The chains moving escalator steps in shopping malls and subway stations require automatic lubrication systems that apply fresh grease every few hours, since cleaning them manually would require shutting down transit for days. These chains carry thousands of pounds constantly moving up and down, with special grease that won't drip onto passengers' clothes or create slippery spots on steps. A single busy escalator can wear through several gallons of lubricant per year from the constant grinding motion of hundreds of chain links moving over metal guides. The grease must also resist breakdown from the heat generated by friction, as an under-lubricated escalator can actually smoke and catch fire from the intense metal-on-metal contact.