Please do not use such an identifier as the primary key for linking records in relationships. Make it a data field in your equipment table so that you can print or display it when needed and so that you can use it is searches and sorts. But use an auto-entered serial number as the unique identifier to use in relationships.
Here's a demo file: https://www.dropbox.com/s/ggjy88cgkescopy/CategorySerialNos.fp7
It's in the older file format so if you are using FileMaker 12 or newer, use Open from FileMaker's File menu to produce a copy of the file converted to the new file format.
The file uses a relationship that matches to all records of the same group (such as all excavators) and then uses the Max function to compute the next number in the series for that group.