Actuator types, Hydraulic and Pneumatic.

Actuators are devices capable of generating a force from liquids, gases, and electrical energy. The actuator receives a command signal from the controller and gives an output necessary to activate a final control element such as valves.

They can be

  • Hydraulic
  • Pneumatic

Hydraulic actuators 

They are used when high power is needed, hydraulics require too much equipment for power supply, as well as periodic maintenance. On the other hand, the applications of pneumatic models are also limited from the point of view of precision and maintenance.

The work done by a pneumatic actuator can be linear movement or rotary.

The linear movement is obtained by piston cylinders (these also provide the rotary movement with a variety of angles by means of rack and pinion type actuators).

We also find pneumatic actuators of continuous rotation (pneumatic motors), combined movements, and even some mechanical transformation of movement that makes it seem of a special type.

Pneumatic actuators

Although the function of the pneumatic and hydraulic actuators are identical, they are classified into

Linear actuators.

Rotary actuators.

Linear Pneumatic Actuators

The pneumatic cylinder consists of a closed cylinder with a piston inside that slides and transmits its movement to the outside through a rod. It is made up of the rear and front covers.

Pneumatic cylinders, regardless of their constructive form, represent the most common actuators used in pneumatic circuits. There are two fundamental types from which special constructions derive.

Single-acting cylinders, with an air inlet to produce a working stroke in one direction.

       Double-acting cylinders, with two air inlets to produce exit and backward work strokes.

Singleacting cylinders

A single-acting cylinder performs work only in one direction. The plunger is made to return by means of an internal spring or by some other external means such as loads, mechanical movements, etc. Its work can be of the “normally in” or “normally out” type.

Single-acting cylinders are used for

clamping,

marking,

ejecting, etc.

They consume less air than a double-acting cylinder of the same size. However, there is a reduction in momentum due to the counterforce of the spring, so a somewhat larger internal diameter may be necessary to achieve the same force.

Also, the adaptation of the spring results in a longer overall length and a limited stroke length, due to dead space.

Types of single-acting cylinders:

Piston cylinders,

diaphragm cylinders,

roll-up diaphragm cylinders.

Double-acting cylinders 

Double-acting cylinders are those that perform both their forward and backward strokes by the action of compressed air. Its name is due to the fact that they use both faces of the plunger (air in both chambers), so these components can do work in both directions.

Its internal components are practically the same as those of simple effect, with small variations in its construction.

Plunger operated Pneumatic Cylinder

The field of application of double-acting cylinders is much broader than that of single-acting cylinders, even when no effort is needed in both directions. This is because, as a general rule (depending on the type of valve used for control), double-acting cylinders always contain air in one of their two chambers, so positioning is ensured.

In order to carry out a certain movement (forward or backward) in a double-acting actuator, a pressure difference must exist between the chambers. In short, we can say that double-acting linear actuators are the most common components in pneumatic control. This is due to:

  •  It is possible to carry out work in both directions (forward and backward strokes).

  •  No force is lost in the actuation due to the absence of spring in opposition.

  •  For the same cylinder length, the double-acting stroke is greater than in the single-acting arrangement, as there is no housing volume.

Other types of cylinders:

Pneumatic bellows cylinder.

Also known as a bellows air motor, it incorporates a double-acting cylinder, a directional control valve actuation system, and two forward and reverse speed regulation screws.

·Pneumatic impact cylinder

The rod of this cylinder moves at a high speed of the order of 10 m / s and this energy is used to carry out marking work on engine benches, wooden profiles, electromechanical components, and work on time-stamping dams, stamping, riveted, bent, etc.

Rod less Pneumatic Cylinder

When the available cylinder space is limited, the pneumatic rod less cylinder is the choice. It can have a relatively long stroke of about 800mm and greater.

Guided pneumatic cylinder

One of the problems that conventional cylinders present is the rotational movement that the rod can suffer, since the piston, the rod, and the cylinder liner are circular in section, so none of them prevent rotation. In some applications, free rotation is not tolerable, so an anti-rotation system is necessary.

One of the systems that, apart from the anti-rotation function, has other advantages is the guided pneumatic cylinder that contains two or more pistons with their rods, which gives rise to a force twice that of conventional cylinders.

They consist of two or more double-acting cylinders coupled in series. Two cylinders with different strokes make it possible to obtain four different piston rod positions.

Tandem cylinders

It is made up of two double-acting cylinders that form a unit. By simultaneously applying pressure to the two pistons, a force of almost twice that of a normal cylinder for the same diameter is obtained on the rod.

