Bridges, pedestrian walkways and overhead piping move due to expansion and contraction from weather and also from seismic motion, or movement of the earth. These structures contain expansion joints to permit movement, and bearing surfaces or slide plates to minimize friction damage. A slide plate is a combination of a flat steel plate and a low-friction material that permits movement with little wear of the structure.
Bearings are used to reduce wear in most moving equipment, but static or non-moving buildings, bridges or other structures also need to move. The weight of traffic passing over a bridge or changes in temperature can cause the bridge structure to move in different directions. Metal beams in direct contact and under load may not move correctly, or the high friction can cause sudden movements or releases that can damage the structure.
Adding a slide plate between the beams creates a low-friction bearing surface that permits smooth movement of the structure. A common plate covering is a slippery fluoropolymer fused to the bearing surface on both beams. The result is two polymer surfaces in contact, resulting in a low-friction bearing requiring no lubrication and little maintenance.
Slide plates can also be used in piping supports, which are used to hold piping where it is elevated above ground. Pipes can expand and contract due to temperature changes, and a slide plate can be added to the pipe support beam. The bearing surface does not move, but allows movement of the piping resting on it.
Geological conditions can also cause movement of buildings and bridges. In an earthquake zone, bearing surfaces may be installed under buildings to separate the structure from the foundation below. During an earthquake, the slide plate design permits the building to move very little while the ground shifts beneath it.
Some applications are not suitable for polymer bearings due to high loads or operating temperatures. A different slide plate design uses bronze or other metals that contain graphite carbon or lubricating oil. The graphite is often contained in the metal as small buttons with a slightly higher profile than the plate. Oil-filled bearings and plates are formed by small beads fused together, called sintering, which contain many channels and voids filled with oil.
Slide plates can also be used as wear surfaces for moving parts. Drawbridges open and close repeatedly to allow boat traffic to move through rivers and channels. The end of the drawbridge can rest on a slide plate that minimizes wear to the bridge. It also provides a bearing surface for any thermal expansion or contraction of the bridge when it is closed.