Just clean the spring section, that means dis-assembly though and taking the curtain off the lower roller.

My experience is they are quite robust, I've yet to have one fail and I'm into 3 figure numbers restoring these shutters. I get the roller out flush with WD-40, wash with hot water and detergent, some times Vim or Cif, then use meths to clean followed by IPA (alcohol nit beer). I usually then use a slight trace of oil in alcohol to lubricate. All the TP shutters I've restored would tension well past 1/90th. This can be done without taking the roller spindle itself apart.

These shutters were made up until the early 1960's so your's might nit be that old

Ian