features very low ground clearance.
Hydraulic filters are designed to protect hydraulic system components from damage due to oil contamination. It’s estimated that a large percentage of fluid power failures are a result of contamination problems. Fluid can become contaminated for a variety of reasons. They include general wear of the components, substandard plumbing and the introduction of new fluid or components. Therefore, hydraulic filters are crucial in maintaining contaminant-free systems.
A variety of filter types offer different configurations and uses. A bag filter utilises a cloth bag to push the hydraulic fluid through the system. In turn, solid contaminants are stopped from flowing through the bag with it. This method is successful for filtering rust, dirt and any particles from cylinder rods entering the system. Magnetic filters use magnetically charged plates to attract metallic contaminants. For screen filters, small wires are woven together to produce a metallic cloth that’s designed to fit exact pore sizes. This method is particularly useful in the process of choosing the right filter when the size of the contaminant is already identified.
Additionally, choosing the right filter should be based on the overall construction, including the filter and the alignment. Before buying a filter, it’s crucial to know whether or not the system requires hydraulic filter housing. In terms of alignment, there are several options available. They include:
- In-line alignment refers to a system whereby the inlet, outlet and filter are lined up together.
- Off-line alignment is when the hydraulic filter isn’t on the main systems loop.
- Duplex configuration is when two filters are used and combined for ease of maintenance.
- Return-line configuration ensures all contaminants are caught upon entering the system.