The Hagerstown plant builds Mack T-200 transmissions, which are more common in dump trucks, cement mixers and other work trucks that take more abuse and pull more weight, Rhoads said.
The plant's 1,200 employees also build Mack engines.
Before the Eaton agreement, highway truck customers were requesting transmissions made by the Cleveland, Ohio, company, he said.
Customers can still request Mack transmissions, he said.
The editor of Heavy Duty Trucking magazine in Irvine, Calif., said he doesn't think the agreement will take much business away from the Hagerstown plant.
"There are a lot of die-hard Mack users who want everything Mack in their trucks," Andrew Ryder said.
With the Eaton agreement, Mack is simply trying to keep up with a market that demands more choices, he said.
"For the future, we are committed to the T-200 transmission," said Paul L. Vikner, Mack executive vice president for sales and marketing, in a prepared statement.
Mack holds the third-largest market share in the trucking industry, Ryder said.
News of the agreement with Eaton had touched off some concerns among Mack employees about using other companies' products.
More rumors of "outsourcing" - the use of outside contractors to perform some tasks - started to fly this week when a machine in the transmission shop broke down and the company temporarily bought parts elsewhere, Rhoads said.
"It's a standard procedure," and does not signal the rise of outsourcing, he said.
Outsourcing is a sensitive subject for Mack workers. In the mid-1980s, the company lost hundreds of jobs when its rear axle carriers were outsourced.
Jim Stewart, president of United Auto Workers Local 171, could not be reached for comment Wednesday.