Leakage from a hydraulic system can be a major problem leading to loss of efficiency, increased power usage, temperature rise, environmental damage, and safety hazard. Hydraulic seals must be used in order to prevent these losses and improve the efficiency of the system.
External leakage, on the other hand, is always serious. The principal cause of external leakage is improper installation or sealing failure.
Types of Hydraulic Seals
- Bases on sealing (Positive seals and Non-positive seals)
Positive seals do not allow any leakage whatsoever it may be (Internal or External) whereas non-positive seals permit a small amount of internal leakage.
- Based on applications (Static seals and Dynamic seals)
Static seals are used to seal two mating parts that have no relative motion between them. Whereas the dynamic seals are assembled between mating parts that move relative to each other. Hence dynamic seals are subjected to wear.
- Based on the shape of seals
These seals include O rings, Quad ring seals, V packing, Cup packing, U packing, etc.
O ring gives very effective sealing at high pressure.
O rings are usually molded from rubber compounds however they can be molded or machined from plastic materials also.
|O ring seal|
An O ring consists of an O ring mounted in the gland so that the O ring’s cross-section is compressed slightly when the gland is assembled.
Quad ring seals
This is a modified square type cross-section made up from homogeneous synthetic rubber.
|Cross Section of Quad Ring Seal|
The quad ring is quite versatile and can be used as a static hydraulic seal as well for rotary and reciprocating motion. It is shown in the figure.
|Dynamic & Static Application of Quad ring seal|
As opposed to an O ring, a quad ring seal has a more square cross-sectional shape with rounded corners.
The quad ring seal design offers more stability than the O ring design and practically eliminates the spiraling or twisting that is sometimes encountered with the O ring.
Fabricated V-ring packings are capable of holding almost all pressure. V-ring is the part of the packing that does the sealing. It has a cross-section resembling the letter V from which its name is derived.
To achieve a seal, the V ding must be installed as part of a packing set or stack, which includes one male adapter, one female adapter, and several V rings.
The male adapter is the first ring on the pressure end of the packing stack and is flat on one side and wedge-shaped on the other to contain the V of the adjacent V-ring.
The female adapter is the last ring of the stack.
U Ring Seal
The fabricated U packing is also used in various dynamic sealing solutions. U packing is always used with a support ring as shown.
U packings are balanced packings; it seals on both the outer diameter and inside diameter, and are applied individually, not in stacks like V rings.
Cup packings resemble a cup or deep dish with a hole in the center for mounting.
Cup seals are used exclusively to seal pistons in both low-pressure and high-pressure hydraulic and pneumatic applications.
They are produced from leather, homogeneous synthetic rubber, and fabric-reinforced synthetic rubber.
Although the cup packing lip flares outwards, the rubbing contact is made at the lip only when the fluid pressure is low.
As the fluid pressure increases, the cup heel expands outwards until it contacts the cylinder wall, at which point high-pressure sealing is in effect.
As the pressure loading shifts the sealing line to the cup heel, the lip is actually pulled into the cup and away from the cylinder wall.
On the return stroke when the pressure is relaxed, the heel will shrink slightly, leaving only the lip in contact with the wall, avoiding unnecessary wear at the heel.
For reciprocating pistons, two cups installed back to back in separate glands are required.
These types of hydraulic seals are a combination of various types and are available in the form of sets. They are designed for special applications. A typical minimum friction seal set commonly used for cylinders is shown in the figure.
It consists of five rings one central rubber ring and four adjacent plastic rings. The complete set when assembled acts as a single dynamic seal.