Upon retirement of a member for nonservice connected disability, he shall receive an annual pension, payable in monthly installments, dependent upon the aggregate length of his service, as follows:
After 10 years but less than 12 years of service, fifty dollars ($50) a month.
After 12 years but less than 14 years of service, fifty-five dollars ($55) a month.
After 14 years but less than 16 years of service, sixty dollars ($60) a month.
After 16 years but less than 18 years of service, sixty-five dollars
($65) a month.
After 18 years but less than 20 years of service, seventy dollars ($70) a month.
After 20 years of service, seventy-five dollars ($75) a month.
A pension for nonservice connected disability shall not exceed one-half of the terminal salary of the pensioner.