Types of bearings

Types of bearings:

Bearings can be classified as fixed bearings and extension bearings. Fixed bearings allow rotation but restrict translation movements. Extension bearings allow rotation and translational movement. There are several types of bearings available. The main types of bearings currently in use are as follows.

Sliding Bearings

The sliding bearing uses one plane metal plate to slide against another. The sliding bearing surface produces frictional force, which is applied to the superstructure, substructure, and bearing. To reduce this friction force, PTFE (polytetrafluoroethylene) is often used as a sliding lubricant. PTFE is sometimes called Teflon, the widely used PTFE brand, or TFE AASHTO [1] and has appeared in other design standards. In its usual application, a steel plate coated with PTFE slides against another plate, usually stainless steel.

Sliding bearings can be used alone or in other types of bearings. Pure sliding bearings can only be used when the deflection in the supports is negligible. They are therefore limited by ASHTTO to 15 m or less [1]. A guide system can be added to the sliding bearing to control the direction of movement. This can be fixed by passing the anchor bolts through the plates.

Rocker and Pin Bearings

Rocker bearing is a type of expansion bearing that has a high variety. It usually consists of a pin at the top that facilitates rotation and a curved surface at the bottom to accommodate translation movements (Figure 1.1a). The top pin is made up of top and bottom semicircular reverse surfaces and a solid circular pin is placed between them. Generally, there are caps at both ends of the pin to prevent the pins from slipping out of the seat and lifting loads if necessary.

The top plate is connected to a single plate by bolting or welding. The lower curved plate sits on a stone plate. To prevent the rocker from happening, the keys are used to keep the rocker in place. One key may be a portal, a small trapezoidal steel bar that is tightly attached to a stone plate at one end and a loose rocker at the other end to the rocker bottom plate. Or it could be the anchor bolt that passes through the slot hole on the bottom rocker plate.

Pin bearing is a type of fixed bearing that adjusts rotation through the use of a steel pin. The unique configuration of the bearing is similar to the rocker described above, and the bottom curved rocker plate is now flat and attached directly to the concrete pier (Figure 1.1b). Rocker and pin bearings are used primarily in steel bridges. They are only suitable for applications where the direction of displacement is well defined because they can only accommodate translations and/or rotations in one direction.

They can be designed to support relatively large loads but usually require more vertical clearance when the load or displacement is large. The experimental limits of load and displacement are approximately 1800 kN and ± 100 mm, respectively, and several degrees of rotation can be achieved [3].

Generally, the moment and lateral forces caused by the movement of these bearings are small and insignificant. However, metal bearings are prone to corrosion and deterioration. The rusted joint can produce much larger forces. Therefore, regular inspection and maintenance is required

Roller Bearings

Roller bearings are composed of one or more rollers between two parallel steel plates. Single roller bearings can facilitate both longitudinal rotation and translation, but the group of rollers only allows longer translations. In the latter case, rotation is provided by combining the rollers with the pin bearing (Figure 1.1c).

Roller bearings are used in steel and concrete bridges. Single roller bearings are relatively cheap to manufacture, but they only have a very limited vertical loading capacity. On the other hand, multiple roller bearings may be able to support very large loads, but they are more expensive.

Like rocker and pin bearings, roller bearings also suffer from corrosion and deterioration. Regular inspection and maintenance is essential.

Elastomeric Bearings

Elastomeric bearing is made of elastomer (natural or synthetic rubber). This allows for translational and rotational motions through the deformation of the elastomer. The elastomer is soft in shear but very hard against volumetric change. Under the compressive load, the elastomer extends laterally. To maintain a large load without excessive deflection, reinforcement is used to prevent lateral elasticity of the elastomer. This leads to the development of many types of elastomeric bearing pads – simple, fiberglass-reinforced, cotton duck-reinforced and steel-reinforced elastomeric pads. Figure 1.2a shows a steel-reinforced elastomeric pad.

Simple elastomeric pads are very weak and flexible because they only prevent bulging by friction forces. They are commonly used in low-to-medium-range bridges where pressure is low. Fiberglass-reinforced elastomeric pads contain alternating layers of elastomer and fiberglass reinforcement. Fiberglass inhibits the lateral deformation of the pads under pressure loads so that a large loading capacity can be achieved.

Cotton-reinforced pads Elastomeric pads are reinforced with close-spaced layers of cotton duck. They exhibit high compressive hardness and strength but have very limited rotational capabilities. Thin layers cause greater shear stiffness, which leads to larger forces in the bridge. Sometimes they are combined with a PTFE slider on the pad to accommodate translations (Figure 1.2b).

Steel reinforced elastomeric pads are constructed from vulcanized elastomer to thin steel plates. They have a high loading capacity on a variety of elastomeric pads, which is limited by the manufacturer’s ability to uniformly vulcanize a large amount of elastomer. All of the aforementioned pads can be produced on a large sheet and cut to any specific application, except for steel-reinforced pads. Steel reinforced pads should be custom-built for each application due to the edge cover required for stainless steel protection.

Steel-reinforced pads are the most expensive but simple elastomeric pads are cheaper. Elastomeric bearings are generally considered the preferred type of bearings because they are low-cost and almost maintenance-free. In addition, elastomeric bearings are extremely forgiving of loads and movements beyond design values.

Curved Bearing

The curved bearing consists of two adjustable curved discs and sliding against the other to accommodate rotation. The curved surface can be cylindrical, which is simply a rotation or sphere around one axis that allows the bearing to rotate around any axis.
The lateral motions are restricted to a pure curved bearing and limited lateral resistance can be developed by a combination of curved geometry and gravity loads.

To adjust lateral movement, the PTFE slider must be attached to the bearings. Keeper plates are often used to move the superstructure in one direction. Large load and rotation capabilities can be designed for curved bearings. Vertical capacity is limited by its size, which largely depends on machine capabilities. Similarly, the rotational capacity is limited by the clearance between the components. Figure 1.3a shows a typical expansion curved bearing. The lower convex steel plate, which has a stainless steel alloy surface, is retracted on a stone plate.

The upper concave plate with a matching PTFE sliding surface is for rotation on the bottom convex plate. Between the single plate and the upper concave plate, there is a flat PTFE sliding surface that can accommodate lateral movements

Pot Bearings

The pot bearing consists of a simple elastomeric disc that is confined to a shallow steel ring or pot (Figure 1.3b). Vertical loads are passed through a steel piston that fits closely to the steel ring (pot wall). Flat sealing rings are used to hold the elastomer inside the pot. The elastomer bearing acts as a viscous fluid inside the pot. Because elastomeric pads are limited, they can carry more weight than conventional elastomeric pads.

The translational motions are restricted to a clean pot bearing, and the lateral loads are passed through a steel piston that runs against the pot wall. To adjust the translational motion, the PTFE sliding surface must be used. Keeper plates are often used to move the superstructure in one direction.

Disk Bearings

Disk bearing, as illustrated in Figure 1.3c, uses a hard elastomeric (polyether urethane) disk to support vertical loads and a metal key in the middle of the bearing to resist horizontal loads. The rotational motion is applied through the deformation of the elastomer. To accommodate translation movements, however, a PTFE slider is required. In these types of bearings, the polyether urethane disk should be rigid enough to withstand large vertical loads without excessive deformation, and soft enough to easily rotate.

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