Table of contents
Sealing has a decisive influence on the function and operating life of a bearing
In order to make optimum use of the potential life of a rolling bearing, the egress of lubricant (grease, oil) must be reliably prevented and the bearing must be securely protected against the ingress of solid and liquid contaminants (e. g. dust, sludge, water, cleaning liquid) ➤ Figure.
Principle of a sealing position
Contaminant particles can damage the bearing
If contaminant particles enter the bearing, overrolling of the particles at the rolling contact forms indentations that cause running noise and may constitute initial points for rapidly propagating material fatigue.
Contaminants with an abrasive action
Where contaminants that have an abrasive action are present in the bearing, the rolling contact surface partners undergo wear and there is a progressive increase in the bearing clearance. With an increasing operating duration, the running accuracy of the bearing in particular is reduced until it fails.
Liquid and vaporous media
If liquid or vaporous media enter the bearing, the lubrication conditions at the rolling contacts are disrupted. If corrosive media are present, oxidative damage of the surfaces will also occur. Both of these situations lead to a considerable reduction in the operating life of bearings.
Effective sealing of the bearing position is thus decisive for the long operating life of a rolling bearing.
Non-contact or contact seals
A basic distinction is made between contact and non-contact seals in the adjacent construction and the bearing.
Non-contact seals are free from friction
Non-contact seals are particularly suitable for applications with high speeds and high requirements for freedom from friction and low heat generation. They are free from friction, except for a small amount of lubricant friction in the sealing gap. In general, non-contact seals are free from wear and have an almost unlimited operating life.
Measures which prevent the ingress of liquids into the bearing while stationary
In order to prevent the ingress of liquids while stationary, additional sealing measures are necessary, such as the supply of barrier media. Where higher requirements for sealing integrity are present, a relatively large design envelope is required for labyrinth designs.
Grease sealing by means of baffle plates and sealing shields
Stationary baffle plates prevent the egress of grease
In the case of grease-lubricated bearings, the egress of grease can be prevented in many cases on the stationary bearing ring by the use of simple baffle plates. Depending on the mounting and operating situation, the plates are braced on either their outer or inner edge ➤ Figure. The grease collar that forms at the sealing gap gives protection against slight contamination impact from outside.
Compact solutions in the form of bearings with sealing shields integrated on one or both sides
For designs in which there is no space adjacent to the bearing for a seal, there are rolling bearings with sealing shields integrated on both sides ➤ Figure; see product chapter. These bearings are supplied with a grease charge. Bearings with only one sealing shield on one side are also commercially available.
Baffle plates and integrated sealing shields
Narrow gap between shaft and housing
A narrow gap between the shaft and housing is a simple and, in many cases, adequate means of sealing against the egress of grease and slight contamination impact from outside ➤ Figure. The sealing gap can be held to a relatively narrow size.
Grooves in the housing increase the sealing action if grease lubrication is used
If grease lubrication is used, the sealing action of the gap can be increased by several grooves in the through bore of the housing ➤ Figure. The grooves act as storage chambers and hinder the ingress of contaminants as well as the egress of lubricant.
Helical grooves are used for oil return
Where oil lubrication is used and the shaft is horizontal, the through bore in some housings has a helical groove ➤ Figure. Depending on the spiral direction relative to the rotational direction of the shaft, oil creeping along the shaft is returned to the housing or contaminants are transported from the gap to the outside. A similar sealing action is achieved if the helical groove is located on the shaft.
Splash rings and flinger shields
The sealing action is achieved by means of rotation and centrifugal force displacement
In the case of splash rings and flinger shields, sealing action is achieved by means of rotation and centrifugal force displacement ➤ Figure and ➤ Figure. Splash rings can give effective sealing, for example, where oil lubrication is used and the shaft is horizontal ➤ Figure. The oil creeping along the bush is propelled away by the splash rings and flows back into the housing via a drain hole.
The sealing gap is protected by an outer flinger shield
If heavy contamination impact or direct impact by liquids is expected at the sealing position, the sealing gap can be protected on the outside by means of a flinger shield ➤ Figure. The same purpose is fulfilled by a simple rubber shield that – without additional means of location – is seated under tension on the shaft or shaft bush ➤ Figure. At higher speeds, however, there is a risk that the rubber shield will become detached from the shaft under the influence of centrifugal force. For this reason, vulcanised flinger shields with sheet metal reinforcement are available that are used very effectively in, for example, passenger car wheel bearing units of the 1st Generation or shaft support bearings ➤ Figure.
Protective or collector labyrinths provide very good sealing results
A considerably greater sealing action than with gap seals is achieved by labyrinths incorporating gaps filled with grease. A distinction is made between protective labyrinths and collector labyrinths.
