Mechanic's Toolbox: Thermite Welding

A transportation network as complex as ours offers some unique challenges for repairing the equipment, machinery, and vehicles that serve our system. This week's photo excursion takes a look at one type of repair that is a little more exciting than your average nuts and bolts fix.

Welding Broken Shaft at Washington Mason Powerhouse | September 15, 1936 | U16182

This 1936 shot catches a shower of fire, smoke, and molten metal pouring out of a welding box during a repair to the main cable-winding machinery. This huge machine, very similar to the ones still in use today at the Washington-Mason Cable Car Powerhouse, is used to pull the wire rope (cable) that drives the Cable Car system.

The main shaft of the machine broke after years of almost continual use and stress, so mechanics employed a unique repair method to mend the shaft without disassembling the machine itself, which is an enormous task. Using a technique called thermite welding (more info on Wikipedia here), where a mixture of powdered metals (typically aluminum and iron oxide) are ignited to melt the parts back together.

Interior Shot of Welded Shaft at Washington Mason Powerhouse | September 15, 1936 | U16183

The result is a permanent repair that is as strong as the original metal. Thermite welding is still used today by the SFMTA track crews to bond sections of track together at joints for seamless and smooth operation of light rail vehicles and streetcars system-wide.

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