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External sealing arrangement

External seals also have two main functions: to prevent lubricating oil from leaking out, and to prevent dust, water and other contaminants from entering the bearing. Similar to internal bearing seals, the external bearing sealing devices also divided into two types, non-contact type and contact type.

Non-contact seals: Non-contact radial shaft seals function by virtue
of the sealing effect of a narrow, relatively long gap which can
be arranged axially, radially or in combination. Non-contact seals,
which range from simple gap-type seals to multistage labyrinths
are practically without friction and do not wear, making them suitable
for high speed applications. In order to improve sealing
capability, clearance spaces are often filled with lubricant.
Contact seals:Seals in contact are used to seal passages between
machine components. These dynamic seals accomplish their sealing
action through the contact pressure of a resilient part of the seal
(the lip is often made of synthetic rubber) and the sealing surface.
These contact seals are generally far superior to non-contact seals
in sealing efficiency, although their friction torque and temperature
rise coefficients are higher.

The following chart lists the special characteristics of seals and other points to be considered when choosing an appropriate seal.

Type Seal construction Name Seal characteristics Selection considerations
Non-
contact
seals 		Clearance seal This is an extremely simple seal design with a small radial clearance. Cautionary points regarding selection
●In order to improve sealing efficiency, clearances between the shaft and housing should be minimized. However, care should be taken to confirm shaft/bearing rigidity and other factors to avoid direct shaft-housing contact during operation.
Oil groove seal
(oil grooves on housing side) 		Several concentric oil grooves are provided on the housing inner diameter to greatly improve the sealing effect.

Oil groove clearance (reference)

Shaft diameter mm Clearance mm
Up to 50
50 or above 		0.2 ~ 0.4
0.5 ~ 1.0
Oil groove seal
(oil grooves on shaft and housing
side) 		Oil grooves are provided on both the shaft outer diameter and housing inner diameter for a seal with even greater sealing efficiency. ●Oil groove width, depth (reference)
width : 2 5 mm
depth : 4 5 mm
●Three or more oil grooves should be provided.
●Sealing efficiency can be further improved by filling the oil groove portion with grease of which the viscosity grade is 150 to 200.
Axial labyrinth
seal 		This seal has a labyrinth passageway on the axial side of the housing.

Labyrinth clearance (reference)

Shaft
diameter
mm 		Clearance mm
Radial direction Axial direction
Up to 50
50 200 		0.2 ~ 0.4
0.5 ~ 1.0 		1.0 ~ 2.0
3.0 ~ 5.0
Radial
labyrinth seal 		A labyrinth passageway is affixed to the radial side of the housing. For use with split housings. This offers better sealing efficiency than axial labyrinth seals. ●Grease is generally used as the lubricant for labyrinth seals,
●Sealing efficiency can be further improved by filling the labyrinth passageway with grease of which the viscosity grade is 150 to 200.
●Labyrinth seals are suitable for high speed applications.
Internal slinger By providing a slinger inside the housing, centrifugal force guides the lubricant flow back on the bearing and helps prevent it from dirtying the work environment.

Cautionary points regarding selection

●By installation on the revolving shaft, these seal types make use of centrifugal force to aid lubrication, seal in lubricant and prevent the entrance of contaminants.
●Installation of a slinger inside the outside of the housing further enhances the sealing in of lubricants.
●Installation of a slinger slinger on the outside of the housing will provide greater protection against dust and other bearing contaminants.

External slinger By mounting a slinger on the outside of the housing, centrifugal force helps to prevent dust and other solid contaminants from entering.
Contact
seals 		Z grease seal In cross section resembling the letter "Z," this seal's empty spaces are filled with grease. The seal is commonly used with a plummer block (bearing housing).

Shaft surface roughness (reference )

Peripheral
speed m/s 		Surface roughness
Ra Rmax
Up to 50
5 10
10 		0.8a
0.4a
0.2a 		3.2s
1.6s
0.8s

Shaft material (reference )

Material Machine structural carbon steel Low carbon alloy steel Stainless steel
Surface
hardness 		HRC 40 or higher necessary HRC 55 or higher advisable
Processing
mothod 		Final grinding without repeat (moving), or buffed after hard chrome plating

Cautionary points regarding selection
When the oil seal and the bearing are in very close proximity, internal bearing clearances are sometimes too small to accommodate the additional heat generated by friction between the seal and shaft. In addition to considering the heat generated by contact seals at various peripheral speeds, internal bearing clearances must also be selected with caution.

V-ring seal This design enhances sealing efficiency with a lip that seals from the axial direction. With the aid of centrifugal force, this seal also offers effective protection against dust, water, and other contaminants entering the bearing. Grease can be used on both sides of the seal.

Oil seal

Oil seals are widely used, and their shapes and dimensions are standardized under JIS B 2402. In this design, a ring-shaped spring is installed in the lip section. As a result, optimal contact pressure is exerted between the lip edge and shaft surface, and sealing efficiency is good.
  Depending upon the direction in which the lip faces (in toward the bearing or away from the bearing) contact oil seals are very effective at preventing lubricant leakage from the housing or contaminants from infiltrating the bearing.

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