Are You Damaging Your Gearbox when Changing Gear?
Clutch Dog Gearboxes
Outboard motors and most stern drive gearboxes (like Mercruiser's Alpha drives)
use a Dog Clutch set up for selecting and shifting gear.
If you select gear slowly and carefully with these gearboxes, you will actually
be damaging the gearbox.
Do you hear a grinding or chattering noise when selecting either forward or
reverse gear?
If you do, something needs to change or one day your lower unit will not have
any drive and it will constantly jump out of gear.
Firstly I will explain how the lower unit/gearbox works.
They have an input shaft that is driven by the engine or 'powerhead'. (This the input shaft is called the driveshaft. The driveshaft has a gear connected to
the bottom of it called the Pinion gear. The pinion gear is always in constant
mesh with the forward and reverses gears. Whenever the motor is running, the
pinion gear is rotating the forward and reverse gears. Between the forward and
reverse gear, there is the mechanism that selects the gear. This is called the
Clutch Dog.
The Clutch Dog sits on the propeller shaft that protrudes out the back of the
gearbox and drives the propeller. The Clutch Dog has an internal splint that is
in constant mesh with the propeller shaft.
Picture the Clutch Dog having a front and a back. At each end, it has large,
square teeth. When you select the gear on the motor you move the Clutch Dog
forwards and backward along the propeller shaft.
When the Clutch Dog is moved to select gear it engages with the same sized
square teeth on the particular gear that is being engaged.
Remember that both of the output gears (forward and reverse) are rotating as
they are in constant mesh with the pinion gear (input gear).
If you select gear slowly, the Clutch Dog will slowly move into mesh with the
gear. This slow movement causes the teeth of the gear to 'skip' over the teeth
on the Clutch Dog. This continuous skipping wears away the nice sharp edges of
the teeth and will eventually cause the motor to jump out of gear.
How to Select Gear. The way to avoid doing damage to your gearbox is to
positively select gear. This means to literally bang the engine into gear so
you hear one clean engagement of the gearbox, not chatter. You should also
pause in neutral when shifting from forward to reverse or vice versa. Mercruiser
alpha allows the propeller time to stop spinning. Do not shift into reverse when you
are moving forward quickly. It creates a huge amount of unnecessary force on
the gearbox (and engine). Slow the boat down off the plane, come back to an
idle and pause in neutral to let the boat slow right down. The reverse is
purely a reverse, not a break...Another cause for the damage being done to the
Clutch Dog is your idle speed might be too high. If your motor is idling higher
than around 900rpm in neutral you will be doing damage every time you are
selecting gear. Make sure your idle speed is set to what the manufacturer
recommends! Positively select year pause in neutral between gear changes. Let
the boat slow down before changing into reverse have the gear oil changed every
100 hours or annually, whichever comes first. Engine Idle Speed Too High
So...
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