Illinois Tool Works (NYSE:ITW) released sales figures today that appeared to show that the diversified manufacturer's business lines continue to benefit from the strengthening global economy.

The company reported that for the three months ended in May, operating revenue increased 21%. Although acquisitions and currency gains accounted for most of the gain, the 10% increase in base sales seems impressive for such a large player. Illinois Tool is confident that this momentum will continue because it increased its second-quarter and full-year earnings forecasts.

While the company's revelations appear to be good news, investors responded by trading the stock down on fairly heavy volume. One factor that may be causing concern is the firm's international business. From March to May, Illinois Tool's North American sales continued to show momentum compared with the first-quarter period from February to April, but growth in the international revenue appeared to tail off a bit.

Illinois Tool's second-largest business line, engineered products, is concentrated in the construction and automotive areas. The European automotive business, in particular, could present a continued challenge for the company. While North American vehicle sales grew at a fairly brisk 3.4% in May, European sales registered a more meager gain of 1.2%. The shaky recovery in Europe is dogging many carmakers. General Motors (NYSE:GM), for example, recently disclosed that it expects its European unit to lose money again this year, a trend that may prompt the automaker to restructure its operations on the continent.

Nevertheless, much of Illinois Tool's business looks to be on the upswing. The strong recovery in North American manufacturing should be an especially big boost for the firm's specialty systems offerings, which include industrial packaging and welding equipment. With the company's stock trading at 25 times trailing earnings, though, investors seem to be feeling cautious.

Fool contributor Brian Gorman is a freelance writer living in Chicago, Ill. He does not own shares of any companies mentioned here.