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Overview of the FTI RailTec CBX Process

Imagine a rock sitting in a fast flowing stream; the smoother the rock, the faster and smoother the water flows around it. Cyclical stress works in the same way around a bolt hole. The bolt hole is a stress concentrator; if there is any flaw in the hole, cracks can appear and lead to its failure.

Bolted rail joints are a known cause of derailments and require routine inspection. When cracks are detected in rail joints, it can mean imposed speed and weight restrictions and lead to expensive unscheduled maintenance costs.

The FTI Split Sleeve Cold Expansion Process (RailTec) is superior to any other method of cold working holes process in current use. It generates a more controllable distribution of compressive stress around the hole without damaging its integrity.

Split Sleeve Cold Expansion is accomplished by pulling a tapered mandrel, pre-fitted with a lubricated split sleeve, through a hole in the rail. The disposable sleeve smoothes the mandrel pull force, ensuring uniform radial expansion of the hole and allows one-sided processing.

Cold expansion counteracts the fatigue-prone characteristics of a hole by creating a compressive residual stress field around the hole. This effectively shields the hole from the cyclic tensile stress loads that cause cracks to form and grow, thereby improving the fatigue and damage tolerance of the rail.

 

 

Zone of compressive residual stress surrounding a cold expanded hole as seen through a polarised filter.

 
       

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