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DESIGN OF SHAFTS AND HOUSINGS

11.3.2 Contact Type Seals seals are installed. It is also desirable for the lubricant (2) Felt Seals for grease lubrication, primarily to prevent dust and
inside the housing to spread a little between the sliding other foreign matter from entering. Felt seals are not
The effectiveness of contact seals is achieved by the surfaces. However, please be aware that ester-based Felt seals are one of the simplest and most common suitable for circumferential surface speeds exceeding
physical contact between the shaft and seal, which grease will cause acrylic rubber material to swell. Also, seals being used for transmission shafts, etc. 4 m/sec; therefore, it is preferable to replace them with
may be made of synthetic rubber, synthetic resin, low aniline point mineral oil, silicone-based grease,and However, since oil permeation and leakage are synthetic rubber seals depending on the application.
felt, etc. Oil seals with synthetic rubber lips are most silicon-based oil will cause silicone-based material to unavoidable if oil is used, this type of seal is used only
frequently used. swell. Moreover, urea-based grease will cause fluorine-
based material to deteriorate. 12. LUBRICATION
(1) Oil Seals The permissible circumferential speed for oil seals
varies depending on the type, the finish of the shaft 12.1 Purposes of Lubrication 12.2 Lubricating Methods
Many types of oil seals are used to prevent lubricant surface, liquid to be sealed, temperature, shaft
from leaking out as well as to prevent dust, water, and eccentricity, etc. The temperature range for oil The main purposes of lubrication are to reduce friction The various lubricating methods are first divided
other foreign matter from entering (Figs. 11.8 and seals is restricted by the lip material. Approximate and wear inside the bearings that may cause premature into either grease or oil lubrication. Satisfactory
11.9) circumferential surface speeds and temperature failure. The effects of lubrication may be briefly bearing performance can be achieved by adopting
In Japan, such oil seals are standardized (Refer to JIS permitted under favorable conditions are listed in Table explained as follows: the lubricating method which is most suitable for the
B 2402) on the basis of type and size. Since many 11.6. particular application and operating condition.
oil seals are equipped with circumferential springs to When oil seals are used at high circumferential surface (1) Reduction of Friction and Wear In general, oil offers superior lubrication; however,
maintain adequate contact force, oil seals can follow speed or under high internal pressure, the contact Direct metallic contact between the bearing rings, grease lubrication allows a simpler structure around the
the non-uniform rotational movement of a shaft to surface of the shaft must be smoothly finished and the rolling elements and cage, which are the basic bearings. A comparison of grease and oil lubrication is
some degree. shaft eccentricity should be less than 0.02 to 0.05 mm. components of a bearing, is prevented by an oil film given in Table 12.1.
Seal lip materials are usually synthetic rubber including The hardness of the shaft's contact surface should be which reduces the friction and wear in the contact
nitrile, acrylate, silicone, and fluorine. Tetrafluoride made higher than HRC40 by means of heat treatment areas. Table 12. 1 Comparison of Grease and Oil Lubrication
ethylene is also used. The maximum allowable or hard chrome plating in order to gain abrasion (2) Extension of Fatigue Life
operating temperature for each material increases in resistance. If possible, a hardness of more than HRC The rolling fatigue life of bearings depends greatly upon Item Grease Lubrication Oil Lubrication
this same order. 55 is recommended. the viscosity and film thickness between the rolling
Synthetic rubber oil seals may cause trouble such as The approximate level of contact surface finish required contact surfaces. A heavy film thickness prolongs the Housing Structure and Simple May be complex, Careful
overheating, wear, and seizure, unless there is an oil for several shaft circumferential surface speeds is fatigue life, but it is shortened if the viscosity of the oil Sealing Method maintenance required.
film between the seal lip and shaft. Therefore, some given in Table 11.7. is too low so the film thickness is insufficient.
lubricant should be applied to the seal lip when the (3) Dissipation of Frictional Heat and Cooling Speed Limiting speed is 65% Higher limiting speed.
Circulation lubrication may be used to carry away to 80% of that with oil
frictional heat or heat transferred from the outside to lubrication.
prevent the bearing from overheating and the oil from
deteriorating. Cooling Effect Poor Heat transter is possible
(4) Others using forced oil
Adequate lubrication also helps to prevent foreign circulation.
material from entering the bearings and guards against
corrosion or rusting. Fluidity Poor Good

Full Lubricant Sometimes difficult Easy
Replacement
Table 11. 6 Permissible Circumferential Surface Speeds
and Temperature Range for Oil Seals Removal of Foreign Removal of particles Easy
Matter from grese is
impossible.
Permissible Operating
Seal Materials Circumferential Temperature External Surroundings seldom Often leaks without
Nitrile Rubber Speeds(m/sec) Range( °C)(1) Contamination due to contaminated by leakage. proper countermeasures.
Leakage
Under 16 – 25 to +100 Not suitable if external
contamination must be
avoided.

Fig. 11.8 Example of Application Synthetic Acrylic Rubber Under 25 – 15 to +130 12.2.1 Grease Lubrication
Fig. 11 .8 of Oil Seal (1) Rubber Silicone Rubber Under 32 – 70 to +200
Under 32 – 30 to +200 (1) Grease Quantity
Fluorine- Under 15 – 50 to +220 The quantity of grease to be packed in a housing
containes Rubber depends on the housing design and free space,
grease characteristics, and ambient temperature. For
Tetrafluoride Ethylene Resin example, the bearings for the main shafts of machine
tools, where the accuracy may be impaired by a small
Note (1) The upper limit of the temperature range may be temperature rise, require only a small amount of
raised about 20 °C for operation for short intervais. grease. The quantity of grease for ordinary bearings is
determined as follows.
Table 11. 7 Shaft Circumferential Sufficient grease must be packed inside the bearing
Surface Speeds and Finish including the cage guide face. The available space
of Contact Surfaces inside the housing to be packed with grease depends
on the speed as follows:
Circumferential Surface Surface Finish 1/2 to 2/3 of the space … When the speed is less than

Speeds(m/s) Ra (µm) 50% of the limiting speed.
1/3 to 1/2 of the space … When the speed is more than
Fig. 11.9 Example of Application Under 5 0.8
Fig. 1 1.9 of Oil Seal (2) 5 to 10 0.4 50% of the limiting speed.
Over 10 0.2

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