HYDRAULIC CYLINDER - HYDRONOUS

HYDRAULIC CYLINDER

1.1       HYDRAULIC CYLINDER

A Hydraulic cylinder (also called a linear hydraulic motor) is a mechanical actuator that is used to give a linear force through a linear stroke. It has many applications, notably in engineering vehicles.

1.2     OPERATION

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. These ends are known as the cap end. The cylinder head where the piston rod comes out of the cylinder. The piston has sliding rings and seals. The function of the sliding rings is to prevent the hudraullic oil to move from one side of the cylinder to the other. The seal provides protection to the oil from coming out of the cylinder. 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. This pressure build in the cylinder with the help of the pump acts on the piston head and force is created by the piston. The force generated by the hydraulic cylinder can be easily calculated by the following formulae.

P =   F

        A

Where,

P = Pressure given to the  cylinder

F = Force applied by the piston,

A = Area of the piston head

Figure-1.1

Flanges, trunnions, and/or clevisses are mounted to the cylinder body. The piston rod also has mounting attachments to connect the cylinder to the object or machine component that it is pushing.

A hydraulic cylinder is the actuator or "motor" side of this system. The "generator" side of the hydraulic system is the hydraulic pump which brings in a fixed or regulated flow of oil to the bottom side of the hydraulic cylinder, to move the piston rod upwards. The piston pushes the oil in the other chamber back to the reservoir. If we assume that the oil pressure in the piston rod chamber is approximately zero, the force on the piston rod equals the pressure in the cylinder times the piston area (F=P/A). The piston moves instead downwards if oil is pumped into the piston rod side chamber and the oil from the piston area flows back to the reservoir without pressure. The pressure in the piston rod area chamber is (Pull Force) / (piston area - piston rod area).

1.3     Types of Cylinder

1.3.1   SINGLE ACTING CYLINDER:

The single-acting piston-type cylinder is similar in design and operation to the single-acting ram-type cylinder. The single-acting piston-type cylinder uses fluid pressure to provide the force in one direction, and spring tension, gravity, compressed air, or nitrogen is used to provide the force in the opposite direction. Figure shows a single-acting, spring-loaded.  Piston type actuating cylinder. In this cylinder the spring is located on the rod side of the piston.  In some spring-loaded cylinders the spring is located on the blank side, and the fluid port is on the rod side of the cylinder.

A three-way directional control valve is normally used to control the operation of the single-acting piston-type cylinder. To extend the piston rod, fluid under pressure is directed through the port into the cylinder. This pressure acts on the surface area of the blank side of the piston and forces the piston to the right. This action moves the rod to the right, through the end of the cylinder, thus moving the actuated unit in one direction.  During this action, the spring is compressed between the rod side of the piston and the end of the cylinder. The length of the stroke depends upon the physical limits within the cylinder and the required movement of the actuated unit. To retract the piston rod the directional control valve is moved to the opposite working position, which releases the pressure in the Figure.

Figure-1.3

The spring tension forces the piston to the left, retracting the piston rod and moving the actuated unit in the opposite direction. The fluid is free to flow from the cylinder through the port, back through the control valve to the return line in hydraulic systems or to the atmosphere in pneumatic systems. The end of the cylinder opposite the fluid port is vented to the atmosphere. This prevents air from being trapped in this area. Any trapped air would compress during the extension stroke, creating excess pressure on the rod side of the piston.

             This  would  cause  sluggish  movement  of the piston and could eventually cause a complete lock,  preventing  the  fluid  pressure  from  moving the  piston. The spring-loaded cylinder is used in arresting gear systems on some models of carrier aircraft. To rise (retract) the arresting hook, fluid pressure is directed through the arresting hook control valve to the rod side of the cylinder. This force moves the piston, which, through the rod and mechanical linkage, retracts the arresting hook. The arresting hook extends when fluid pressure is released from the rod side of the cylinder, allowing the spring to expand. Leakage between the cylinder wall and piston is prevented by adequate seals. The piston in figure contains V-ring seals.

1.3.2   DOUBLE ACTING CYLINDER

Most piston-type actuating cylinders are double-acting, which means that fluid under pressure can be applied to either side of the piston to apply force and provide movement. One design of the double-acting cylinder is shown in figure. This cylinder contains one piston and piston rod assembly. The stroke of the piston and piston rod assembly in either direction is produced by fluid pressure. The two fluid ports, one near each end of the cylinder, alternate as inlet and outlet ports, depending on the direction of flow from the directional control valve. This actuator is referred to as an  unbalanced actuating cylinder because there is a difference in the  effective  working  areas  on  the  two  sides  of the  piston. Therefore,  this type of cylinder is normally installed so that the blank side of  the piston carries the greater load; that is, the cylinder carries  the  greater  load  during  the  piston  rod extension  stroke. A four-way directional control valve is normally used to control the operation of this type of cylinder. The valve can be positioned to direct fluid under pressure to either end of the cylinder and  allow  the  displaced  fluid  to  flow  from  the opposite end of the cylinder through the control valve  to  the  return  line in hydraulic systems or to the atmosphere in pneumatic systems. There are applications where it is necessary to move two mechanisms at the same time. In this case, double-acting piston-type actuating cylinders of different designs are required. See figures. Figure shows a three-port, double-acting piston-type actuating cylinder.

Figure- 1.4

This actuator contains two pistons and piston rod assemblies. Fluid is directed through port A by a four-way directional control valve and moves the pistons outward, thus moving the mechanisms attached to the pistons’ rods. The fluid on the rod side of each piston is forced out of the cylinder through ports B and C, which are connected by a common line to the directional control valve.  The displaced fluid then flows through the control valve to the return line or to the atmosphere. When fluid under pressure is directed into the cylinder through ports B and C, the two pistons move inward, also moving the mechanisms attached to them. Fluid between the two pistons is free to flow from the cylinder through port A and through the control valve to the return line or to the atmosphere. The actuating cylinder shown in figure is a double-acting balanced type. The piston rod extends through the piston and out through both ends of the cylinder.  One or both ends.

1.4     PARTS OF HYDRAULIC CYLINDER

A hydraulic cylinder consists of the following parts:

1.4.1 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.

1.4.2   Cylinder Bottom 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 Cylinders" below) In this type the barrel can be disassembled and repaired in future.    

                                                                                         

                                                                       Figure-1.6

1.4.3   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.

Figure-1.7

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.

1.4.4 Piston

The piston is a short, cylinder-shaped metal component that separates the two sides of the cylinder barrel internally. The piston is usually machined with grooves to fit elastomeric or 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.

Figure- 1.8

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.

1.4.5   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.

Figure-1.9

1.4.6    Other parts

• Cylinder bottom connection
• Seals
• Cushions

A 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. For this reason, the ideal connection of a 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.

Figure-1.10

•           How Stuff Works                     This commercial website provides no technical  

                                                              explanations of technical topics.

•           www.wikipedia.com                Working of single acting and double acting cylinder.

•           www.wikipedia.com                Working of  single acting and double acting pumps.

•           www.festo.com                        This site is a good free source for information about the

                                                              calculations and designing of cylinder & pumps.

•           www.wikipedia.com                 Piston seals.

•           www.google.com                      Hydraulic Fluid.

Books Considered :

•           Strength of materials                 By  Dr. Sadhu Singh, Art. 12.9 Beam Column, pg. 566.

•           Machine Dersign                       By  R.S.Khurmi, Art.4.3,4.4,4.9  pg.88.