Rocker and Roller Bearing
For railway bridges with spans in excess of 30.5m, where open web through girders are generally provided, the amount of movement needed and the vertical load transmitted through each bearing is too large to be catered by the sliding bearings. It is common, on Indian railways, to provide rocker and roller bearings at the free end of the open web through girders, and rocker bearings at the fixed end.
The roller bearing consists of a base plate, two or more rollers, and a top plate. The rocker and roller end is made by providing a saddle and knuckle plate on top of the rollers whereas the same arrangement except rollers is at the rocker end. The rocker and roller end of bearing permits translation as well as rotation, whereas the rocker end permits only rotation.
A moderate amount of maintenance can keep the bearings intact and can continue to work properly for a long period of time.
Rocker and Roller Standard Bearings
Rocker bearings are of various types and styles which are only limited by the imagination of the engineer or the designer. Some railroad companies still have the standard types and sizes. The standard styles are Expansion Rocker Bearings and Fixed Rocker Bearings.
Connections of Rocker and Roller Bearing
These bearings are built up by welding two relatively large and thick pieces of plates. The top plate or saddle plate is connected to the end of the bottom chord. Since this connection is crucial for transmitting the horizontal thrust from the bottom chord to the bed block, it must be made a tight fit. The top plate is generally installed to the underside of the bottom chord in situ.
However the joint is not amenable for riveting for want of adequate space. The number of rivets required and their lengths will also be very large. The joint is therefore made with turned and fitted bolts. The reaming of the holes in the bottom chord and the saddle is therefore required to be done by assembling them together.
The barriers are designed with several welding practicalities taken into account, and it also allows the welder to access all the joints that are needed to be welded together.
Excessive weld sizes of the design of partial and full penetration groove weld when unnecessary needs to be avoided. Over-done or unexpected welding can add extra cost to the bearings and add reduced durability or service life to the bearing. These are especially recommended on railroad structures where large impact loads are sustainable.
The mild steel to be used for the components of the bearings IS 2062-1192 shall comply with the IS 2062-1192. The steel for forging to be used for the components of the bearings shall comply with class 3, 3A, or 4 of IS: 1875 and the steel forging shall comply with class 3, 3A, or 4 of IS 2004.
The pins which are used in rocker bearings can be simple cold-rolled steels like C1018 or can be heat-treated forged with other materials such as ASTM 688 Classes B through F Nuts (Recess Pin Nuts or Plain Pin Nuts). These nuts and recesses should be of the same material as the steel plates.
The grease on the pins and mating parts should be waterproof with medium-consistent grease. Grease fittings for the lubrication of hidden parts can be provided when there is a need for the structure.
The coating on the bearings is high-performance paints which include high-solids zinc primers or the bearing assemblies can be metalized with zinc thermal spray coatings. Galvanized or carbon steel components can be considered for the paint coating on demand. The coating recommendations can be made from any long-lasting products irrespective of the environmental conditions.
The design of rocker and roller bearing involves the selection of roller length, its diameter, the radius of the contact surface of saddle/ knuckle, thickness and plan size of the base plate and number and size of anchor bolts.
Simple design rules are provided in the IRS: steel bridge code to obtain these values. Maintenance crews need to have better access to all the lubrication points and grease fittings. These bearings rock easily, and special fixtures or temporary constructions are often needed to prevent any unwanted movements during the building of all structural elements.