Oil Seals (Part 2): How to select the right oil seal

In Part 1, we explained the structure, functions, and types of oil seals.

You can find more information on our web, so please take a look.

Oil Seals (Part 1): The structure, functions, and types of oil seals

Oil seals come in various shapes to fit the machines and substances to be sealed.
For this reason, when designing a machine, it is important to select the oil seal that is right for that machine.

That's where this column comes in.
We will explain the key points for selecting the oil seal that is right for your machine.

1. Criteria for selecting oil seals

Oil seals come in a wide range of types, and they also have various sizes.
When selecting the right oil seal for your machine from among these many varied types of oil seals, the following two criteria are very important.

• Criterion 1: It should be appropriate for the machine's usage environment and the operating condition that is being demanded of the oil seal
• Criterion 2: It should be easy to acquire replacement oil seals and it should facilitate maintenance/inspection of the machine

If these criteria are met, damage of the machine can be reduced, the time needed to replace the oil seals when performing repairs can be shortened, and the machine can be used for a longer period of time.

In this way, selecting the appropriate oil seal will lead to machine design that is economically superior!

2. How to select the right oil seal

In general, oil seals should be selected in the order of priority indicated in Table 1.

However, when you actually select the oil seal to use, the most important factors are past success history and points of improvement, so it is not necessary to follow this order to the letter.

Table 1: The order of priority for selecting oil seals

No. Examination item 1 Seal type 2 Rubber material 3 Metal case and spring material

1) Seal type

Select your oil seal type according to Table 2.

Table 2: How to select the seal type

No. Examination item Flowcharts 1 O.D. (outside diameter) wall material Figure 1 2 Necessity of spring Figure 2 3 Lip type Figure 3

Figure 1: O.D. (outside diameter) wall material

Figure 2: Necessity of spring

Figure 3: Lip type

 

＜Seal selection example＞
Based on the above flowcharts, the oil seal type that meets the requirements shown in Table 3 would be the type code MHSA or HMSA shown in Table 4.

Table 3: Requirements

No. Requirements 1 Housing Made of steel, one solid design, housing bore surface roughness 1.8 μmRa 2 Substance to be sealed Grease 3 Pressure Atmospheric 4

Shaft surface speed

(peripheral speed)

6 m/s 5 Air-side condition Dusty

Table 4: Type of selected seal

Type 1 Type 2 O.D. wall material Rubber O.D. wall Metal O.D. wall Necessity of spring Spring required Spring required Lip shape Minor lip required Minor lip required
Type (type code)

For a more detailed discussion of seal types and type codes, please see the following:

2) Rubber material

The rubber material used in the oil seal should be selected based on the operational temperature and substance to be sealed.
Table 5 lists the major rubber materials along with their operational temperature ranges.
Note that it is necessary to check the compatibility with fluids.
＜N.B.＞
Extreme pressure additives are compounds added to the lubricant. They are activated by heat and chemically react against rubber, which deteriorates rubber properties. For this reason, it is necessary to check for compatibility with rubber materials.

Table 5: Major rubber materials and their operational temperature ranges

Rubber material
(ASTM*1 code) Grade Features Operational temperature range (°C) Compatibility with fluids

Nitrile rubber (NBR)

Standard type

Well-balanced in terms of resistance to abrasion and high and low temperatures

-30～

100

Necessary to check compatibility with fluids
(See *2)

Fluids
• Fuel oil
• Lubricating oil
• Hydraulic fluid
• Grease
• Chemicals
• Water

High- and low-temperature-resistant type Highly resistant to both high and low temperatures -40～

110

Hydrogenated nitrile rubber (HNBR)

Standard type

Compared with nitrile rubber, superior in resistance to heat and abrasion

-30～

140

Acrylic rubber (ACM)

Standard type High oil resistance and good abrasion resistance -20～

150

High- and low-temperature-resistant type Improved low temperature resistance and same level of heat resistance as the standard type -30～

150

Silicone rubber (VMQ)

Standard type Wide operational temperature range and good abrasion resistance -50～

170

Fluoro rubber (FKM)

Standard type The most superior in resistance to heat, and good abrasion resistance -20～

180

Notes
*1 ASTM: American Society for Testing and Materials
*2 For more details on fluid compatibility, please see the following:

Rubber materials, operational temperature ranges and their compatibility with fluids

3) Metal case and spring material

The metal case and spring material used in the oil seal should be selected based on the substance to be sealed.
Table 6 shows how to select the metal case and spring materials.

