propel drive fix

can someone tell me what the name of the special tool is to tighten this "nut" holding the gear?  Mine keeps coming loose.  Makes and ugly racket.  The "solution" was to put another identical one on top of this one and tighten as a lock nut, but they both come loose...

I can't get the other side of the drive off.  Is it not supposed to come off?

Spanner wrench?

Hi there, sorry I didn't see this.  the tool is a spanner wrench.  If you purchased from a dealer I would contact them first.

The other side plate is removable, but will be tough to remove coming out of the factory.