A locking mechanism for a headgear suspension using two racks, a pinion, a driver and a lockable knob. The two racks are positioned on the suspension such that they overlap and permit engagement of the gears on the driver with teeth on the rack such that when the lockable knob is in the unlocked position the driver tightens or loosens the racks to adjust the diameter of the suspension to the user's head size. The lockable knob of the invention operates on a pull, rotate, push and lock system. There are no metal parts and the lockable knob can be locked and unlocked with one hand, by the user.