Table 6: Selection of metal case and spring materials

Substance to be sealed Material Metal case Spring

Cold rolled carbon steel sheet
(JIS* SPCC)

Stainless steel sheet
(JIS* SUS304)

High carbon steel wire
(JIS* SWB)

Stainless steel wire
(JIS* SUS304) Oil ○ ― ○ ― Grease ○ ― ○ ― Water × ○ × ○ Seawater × × ○ Water vapor × ○ × ○ Chemicals × ○ × ○ Organic solvent ○ ○ ○ ○

Notes
* JIS: Japanese Industrial Standard
✓: Compatible
✗: Incompatible
―: Not applicable

 
3. Shaft and housing design

Oil seals can show good sealing performance in combination with properly designed shafts and housings.

1) Shaft design

Table 7 shows the shaft design checklist.

Table 7: Shaft design checklist

No. Examination item Major points to confirm Remarks 1 Material Use one of the carbon steels for machine structural use, low-alloy steel, or stainless steel. Soft materials (brass and so on) are not suitable. 2 Hardness Shaft hardness should be at least 30 HRC. In usage conditions where wear can occur easily because of dust or contaminated oil, hardness should be 50-60 HRC. 3 Shaft diameter tolerance This should be h8 (seals are designed to suit shafts with a tolerance of h8). 4 Shaft end chamfer "Provide a chamfer on the shaft end.
(This prevents failure during mounting.)" See Figure 4. 5 Surface roughness and finishing The shaft surface to be in contact with the lip should be finished to
0.1 to 0.32 μmRa and 0.8 to 2.5 μmRz
and the lead angle to no greater than 0.05°. (There is a risk that the lead marks will impede the sealing performance of the oil seal: see Figure 5.)

Nominal shaft diameter
d1, mm d1-d2, mm を超え 以下 ― 10 1.5 min. 10 20 2.0 min. 20 30 2.5 min.

Figure 4: Shaft end chamfer

a) Good finished surface
(no lead marks) b) Undesirable finished surface
(visible lead marks)

Figure 5: Shaft surface with and without lead marks

2) Housing design

Table 8 shows the housing design checklist.

Table 8: Housing design checklist

No. Examination item Major points to confirm Remarks Material Steel or cast iron is generally used as the housing material.
Aluminum alloys and resin (materials with a large difference between the linear expansion coefficients) demand sufficient consideration (as there is a risk of failure due to the increased clearance with the oil seal at high temperatures). 2 Bore diameter tolerance 1. If the nominal bore diameter is 400 mm or less:
H7 or H8
2. If the nominal bore diameter exceeds 400 mm:
H7 3 Bore inlet chamfer Provide an appropriate chamfer with rounded corners.
(This facilitates mounting.) See Figure 6. 4 Shoulder diameter
(if the housing bore has a shoulder) Set appropriate shoulder diameter. See Figure 7. 5 Bore surface roughness 1. For metal O.D. wall type:
0.4 to 1.6 μmRa,
1.6 to 6.3 μmRz
2. For rubber O.D. wall type:
1.6 to 3.2 μmRa,
6.3 to 12.5 μmRz
(Firmly affixes the oil seal and prevents leakage through the seal O.D.)

If you are looking for more details, kindly visit NFJ.

Nominal seal width
b, mm

B1 min.
mm L
mm Over Up to ― 10 b + 0.5 1.0 10 18 1.5 18 50 b + 0.8

Figure 6: Recommended housing bore chamfers (shouldered bore)

Nominal seal O.D.
D, mm

F

mm Over Up to ― 10 D - 4 10 18 D - 6 18 50 D - 8

Figure 7: Recommended housing shoulder diameters

3) Total eccentricity

When the total eccentricity is excessive, the sealing edge of the seal lip cannot accommodate shaft motions and leakage may occur.
Total eccentricity is the sum of shaft runout and the housing-bore eccentricity.
Total eccentricity, shaft runout and housing-bore eccentricity are generally expressed in TIR (Total Indicator Reading).

A) Shaft runout
As shown in Figure 8, shaft runout is defined as being twice the eccentricity between the shaft center and center of shaft-center rotation trajectory.

Figure 8: Shaft runout

B) Housing-bore eccentricity
As shown in Figure 9, housing-bore eccentricity is defined as being twice the eccentricity between the housing-bore center and shaft rotation center.

Figure 9: Housing-bore eccentricity

4) Allowable total eccentricity

The allowable total eccentricity is the maximum total eccentricity at which the sealing edge can accommodate shaft rotation and retain adequate sealing performance. The oil seal's allowable total eccentricity is affected by the design of the oil seal, the accuracy of the shaft, and the operating conditions.

