SEATTLE — Factories at Boeing Co. were due to start humming again after Machinists union members voted to end a costly eight-week strike that clipped profits and stalled deliveries by the world's No. 2 commercial airplane maker.

Workers returned Sunday night to Boeing's commercial airplane factories, which have been closed since the Sept. 6 walkout. The strike cost an estimated $100 million a day in deferred revenue and production delays on the company's highly anticipated next-generation passenger jet.

Machinists union members ended their walkout on Saturday by ratifying a new contract with Boeing. Members of the union, which represents about 27,000 workers at plants in Washington state, Oregon and Kansas, voted about 74 percent in favor of the proposal five days after the two sides tentatively agreed to the deal and union leaders recommended its approval.

The union has said the contract protects more than 5,000 factory jobs, prevents the outsourcing of certain positions and preserves health care benefits. It also promises pay increases over four years rather than three, as outlined in earlier offers.

The union members, including electricians, painters, mechanics and other production workers, have lost an average of about $7,000 in base pay since the strike began. They had rejected earlier proposals by the company, headquartered in Chicago.