Caterpillar Inc. opted not to increase its 2018 earnings forecast this quarter, disappointing investors on Tuesday after two straight quarters of raised expectations, but the heavy-duty equipment maker did report quarterly profit that beat market estimates.

The company kept unchanged the 2018 adjusted profit per share outlook of $11.00 to $12.00 per share, which didn't go down well with investors who were expecting yet another upgrade in the earnings guidance.

This led to a sell-off in the heavy-duty equipment maker's shares in pre-market trading. The stock was last trading down 6 percent before the opening bell on Tuesday.

"Earnings were fine," said Stephen Volkmann, equity analyst at Jefferies. "But, clearly, people were expecting another strong quarter and this one was not."

Caterpillar (CAT.N) reported an adjusted profit of $2.86 a share in the third quarter, compared with $1.95 a share, last year. Analysts on average had expected earnings of $2.85 a share, according to Refinitiv.

Net profit for the quarter through September came in at $2.88 per share, compared with $1.77 last year.

The Deerfield, Illinois-based company has boosted the full-year profit outlook twice this year, betting on a global economy that is poised to record its strongest growth since 2011.