What a Railway Screw Spike Does
A railway screw spike is a threaded fastener driven into a timber sleeper to hold a tie plate or sole plate against the sleeper, which in turn locates the rail foot and preserves gauge. Unlike a smooth shank dog spike, a screw spike is installed into a pre-bored hole and grips the wood fibres along the full length of the thread, so it resists extraction far better under repeated wheel load and vibration.
Screw spikes are used wherever timber sleepers remain in service: secondary lines, yards, bridges with timber decks, turnout areas and maintenance work where existing sleeper holes must be reused.
Geometry and Thread Forms
A typical screw spike consists of a forged or rolled head, a plain shank and a tapered thread. Two thread families are common in the trade: a coarse wood screw thread with a sharp root taper, and a lag thread that resembles a machine screw thread with a cylindrical core. The larger core diameter of a lag thread raises pull out resistance, while the coarse thread cuts more freely in dense hardwood. Common shank diameters range from 16 mm to 24 mm with lengths from 150 mm to 260 mm, and oval neck heads, square heads and countersunk heads are all produced. Gauge and hole position tolerances on the tie plate are as important as the spike itself, since a misaligned plate forces the spike to bend during driving.
Materials and Corrosion Protection
| Item | Typical specification | Notes |
|---|---|---|
| Base material | Q235 to GB/T 700, S235JR to EN 10025-2 | Carbon structural steel with adequate ductility for cold driving |
| Alternative material | Medium carbon steel such as 45 to GB/T 699 | Higher strength where pull out loads are critical |
| Hot dip galvanising | ISO 1461 | Minimum local coating 70 micrometres and minimum average 85 micrometres for steel 6 mm and above |
| Mechanical zinc | ASTM B695 class 50 | Nominally 50 micrometres, used where hot dip temperatures are unsuitable |
| Salt spray assessment | ISO 9227 | Neutral salt spray used to support corrosion claims |
Coating choice is driven by the environment. Dry inland track can use plain oiled spikes; coastal, tunnel and wet cutting locations justify hot dip galvanising or an approved alternative coating. Thread surfaces must remain free of heavy coating build up so that driving torque stays within the range the sleeper can accept.
Installation Practice
Correct installation begins with a pilot hole. For dense hardwood the hole diameter is normally set between about 0.6 and 0.8 times the shank diameter, and it is drilled to slightly more than the threaded length so that the plate can be drawn down without splitting the sleeper. Pre-boring is essential on dry, seasoned hardwood, where driving without a pilot hole splits the sleeper and destroys the grip. The spike is then driven or power turned until the head seats on the plate, without over-torquing: excessive torque shears the wood fibres around the thread and reduces pull out capacity. On relaid track, hole positions are offset from previous holes so that every spike develops fresh wood grip.
Failure Modes and Inspection
Field failures fall into four groups. Thread stripping in the wood reduces restraint and shows up as plate movement and gauge variation. Corrosion at the shank to head transition eventually causes head loss. Fatigue cracking appears where the spike vibrates against an oversized plate hole. Bending occurs when the plate is not bedded or when a spike is driven into an obstruction. Inspection intervals should therefore include a check of plate bearing, spike head seating, visible corrosion and gauge in the vicinity of every suspected location.
Frequently Asked Questions
Q: What is the difference between a screw spike and a dog spike?
A: A dog spike has no thread and relies on friction in the wood, while a screw spike is turned into a pre-bored hole and uses its thread to resist extraction, giving much higher pull out resistance under vibration.
Q: Why must a pilot hole be drilled?
A: Boring prevents splitting of seasoned or dense timber and allows the spike to cut a clean thread rather than crushing the fibres, which would reduce grip.
Q: Which coating is used for coastal track?
A: Hot dip galvanising to ISO 1461 is the usual choice, with a minimum average coating thickness of 85 micrometres for steel 6 mm and above. Mechanical zinc to ASTM B695 class 50 is an alternative where hot dip processing is unsuitable.
Q: What causes a screw spike to lose grip?
A: The usual causes are an oversized pilot hole, over-torquing during installation, split sleepers, or re-driving into old holes. Correct hole sizing and fresh hole positions restore the required restraint.
Q: How are screw spikes tested after production?
A: Dimensional and thread gauging, hardness checks, coating thickness measurement, and where agreed, pull out or proof load testing on samples from each batch.
Q: Can screw spikes be reused?
A: Reuse is normally limited to maintenance work in the same hole size and is not recommended where gauge restraint is critical, because a previously stressed thread and a worn hole both reduce pull out capacity.
