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Selecting an appropriate seal design and material depends on the operating conditions of the application such as:

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HMS5 seal

Because the inluence of one operating condition typically dominates the seal selection process, there are no universal rules for determining the appropriate seal type or design for a given application This section describes how operating conditions affect seal performance and service life and provides guidance on selecting the most appropriate seal for a given application.

Pages 103 to 105 and 174 to 183 show the standard SKF radial shaft seals and their main features and permissible operating conditions

CRW1 seal

Grease retention

Greases have a relatively high viscosity and are relatively easy to retain in a bearing arrangement In many grease lubricated applications, a non-spring-loaded sealing lip design or a V-ring can adequately retain the grease (Fig. 1)

However, more demanding applications may require HMS5 or CRW1 spring-loaded radial shaft seals (Figs. 2 and 3)

When frequent relubrication is required, the lip of at least one of the seals in the sealing arrange ment should be directed toward the air side so that excess grease can escape via the sealing lip (Fig. 3) This avoids grease build-up, which can retain heat and limit heat dissipation For grease lubricated applications, SKF recommends calculating the permissible circumferential speed for oil and halving the result.

HMS5 seal

Oil retention

Lubricating oils, particularly relatively low-viscosity oils, are much more dificult to retain than greases Therefore, HMS5 or CRW1 spring-loaded radial shaft seals (Figs. 4 and 5) are recommended in order to achieve the neces sary radial load and resistance to dynamic runout and shaft-to-bore misalignment for a satisfactory sealing performance.

Standard HMS5 seals have a straight lip while CRW1 seals are designed with SKF Wave lips to provide improved pumping ability, regardless of the direction of shaft rotation (Fig. 5 on page 21) Another way of increasing a seal’s pumping ability is to add a helix pattern, i e hydrodynamic features, to the sealing lip design.

The rubber outside diameter, like the one found on HMS5 seals, helps compensate for small imperfections in the housing bore surface and is therefore recommended when the required housing bore surface is questionable.

For very tough operating conditions, where circumferential speeds are relatively low, metal face seals, like the HDDF seal (Fig. 6), can be used for both oil or grease retention.

V-rings (Fig. 7) may also be used to retain oil, provided they are installed on the oil side and supported axially on the shaft.

CRW1 seal

HDDF metal face seal


CRW1 seal

Contaminant exclusion

Radial shaft seals that are primarily used for contaminant exclusion should be installed with the lip pointing outward When additional protection is needed, SKF recommends a seal design that incorporates an auxiliary lip, for ex ample the HMSA10 or CRWA1 seals.

For tough operating conditions, SKF Wave seals (Fig. 8) with hydrodynamic features are recommended To further enhance sealing eficiency, two single-lip seals can be arranged in tandem (Fig. 9) or a double-lip seal, like the HDSE1 seal, can be used (Fig. 10).

V-rings (Fig. 11) are used primarily to exclude contaminants These seals, which act as lingers, rotate with the shaft and seal against a surface that is perpendicular to the shaft.

V-rings and axial clamp seals are often used as secondary seals to protect the primary seals from coarse contaminants.

V-ring seal arrangements are not intended for oil retention.

CRW1 seals in tandem

HDSE1 seal


HMSA10 seal

Retention and exclusion

In many applications, the exclusion of contaminants is just as important as lubricant retention Seals with an auxiliary lip, like the HMSA10 seals (Fig. 12), are appropriate for these applications.

Another option is to use two seals installed in opposite directions (Figs. 13 and 14) or two opposing V-rings (Fig. 15) with a spacing washer.

Under extremely tough operating conditions, SKF recommends using HDDF metal face seals (Fig. 6 on page 23), provided that the sliding velocity of the mating surfaces lies within the permissible range.

Two seals in opposite direction

Two seals in opposite direction


Separating two liquids

When an application has to keep two liquids from coming into contact with each other, there are two suitable solutions These solutions, which depend on the availability of space and required eficiency, include.

  • the use of two separate seals (Figs. 16 and 17), positioned with their lips facing in oppos ite directions or
  • the use of HDSD2 double-lip seals (Fig. 18)

In both alternatives, the sealing lips must be spring-loaded When using an HDSD seal, it is very important to provide a means to lubricate the sealing lips, i e the cavity between the sealing lips must be illed with grease prior to installation.

CRW1 seals

HMS5 seals

HDSD2 seal

Circumferential and rotational speed

The permissible speed of a seal is determined by its design and sealing lip material as well as the material and condition of the shaft All of these factors inluence the heat generation at the seal counterface Lubrication of the sealing lip and the characteristics of the lubricant also have a direct inluence on heat generation because they have a direct impact on heat dissipation.

