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Automakers are pushing factories and workers to the limit to try to meet burgeoning demand for new vehicles.

USA Today reported that some plants are adding third work shifts. Others are piling on worker overtime and six-day weeks. Ford Motor and Chrysler Group are cutting out or reducing the annual two-week July shutdown at several plants this summer to add thousands of vehicles to their output.

“We have many plants working at maximum capacity now,” says Ford spokeswoman Marcey Evans. “We’re building as many (cars) as we can.”

The automakers’ problem now is one they welcome: It’s hot demand. Sales for 2012 are estimated at 14.3 million vehicles, according to IHS Automotive, up from 12.8 million last year.

Since the boom years when the industry made about 16 million vehicles a year, automakers have slimmed down for the new reality. They have no excess capacity but don’t want to open new plants and risk having to repeat the recent painful and expensive closings if demand falters. They vow never again to have too many factories making too many cars that then required huge sales incentives to move.

As demand rises, they push existing plants and workers harder. “Some of the folks are working 60 hours a week, week after week,” says Kim Hill, director of economic development strategies for the Center for Automotive Research.”When do you back up against a wall and you can’t run your workers any longer?”