Seals are classified into static and dynamic seals.  A static seal is installed between two components that are connected rigidly (without movement).  A dynamic seal is used to prevent leakages between hydraulic components that move relatively against each other.  Therefore, dynamic seals are influenced by frictions, while static seals are free from frictions.

A seal is defined as a piece of material used for preventing the decrease of a fluid flow between two surfaces.

A seal functions to:

  • Prevent lubricating oil from leaking.
  • Protect the hydraulic system from dirt.
  • Separate fluids such as lubricating oil and water.
  • Prevent leakages when there are vibrations or frictions between components.
  • Prevent the components that are more expensive than the seal itself from wearing.

In general, there are two types of seal:

  • Static Seal
  • Dynamic Seal

The above figure shows some examples of static seal: O-Ring, gasket, or liquid gasket material.

The above figure shows some examples of static seal, including packing ring , duo-cone seal dan lip-type seal.


Static Seals

Static seals are free from any friction between two surfaces of hydraulic components.  They are used to seal two stationary surfaces.  When a seal is compressing, the coarse parts of the surface are filled with the seal in order to prevent leakages.  A seal must be used in accordance with its application.  If a seal requires a bolt, tighten the bolt to a proper torque in order to reach a proper sealing.

Some examples of correct applications of seals:

  • Between the cylinder and the block by using a gasket
  • Between the block and the oil by using a gasket
  • Between the hose coupling and the hydraulic cylinder by using an O-ring.

Dynamic Seals:

Dynamic seals are used between two surfaces that move against each another.  A dynamic seal has the same function as a static seal on small components, but they work at a very different pressure.  Wears can result from high pressure, frictions, dirt, vibrations, and insufficient lubricating oil.  Dynamic seals are used in many applications, such as in hydraulic cylinders.

One important reason why you should study seals is to prevent possible damages to them that can cause:

  • Fire
  • Damages to engine components
  • Oil leakages
  • Lost time.

O-Ring Seals

An O-Ring seal takes the form of a ring with smooth surface and is made of natural rubber, synthetic materials, rubber or plastic.

Such materials are cheap, easy to install, and usable for many kinds of application.

One important thing to remember is that you cannot use color codes to identify O-ring seals.  An O-ring seal has its part number printed on the ring.  Use the part number to get the correct seal.

The purpose of an O-ring seal is to prevent the movement of gas or liquid between two parts by changing its shape and compression on pressure.  An O-ring seal is classified as compression seal.

Method of installing an O-ring seal on a cylinder

It is important to install an O-ring seal in a correct manner in order to completely prevent leakages.  Follow the following procedure for installing an O-ring seal.

  • Make sure you have got an O-ring seal with the correct part number. Some types of seal will be damaged if they are used for sealing a high temperature fluid.

  • Clean the surfaces of the O-ring seal and the component properly before installation.  The surfaces must be free from dust or dirt

  • Make sure that the O-ring seal is completely packed before installing it

  • Make sure that the O-ring seal is free from dirt, scratch, or any other defect before installation. Any O-ring seal with dirt, scratch or defect should not be used.

  • For easy installation, it is necessary to apply lubricating oil on the surface that will be sealed, not on the the surface of the O-ring seal itself.

  • If the O-ring seal is to be used in a dynamic application such as on cylinder, it must be installed accurately and properly. The end of the cylinder or the seal groove can cause damages to the smooth surface of the O-ring.

  • Make sure that the backup ring is installed correctly.  This method can be read on the manual provided.

  • Backup ring must be installed at the side of the O-Ring opposing the pressure.  Some applications require two backup rings, to be installed on each side of the O-ring seal.  This type is used in case there are extreme pressure variations on each side of the O-ring.

  • Do not cut or distort the O-ring when installing it. If you need to press the O-ring seal, use a tap or seal installation equipment (collar) that will control the seal without damaging it.

  • Remove the twist during the installation of the O-ring seal as the twist can cause leakages.

  • Do not pull the O-ring seal too far during installation.  Pulling it too far can cause leakages.  After the O-ring is installed, check the position.  The seal must be installed tightly.

  1. Do not pull the O-ring too far to avoid the seal from expanding
  2. This seal is installed incorrectly. Incorrect installation can cause the seal to expand.

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