How the SKL System Works
The SKL rail fastening system is a complete elastic fastening solution for concrete sleepers. Its working principle is simple: a spring clip is deflected over the rail foot and held in place by the geometry of the guide plate and shoulder, so that the clip stores elastic energy and presses the rail down with a controlled toe load. The screw spike, anchored into a plastic dowel cast in the sleeper, holds the whole assembly against vertical and lateral forces. Because every part is replaceable, maintenance crews can swap a damaged clip or pad without disturbing the sleeper or the rail.
The Five Core Components
SKL tension clip: the elastic element that creates the clamping force on the rail foot. Available as SKL1, SKL3, SKL12 and SKL14 with different toe loads and leg geometries.
Screw spike: the anchor that holds the system to the concrete sleeper, in grade 4.6-8.8 carbon steel.
Plastic dowel: a sleeve cast into the sleeper hole (HDPE or reinforced nylon PA66) that receives the screw spike and protects the concrete thread.
Guide plate: sits under and beside the clip, positions the rail foot and transfers lateral forces to the sleeper; made of reinforced PA66 for UIC54, UIC60, 50 kg/m and 60 kg/m rail.
Rail pad: an elastic sheet between the rail and the sleeper seat that absorbs impact, reduces noise and provides electrical insulation; EVA or rubber as ordered.
Specifications for One Unit of SKL System
| Item | Quantity | Type | Material | Remark |
|---|---|---|---|---|
| SKL tension clip | 2 | SKL1, SKL3, SKL12, SKL14 | 60Si2CrA, 38Si7 | Hardness 42-47 HRC; standards DIN 17221, GB/T 1222 |
| SS35 screw spike | 2 | M24x150 mm | Q235 (4.6), 35# (5.6), 45# (8.8) | Standard ISO 898-1, UIC 864-1, NF F50-050 |
| Flat washer | 2 | Uls7 | Q235 | Standards EN 10025, EN 10139 |
| Plastic dowel | 2 | For screw spike size | HDPE, reinforced PA66 | Inserted into concrete sleeper |
| Guide plate | 2 | UIC54, UIC60, 50 kg/m, 60 kg/m | Reinforced PA66 | Different designs available |
| Rail pad | 1 | As ordered | EVA or rubber | Size matched to rail base |
Material Selection in the System
Clip steel is chosen for fatigue resistance: SKL1 and SKL3 use 60Si2CrA, SKL12 uses 38Si7, and all are quenched and tempered to 42-47 HRC so the toe load stays stable over millions of cycles. Screw spikes follow the ISO 898-1 property classes: grade 4.6 (Q235) for light traffic, grade 8.8 (45#) for heavy haul. Plastic dowels must be dimensionally stable in the sleeper concrete; PA66 with glass reinforcement gives higher pull-out values than HDPE and is preferred for heavy-load lines. The guide plate geometry determines the toe load of the assembled system, so plates and clips must be ordered as a matched set.
Installation and Maintenance
The system is installed by placing the pad and guide plate on the sleeper seat, setting the rail, dropping the clip into position, and pressing or driving the clip over the rail foot; the screw spikes are then torqued to the drawing value. No special tools beyond a clip press and a torque wrench are needed. For maintenance, a clip is released with the same press and replaced in minutes; the pad is inspected for cracks and the spike torque is checked at the same visit. On lines with extreme weather, use the environmental variants: low-temperature clip steel and neoprene pads for cold regions, zinc flake-coated spikes for coastal sites.
Ordering a Complete System
State the rail section (UIC54, UIC60, 50 kg/m, 60 kg/m) to match the guide plate.
State the sleeper type and hole spacing for the dowel size.
Choose the clip type by axle load: SKL1/SKL3 for standard, SKL12 for heavy haul.
Specify spike grade and surface treatment for the environment.
Request the pad grade (EVA or rubber) and any insulation requirement.
Frequently Asked Questions
Q: What is the difference between SKL1 and SKL12 clips?
A: SKL1 and SKL3 are lighter clips for standard lines, while SKL12 and SKL14 are heavier variants with higher toe load and thicker legs, specified for 75 kg/m heavy-haul rail and 30 t axle loads.
Q: Why is a plastic dowel used in the concrete sleeper?
A: The dowel provides a clean, reusable thread seat in the concrete, protects the concrete from spalling, and lets the screw spike be removed and re-inserted many times during maintenance.
Q: Can the SKL system be used on wooden sleepers?
A: The standard SKL system is designed for concrete sleepers with cast-in dowels. For wooden sleepers, other fastening systems with different plates and spikes are normally used.
Q: What torque is applied to the screw spikes?
A: The torque is defined in the system drawing, typically in the range of 150-250 N·m depending on spike diameter and grade. Over-torquing can crack the dowel; under-torquing reduces the toe load.
Q: How is the rail pad insulation tested?
A: Insulating pads are tested for electrical resistance between the rail and the sleeper anchor; typical requirements are in the order of several kilohms per rail seat for track-circuit compatibility, verified with a megohmmeter before traffic opens.