The friction between the moving and stationary rings of the mechanical seal will occur according to different conditions:
(1) Dry friction:
No liquid enters on the sliding friction surface, so no liquid film exists, only dust and oxide layers, and adsorbed gas molecules. When the moving and stationary rings are running, the result is necessarily that the friction surface is heated and the wear is aggravated, resulting in leakage.
(2) Boundary lubrication:
When the pressure between the moving and stationary rings increases or the ability of the liquid to form a liquid film on the friction surface is poor, the liquid will be squeezed out of the gap. Since the surface is not absolutely flat, but uneven, there is contact wear in the bulge, while in the recess the liquid lubrication is maintained, resulting in boundary lubrication. The degree of wear and heat generation of the boundary lubrication is medium.
(3) Semi-liquid lubrication:
There is liquid in the pits on the sliding surface, and a thin liquid film is maintained between the contact surfaces, so the heat and wear are good, because the liquid film between the moving and stationary rings has surface tension at the exit. Therefore, the leakage of the liquid is restricted.
(4) Complete liquid lubrication:
When the pressure between the moving and stationary rings is insufficient, and the gap is increased, the liquid film is thickened, and there is no solid contact at this time, so there is no friction phenomenon. However, since the gap between the moving and stationary rings is large in this case, the sealing effect is not obtained and the leakage is serious. This is generally not allowed in practical applications (except for controlled membrane mechanical seals).
Mechanical seals work between static and static rings, most of which are in boundary lubrication and semi-liquid lubrication, while semi-liquid lubrication can achieve the best when the friction coefficient is the smallest, that is, the wear and heat are satisfactory. Sealing effect.
To make the mechanical seal work under good lubrication conditions, it also needs to consider the characteristics of the medium, pressure, temperature, sliding speed and other factors, but choose the appropriate pressure between the moving and static rings, reasonable lubrication structure and improve the movement, The static ring friction surface quality is also an important factor to ensure the effective operation of the seal.
Several types of lubrication-enhancing structures
1. End face eccentricity:
Generally, the mechanical seal, the center of the moving ring, the center of the stationary ring, and the center line of the shaft are all in a straight line. If the center of the end face of one of the moving ring or the stationary ring is offset from the center line of the shaft by a certain distance , the ring can be continuously brought into the lubricating fluid to rotate between the sliding surfaces to rotate.
It should be noted that the eccentric size should not be too large, especially for high pressures, the eccentricity will cause the pressure on the end faces to be uneven, resulting in uneven wear. For high-speed sealing, it is not advisable to use the moving ring as the eccentric ring, otherwise the machine will vibrate due to the balance of centrifugal force.
2. End groove:
It is difficult for high-pressure and high-speed machines to maintain a liquid film between the friction surfaces, and the liquid film is often destroyed by the frictional heat generated by high pressure and high rotational speed. In this case, it is very effective to take the form of slotting to strengthen the lubrication. Both the moving ring and the stationary ring can be slotted, typically on a more wear resistant material. The moving ring and the stationary ring should not be grooved at the same time, as this will reduce the lubrication effect. In order to prevent the dirt or the grinding debris from entering the friction surface as much as possible, the liquid should be sealed in the direction of the centrifugal force (outflow type), and the groove should be opened on the static ring to prevent the centrifugal force from introducing the dirt into the friction surface. Conversely, when the liquid flows in the direction of the reverse centrifugal force (inflow), the groove should be opened on the moving ring, and the centrifugal force helps to remove the dirt from the groove.
The small grooves of the friction surface have a rectangular shape, a wedge shape, or other shapes. The tank should not be too much or too deep, otherwise it will increase the leakage.
3. Static pressure lubrication:
The so-called static pressure lubrication is to directly introduce the pressurized lubricating liquid into the friction surface to play the role of lubrication. The introduced lubricating fluid is supplied by a separate liquid source such as a hydraulic pump. This pressurized lubricant is counteracted by the fluid pressure within the machine, which is also commonly referred to as hydrostatic pressure seals.
The mechanical seal of the gas medium should take measures to establish the lubrication of the film. If the gas static pressure controlled membrane mechanical seal or solid lubrication is used, the self-lubricating material is used as the moving ring or the static ring. Whenever possible, the gas medium conditions should be converted to liquid medium conditions as much as possible, which is both easy to lubricate and easy to seal.
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