The time has come. This specific gearbox has possibly been around before you started working at your facility. It's always having issues like mechanical failures and needs more maintenance than your team has time to complete.
The CAD drawings might be missing, the manufacturer might be no longer in business to replace the gearbox or components, or you've been ready to switch brands altogether. If this sounds like your situation, you'll need to use this gearbox replacement checklist before replacing your gearbox.
It would be best if you had some more reliability--and those "other" gearboxes and reducers can't handle your heavy-duty application. Kick that obsolete unit to the curb today.