Falling profits, muted outlook, aggressive restructuring -- that pretty much sums up Cummins' (NYSE:CMI) last year. Or should I say, it sums up 2015 for most heavy-machinery companies.

But Cummins has been among the biggest losers in the industry over the past year, falling more than rival Caterpillar (NYSE:CAT) despite a smaller exposure to the struggling mining and oil and gas sectors. In fact, Cummins also took a bigger hit than PACCAR (NASDAQ:PCAR), its key customer in the trucking industry.

CMI Chart

CMI data by YCharts.

Cummins is clearly under tremendous pressure, and there's little to suggest that things will improve anytime soon. Take a look at the slideshow below to learn about the three biggest hurdles to the company's growth this year.