DRESSTA TD-25M EXTRA, Service Manual

Page: 55 / 595

DRIVE TRAIN
SECTION 7
Page 3
DRESSTA TD-25M EXTRA
OPERATION
When the control lever is moved forward, the pivot ring depresses the Forward direction spool
directing oil to the “F” port at the bottom of the valve. The “F” pressurized oil is also directed
internally to the base of the Reverse spool which pushes the Reverse spool up to the pivot ring.
This action creates a hydraulic detent holding the lever forward.
When pulling the control lever to neutral or reverse, the pivot ring pushes down on the Reverse
spool which displaces the hydraulic detent oil. At the same time the Forward spool follows the pivot
ring up and mid way through its stroke the forward pressure is cut off as well as the reverse
hydraulic detent, allowing the valve to go into neutral. If the lever is pulled all the way to reverse the
same sequence of events happens for the reverse hydraulic detent.
Steering is accomplished in two ways: by moving the control lever to the left or right or by shifting
the HI-LO speed range switches. The two steering methods can be used together and in any
combination.
When the machine is started with both speed range switches in LO-LO positions both tracks are
in LO-LO. With the lever pushed to forward the machine travels in FWD, LO-LO (both tracks
in LO-LO). Initial movement of the lever to the right drops right LO clutch steering pressure causing
the right steering clutch to be in a neutral condition. The neutral condition is indicated by a “feel
point” or effort increase in the control lever movement. Further movement of the lever further
decreases pressure at the right brake, which is moved to the end of the lever stroke will result in a
pivot turn to the right. By shifting both range switches to their upper position both tracks shift up to
HI-HI (both tracks in HI-HI). If traveling in HI-HI and the lever is pulled to the right the following
occurs. The initial lever movement lowers signal pressure causing a sequence valve to
automatically shift the right track from HI to LO. With further lever travel to the right the right LO
track will go to the neutral condition as indicated by the “feel point” as discussed earlier. Further
lever movement will result in a pivot turn. As the lever is moved back to the upright position the LO
clutch engages and then up into HI. Speed range switch steering results in full power turns.
With the lever pushed to forward and both speed range switches in LO-LO positions the machine
travels in FWD, LO-LO (both tracks in LO-LO). By shifting the right switch to HI position, the right
track shifts up to HI. Both machine tracks are engaged, while the right track features the higher
speed and, as a result, the machine will make a gradual turn to the left. To cause the machine to
make a gradual turn to the right proceed in similar manner by controlling the left hand track power.
The machine will make a gradual turn so long as one of the speed range switches gets reactivated.
Then the machine will revert to straight travel in a range it was engaged in prior to the gradual turn
activation.
A transmission range gear is selected by pushing the drive train control lever up-shift and
down-shift buttons.
For a complete Machine Speed and Direction Changes description and operation, refer to
OPERATOR’S MANUAL.
Two Speed Steering Valve (Fig.’s 7.4, 7.5 and 7.6)
The two speed steering valve contains three valves. Two of them: upper LC, RC, FB and lower
HI-LO are mounted above rear main frame cover and one - manifold J under this cover.
The upper valve consists of three basic sections: LC left clutch section, RC right clutch section
and the FB foot brake section.
The LC and RC clutch sections control the flow of oil to the clutch packs to drive each track
individually. Both of the sections contain a metering spools which are each held in the rear drive
position by pilot pressure CL and CR respectively from the drive train control valve.