Abstract: An arbor assembly for use with rotary mowers, and the combination of such arbor assembly with a mower blade subassembly and with a pulley subassembly. The arbor assembly comprises a shaft having reduced diameter end portions defining upper and lower shoulders against which are seated the inner races of upper and lower antifriction bearings. The outer races of the upper and lower antifriction bearings are supported by bearing retainers supported by the mower deck. The opposite ends of the shaft are splined. The opposite ends of the shaft are also internally axially tapped. A mower blade subassembly is adapted to be mounted by means of a cap screw which engages the tapped lower end of the shaft. The mower blade subassembly comprises a blade adaptor and a mower blade which bears against the under surface of the blade adaptor. The mower blade is free to rotate relative to the shaft if the blade strikes an obstruction.