For details on shaft and housing design, please see the following:
Examples of allowable total eccentricity for oil seals

4. Seal characteristics

Oil seal performance is affected by not only the type and material of the selected oil seal, but also a variety of other factors, such as operating conditions, total eccentricity, rotational speed, the substance to be sealed, and lubrication conditions.
Figure 9 shows items relating to oil seal characteristics.

Figure 9: Items relating to oil seal characteristics

No. Item Content Major factors 1 Sealing property Lip pumped volume
(the volume of oil, etc., pushed back at the lip contact area per unit of time) • Shape
(hydrodynamic ribs)
• Rotational speed
• Oil viscosity, etc. 2 Oil seal service life Wear on the rubber material
Loss of lip sealing function • Operational temperature
• Total eccentricity
• Rotational speed
• Substance to be sealed
• Lubrication conditions, etc. 3 Lip temperature Temperature rise due to sealing edge friction heat because of the shaft rotation • Rotational speed, etc. 4 Allowable peripheral speed When shaft rotation is extremely fast, the sealing performance deteriorates. • Total eccentricity
• Rubber material
• Seal type, etc. 5 Allowable internal pressure Internal pressure is a factor that may deteriorate seal performance. • Total eccentricity, etc. 6 Seal torque The seal torque is large. • Lip radial load
• Lubricating oil
• Rotational speed
• Shaft diameter, etc.

For a more detailed discussion of seal characteristics, please see the following:
Seal characteristics

5. Conclusion

When selecting the oil seal that is right for your machine, it is important that the oil seal be appropriate for the requirements of the usage environment and that it be easily acquired for replacement.
In this month's column, "How to select the right oil seal," we conveyed the following points:

1) Oil seal shape and material should be selected based on the housing, substance to be sealed, pressure, rotational speed, total eccentricity, and air-side conditions.
2) Oil seals can show good sealing performance in combination with properly designed shafts and housings.
3) Oil seal performance is affected by not only the type and material of the selected oil seal, but also a variety of other factors, such as operating conditions, total eccentricity, rotational speed, the substance to be sealed, and lubrication conditions. For this reason, diligent care is required in oil seal selection.

In order for the sealing property of the oil seal you selected to really shine, attention needs to be paid to how it is handled.
In the event of seal failure, it is necessary to take effective countermeasures.

We will cover these points in the next column, "Oil Seals (Part 3)".

If you have any technical questions regarding oil seals, or opinions/thoughts on these "Bearing Trivia" pages, please feel free to contact us using the following form:

O-rings and seals are used in a wide range of industries to help you tightly seal the connections in pipes, tubes, and other elements of complex hydraulic and pneumatic systems. Due to many applications, there is also a wide variety of O-ring material choices available. Nitrile (Buna), Neoprene, Ethylene Propylene (EPDM Rubber), Silicone, Fluorocarbon (Viton), and PTFE (Teflon) are among the most commonly used compounds for O-rings and seals.

To answer many of your questions about O-rings, The Hope Group has created an advanced O-ring material selection guide, where we will look at the properties and compatibility as well as the temperature range and hardness of each O-ring material.

Factors to Consider When Picking O-rings

When picking the right O-ring for your specific application, there are many factors to consider. They include but are not limited to operating conditions, chemical compatibility, sealing pressure, temperature, durometer, size, and cost. Depending on the specific situation, you may also look at abrasion, tear, ozone, electrical resistance properties. Additionally, you can perform appropriate field tests to ensure the fluid, temperature, pressure, and environmental conditions are compatible with the O-ring of your choice.

O-ring Material Selection Guide

In order to accommodate a large variety of applications, manufacturers make O-rings and seals using various elastomers with different physical and chemical properties. Let’s look at some of them below:

Nitrile Butadiene Rubber (NBR)

Resistant to: Water, Petroleum Oils & Fluids, and Hydraulic Fluids

Not recommended for: Phosphate ester base hydraulic fluids, automotive brake fluids, ketones, strong acids, ozone, freons, halogenated hydrocarbons, and methanol

Temperature Range: -40° to +250°F, although that’s an average for the lower and upper tolerances for the various nitrile butadiene rubber (NBR) compounds manufactured by Parker. Parker’s Buna-N compound, which ranges from 70 to 90 durometer hardness nitrile, withstands temperatures from -30°F up to 250°F which includes compounds N0674

Hardness: 40 to 90 durometers Shore A

• Buna Nitrile

Most popular elastomer O-ring material. Parker Hannifin uses 70 durometer hard nitrile (Buna-N) for most of its standard O-rings supplied, with 90 durometer available for tube fittings and adapters. Seal professionals value Buna-Nitrile elastomer for competitive price and excellent resistance properties to petroleum-based oils and fuels, silicone greases, hydraulic fluids, water, and alcohols.

