Theory of operation – John Deere stx38 User Manual

Page 245

3/21/97

6 - 13

HYDROSTATIC POWER TRAIN

THEORY OF OPERATION

Theory of Operation:

When the brake pedal (A) is depressed, the bellcrank
(B) and frame rod (F) rotate forward. This motion
causes the brake rod (H) and neutral return strap (G),
connected at the same point on the bellcrank, to move
forward at the same time. As the brake pedal is
depressed further, spring strap (N) compresses the
compression spring (M). As the spring is compressed,
the brake lever (K) rotates forward forcing its friction
surface (Q) against the brake disc (R). Continued force
pushes the disc against the shimmed friction puck (P),
captured in the housing. As the brake pedal is
depressed further, more-and-more force is applied until
the disc stops turning. This works in conjunction with
the neutral return linkage, stopping the tractor.

As this is happening, the neutral return strap (G) is
pulled forward, forcing the cam roller (U) to roll up one
of the ramp contours of shifter bracket (L). This action
moves the shifter bracket (L) and lever (D) back
towards the neutral position. At the same time, this
returns the shift linkage (I) into neutral position and the
internal, variable pump swashplate into its level (non-
pumping) position. This works in conjunction with the
brake linkage, stopping the tractor. As-soon-as the shift
lever reaches either of the radii of the neutral slot (V),
the return spring (S) snaps the shift lever back into
neutral position.

When the brake pedal is fully depressed, the brake can
be locked into park position with the park lever (C) and
locking rod (D). The locking spring (E) holds the locking
rod in place until the operator eases up on the pedal
and the force of compression spring (M) locks the
bellcrank (B) against the locking rod (D). To release the
park brake, the operator must again depress the brake
pedal fully and pull the park lever (C) and locking rod
(D) into the disengaged position. The brake rod neutral
spring (J), maintains just enough air gap between the
brake lever friction surface (Q), friction puck (P), and
the brake disc (R) when the brake linkage is
disengaged.

M58258

M58259

M58252

M55800

M55850