Factors Affect Sealing
- Factors affect sealing
Application DesignApplication design has a direct relationship on seal performance. To select the right oil seals for sealing application is necessarily determined by the operating conditions under which the seal must function. Incorrect application design failed in sealing.
Medium To Be SealedChemical effects of media being sealing have a determining influence, particularly as operating temperature increase. The sealing compound may either harden or soften depending upon the medium's influence. Hardening results from:Aging phenomena, especially at elevated temperature, caused by media to be sealed. Softening results from :Swelling due to media to be sealed.
TemperatureThe temperature directly at the sealing edge has a marked influence on the life of a seal , In every case this temperature depends on the heat dissipation conditions at the location where the seal is installed. The excess temperature at sealing edge rises with increasing shaft revolution. Material used for sealing has its operation temperature limit. In choosing the correct sealing material is an important consideration.
Shaft FinishSince the sealing lip contact directly to the shaft, the shaft finish is important for proper seal performance. The shaft finish should be 10-25 micro-inches AA with no machine lead. This shaft finish can be obtained by plunge grinding. Machine lead (helical scores) causes abrasion to sealing lip and possibly act as a pump that give rise to leakage.
Shaft Surface HardnessSealing life depends on the contact surface hardness of the shaft. Surface hardness should be at least Rockwell "C" 30. There is little indication that hardening above this value will improve the wear resistance of the shaft. When contaminated media or dirt may enter from the outside or when surface speed exceed 15 fps or in situations where the shaft can be damaged (nicked, scratched, etc ) prior to or during assembly, minimum hardness must be Rockwill "C" 45. This will help protect the shaft from possible damage, thus maintaining a flaw-free surface for the seal to run on. Surface hardness should penetrate to a depth of at least 0.3mm. Where shafts can not be sufficiently hardened, a wear sleeve may solve the problem.
Shaft EccentricitySeal performance can be impaired by two types of eccentricity:
1. Shaft to bore misalignment:
Shaft to bore misalignment is the distance that the center of shaft rotation is from the center of the bore. Misalignment cause uneven wear of the sealing lip resulting in a shorter life.
2. Dynamic runout:
Dynamic runout is twice the distance the enter of the shaft displaced from the actual center of rotation. Runout usually is caused by misalignment, shaft imbalance, or actual flexing of the shaft in the application. Shaft runout should be avoid if at all possible or should be kept at minimum limits. Greater runout may cause the sealing lip to lift at high RPM, due to inertia, causing media leakage. The shaft seal must be located next to the bearing and bearing play must be minmized. A spring loaded seal with and adequate flex section will operate satisfactorily if the total eccentricities do not exceed the limits.