First, generate a key without password:
root@example:~# ssh-keygen -f ~/.ssh/id_rsa -q -P ""
You will see id_rsa and id_rsa.pub
root@example:~# ls -la .ssh total 20 drwx------ 2 root root 4096 Nov 18 15:19 . drwx------ 7 root root 4096 Apr 14 2018 .. -rw------- 1 root root 1679 Nov 18 15:19 id_rsa -rw-r--r-- 1 root root 401 Nov 18 15:19 id_rsa.pub -rw-r--r-- 1 root root 444 Sep 22 2017 known_hosts