Savvy patients know to seek the appropriate specialist when addressing medical concerns. When industrial sectors fall ill within an impaired economy, Fools must find the specialists that may have discovered a cure.

Let's begin with a glance into third-quarter earnings for two heavy hitters among the manufacturers of heavy equipment.

Tyrannosaurus Terex
In a global economy that appeared to be firing on all cylinders, diversified equipment maker Terex (NYSE:TEX) blossomed. From 2003 through a sharp peak in late 2007, Terex shares multiplied more than 13-fold. Once the collapse in construction demand from U.S. and European markets set in, however, Terex's diverse product line shifted from an asset to a liability in this Fool's view. The company's third-quarter earnings continue to tell the same scary story.

Terex lost $103 million, reflecting a monstrous 51% revenue decline. The backlog of equipment orders over the next 12 months contracted by 58% to $1.52 billion, with new orders falling in all four of the company's business segments. Net sales experienced their biggest percentage declines in the aerial work platforms unit, down a gut-wrenching 66.5% year over year. Construction equipment logged a 55.9% decline, while the cranes and materials processing units recorded lesser (though substantial) relative weakness.

Meanwhile, Terex investors will recall that shares were diluted by more than 10% back in May, when the company issued 11 million new shares to shore up its beleaguered balance sheet. Along with $450 million in notes, these capital raising efforts doubled Terex's cash balance to more than $1 billion, while contributing to rising long-term debt that now stands at $1.9 billion. Through it all, the manufacturer's net debt-to-capitalization ratio has barely budged ... standing at 34.8% as of Sept. 30.

Bucyrus has no virus
The third-quarter results from mining equipment specialist Bucyrus (NASDAQ:BUCY) tell an entirely different story. The company's backlogs are indeed declining, as a reduced rate of new equipment orders permits the company to work through the backlog. But noteworthy strength in demand for after-market parts to maintain the existing fleet of Bucyrus' monster machines has bolstered earnings results considerably.

Bucyrus earned a very respectable $92.1 million in the third quarter, representing a 43.5% increase over the prior-year period. Sales grew by a more modest 5%, highlighting a 500-basis-point increase in gross profit margin year over year. Sales from the company's underground mining segment posted the greatest strength, marking a 17.5% gain and cementing the accretive contribution of Bucyrus' 2007 acquisition of DBT. The underground unit now accounts for 54% of total sales.

To be sure, Bucyrus has not been spared the impacts of an impaired global credit environment, nor near-term weakness from within markets like that for domestic thermal coal. Bucyrus' backlog has receded by more than 22% since the start of the year, to $1.94 billion. New orders logged during the third quarter were a full 36% off their prior-year pace, including a whopping 53% decline in orders for original equipment.

Clearly, customers are deferring new equipment purchases in favor of extending the operating lives of fleets through rising investments in aftermarket parts. That distinction is visible in Bucyrus' sales results as well, where original equipment sales declined 1.8%, while aftermarket sales surged by 11.8%. Terex, referring to its aerial work platform segment, confirmed that "Rental customers in the North American and European markets continued to age and reduce their fleets, deferring the purchase of new products."

Bucyrus' balance sheet remains solid, with a long-term debt of about $500 million that has barely budged in 2009. For context, consider that Bucyrus' inventories are valued at more than the company's debt, and the same can be said for accounts receivable. Bucyrus' net debt-to-capitalization ratio is just about 14%, or less than half Terex's figure.

Can you hear me now?
One year ago, I asked Fools to consider the comparative business profiles of these two manufacturing titans as they pertained to the specific and identifiable challenges facing the U.S. and European economies in particular. I predicted that mining equipment specialists such as Bucyrus and Joy Global (NASDAQ:JOYG) would fare better than their more diversified counterparts, because global mining activity would likely recover more quickly than construction and other consumer-dependent drivers of demand. Checking in with the group last February, I urged Fools to consider relative debt burdens carefully when selecting their picks in this sector. Over the past year, these two equities have outperformed their diversified peers by a wide margin.

The dual drags of diversification and debt have weighed heavily upon heavy equipment manufacturers like Terex and Manitowoc (NYSE:MTW). I still believe that these diversified players may again flourish on the heels of a broad-based industrial recovery, but recent indications from coal miners like Peabody Energy (NYSE:BTU) and steelmakers like Nucor (NYSE:NUE) reveal that we are not there yet. I continue to view this economic environment as far better suited to the mining equipment specialists, and I offer Caterpillar (NYSE:CAT) as a final addition to my short list of compelling long-term investments in industrial equipment.

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Fool contributor Christopher Barker is the Nat King of coal and the wild boar of iron ore. He can be found blogging actively and acting Foolishly in the CAPS community under the name TMFSinchiruna. He tweets. Chris owns shares of Peabody Energy. The Fool owns shares of Terex. Try any of our Foolish newsletters today, free for 30 days. The Motley Fool's disclosure policy is busy learning Mandarin.