Cheaper oil is creating winners and losers among America’s factories. Manufacturers who use oil and products derived from it are saving big money. But energy development has been a hot spot in the post-recession U.S. economy, benefiting producers of everything from steel pipes and valves to earthmovers. Those companies are now being squeezed. Among those feeling the pinch is Dover Corp. , which earns a quarter of its revenue from selling pumps and other heavy equipment to energy companies. “Clearly there are near-term challenges in front of us,” Chief Executive Bob Livingston told investors in New York on Monday. Dover said it hasn’t yet seen a pullback in oil drilling, but the company is cutting costs and re-evaluating some investments in anticipation of reduced oil exploration in the U.S. next year. General Electric Co. in October cut projections for future growth in its oil and gas business, adding it was monitoring the sector with “caution.”