The Thermal Environment of a Railway Track
A rail clip does not work at a fixed temperature. Rail surface temperature in service routinely swings from about -40 deg C on cold-region networks to more than +60 deg C on the sunlit rail of a hot-climate line, and the daily range can exceed 30 deg C. The rail expands and contracts with this swing, while the clip must keep holding the rail foot with an adequate clamping force throughout. Designers therefore treat temperature as a load case: the fastening system must resist the longitudinal forces generated by thermal movement, keep the toe load within specification after years of temperature cycling, and avoid brittle failure of the spring steel in deep winter. These three demands, thermal restraint, clamping force stability and low-temperature toughness, define most of the practical engineering around temperature and clips.
How Temperature Changes Clip Clamping Force
Clamping force, or toe load, is produced by deflecting the clip into its working position. Spring steel such as 60Si2MnA (GB/T 1222) has a rated service range of roughly -40 to +80 deg C. Within this range the elastic modulus of steel falls by only about one percent from 20 to 60 deg C, so the immediate loss of clamping force from heat is small. The real long-term effect is stress relaxation: at elevated temperature a deflected spring slowly converts elastic strain into permanent set, so after a hot season the clip sits with a slightly smaller working deflection and a lower toe load. In cold weather the failure mode changes from relaxation to fracture. Below roughly -20 deg C the Charpy impact energy of ordinary silicon-manganese spring steel drops noticeably, and a clip with a rolling seam, grinding mark or corrosion pit can crack when struck by a wheelset or during tamping. For this reason cold-region specifications require impact-tested material and controlled surface finish rather than a simple strength check.
Rail Expansion and the Forces a Clip Must Resist
The longitudinal movement of the rail is set by the coefficient of linear expansion of steel, about 11.5 x 10^-6 per deg C. The table below shows the length change for common rail section lengths; these are the movements the fastening system must control at joints, at the ends of continuously welded rail (CWR) and in rail expansion devices.
| Rail length | Temperature swing | Length change |
|---|---|---|
| 12.5 m | 30 deg C | 4.3 mm |
| 12.5 m | 50 deg C | 7.2 mm |
| 25 m | 30 deg C | 8.6 mm |
| 25 m | 50 deg C | 14.4 mm |
| 100 m | 30 deg C | 34.5 mm |
| 100 m | 50 deg C | 57.5 mm |
In hot weather the rail is in compression. If the fastening system loses grip, the longitudinal restraint drops and the rail can push itself sideways into a buckle, a sun kink, on lines where the ballast or sleeper resistance is also weak. In cold weather the rail is in tension and the risk moves to pulled joints or, on CWR, to a broken rail opening a wide gap. The clip therefore works together with the whole fastening system: its clamping force feeds the longitudinal rail restraint measured under EN 13146-1, and the toe load itself is verified under EN 13146-7 and matched to the system requirements of EN 13481-1. Clips on curves and on gradients see higher longitudinal force and need the same toe load as on straight track, which is why curve sections show loosening first.
Selecting Clips and Spring Steel for Hot and Cold Regions
For most projects one spring steel grade, 60Si2MnA quenched and tempered to a tensile strength of at least 1568 MPa, covers the standard range of -40 to +80 deg C. Where continuous temperatures above 80 deg C are expected, for example on heavy-haul desert lines, a chromium-alloy spring steel with better relaxation resistance is worth specifying, and the toe load should be set with the hot-season working deflection in mind. For very cold regions below -40 deg C, the purchase specification should add an impact test at the minimum service temperature and require surface defect-free material, because fatigue and impact performance, not static strength, decide winter survival. Stainless steel clips are sometimes proposed for temperature reasons; they are unnecessary for temperature alone and their lower yield strength compared with hardened spring steel means they should only be chosen for corrosion-driven applications with a confirmed strength calculation.
Seasonal Inspection and Re-tightening Practice
Maintenance engineers get the most value from two seasonal checks. Before winter, verify that toe load and clip position are within the system drawing, because a clip working with reduced deflection has no reserve to absorb impact in brittle conditions. After the hot season, re-tighten or re-set clips that show relaxation, and pay special attention to curves, gradients, joints and expansion device zones where the thermal load is highest. Use a calibrated torque wrench for systems that are torque-set, or a toe-load gauge where the system specifies one, and never re-use a clip that has taken a permanent set, been hit by a tamping tool or shows any crack. On new lines, schedule the first re-tightening one to three months after opening, when initial settlement has stabilised, and log the measured values so the attenuation trend of the section is visible before it becomes a geometry problem.
Frequently Asked Questions
Q1: Does a hot day directly reduce clip clamping force?
Immediately, by only about one percent, because the elastic modulus of spring steel changes little up to 60 deg C. The practical danger is long-term stress relaxation over many hot seasons, which slowly lowers the toe load. The correct response is periodic toe-load checking, not seasonal re-torqueing of healthy clips.
Q2: Why do clips fracture more in winter?
Because the toughness of carbon spring steel falls as temperature drops below about -20 deg C. A clip that already has a small crack, a grinding burn or a corrosion pit can then propagate to fracture under impact. Specify impact-tested material and inspect for surface defects before winter.
Q3: How much does a rail actually move with temperature?
Steel expands about 0.115 mm per metre per 10 deg C. A 25 m rail with a 50 deg C seasonal swing changes length by about 14 mm. Joints, expansion devices and the longitudinal restraint of the fastening system must absorb this movement.
Q4: What is a sun kink and how do clips prevent it?
A sun kink is the lateral buckling of CWR in extreme heat when the compressive force in the rail exceeds the combined restraint of fastenings, sleepers and ballast. A clip system with the specified toe load keeps longitudinal restraint high, which is a first line of defence against buckling.
Q5: Should torque values change with the season?
No. The torque or deflection target in the system drawing is valid all year. What changes is the inspection frequency: check before winter and after the hot season, and re-tighten only clips found below the target, using the tightening procedure of the fastening system.
Q6: Are stainless steel clips better in extreme temperatures?
Not for temperature. Austenitic stainless grades keep toughness at low temperature, but their yield strength is far below hardened spring steel, so a stainless clip needs a heavier section for the same toe load. Choose stainless only for corrosion-driven locations and verify the design with calculation.