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Sealing ball bearings – Seals, sealing washers and cover washers
Ball bearings are critical components in many machines and equipment and play a central role for low-friction movement of parts. Design-based protection of rolling bearings against overload and failure due to overheating, contamination and external influences is therefore one of the fundamental tasks of a design engineer. Ball bearings can for example be sealed or one can use roller bearings already sealed by the factory to achieve the longest possible service life. To minimize friction and wear, it is necessary to ensure that bearings are sufficiently lubricated. As part of maintenance, the ball bearings must therefore be checked and relubricated as needed. However, modern industrial technology increasingly uses self-lubricating or non-serviceable sealed rolling bearings, which are already equipped by the manufacturer with corresponding sealing elements and lubricant deposits.
What are seals and cover washers for ball bearings?
Seals and cover washers are specially designed components that are intended to protect the bearing interior against external contaminants in general while also preventing lubricant from escaping. Cover washers are non-abrasive (contactless) rolling bearing seals. Cover washers are usually pressed into the outer ring of the ball bearing. There is a gap between the inner ring and the outer ring, which is why complete sealing is not possible. Cover washers should not be confused with sealing washers (including sealing washers made of rubber) and baffle plates. Sealing washers are available in various versions. Depending on the material used and the shape of the sealing washer, these can also be designed as contact seals. This usually results in a better bearing seal. However, a tight seal will cause friction, resulting in friction-related energy losses and higher bearing temperatures.
- 1 - Bore
- 2 - Baffle plate
- 3 - Shaft
In contrast to cover washers, baffle plates are generally not integrated directly into the rolling bearing, but are placed stationary next to the bearing. They keep the grease close to the bearings and reduce leakage of grease even at higher circumferential speeds or as lubricant viscosity decreases in high temperature ranges.
Why are seals important?
Open bearings cannot be used in all situations, e.g. in environments characterized by high contamination. Bearings with rubber seals are recommended in these cases. For ball bearings, seals and cover washers have the main function of protecting the inside against ingress of dust, foreign particles and moisture, which reduces rotational blocking due to seized or rusting foreign particles and improves durability. Seals also protect against unwanted leakage of lubricants, thus maintaining the lubrication effect in the ball bearing.
Types of seals
In general, seals can be divided into two categories: contacting and non-contacting seals. The following types of seals are also distinguished depending on how the parts to be sealed move in relation to each other: Static seals (no motion), translational seals (linear motion), and rotational seals (rotary motion). The following table provides an overview of the advantages and disadvantages of contacting and non-contacting seals, as well as some examples:
| Non-contacting (contactless) | Contacting (grinding) | |
|---|---|---|
| Advantages | Generates no friction, reduces wear and increases energy efficiency Reduced susceptibility to overheating and abrasion Suitable for higher torque values |
Simple design Resistant to contamination Located directly at the sealing point and therefore have a high sealing effect |
| Disadvantages | Sensitive to contamination | Generated friction can lead to energy losses |
- 1 - Open deep groove ball bearing
- 2 - Cover washer on one side (e.g. Z)
- 3 - Cover washer on both sides (e.g. ZZ / 2Z)
- 4 - Non-contacting seal (e.g. VV or 2RZ)
- 5 - Contacting seal (e.g. DDU or 2RS)
Different seals can be used for ball bearings. In this context, MISUMI offers a wide range of shaft sealing rings and ball bearings with different sealing options. Special shaft sealing ring tables are available to assist with the selection.
Grooved ball bearings with various seals
Different letter combinations are used to refer to grooved ball bearings based on the type of seal. The following overview shows deep groove ball bearings with various seal types and their advantages and disadvantages:
Z / 2Z (ZZ)
Z or 2Z grooved ball bearings have a non-contacting cover washer installed on one side (Z) or both sides (2Z). Grooved ball bearings with this designation are primarily used where absence of friction is the priority and the risk of contamination is relatively low. ZZ ball bearings are factory filled with grease and are thus ready for installation.