Protective labyrinths are centrifugal force seals. They protect sealing positions subjected to little external load due to liquids against the egress of grease and the ingress of contamination.
Collector labyrinths are labyrinths of highly complex design with collector grooves and drain holes in the housing that are also suitable for the sealing of sealing positions subjected to very high impact by liquids.
Labyrinths are configured with radially or axially aligned crosspieces
Depending on the contamination load, labyrinths are configured with one crosspiece or several crosspieces ➤ Figure. Labyrinths with several radially aligned crosspieces are only considered, for reasons of mounting, for split housings ➤ Figure. Their advantage is that the grease propelled outwards is held in U-shaped housing slots, which thus improves the sealing action.
In order to improve sealing, labyrinths are frequently filled with grease
In practice, labyrinths filled with grease are frequently used in order to achieve better sealing against contamination. In a highly contaminated environment, fresh grease is occasionally pressed into the sealing gap, which has the effect of pressing contaminated grease out of the sealing position.
If significant angular deviations of the shaft are expected in the case of spherical roller bearings, for example, there is a risk that grazing of the crosspieces will occur in labyrinths in accordance with ➤ Figure. In this case, labyrinths with bevelled crosspieces are used. The bevel is matched to the motion about the centre point of the bearing ➤ Figure.
Examples of labyrinth seals
Labyrinth seals integrated in the bearing
Where there is heavy liquid impact and sealing positions are inundated by surge flows, so-called collector labyrinths are used. These seals effectively repel liquids if the sealing position is not subjected to prolonged inundation while stationary.
The contact pressure at sliding contact causes friction in contact seals at the sliding surface
Contact seals are in contact at their sliding surface under a certain contact pressure. In many cases, they represent the most favourable solution in terms of design envelope and costs. The general disadvantage, however, is the loss of energy due to friction at the sealing contact.
Felt ring seals
Felt rings are simple sealing elements used with grease lubrication
Felt rings are simple sealing elements that have proved effective primarily with grease lubrication ➤ Figure. After a short running-in time, the felt forms a sealing surface in preload-free contact with excellent sealing action against dust. Before mounting, the rings are soaked with oil.
Felt ring seals
Several felt rings can also be arranged adjacent to each other
A felt ring is sufficient if the level of contamination is not too high. If the environmental conditions are less favourable, several felt rings can be arranged adjacent to each other.
Operating temperatures up to +100 °C are possible. At higher temperatures, sealing rings made from wound yarns are required that comprise PTFE, graphite or aramide and glass fibres, and are impregnated with PTFE or graphite.
The dimensions of felt rings and annular slots are standardised and stated in DIN 5419.
Metallic sealing washers
Sealing washers made from thin sheet metal provide good sealing for grease lubrication
When using grease lubrication, effective sealing can also be achieved by axially sprung metallic sealing washers ➤ Figure. These thin sheet metal seals are braced in the end face of the inner ring or outer ring and are in spring contact with the other bearing ring. In order to prevent fretting of the seal tongues during running-in, these washers are greased before mounting.
Metallic sealing washers are suitable if no static or pressure-resistant sealing is required
At the sealing contact, the sprung sealing washer beds into the bearing ring during the running-in process and the preload decreases. A groove is formed in the bearing ring that, together with the seal edge, forms a small "microlabyrinth" under light contact. Due to this principle, these seals are suitable for bearing positions that require no static or pressure-resistant sealing of the bearing.
In washers in double arrangements, the intermediate space is filled with grease
In addition to simple sprung sealing washers, there are also washers in double arrangements. The intermediate space is additionally filled with grease at the mounting stage. Due to their small section width, these seals can normally be fitted retrospectively and without design modifications to the adjacent construction.
Metal sealing washers
Sealing washers with elastomer seal lip
Sealing washers integrated in the bearing are used for small design envelope
Where the design envelope is small, rolling bearings (e. g. ball bearings, roller bearings, spherical roller bearings, toroidal roller bearings) with integrated sealing washers are frequently used; see product chapter. These sealing washers comprise a sheet metal washer for reinforcement and have a vulcanised elastomer seal lip.
Various elastomers are available for selection
Various elastomers can be selected in accordance with the chemical and thermal requirements. In general, nitrile butadiene rubbers NBR are used. For applications with higher temperatures or speeds, sealing washers with lips made from fluoroelastomers FKM are frequently used.
The seal lip can be in radial or axial contact
Sealing washers are available with seal lips for radial or axial contact ➤ Figure. They are suitable for sealing against loss of grease and ingress of contamination at low pressure differentials.
Sealing washers with one or more radial seal lips are also suitable for the sealing of rolling bearings with axial relative motion between the inner ring and outer ring (e. g. in spherical roller bearings or toroidal roller bearings).
