Private equity firm Key Principal Partners has invested $40 million in the subordinated debt of Western Express Inc. to facilitate the trucking company’s acquisition of Smithway Motor Xpress.

Key Principal Partners said Western Express is a truckload carrier founded in 1991 by Wayne Wise. The company has grown to become a top-15 truckload carrier in the United States through a combination of internal growth and acquisitions.

Smithway Motor Xpress, based in Fort Dodge, Iowa, is a flatbed and van truckload carrier that serves customers primarily in the building materials, steel and heavy equipment industries.

Key Principal Partners managing partner Leland Lewis said in a statement that Western Express “has expanded significantly over the past several years through strategic acquisitions like this one, and we’re confident that the company is well positioned for continued growth.”