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Image source: Caterpillar.

2015 was a terrible year for heavy-equipment specialist Caterpillar (NYSE:CAT). Constant struggles from customers who increasingly found themselves unable to afford major capital expenditures weighed on Caterpillar's ability to keep its own revenue up, and the weakness in the energy sector was only the most recent in a long series of setbacks for the industries that the equipment-maker serves. Amid all the gloom, few headlines seemed more troubling than the company's own financial results, which seemed to steadily worsen over the course of the year. Let's look more closely at Caterpillar's worst headlines in 2015.

First-quarter results: Looking ahead to tougher times
The one ray of sunshine in Caterpillar's financial picture in 2015 came early in the year, when the company reported its first-quarter results. The company boosted its earnings per share by more than 25%, beating expectations, and it raised its guidance for full-year earnings by $0.10 per share.

However, Caterpillar foreshadowed tougher times ahead. A big part of the first quarter's performance came from the one-time sale of its logistics business. Moreover, Caterpillar noted that the worst impact from lower oil prices hadn't yet surfaced, and executives predicted "an uncertain and challenging year" ahead for the company. Given the weak results in Brazil and China and ongoing uncertainty about Europe's ability to recover, the first quarter set the stage for what would become a disappointing year.

Second-quarter results: On their way down
By July, Caterpillar's second-quarter results showed the full extent of the difficulties that the company faced. Sales fell 13%, cutting profits by 25% to 30% depending on whether you include the costs of restructuring Caterpillar's operations in the mix. Across the globe, Caterpillar saw big declines, with the Asia-Pacific and Latin American regions posting the largest revenue drops. Even as energy weighed on the company, construction was the worst performer, with the combination of a strong dollar and weak end-market demand leading to an 18% slump.

Even worse, Caterpillar remained pessimistic about its immediate future. "Many of the key industries we serve remain weak," CEO Doug Oberhelman said in the conference call following the second-quarter report, "and we haven't seen sustained signs of improvement." In particular, the company still believed that pressure in energy would lead to worsening results later in the year.

Third-quarter results: Another quarter, another drop
In October, Caterpillar's third-quarter results proved to be the most concerning yet for the heavy-equipment maker. A 19% drop in revenue sent net income down by two-thirds, and even adding back restructuring costs didn't make the company's financials look much rosier. All of the company's regions suffered considerable sales declines, with Latin America standing out because of its declines of more than 30%. Energy and transportation suffered a 25% drop in revenue, while other segments consistently posted double-digit sales declines of their own.

More extensive restructuring efforts will cost the company about $0.90 per share in earnings in the full 2015 year, but Caterpillar also believes that 2016 will continue to be weak. Overall, the company guided revenue down about 5% from 2015 levels. Sector by sector, construction should hold up best, with the potential to post flat sales if things go well. In energy and transportation, though, the company expects a 5% to 10% drop, and Caterpillar thinks that the resource industry will suffer a 10% decline.

Through all of this bad news, Caterpillar has tried to stay focused on things that it can control. Its restructuring efforts have resulted in massive savings from job cuts and productivity-enhancing initiatives. Nevertheless, with none of its optimism showing up in its financial results during the year, it's been hard for Caterpillar to generate much excitement among its investor base that any near-term rebound is likely to happen.

Dan Caplinger has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.