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Molded thermoset composite replaces cast iron
New third-rail supports insulate and resist corrosion
By Robert N. Boggs, Southern Technical Editor -- Design News, November 19, 1995
Garner, NC—A cast-iron bracket supports the third rail of several electrified rail lines. Bolted to an extended railroad tie, the bracket supports an insulator, which in turn supports the contact rail.
Metro-North Commuter Rail Road is the second most densely travelled passenger rail line in the U.S. The company experienced failure of some of their cast iron brackets in the Park Avenue tunnel. Engineers traced the cause to salty water from the street above corroding the brackets. They also suspected the cast iron brackets of undesirable and possibly dangerous current leakage, even though--theoretically--they are not in the electromagnetic circuit.
As an alternative to cast iron, Penn Compression Moulding adapted composite molding compound technology to produce support brackets. Identical mounting dimensions make the composite brackets direct replacements for cast iron. The material is a glass reinforced vinyl ester thermoset composite. In addition to the necessary structural strength and corrosion resistance, it supplies extra insulation not provided by the cast iron.
Two types of physical tests compared the strength characteristics of the cast iron and composite brackets. In one, a hydraulic cylinder applied a lateral force to failure. In the other, a falling weight supplied an impact force in the same lateral direction. When engineers apply force slowly, the cast iron exhibits higher strength values. In the impact mode, however, the composite brackets absorb more than twice as much energy as the cast iron.
In both cases, the cast iron failed catastrophically with practically no deflection before brittle failure. The composite brackets yield considerably before delamination begins. In addition, the composite brackets do not totally separate from the base. At lower temperatures, the cast iron exhibits lower strength values, while the composite values are higher.
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"These composite materials really perform in high-corrosion, high-stress environments," says Robert Farrell, corporate engineering manager for Penn. "They provide both electrical and thermal insulation in structural applications."
Composite brackets cost almost 12% more than the cast iron. Self-extinguishing and impervious to the effects of arcing and chemicals, the material is UV-stable. The brackets weigh about one-third less than cast iron brackets. Minimum life expectancy is 25 years.
Additional details...Contact Robert G. Farrell, Penn Compression Moulding, Inc., 1027 Highway 70W, Garner, NC 27529, (919) 779-4474.
