The shaft surface finish is of primary importance for efficient sealing and achieving adequate seal life. The shaft hardness should increase with increasing peripheral speed. A peripheral speed of 4 m/s, the hardness should be minimum 45 HRC and at 10 m/s 55 HRC. Recommended minimum hardness depth is 0.3 mm if shafts are not fully hardened.
Inadequate shaft finish or grooved, pitted surfaces generate higher friction, hence higher temperatures reducing seal life or overall seal-ability. Machining defects, scratches or existing groove marks on the shaft must be avoided. The shaft tolerance and run-out allowance must be in compliance to achieve proper sealing.
Even very small defects could be sufficient to increase the film thickness, eventually rupturing the meniscus and causing leakage. It is also important to avoid spiral grinding or groove marks. This could cause a pumping effect or a dry run condition resulting in leakage or failure.
Below are the DIN 3760 standards for shaft tolerance.
|SHAFT TOLERANCE INCH (in)||SHAFT TOLERANCE IN METRIC (mm)|
|Shaft Diameter||Shaft Tolerance||Shaft Diameter||Shaft Tolerance|
|Up to 4.000||+/-0.003||Over 0 to 3||0/-0.06|
|4.001 to 6.000||+/-0.004||Over 3.1 to 6||0/-0.075|
|6.001 to 10.000||+/-0.005||Over 6.1 to 10||0/-0.09|
|10.001+||+/-0.006||Over 10.1 to 18||0/-0.110|
|Over 18.1 to 30||0/-0.130|
|Over 30.1 to 50||0/-0.160|
|Over 50.1 to 80||0/-0.190|
|Over 80.1 to 120||0/-0.220|
|Over 120.1 to 180||0/-0.250|
|Over 180.1 to 250||0/-0.290|
|Over 250.1 to 315||0/-0.320|
|Over 315.1 to 400||0/-0.360|
|Over 400.1 to 500||0/-0.400|