of screw 50 continuously engages one side of the thread of nut 52 against the spring bias applied to face 14 this is clearly shown in fig2 where side 50a of the thread of screw 50 engages side 52a of the thread of nut 52 lubricant if used would collect in the space 55 between the threads the unidirectional loading of the threads caused by the spring bias plus the high humidity environment caused by proximity to the evaporator coil not illustrated would ordinarily cause corrosion of the screw 50 and or nut 52 with the collection of the corrosion products in space 55 where they would constitute an abrasive causing wear of the screw 50 and or nut 52 however according to the teachings of this invention the screw 50 is coated with a 0 8 to 1 2 mil layer of teflon 51 as is best shown in fig3 and nut 52 is made of nylon because screw 50 is overed with teflon layer 51 which is impervious to the atmosphere it is protected from the environment does not require lubrication since teflon is self lubricating and has the mechanical strength of the underlying steel screw 50 the teflon layer 51 would be peeled off but for the yielding of the nylon nut 52 which is of a compatible hardness relative to the teflon and which deforms so as to spread the loading over all of its.