Ethylene-Propylene (EPDM)

EPDM has a spotless reputation in the sealing world because of its excellent resistance to heat, water and steam, alkali, mild acidic and oxygenated solvents, ozone, and sunlight (UV). Nevertheless, experts do not recommend EPDM compounds for gasoline, petroleum oil and grease, and hydrocarbon environments.

Resistant to: Extreme cold, steam, hot water, sunlight and UV, dilute acids, ketones, alkalis

Not recommended for: Petroleum base oils and di-ester base lubricants

Temperature Range: -65° to +450°F

Hardness Range: 40 to 90 durometers Shore A

Neoprene (CR)

Neoprene is a general-purpose elastomer with moderate resistance properties to petroleum oils and weather (ozone, sunlight, UV, and oxygen). Neoprene O-rings have a relatively low compression set, good resilience, abrasion, and are flex cracking resistant.

Resistant to: Refrigerants (freons, ammonia), high aniline point petroleum oils, mild acids, and silicate ester lubricants

Not recommended for: Phosphate ester fluids and ketones

Temperature Range: -45° to +250°F

Hardness: 50 to 80 durometers Shore A

Fluorocarbon (Viton)

When we talk about fluorocarbon O-rings, Viton is a popular trade name that may come into your mind. Fluorocarbon (FKM) compounds combine high-temperature resistance with excellent chemical resistance. These properties make them a popular choice for many applications, including aircraft and automotive industries.

Resistant to: Petroleum base oils and fluids, some phosphate ester base fluids, silicone and silicate ester base lubricants, acids and halogenated hydrocarbons

Temperature Range: Standards -15°F to +400°F, but some Parker FKM Viton compounds can tolerate temperatures down to –65F and up to +450F.

Hardness: 50-95 Durometers Shore A

Perfluoroelastomers (FFKM) (Parfluor)

Perfluoroelastomers (FFKM) are an extension of the Fluorocarbon FKM elastomers extending the compatibilities of the FKM while at the same time extending the upper temperature limits of the materials while compromising the lower temp limits. FFKMs are the cleanest/purest compounds available on the market. They are the first choice for clean applications and are particularly popular in the semiconductor industry.

Silicone

Silicone O-rings have many outstanding properties, including excellent flexibility and fatigue life, strong ozone, and UV radiation resistance. Despite the abovementioned characteristics, experts do not recommend silicone O-rings for dynamic applications. The low strength and poor abrasion resistance as well as high gas permeability, make them not compatible with the most petroleum fluids, ketones, water, and steam.

Resistant to: Dry heat (air to 400°F) and high aniline point oils

Not recommended for: Most petroleum fluids, ketones, water and steam

Temperature Range: -175F to +450°F

Hardness: 40-80 Durometers Shore A

Polytetrafluoroethylene (PTFE)

Polytetrafluoroethylene (PTFE) O-rings are designed to be used in harsh environments with temperatures ranging from -450°F to 600°F. PTFE O-rings are compatible with the widest range of chemicals, such as acetone, isopropyl, methyl, etc. Furthermore, they have low gas permeability and low absorption. Unfortunately, due to polytetrafluoroethylene material properties, pure PTFE O-rings are very rigid and hard to apply. Therefore, manufacturers, including Parker, solve this problem by mixing PTFE material with various fillers to provide users with more flexibility. PTFE seals are often used in food, pharmaceutical, and medical industries.

Resistant to: Most chemicals, excluding alkali metals, fluorine, a few fluoro-chemicals such as chlorine tri-fluoride and oxygen difluoride

Not recommended for: Applications requiring O-Ring stretch and compression

Temperature Range: -260°F to 300°F

Hardness: 55 to 60 durometer Shore D

All Elastomer Families:

All of the above elastomer families as well as those specials not listed are available in many specialty formulations. There are FDA, USP Class VI, Nuclear Grades and compounds that meet AMS, Mil standards as well as other specifications. There are colored and translucent materials as well as internally lubricated materials to meet special needs. There should be a material fit for your application.

Standard vs. Custom O-ring Materials

As an authorized Parker distributor in addition to standard O-ring materials mentioned above, The Hope Group also offers custom seals that are designed exclusively for your specific application. We take care of any functional requirements, gland limitations, installation improvements, etc. Contact us to learn more or speak to our seal specialists.

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