Lubrication of mechanical seal
Mechanical seals working in liquid medium generally rely on the liquid film formed by the liquid medium between the dynamic and static ring friction surfaces for lubrication. Therefore, maintaining the liquid film between the friction surfaces is essential to ensure the stable operation of the mechanical seal and extend the service life.
According to different conditions, the friction between the dynamic and static rings of the mechanical seal will be as follows:
(1) Dry friction:
No liquid enters on the sliding friction surface, so no liquid film exists, only dust and oxide layer, and adsorbed gas molecules. When the dynamic and static rings are running, the result is necessarily to cause the friction surface to heat up and aggravate and cause leakage.
(2) Boundary lubrication:
When the pressure between the moving and static rings increases or the liquid has a poor ability to form a liquid film on the friction surface, the liquid will be squeezed out of the gap. Because the surface is not absolutely flat, but is uneven, there is contact wear when the protrusion is raised, and the lubricating performance of the liquid is maintained in the concave portion, resulting in boundary lubrication. Boundary lubrication has moderate wear and heat generation.
(3) Semi-liquid lubrication:
There is liquid in the pit on the sliding surface, and a thin liquid film is maintained between the contact surfaces, so the heat generation and wear are better. Because the liquid film between the dynamic and static rings has surface tension at its outlet, Therefore, the leakage of liquid is limited.
(4) Complete liquid lubrication:
When the pressure between the dynamic and static rings is insufficient, and the gap is increased, the liquid film is thickened. At this time, there is no solid contact, so there is no friction. However, in this case, the gap between the dynamic ring and the static ring is large, so the sealing effect cannot be achieved and the leakage is serious. In practical applications, this situation is generally not allowed (except for controlled membrane mechanical seals).
The working conditions between the dynamic and static rings of the mechanical seal are mostly in the state of boundary lubrication and semi-liquid lubrication, and semi-liquid lubrication can obtain the best under the condition of minimum friction coefficient, that is, satisfactory wear and heat generation. Sealing effect.
In order to make the mechanical seal work under good lubrication conditions, it is necessary to comprehensively consider factors such as medium characteristics, pressure, temperature, sliding speed, etc., but select the appropriate pressure between the dynamic and static rings, the reasonable lubrication structure and improve the dynamic, The quality of the static ring friction surface is also an important factor to ensure the effective operation of the seal.
Several enhanced lubrication structures
1. End face eccentricity:
In general mechanical seals, the center of the moving ring, the center of the stationary ring and the center line of the shaft are all on a straight line. , It can make the ring constantly bring lubricating fluid to the sliding surface for lubrication when rotating.
It should be noted that the size of the eccentricity should not be too large, especially for high pressure, the eccentricity will make the pressure on the end face uneven and cause uneven wear. For high-speed seals, the moving ring should not be used as an eccentric ring, otherwise the machine will vibrate due to the balance of centrifugal force.
2. Slotted end face:
It is difficult for high-pressure and high-speed machines to maintain the liquid film between the friction surfaces. The liquid film is often destroyed by the friction heat generated by high-pressure and high-speed machines. In this case, it is very effective to adopt the slotted form to strengthen lubrication. Both the moving ring and the static ring can be slotted, generally on more wear-resistant materials. The moving ring and static ring should not be grooved at the same time, because this will reduce the lubrication effect. In order to prevent dirt or abrasive debris from entering the friction surface as much as possible, the liquid should be sealed in the direction of centrifugal force (outflow). The groove should be opened on the static ring to prevent the centrifugal force from introducing dirt into the friction surface. Conversely, when the liquid flows in the reverse centrifugal force direction (inward flow), the groove should be opened on the moving ring. The centrifugal force helps to throw dirt out of the groove.
The small grooves on the friction surface are rectangular, wedge-shaped, or other shapes. The groove should not be too much or too deep, otherwise leakage will increase.
3. Static pressure lubrication:
The so-called static pressure lubrication is to introduce pressure lubricating liquid directly into the friction surface to play the role of lubrication. The introduced lubricating liquid is supplied by a separate liquid source, such as a hydraulic pump. This pressured lubricating fluid is resistant to the fluid pressure in the machine. This form is also commonly referred to as a hydrostatic pressure seal.
The mechanical seal of the gas medium should take measures to establish gas film lubrication, such as the use of gas static pressure controlled membrane mechanical seal or solid lubrication, that is, the self-lubricating material is used as the moving ring or static ring. As long as the conditions permit, the gas medium conditions should be converted to liquid medium conditions as much as possible, so that it is easy to lubricate and seal.
