QD is for Quick Disconnect.   
COPIED from Baldor: To install Baldor QD Sheaves, the cap screws are used as pull-wrench only; no additional leverage is necessary. To remove QD Sheaves, the cap screws are taken out and used as jack screws ( pushers in the other 2 holes - SeniorCitizen ). A few quick turns on each screw, and the tight grip of the bushing on the shaft is easily broken.
 
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Before doing anything toward dis-assembly measure or mark the amount of shaft protruding above the bushing.  You'll need that to re-assemble. 
The OD of the bushing is tapered and matches the taper of the sheave ID.  I couldn't find torque specifications for re-assembly but it isn't much.  For that size possibly 10-15 ft. lbs but that's a guess.  The torque comes with the installation instructions when they are purchased new.  I'll keep looking for that.  You should find the size by a letter designation on the bushing and sheave , like JA as an example.
If the tapered surfaces are rusty clean with fine emery cloth or sandpaper.  Wash with a liberal amount of WD-40 and wipe clean until you can feel nothing (grit) with your hands.  Install dry, no never sieze, no oil , no nothing , just that residue from the WD-40 after wiping dry. 
NOTE: during dis-assembly and after the sheave drops from the tapered bushing, if the bushing doesn't easily slide off the shaft, lightly drive a screwdriver in the bushing split slot to expand it just a little.