About The Variable Speed Drive System - MTD 700 Series Professional Shop Manual

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700 Series Lawn Tractor

About the variable speed drive system

There are two drive belts in the system. A long drive
belt transfers power from the engine crankshaft to the
lower sheave of the variable-speed pulley. A shorter belt
fits in the upper sheave of the variable speed pulley; trans-
ferring power from there to the input pulley on the tran-
saxle. See Figure 5.1.
• The drive control pedal tensions the front drive
belt using a pivoting double idler bracket.
See Figure 5.2.
• The center partition of the variable speed pulley
separates the lower (front) belt from the upper
(rear) belt.
• As the front belt is tensioned, it is drawn deeper
into its sheave. The effective circumference of
the driven pulley shrinks.
• As the front belt is drawn deeper into its
sheave, the center partition is forced upward,
making the upper sheave narrower.
• As the upper sheave pinches-down on the rear
drive belt, the belt is forced outward in the
sheave. The effective circumference of the pul-
ley driving the rear belt grows.
• As the upper and lower sheaves change size, the drive ratio shifts increasing the speed of the rear belt.
• The upper drive belt is kept under constant tension by a pivoting idler arm with a pulley and a tension
spring.
• When the clutch/brake pedal is applied it takes tension off the front double idler bracket, releasing tension
on the belt and applying the brake on the transmission.
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Variable Speed Pulley
Upper Drive Belt
Figure 5.1
Double Idler Bracket
Figure 5.2
Lower Drive Belt
Drive pedal bracket