Rotary pneumatic actuators

Rotary or rotary actuators are responsible for transforming pneumatic energy into rotational mechanical energy. Depending on whether the turning mobile has a limited angle or not, the two large groups to be analyzed are formed:

Limited turn actuator

They are those that provide turning movement but do not produce a revolution (except for some particular mechanics such as rack and pinion). There are single and double effect arrangements for turning angles of 90º, 180º …, up to a maximum value of about 300º (approximately).

Pneumatic motors

They provide a constant rotary motion. They are characterized by providing a high number of revolutions per minute.

LIMITED ROTATION ACTUATORS

Vane actuator

The vane-type rotary actuator is perhaps the most representative of the group of limited-turn actuators. These actuators perform a turning movement that rarely exceeds 270º, incorporating mechanical stops that allow the regulation of this turn.

They are made up of a casing, inside which is a palette that delimits the two chambers. Integrated with this pallet is the shaft, which runs through the outer casing. It is precisely on this axis that we get the work, in this case in the form of limited angular motion.

As we can see in the below figure, the operation is similar to that of double-acting linear actuators. When applying compressed air to one of its chambers, the blade tends to rotate on the axis, as long as there is a pressure difference with respect to the opposite chamber (generally communicated with the atmosphere). If the position is reversed, a turning movement in the opposite direction is achieved.

These components have advantages inherent to the latest generation components, such as damping at the end of travel, the possibility of magnetic position detection (mechanical or magnetic), etc. The mechanical detection is carried out by means of external mobile elements adjustable in degree by means of the graduated vernier.

The cylinders that function as rotary actuators, of limited turn, are the rotary piston-rack-pinion cylinder in which the linear movement of the piston is transformed into a rotary movement by means of a rack and pinion assembly and the Rotary vane cylinder of double-acting for angles between 0 ° and 270 °. In the following figure the piston-rack-pinion cylinder:

LINEAR HYDRAULIC ACTUATORS

Linear motion hydraulic cylinders are commonly used in applications where the thrust force of the piston and its displacement are high.

Hydraulic cylinders can be

Single-acting,

Double-acting

Telescopic.

Single-acting hydraulic cylinder

In the first type, the hydraulic fluid pushes the cylinder piston in one direction and an external force (spring or gravity) retracts it in the opposite direction. The cylinder body is the tubular outer case and contains the piston, piston seal, and rod. “Caliber” is the term used to indicate the diameter of the piston. The piston end of the cylinder (sometimes called the “blind end”) is known as the head end. The end from which the stem extends and retracts is known as the stem end.

The double-acting cylinder

It uses the force generated by the hydraulic fluid to move the piston in both directions, using a solenoid valve. The double-acting cylinder is the most common hydraulic actuator used today and is used in implement, steering, and other systems where the cylinder is required to operate in both directions.

A cylinder bore is a term that indicates the internal diameter of the cylinder. A large-bore cylinder produces a greater volume per unit length than a small bore cylinder. To move a piston the same distance, a large-bore cylinder needs more oil than a smaller bore cylinder. Therefore, for a given flow regime, a large-bore cylinder moves more slowly than a small bore cylinder.

The effective area of a cylinder is the area of the piston and piston seal on which the oil acts. Because one end of the rod is attached to the piston and the opposite end extends out of the cylinder, the effective area of the end of the rod is less than the effective area of the head end. The oil does not act against the area of the piston covered by the rod joint.

Its design allows the oil pressure to extend the seal against the cylinder wall, so that the higher the pressure, the greater the sealing force. The head end seal (ring seal) prevents oil from escaping between the stem neck and the cylinder wall.

The seals are made of polyurethane, nitrile, or Viton. The material must be compatible with the fluids used and the operating conditions.

Shock absorbers

The figure shows a cylinder with shock absorbers.

When a moving cylinder comes to a dead-end (such as at the end of the cylinder’s stroke), the action it experiences is known as “shock loading.” When a cylinder is subjected to a shock load, shock absorbers are used to minimize the effect.

As the piston approaches the end of the stroke, the damper moves within the return oil passage and restricts the flow of return oil from the cylinder. The restriction generates an increase in the return oil pressure between the return oil passage and the piston. The increase in oil pressure produces a “damping effect” that reduces the movement of the piston and minimizes the shock that occurs at the end of the stroke.

Some cylinders may require a head-end damper, while others may require both head end and rod end dampers.

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