Operations personnel who know your machinery backwards and forwards are sometimes more important and far more critical than the sales representatives who sells those products in the marketplace. These important people are being recruited by other companies all of the time, so if you are dependent upon one individual for a certain product line or service, you have put yourself at risk.
Pay good knowledgeable sharp operators well. Pay the current operator well and be glad to pay him a premium for cross training the second and third backup operators. Make it worth his time to cross train others and reassure him of his continuance with the firm.