Diagram 1 compares the permissible circumferential speeds for various seal designs assuming normal seal operation, grease or oil retention and no pressure differential across the seal.

CRWA5 seal

Pressure differentials

When subjected to a pressure differential, the seal must resist the additional radial load generated by the pressure If the seal is not designed to resist the pressure, it will be forced against the shaft, increasing the radial load, underlip temperature, friction and wear of the seal and the counterface, resulting in shortened service life.

Standard seals are rated for no more than 0,07 MPa at 5 m/s (10 psi at 1 000 ft/min), but SKF offers CRW5 and CRWA5 pressure proile seals that can accommodate 0,34 MPa at 5 m/s (50 psi at 1 000 ft/min) Beyond 0,34 MPa (50 psi), SKF offers a line of special order PTFE seals that can accommodate more than 3,5 MPa (500 psi).

In applications with pressure differentials, shaft seals should be secured axially from the low-pressure side to prevent them from moving axially This can be accomplished by installing the seal into a counterbore (Fig. 19) or by using a retaining ring.

Limited space

In many cases, the available space is insuficient for a radial shaft seal having dimensions in accord ance with ISO 6194-1 or DIN 3670 In these situations, special radial shaft seal designs must be used (Fig. 20).

V-rings (Fig. 21) are also suitable for appli cations with limited space because they can be positioned outside the actual seal pos ition V-rings seal axially by exerting light pressure against the counterface that can be a station ary or rotating machine component.

In applications with large shaft diameters, HS8 seals are an appropriate choice when space is limited (Fig. 22).

Special seal design

V-ring seal

HS8 seal

V-ring seal

V-ring seals

Installation restrictions

In applications where the seal cannot be installed via the shaft end, a V-ring or any of the split HS, HSS or HRS designs can be used († pages 192 to 199).

After being positioned on the shaft, HS, HSS and HRS seals are held together by a spring and spring connector These seals should be retained axially in the housing bore by a one-piece or split cover plate.

Split HS radial shaft seals are suitable for circumferential speeds up to 7,5 or 10 m/s (1 480 or 1 970 ft/min), depending on their design, and are available for shaft diameters up to approximately 4 570 mm (180 in).

Since V-rings are elastic, they can be stretched and are therefore easy to install, even in applications where they have to be passed over other components (Fig. 23) However, in the event that replacing a V-ring would equire the time consuming removal of several components, it is advantageous to install one or two replacement V-rings on the shaft from the outset (Fig. 24) When the time comes to replace a worn V-ring, it can be cut and removed and the replacement V-ring can be pushed into position.

V-ring seal


Seals installed on vertical shafts are usually more exposed to contaminants like rain water than seals on horizontal shafts Oil retention is also more challenging for seals installed on vertical shafts In general, however, all seals listed in the product tables are suitable for use on both horizontal and vertical shafts.

V-rings (Figs. 25 and 26) have an interference it on the shaft and rotate with it They act as lingers and are therefore particularly suitable as both primary and secondary seals on vertical shafts Highly eficient sealing arrangements, like those found in submersible pumps, can be achieved using radial shaft seals in tandem with a V-ring for additional protection against contaminants (Fig. 27).

At relatively low speeds, HDDF metal face seals (Fig. 28) effectively retain grease or oil and prevent the ingress of contaminants on vertical shafts.

V-ring seal

CRW5 seals + V-ring seal

HDDF metal face seal

Counterface design

The service life and performance of a seal are largely inluenced by:

  • Shaft material and hardness
  • Shaft surface inish and tolerance grade
  • Dynamic runout and shaft-to-bore misalignment

A shaft surface that is too smooth can lead to lubricant starvation, while a shaft surface that is too rough can accelerate sealing lip wear The shaft surface should be machined without direction ality as directionality can cause leakage depend ing on the direction of rotation Dynamic runout and shaft-to-bore misalignment cause an uneven radial load on the circumference of the sealing lip As a result, the sealing lip, particularly at high speeds, will not be able to follow the shaft This, in turn, will result in a gap between the sealing lip and the shaft, causing reduced sealing ability.

Unlike radial shaft seals, V-rings and axial clamp seals are not affected by normal coaxiality deviations or runout.

Axial movement

Axial movement

Axial movement of the shaft relative to the housing bore does not detract from the sealing ability of radial shaft seals (Fig. 29), provided that the total surface in contact with the lip has the same quality with respect to hardness and surface finish.

The amount of axial movement that can be accommodated by V-rings, axial clamp seals and HDDF seals is limited by the permissible displace ment of the seal relative to its counterface.