Masport ROTARY MOWER, Servicing Manual, Page: 13 / 20

13
Rear Axle Unit - Rear Wheel Drive
Models
DESCRIPTION
All rear wheel drive mower models have a similar construction,
although there are some variations in the components. The output
shaft of the gearbox has a slot at each end to accept sliding pawls
whose ends engage with the three-lobed ramps inside the pinion
gears. The sliding pawls oscillate back and forth inside the pinions so
the pinion can rotate freely on the shaft in one direction. But rotation
in the opposite direction is prevented by the pawls engaging on the
square faces of the pinion ramps. This engagement transmits the drive
to the wheel gears. The action of ‘free-wheeling’ in one direction allows
the mower to be pushed forward, and to be steered around corners.
When the gearbox clutch is engaged, the output shaft rotates the
pinions to drive the mower forward. Note that the pinion gears
must
have interior ramps of opposite hand, one for each side of the mower.
The gears carry the mark ‘RH’ or ‘LH’ inside, adjacent to the ramp,
to show whether they are for the Right Hand or Left Hand side of the
mower. Remember, right and left are as seen by the operator when
standing at the handle.
While the same pawl can be used on either side of the mower, it MUST
be installed in the slot so that the rounded corners will run smoothly up
the ramps and the square corners will give positive engagement with
the square faces of the pinion ramps.
The pinion gear engages with a larger wheel gear, so the wheel axle
is parallel to the gearbox output shaft but some distance from it. Thus
when the rear axle assembly is rotated about the axis of the gearbox
output shaft, the wheel axle will travel in a circle some distance away
from it. This movement of the wheel axle is used to raise and lower the
back of the lawnmower. A connecting rod to the front of the mower
moves a similarly offset wheel axle assembly about its rotation point,
raising and lowering the front wheels also.
A spring-loaded height adjusting lever mounted on the rear axle
assembly permits the mower height to be raised and lowered, and
the lever drops into a notch in the quadrant to hold the mower at
the desired cutting height. A counterbalance spring attached to the
connecting rod takes some of the mower weight to facilitate raising
and lowering the mower.
The rear axle assembly has a plate at each end carrying a wheel stub
axle on the outside, and a bearing sleeve on the inside. The inside of
the bearing sleeve provides a housing for the sealed ballrace of the
gearbox output shaft, while the outside of the sleeve carries the plastic
pivot bushes on which the axle assembly rotates as the cutting height
is altered.
There are two different methods for mounting the rear axle assembly
to the mower housing. In most aluminium housings there is a part
cylindrical seat provided for the plastic pivot bushes, and they are
held in place by saddles
screwed edgewise into the wall of the
housing. Steel housings, on the other hand, use flanged steel retainer
cups which fit over the pivot bushes and fit against the outer faces of
the housing. The cups have threaded inserts for set screws
passing
through the housing wall to clamp the cups in place. Some
aluminium housings use the flanged steel cups instead of saddles.
(See Step 7 of REMOVING THE REAR AXLE UNIT FROM REAR
WHEEL DRIVE MOWERS).
MODIFIED PINIONS AND GEARS
A manufacturing improvement was made in May 2010. This was a
change to a larger tooth profile on the pinions and gears. The original
pinions had 16 teeth and the matching gears had 60 teeth. Now the
pinions have 12 teeth and the gears have 45 teeth. Old style pinions
and gears cannot be mixed with the new ones. See the illustration
above.
As the gear ratio is unchanged, and the shaft centre distance is also
unchanged, it would be possible to use an old set on one side of
a mower and a new set on the other, but we recommend fitting a
complete set if any of the old pinions or gears need replacing. The
pawls are unchanged. If you are replacing an old gear with a spacing
shoulder behind it, fit spacer No 569479 behind the new gear to bring
the teeth into correct alignment with the pinion.
CORRECT PAWL, PINION AND CAP POSITIONS
REMOVING REAR AXLE UNITS FROM REAR WHEEL DRIVE
MOWERS
Preparation (All models)
We strongly advise that all fuel and oil be drained from the mower
before commencing this procedure as the mower will be tipped over.
This will avoid the risk of fuel and oil reaching vulnerable parts of
the mower. In any case, this could be a sensible time to make an oil
change.
SEE THE NOTE ON ‘POSITIONAL DESCRIPTIONS’ ON PAGE 2.
1. Disconnect the counterbalance spring fitted between the
connecting rod and the housing. Take care, as this spring is very
strong. To minimize the spring tension, set the cutting height to its
highest position. Unhook the spring from the housing by passing
a strong wire loop around the front hook of the spring, just above
the housing. With the wire formed into a closed loop, a suitable
handle through the loop will allow the front hook of the spring to
be pulled forward and out of the hole in the housing. Once clear
of the housing, the tension on the spring can be slowly released
until it is safe to remove the wire loop.