A rail fish plate, also called a joint bar or splice bar, is a steel jointing bar that restores effective sectional strength at rail joints by clamping the rail webs together, allowing loads to be distributed across both rails. Rail joint bars are bolted in pairs to the sides of rail ends, creating structural continuity between rail sections. This article explains how rail joint bars work in railway tracks and why they are critical for jointed track systems.
Structural Connection and Load Distribution
Joint bars act as a bridge between two rail ends, bolted securely through the web of the rail to create a continuous beam. The bars are placed on both sides of the rail web and clamped by fish bolts, so that the two rail ends behave as one structural member. This connection maintains the vertical and lateral alignment of the rails at the joint, preventing vertical dips or horizontal misalignments that could lead to derailment.
By spanning the gap between rails, joint bars transfer vertical, lateral and longitudinal forces from one rail to the next, reducing the impact on the rail joint. The wheel load is shared between the two rail ends through the bars, which reduces the stress at the rail ends and prevents the formation of steps or battered joints. The fish bolts are tensioned so that the friction between the bars and the rail web contributes to the load transfer.
Thermal Expansion Management
Joint bars allow for a small, engineered gap between rail ends, enabling the metal to expand in hot weather and contract in cold weather without causing the track to buckle. The expansion gap is designed according to the rail length and the local temperature range, and the joint bars hold the rail ends in alignment while the gap changes with temperature. This is particularly important in areas with significant temperature variations.
Materials of Rail Joint Bars
Standard steel joints are typically made from hot-rolled steel designed for high tensile strength, often heat treated to improve wear resistance. Common steel grades include 45#, 55# and 50Mn, and microalloyed rail steel such as U71Mn for heavy-duty applications. Insulated joints are made of high-performance fiberglass, polyurethane and thermosetting composites, providing electrical insulation and corrosion resistance for signaling circuits. The material choice depends on the rail section, the traffic load and whether the joint must be electrically insulated.
Typical Joint Bar Dimensions
| Rail type | Length | Width | Thickness | Bolt size |
|---|---|---|---|---|
| QU70 crane rail | 760 mm | 70 mm | 20 mm | M24 |
| QU80 crane rail | 820 mm | 75 mm | 20 mm | M24 |
| QU100 crane rail | 950 mm | 80 mm | 22 mm | M27 |
| QU120 crane rail | 1050 mm | 85 mm | 26 mm | M27 |
Fish plates for standard rails are supplied in 4-hole or 6-hole configurations with lengths such as 24 inch or 36 inch, and the hole spacing and diameter are matched to the fish bolt size and the rail section.
Maintenance and Repair
Joint bars facilitate easier removal and replacement of individual rail sections during track maintenance, because the joint can be dismantled by removing the fish bolts. During inspection, check the fish bolts for loosening, the bars for cracks and corrosion, and the rail ends for wear and batter. Retighten the bolts to the specified torque, and replace worn bars and bolts promptly. Insulated joints require special attention to maintain the insulating properties and prevent electrical leakage.
Common Misconceptions
A common misconception is that rail joints are weak points that should be avoided everywhere. In fact, jointed track with properly installed joint bars is reliable and economical for many lines, and compromise joints are necessary where different rail sections meet. Another misconception is that the fish bolts only hold the bars in place; in reality, the bolt tension generates friction that is essential for load transfer. Some also believe that longer joint bars are always better; the length and hole pattern must match the rail section and the standard, otherwise the joint will not perform correctly.
Frequently Asked Questions
What is a rail joint bar? rail joint bar, also called a fishplate, is a steel plate bolted in pairs to the sides of rail ends to create structural continuity between rail sections.
How do joint bars transfer load? The bars span the gap between the rail ends and transfer vertical, lateral and longitudinal forces from one rail to the next, reducing stress at the joint.
What materials are joint bars made of? High-strength steel such as 45#, 55#, 50Mn and U71Mn, with fiberglass and thermosetting composites for insulated joints.
Why is there a gap between rail ends? The gap allows the rail to expand and contract with temperature changes without buckling the track.
What is an insulated joint used for? Insulated joints separate track circuits electrically while maintaining mechanical continuity, which is essential for signaling systems.
How are joint bars maintained? Inspect the bars and bolts regularly, retighten fish bolts to the specified torque, and replace cracked or corroded bars promptly.