Sealing washers with seal lips in axial running contact are used primarily in bearings with lower axial operating clearance (e. g. in deep groove ball bearings).
Sealing washer RSD with radial sealing action
Sealing washers RSD have a seal lip with radial sealing action and are characterised by low seal friction ➤ Figure. The seal lip is initially in very light contact with the shaft (little or no contact pressure). After running-in, a minimal sealing gap is created.
Sealing washer RSR with radial contact
Seals RSR with radial contact are equipped with a seal lip geometry that is similar to a rotary shaft seal ➤ Figure. The angles of the seal edge are selected such that the pumping action at the sealing contact repels liquids outwards and a small amount of grease is conveyed under the seal edge in order to lubricate the seal lip.
Sealing washer for axial inward sealing
In the case of sealing washers for axial sealing, an additional outer seal lip is often provided ➤ Figure. The outer seal lip (protective lip) is of a non-contact design and, together with the undercut on the inner ring, forms an additional protective labyrinth against the ingress of contamination.
Sealing washer for axial outward sealing
A seal geometry that is designed in particular for applications with a rotating outer ring and high requirements for retention of the grease is the sealing washer with a seal lip in axial contact in accordance with ➤ Figure. This seal has increased sealing action against the egress of grease. An additional non-contact lip that repels the grease provides a grease chamber in front of the main seal lip for axial outward sealing.
Sealing washers with elastomer seal lips
Lip seals provide radial or axial sealing
Double lip seal: the inner seal lip prevents egress of lubricant, the outer seal lip prevents ingress of contamination
➤ Figure shows a double lip seal made from NBR for use in standard plummer block housings. The radially split seal can be easily inserted in the annular slot in the housing. Ingress of contamination is prevented by the outer seal lip, while the inner seal lip prevents the egress of lubricant. The grease between the two seal lips supports the sealing action.
Double lip seal for plummer block housings
V-ring seals are lip seals with axial sealing action
The V-ring is a lip seal with axial sealing action ➤ Figure. The ring is made from elastic rubber NBR. During mounting, it is stretched and slid onto the shaft so that the seal lip is in contact with the housing wall.
At circumferential velocities over 12 m/s, experience shows that the V-ring must be radially located so that it does not become detached due to centrifugal force. Precise circumferential velocities for specific applications must always be agreed in consultation with the sealing ring manufacturer.
Lip seals of three-part construction
Lip seals of a three-part construction with single or multiple lips (comprising an NBR sealing element between two sheet steel washers) are used, for example, in radial insert ball bearings ➤ Figure.
The outer sheet steel washer protects the seal lips against damage
During mounting, this sandwich construction allows concentric alignment of the rubber seal lip with the inner ring. The outer washer protects the seal lips effectively against damage caused by coarse contaminants.
Lip seals of sandwich construction with seal lip protection
Rotary shaft seals
For oil sealing of rotating shafts, rotary shaft seals (RWDR) in accordance with DIN 3760 and DIN 3761 and with spring preload are suitable. Frequently used designs are shown in ➤ Figure. The sealing rings are designed for applications with slight pressure differentials.
Speed limits for rotary shaft seals are given in DIN 3760.
With oil lubrication
Depending on the seal material and the surface structure of the shaft, the geometry of the seal lips generates a pumping action in the sealing gap towards the steep flank of the seal lip. The sealing ring is therefore mounted with the steep flank facing in the direction of the medium against which sealing is required.
With grease lubrication
In the case of grease lubrication, the steep flank of the RWDR is often placed in the direction of grease egress. As a result, some grease passes under the seal lip for lubrication of the sealing edge. The preload force of seal lips – in relation to their contact length – is normally 80 N/m to 150 N/m.
Rotary shaft seals
Springless Schaeffler sealing rings G, GR, SD
Schaeffler sealing rings are highly suitable for a small radial design envelope and in combination with needle roller bearings
Low-friction sealing of bearing positions with a small radial design envelope, such as bearing positions with needle roller bearings, can be effectively achieved using sealing rings G, GR and SD; see product chapter. These sealing rings can be used individually or in a double arrangement ➤ Figure. In the double arrangement, one seal lip faces inwards to seal the lubrication medium, while the second seal lip faces outwards to give protection against contamination. In order to improve the protective function, the space between the seals can be filled with grease. With an extended inner ring, a sealing ring with the same outside diameter as the outer ring can be used, where the seal lip runs on the extended inner ring. Sealing rings give good protection against contamination and spray water as well as against the egress of oil and grease under slight pressure differentials. In order to reduce friction and protect the seal lip against damage, the sealing edge must be lubricated.
Schaeffler sealing rings