RS / 2RS
RS or 2RS stands for a design of the bearing with contacting sealing washers, where RS stands for the design with sealing washer on one side and 2RS for sealing washer on both sides. Other designations for ball bearings with contacting sealing washers are also common. Due to the variety of the shapes of sealing washers, we will discuss only some of them here. Bearings with abrasive sealing washers on one side of the bearing are for example also marked with DU. The design with contacting sealing washer on both sides of the bearing is identified by the DDU marking. Momentary temperatures of up to 120 °C are possible. The very simple assembly and disassembly eliminates any risk of bearing damage. Disadvantages are increased friction resistance and reduced maximum rotational load.
RZ / 2RZ
RZ or 2RZ stands for a design of the bearing with low-contact or non-contacting and armored sealing washers. The one-sided design is called RZ, and 2RZ is the design with a sealing washer on both sides. Low-contacting sealing washers also have further designations, for example the version V or VV, where V stands for the one-sided contact-free sealing disc and VV for the two-sided design.
Shaft seal rings
In addition to seals and sealing washers, there are other design-based solutions such as shaft sealing rings, also called Simmer rings (after their developer Walther Simmer). Shaft sealing rings protect shafts and bearings against dust, liquids, and other contaminants. They also prevent fluids or lubricants from escaping from a rotating shaft or axis. There are different shaft seal ring designs, e.g.:
- Radial shaft seal ring
- Axial shaft seal ring
- Labyrinth seal
- Teflon seal
Radial shaft seal ring
A radial shaft seal is a lip seal for sealing rotating shafts. They are installed with a fixed seat in the housing or housing cover. A radial shaft sealing ring generally consists of a sealing lip, a metallic stiffening ring, and an annular screw tensioning spring.
- 1 - Sealing lip
- 2 - Screw tensioning spring
- 3 - Metal ring
When pushed onto the shaft, the sealing lip is expanded together with the spring washer. The sealing lip lies directly on the surface of the rotating shaft. Radial shaft seal rings are often used for bearings with a two-piece housing.
Typically, radial shaft seals are seals that come into contact with each other, but can also be designed to be non-contacting at high rotational speeds.
Criteria for selecting seals and cover washers
The following criteria should be considered when selecting seals and cover washers:
- Type of seal or cover: Determine the main purpose (prevent liquid from entering or merely cover the opening). Is a non-contacting seal sufficient or does only one contact seal fulfill the purpose?
- Environmental Conditions: Parameters such as temperature, humidity, chemical exposure and pressure must be determined.
- Material selection: The choice of material depends on the media to which the seal is exposed, such as oil, water, acids, alkalis, or aggressive chemicals. Common materials include rubber, elastomers, Teflon, metals and plastics.
- Mechanical loads: Mechanical loads such as vibrations, shock loads and movements that could affect the seal or cover washer must be observed. The seals should be able to handle these loads without damage or leaks.
- Cost-effectiveness: Cost factors (e.g. quantities, expected maintenance costs) must be taken into account.
Material selection for seals
MISUMI offers seals, e.g., made of the following materials:
- Nitrile Rubber (NBR): NBR is particularly oil-resistant (especially against hydraulic oils, mineral oils, etc.), weather-resistant and offers good general resistance to many chemicals. The range of application is -40 to 108°C. NBR is often used in flat gaskets (these are static gaskets installed as sealing elements, e.g. between two flanges).
- Fluoro rubber (FKM - also known as Viton): FKM is characterized by excellent resistance to chemicals, high temperatures and various oils and fuels. FMK also has a low gas permeability. Special FKM seals are particularly resistant to acids, among others
- Polytetrafluoroethylene (PTFE - also known as Teflon): PTFE is characterized by low friction and very high temperature resistance (200 to 260 °C, momentarily up to 300 °C). It can be exposed to aggressive chemicals.
