Even with a lock washer, I continue losing the nut off my mower.

Use a lock nut with a nylon insert. This arrangement will not vibrate loose as readily.

Check your state and local codes before starting any project. Follow all safety precautions. Information in this document has been furnished by the North American Retail Hardware Association (NRHA) and associated contributors. Every effort has been made to ensure accuracy and safety. Neither NRHA, any contributor nor the retailer can be held responsible for damages or injuries resulting from the use of the information in this document.