The difference in linear expansion of the carbon anode and the steel yoke or spider of the anode rod at high temperatures in the reduction cell means that the yoke expands more than the anode, bending the stubs which are anchored in the anode block.
Stubs become bent inwards just above the iron thimble, changing the geometry of the yoke, and stubs can no longer be correcly located in the anode holes during subsequent rodding. Ultimately the stubs will no longer fit into the holes.
Historically, smelters often continued to use toed-in stubs until they would no longer fit into the anode, when the bent stubs would be cut off and replacements welded onto the yoke. Today, many smelters accept that the use of a stub straightener has a cost benefit. Straight stubs reduce operational difficulties and improve the stub-carbon volt drop.
VHE of Iceland has proven solutions suitable for different reduction technologies. For smaller diameter stubs, ambient temperature straightening is an economical approach. VHE has delivered a number of such stub straightening machines to smelters around the world.
For larger stub diameters, hot straightening is often a better solution. Stubs heated to 650°C need considerably less straightening force, and the cost of an induction pre-heating unit is to a large extent off-set by savings in the heavy duty steelwork and hydraulic systems which would otherwise be needed. VHE has delivered such stub straighteners for stubs up to 180mm to a number of smelters internationally.