Bearing arrangements of all types do not only consist of bearings but include the associated components, such as shafts and housings, as well as seals. The performance of the seal is of decisive importance where lubricant cleanliness is concerned and this cleanliness has considerable influence on bearing life, as can be seen, for example, when the SKF rating life is applied.
For the designer, this means that bearings and seals must be considered as constituting a system. When designing the sealing arrangement and selecting the seal, the bearing arrangement and the requisite bearing life must not be neglected.
For SKF, this means that even greater attention is now being paid to the question of how bearings and bearing arrangements should be sealed as well as to the question of sealing in general. For this reason, two important European seal manufacturers as well as the largest seal manufacturer in the USA now belong to the SKF Group.
This general technical section of the catalogue contains information and recommendations regarding the selection and application of seals.
Obviously, it is not possible to cover all the aspects sufficiently in detail to deal with all eventualities so that reference is made in many places to the SKF application engineering service. The higher the technical requirements placed on a seal, the greater the advisability of contacting SKF, particularly if in-house experience is lacking.
Data which are specific to a certain type of seal will be found in the text preceding the relevant product table section.
Radial shaft seals
Radial shaft seals are used between rotating and non-rotating machine components. They are made up of:
a cylindrical outer covering of sheet steel (shell) or elastomer which seals statically against the housing bore and enables the requisite interference fit of the seal in the housing bore to be obtained as well as facilitating proper installation
the sealing lip of elastomer which provides dynamic and static sealing against the shaft. The lip has a sealing edge which is formed by pressing, cutting or grinding and is normally pressed against the counterface on the shaft with a defined radial force by a garter spring. The edge of the sealing lip and the shaft counterface form the most important functional area of a radial shaft seal. The sealing effect of the lip can be enhanced by providing the contact area of the lip with hydrodynamic aids which may be designed for single direction operation, or for alternating directions of shaft rotation.
An additional sealing lip may also be provided to protect the sealing contact proper from dust and other fine solid contaminants. A suitable lubricant in the space between the sealing lip proper (primary lip) and additional (secondary) lip can reduce wear and delay corrosion. Where hydrodynamic aids have been provided it should be remembered that a pressure deficit can be produced in the space between primary and secondary lips which will increase the radial force. This situation can be alleviated by providing breathing holes in the secondary lip. It should also be remembered that contaminants which have penetrated past the secondary lip will eventually cause damage in the counterface region. A build-up of heat can also occur between the two lips; this promotes premature wear.
Radial shaft seals with low cross sectional height
It is not always easy to find commercially available radial shaft seals for bearing arrangements that incorporate needle roller bearings or plain bearing bushings because of their very low cross section. As a result, SKF offers a wide assortment of special low cross section radial shaft seals to fit virtually all of these bearing arrangements. These contact seals are designed without garter spring and available in two designs:
the G design with a single lip
the SD design with a double lip
radial shaft seal
radial shaft seal,
d1 ≤ 7 mm
radial shaft seal,
d1 ≥ 8 mm
The mechanical seals from SKF consist of two identical sealing rings and two similar nitrile rubber Belleville washers. The sealing rings are of wear and corrosion resistant steel and have finely finished sliding and sealing surfaces. Sealing is assured even against thin oils, such as SAE 10W40 oils. The Belleville washers (cup springs) of nitrile rubber provide the necessary uniform face loading and positive sealing at the bore and outside diameters. The outside diameter of the washers adapts to the form of the bore in which they are mounted and there are no special demands in respect of surface finish.
SKF mechanical seals are supplied as ready-to-mount cartridge units. A special retainer ring holds the two sealing ring faces together. This not only facilitates mounting but also protects the seal against damage. The retainer is left in position after mounting and provides extra protection for the sliding surfaces during the initial stages of operation. It slowly wears away.
V-ring seals are mounted on shafts and their thin, tapered lip seals against a surface (counterface) at right angles to the shaft (fig 1a). No special demands are placed on the surface finish of the shaft seat and the V-rings act as flingers (fig 1b), since they have an interference fit on the shaft and rotate with it. Misalignment of the shaft with respect to the counterface (fig 1c) can be tolerated and V-rings also provide reliable sealing if the shaft is out-of-round or rotates eccentrically (fig 1d). It should be remembered when using V-rings that the amount by which the shaft can be displaced axially is governed by the permissible displacement of the V-ring relative to its counterface.
V-rings are made entirely of elastomer without fabric or sheet metal reinforcement. They are, therefore, particularly easy to install. They can be stretched and, depending on size, can be pushed over other components of the arrangement, e.g. flanges, pulleys or even housings. This is a very valuable characteristic, expecially in the case of repairs.
Axial clamp seals
The axial clamp seals are designed for large and very large diameters and are eminently suitable as secondary seals for applications where otherwise the primary seals would be subjected to excessive quantities of particulate contaminants or water. The seals do not rotate but seal axially against a rotating counterface.
These axial clamp seals are made of appropriately profiled strips of non-reinforced nitrile rubber which are held firmly in place by stainless steel screw-type clamps. They are available in the diameter range 150 to 4 600 mm.
The standard range of axial clamp seals basically comprises seals intended for inch-diameter seats. However, as normally the seals are mounted with an approximately 25 mm gap between the ends, they may be used for appropriate metric-size seats. In case of doubt, please contact the SKF application engineering service.
SKF axial clamp seals are produced in two different designs:
CT1 design Axial clamp seals of the CT1 design have a flat face sealing lip and are held in position by a screw-type clamp. The maximum permissible axial displacement (operating interference) with respect to the counterface is +2,4 mm.
CT4 design Axial clamp seals of the CT4 design are extra wide and have a double clamp. The maximum operating interference is +4,8 mm. The sealing lip is flat as for the CT1 design.
Sealing and spacing washers
SKF sealing washers of the Z type form efficient labyrinth seals which can be used in a wide variety of applications. They provide inexpensive solutions to many sealing problems. Their use is recommended in the first instance for simple bearing arrangements subjected to arduous operating conditions. The efficiency of the seal is influenced by the number and design of the washers incorporated. There are two types of sealing washer: the standard design (fig 1) and the flocked design (fig 2) which is intended for difficult operating conditions. Depending on the degree of contamination and the desired relubrication interval, up to four sets of sealing washers may be mounted immediately adjacent to each other. The sealing washers may also be combined with other standard seals.
When it is necessary to relubricate a bearing with sealing washers at both sides, the inclusion of a set of ZW spacing washers (fig 3) enables a free passage for the fresh grease to be achieved. A set of ZW spacing washers consists of one washer to be mounted in the housing bore and one to be mounted on the shaft.
SKF wear sleeves are available in two different designs, depending on size. One is the extremely thin-walled SKF SPEEDI-SLEEVE which allows replacement seals of the same size as the original seals to be used. The SKF SPEEDI-SLEEVE is produced for shaft diameters up to and including 203 mm. For larger shafts, up to approximately 1 150 mm in diameter, SKF produces the LDSLV wear sleeves in two designs: the LDSLV3 with flange, and the LDSLV4 without flange.