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

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

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

1) Seal type

Select your oil seal type according to Table 2.

Table 2: How to select the seal type

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

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

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

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

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

Figure 6: Recommended housing bore chamfers (shouldered bore)

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

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:

Contact Us