Members of The Boeing Co.’s largest union overwhelmingly approved a motion Wednesday in Seattle’s KeyArena to sanction a strike against the company.

The strike sanction vote by the International Association of Machinists and Aerospace Workers District Lodge 751 is not a sign that a strike is imminent, but that the lodge is taking preliminary steps to prepare for one if necessary.

Union spokeswoman Connie Kelliher says around 15,000 members of the Machinists union showed up Wednesday at KeyArena. More than 99 percent voted in favor of the strike authorization.

Kelliher says union members in Wichita, Kan., and Portland also approved strike authorization by similar margins.

A successful strike sanction vote will enable the national union to pay workers strike benefits if necessary and is a shot across Boeing’s bow in negotiations.

Boeing union members who worked at least a partial shift will be excused for the rally but won’t be paid for the time off, the union said. The rally usually slows production at Boeing plants to a crawl.

The union has more than 20,000 members in the Puget Sound area, including workers at Boeing’s plant at Frederickson in Pierce County.

The contract between Boeing and the union is set to expire Sept. 3.

An additional vote by union members would be required before any walkout could occur.

Boeing spokesman Tim Healey said the company has provided the union with both its incentive plan and health care proposals much earlier than normal to allow sufficient time to discuss the possibilities.

Healey said the company is disappointed that the union scheduled its vote for a work day because of the impact that members’ absences have on production schedules.

The company has said it wants to institute a modified pension plan for new hires beginning with the new contract, but it has yet to make a formal proposal on that issue.

That new pension plan proposal will call for a defined contribution rather than a defined benefit plan, which means the amount of money contributed will be specified but not the benefit a retiree would draw.

The union wants substantial wage and benefit increases because of Boeing’s unprecedented backlog of commercial airplane orders and high profits. It wants to retain and enhance the current defined benefit pension plan for all workers.

John Gillie: 253-597-8663

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The Associated Press contributed to this report.