term "hydraulic seals" actually describes a class of
seals that are used in applications with either rotary or reciprocating
motions. Hydraulic seals are exposed to hydraulic fluids such
as hydrocarbon and phosphate ester and are designed for high-pressure
dynamic applications such as hydraulic cylinders. Hydraulic seals
usually need to be higher friction seals than pneumatic seals
but often operate under lower operating speeds.
seals, piston seals, u-cups, vee-cups and flange packings are
just some of the sealing designs that can be used as a hydraulic
seal. Sometimes a composite seals are used as hydraulic seals.
A composite seal is a product which has two or three materials
manufactured into one seal. Often there will be an elastomer ring
and a PTFE ring giving the seal the advantages of both materials.
sealing orientation for hydraulic seals can include internal (rod
seal), external (piston seal), symmetrical or axial seal. With
the internal (rod) seal, the seal is fit into a housing bore and
the sealing lip touches the shaft. With the external (piston)
seal, the seal is placed onto a shaft and the sealing lip touches
the housing bore. Since symmetric seals are the same on both sides
they will function as either a rod or a piston seal. The axial
seal fits axially against the housing.
applications need only one hydraulic seal (single acting) because
it can seal in the one axial direction the application is moving.
However, a reciprocating application will need two hydraulic seals
(double acting), one for each of the directions.
life of hydraulic seals is dependent on many factors including
the maximum operating speed, maximum operating temperature, maximum
operating pressure and the vacuum rating. When ordering hydraulic
seals you will want to know the shaft outer diameter or seal inner
diameter, housing bore diameter or seal outer diameter, the axial
cross section and the radial cross section.
problems come from:
If pressures become too high then extrusion can take place in
the gap between the cylinder and piston, or the gland and the
rod. You will want to have an elastomer that is resistant to extrusion
in high pressure conditions. Anti-extrusion rings and guide rings
can be utilized as part of the configuration to help the elastomer
adhere to the metal surfaces in high pressure conditions.
As the temperature increases the fluid viscosity will decrease
and the effect will be poor lubrication and an increase in the
damage done by friction. This reduces the life of hydraulic seals.
It is important to chose an elastomer that can withstand the temperature
in the application.
When you have a rigid surface and a flexible material and they
are sliding back and forth against each other in the way they
do in hydraulic seals, the elastomer can be damaged by friction
in a very short amount of time. The design needs to take into
account the nature of the elastomer and smoothness of the surface
finish of the rigid material.
Speed is another important consideration. Obviously, the higher
the speed the more potential damage to the seal.