An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which an agent acts upon an environment. The agent can be either an artificial intelligence agent or any other autonomous being (human, other animal, etc.)
Hydraulic actuators or hydraulic cylinders typically involve a hollow cylinder having a piston inserted in it. An unbalanced pressure applied to the piston provides force that can move an external object. Since liquids are nearly incompressible, a hydraulic cylinder can provide controlled precise linear displacement of the piston. The displacement is only along the axis of the piston. A familiar example of a manually operated hydraulic actuator is a hydraulic car jack. Typically though, the term "hydraulic actuator" refers to a device controlled by a hydraulic pump.
Hydraulic cylinders get their power from pressurized hydraulic fluid, which is typically oil. The hydraulic cylinder consists of a cylinder barrel, in which a piston connected to a piston rod moves back and forth. The barrel is closed on each end by the cylinder bottom (also called the cap end) and by the cylinder head where the piston rod comes out of the cylinder. The piston has sliding rings and seals. The piston divides the inside of the cylinder in two chambers, the bottom chamber (cap end) and the piston rod side chamber (rod end). The hydraulic pressure acts on the piston to do linear work and motion.
Cylinder barrel The cylinder barrel is mostly a seamless thick walled forged pipe that must be machined internally. The cylinder barrel is ground and/or honed internally Cylinder base or cap In most hydraulic cylinders, the barrel and the bottom portion are welded together. This can damage the inside of the barrel if done poorly. Therefore, some cylinder designs have a screwed or flanged connection from the cylinder end cap to the barrel. (See "Tie rod cylinder", below) In this type the barrel can be disassembled and repaired. Cylinder head The cylinder head is sometimes connected to the barrel with a sort of a simple lock (for simple cylinders). In general, however, the connection is screwed or flanged. Flange connections are the best, but also the most expensive. A flange has to be welded to the pipe before machining. The advantage is that the connection is bolted and always simple to remove. For larger cylinder sizes, the disconnection of a screw with a diameter of 300 to 600 mm is a huge problem as well as the alignment during mounting.
Piston The piston is a short, cylindrical metal component that separates the two parts of the cylinder barrel internally. The piston is usually machined with grooves to fit metal seals. These seals are often O-rings, U-cups or cast iron rings. They prevent the pressurized hydraulic oil from passing by the piston to the chamber on the opposite side. This difference in pressure between the two sides of the piston causes the cylinder to extend and retract. Piston seals vary in design and material according to the pressure and temperature requirements that the cylinder will see in service. Generally speaking, elastomeric seals made from nitrile rubber or other materials are best in lower temperature environments, while seals made of Viton are better for higher temperatures. The best seals for high temperature are cast iron piston rings. Piston rod The piston rod is typically a hard chrome-plated piece of cold-rolled steel which attaches to the piston and extends from the cylinder through the rod-end head. In double rod-end cylinders, the actuator has a rod extending from both sides of the piston and out both ends of the barrel. The piston rod connects the hydraulic actuator to the machine component doing the work. This connection can be in the form of a machine thread or a mounting attachment, such as a rod-clevis or rod-eye. These mounting attachments can be threaded or welded to the piston rod or, in some cases, they are a machined part of the rod-end.
Rod gland The cylinder head is fitted with seals to prevent the pressurized oil from leaking past the interface between the rod and the head. This area is called the rod gland. It often has another seal called a rod wiper which prevents contaminants from entering the cylinder when the extended rod retracts back into the cylinder. The rod gland also has a rod wear ring. This wear ring acts as a linear bearing to support the weight of the piston rod and guides it as it passes back and forth through the rod gland. In some cases, especially in small hydraulic cylinders, the rod gland and the rod wear ring are made from a single integral machined part. Other parts Cylinder base connection Seals Cushions
Single acting vs. double acting Single acting cylinders are economical and the simplest design. Hydraulic fluid enters through a port at one end of the cylinder, which then moves the piston to extend the rod. An external force returns the piston to its normal position and forces the hydraulic fluid back through the supply tubing to the fluid reservoir. Double acting cylinders have a port at each end, supplied with hydraulic fluid for both the retraction and extension of the piston. They are used where an external force is not available to retract the piston or where high force is required in both directions of travel. An hydraulic cylinder should be used for pushing and pulling only. No bending moments or side loads should be transmitted to the piston rod or the cylinder to prevent rapid failure of the rod seals. For this reason, the ideal connection of an hydraulic cylinder is a single clevis with a spherical ball bearing. This allows the hydraulic actuator to move and allow for any misalignment between the actuator and the load it is pushing.
