Various types of self-propelled riding mowers have been available heretofore.  Except for those relatively large commercial mowers which utilize cutting reels and are typically used on golf courses, most such riding mowers include mower decks housing one or more rotary blades.  Some riding mowers utilize sectional mower decks with several interconnected rotary blades thereacross.  The Toro Company, which is the assignee hereof, has been a leading manufacturer of such mowers for years. 
Sectional mower decks typically include pivotal wing sections which can be selectively raised to present a more narrow profile such as for trimming, storage or transport.  When such wing sections are raised, the cutters must be disengaged for safety purposes.  Belt drive arrangements are typically utilized in such mower decks and there have been various approaches to such disengagement.  For example, U.S. Pat.  Nos.  4,395,865 and 4,429,515 to Davis shows a self-propelled lawn mower having a sectional mower deck wherein a combined clutch and brake mechanism mechanically disengages the drive belt to the blade and simultaneously brings it to a stop when the wing section is raised beyond its cutting range.  U.S. Pat.  Nos.  3,152,431 to Ott and 3,116,583 to Mason are also representative of the prior art in this regard.  As the wing section are raised, belt tension is released to disengage the cutters thereon and is then reapplied later to re-engage the cutters upon lowering the wing sections.  However, the belt drive arrangements of the prior art have tended to suffer from relatively higher belt wear and misalignment, which in turn has caused higher maintenance and repair costs. 
There is thus a need for a new mower deck incorporating an improved belt drive arrangement which minimizes belt misalignment while maintaining substantially constant belt tension for longer wear and less maintenance.