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

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

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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

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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 &#;&#;&#; &#;&#; &#; 10 1.5 min. 10 20 2.0 min. 20 30 2.5 min.

Figure 4: Shaft end chamfer

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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.)

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:

Different Types of Oil Seals

Are you looking to keep your machinery free from any unwanted leakages but aren&#;t sure which rotary shaft seal is right for your needs? This guide will provide you with everything you need to know in order to select the right one for your application.

Oil seals, which are also known as rotary shaft seals, fluid seals or grease seals, play an important role in closing down the gaps between moving and stationary elements of mechanical equipment.

By preventing lubricants from escaping, they protect key components of machinery from being damaged by leaks of various fluids. Everything from car engines to assembly machines use these oil seals to remain free from any harmful interactions that can cause serious and expensive damage to any of their critical parts.

There are a wide range of oil seals to select from for any number of uses, so this guide will break down the most common seals to help you choose the right one for whatever piece of machinery you are working on.

What are rotary shaft seals?

Rotary shaft or oil seals are placed between moving and stationary pieces of machinery to ensure that contaminants, moisture, corrosive materials and abrasives do not damage the various components. They can also prevent unwanted mixing of fluids, including water and oil combining within a machine.

How are Oil Seals made?

First, an elastomer, most often nitrile, is vulcanised to a metal ring. This creates a stiffening effect that includes a specialised metal tension spring directly behind the sealing lip, keeping the oil seal firmly in place against the moving part.

Oil Seal Materials

There are many different materials used to manufacture oil seals.

Leather Oil Seals - Leather Seals, also known as Type L Oil Seals, are most common in components that are subject to dirt and poor lubrication. Since they come pre-lubricated and are able to absorb fluids, leather oil seals are able to provide sealing properties in conditions that synthetic rubber is unable to.

Synthetic Rubber Oil Seals - Styrene Butadiene Rubber oil seals, or just SBR oil seals, offer strong resistance to abrasions and lesions, making them an ideal seal for fast-moving machinery. With the ability to withstand extreme temperatures with its heat-aging qualities, they can be used in outdoor components. They are also seen as more cost-effective oil seals than natural rubber.

Nitrile Oil Seals - Nitrile oil seals, which is the commonly used term for acrylonitrile-butadiene rubber seals, is a very good general-purpose option due to the flexibility of use across a variety of components. The resistance is strong against fats, hot water, gasoline, mineral oils, grease and animal oils, making them the most often-used oil seals. They do not have a wide temperature range, making them a poor choice for machinery that can see extreme changes in temperature.

Viton Oil Seals - A synthetic rubber and fluoropolymer elastomer, Viton is used to make oil seals that provide resistance in both high temperature, up to 250°C and low compression set components. They also offer a high resistance to chemicals and abrasions, so they can be used in elements that regularly interact with petroleum and solvents.

Polyacrylate Oil Seals - Mostly selected for automotive and transmission uses, polyacrylate seals are able to withstand fuel, oil, ozone, sunlight and weather when used. With cars exposed to all these different fluids and elements, they are the perfect choice. However, they should not be used in low temperatures, as their flexibility weakens when cold.

Silicone Oil Seals - Designed to absorb lubricants in order to lessen wear and friction, silicone rotary shafts also offer high thermal resistance and a large temperature range. But, they do not handle abrasions well or interact with oxidized oils.

PTFE Oil Seals - A relatively new and exciting oil seal, the use of polytetrafluoroethylene means that they can withstand dry or unlubricated operations. With a massive thermal range of  -130ºC to +200ºC and a strong resistance to chemicals, they are considered to be the future of rotary shaft seals.

How to Choose the Right Oil Seal

There are several key factors to consider when you are selecting the oil seals for your next project to ensure that you protect your machinery from immediate and long-term damage.

Pressure - Many oil seals can only withstand low-pressure applications, so understanding the compression set of your components is key.

Temperature - Just like pressure, the temperature that your oil seal will be operating in must be known so that you can choose one that can withstand the heat or cold. PTFE have the widest range of temperature, making them useful for machines who can see usage in extreme weather or elements.

Shaft Speed - Considering the speed that the shaft will be moving, the runout, the housing bore and the type of oil being sealed is vital to making sure you select an oil seal that will not suffer from abrasions or spiralling.

Fluid Types - Various oil seals are able to withstand interactions with oils, fuels, grease, water and more. Knowing what type of fluid the rotary shaft seal will be in constant contact with will ensure the longevity of the seal and surrounding components.

Lubricant Amount - Seals will always perform best when lubricated, however in some machines there are more likely to be dry spells. For these cases, selecting a leather or PTFE seal will be beneficial, as both can operate with less lubrication than others.

If you require any assistance in discovering the factors that lead to the right oil seal choices, the Simply Seals Team is happy to assist you!

Find the right Oil Seal for your needs with Simply Seals!

With dozens of specifically-designed oil seals made from a variety of materials, it can be very difficult to understand the best choice for their machinery.

That is why the Totally Seals team is always available to assist you with choosing the correct style, material, and size of oil seal for everything from commercial machinery to hobby car building.

Contact us today to make sure that you select the proper rotary shaft seal that will protect your components and lead to a long, healthy life for all of your machines!

If you want to learn more, please visit our website pu oil seal.