Cylinder type Cylinder actuators provide a fixed length of straight- line motion. They usually consist of a tight-fitting piston moving in a closed cylinder. The piston is attached to a rod that extends from one end of the cylinder to provide the mechanical output. The double-acting cylinder has a port at each end of the cylinder to admit or return hydraulic fluid. A four- way directional valve functions to connect one cylinder port to the hydraulic supply and the other to the return, depending on the desired direction of the power stroke
Limited-rotation actuators are used for lifting, lowering, opening, closing and transferring movements by producing limited reciprocating rotary force and motion. Rotary actuators are compact and efficient, and produce high instantaneous torque in either direction. Hydraulic fluid is applied to either the two end chambers or the central chamber to cause the two pistons to retract or extend simultaneously so that the racks rotate the pinion gear.
Plunger Hydraulic Actuators A plunger actuator is a simple variation of the original device. It is a hydraulic cylinder without a piston and is, therefore, only limited to being used as a pushing device. Such a device is normally employed within crushing machines at garbage disposal yards or machines that are used to crush old cars and other large metallic objects. The pushing force provided by a plunger actuator is much stronger than a simple hydraulic actuator.
A differential actuator works much like the original hydraulic cylinder in regards to pulling an object. However, there is one minute difference. When it comes to pushing down on an object, this device works much faster because it provides more strength and is comparable to the plunger hydraulic actuator. The reason for this is simple. The normal hydraulic pump employs oil to pull onto the pistons and pull objects. When the oil is pumped out, the piston is used to push an object. However, in the case of a differential hydraulic device, the oil is not pumped out while using the piston to push an object, which is why the entire device works faster while pushing. A differential hydraulic actuator is used in winches and cranes as well as the braking systems of vehicles.
A rephasing hydraulic device involves the amalgamation of two or more hydraulic actuator devices. The individual hydraulic actuators can be connected to each other following a series or parallel patterns of connection. In the case of a parallel connection, each hydraulic actuator is similar to each other. When is comes to a series connection, the individual hydraulic actuators should be different from each other. The purpose of this entire contraption is to allow force to flow through each hydraulic actuator in the most efficient manner. A rephasing hydraulic actuator is used within large air conditioning units as well as large water pumps.
Variable hydraulic actuators are widely used as drives of machine tools, rolling mills, pressing and the foundry equipment, road and building machines, transport and agricultural machines, etc. A number of advantages in comparison with mechanical and electric transfers explains such their wide application: • infinitely variable control of gear-ratio in a wide range and an opportunity to create the big reduction ratio; • small specific weight, i.e. the weight of a hydroactuator is in ratio to transmitted capacity (0,2...0,3 kg / kWt); • opportunity of simple and reliable protection of the engine from overloads;
• simplicity of transformation of rotary movement into reciprocating one; • efficiency of a volumetric hydraulic actuator is a little bit lower, than efficiency of mechanical and electric transfers, and during regulation it is reduced; • conditions of operation of a hydraulic actuator (temperature) influence its characteristics; • efficiency of a hydraulic actuator is a little reduced in the process of exhaustion of it resource owing to the increase in backlashes and the increase of outflow of liquid (falling of volumetric efficiency); • sensitivity to pollution of working liquid and necessity of high culture service.