In this video we’re going to look at synchromesh manual transmissions.
The purpose of a transmission is to turn engine power into vehicle motion.
Giving us a range of forward gears, depending on the opposing forces and desired
BLAH BLAH BLAH
Right, in layman’s terms, if we didn’t have gearbox
we would either drive around at 5 miles per hour or wouldn’t be able to get up a hill.
Basically the gearbox matches engine speed to road speed.
Looking inside the gearbox we have 3 important shafts,
the first is the primary also known as the input shaft,
driven by the engine via the splines in the clutch plate inputting power into the gear box.
Permanently meshed with this primary shaft by the first constant meshing gears is the lay shaft
also known as counter shaft.
All the while the primary shaft is turning, its force is transmitted to the lay shaft
and on this lay shaft are a range of gears
that are permanently meshed to gears that are supported on, but not splined or connected to the main shaft.
On the main shaft the synchromesh units are splined to the shaft,
and the rest of the constant mesh gears are free to rotate on the shaft
with the use of needle roller bearings.
A constant mesh gear will only drive when connected to a synchromesh unit
which is in turn is splined to the main shaft.
In the neutral position, drive comes in on the primary shaft
and drops down to the layshaft through constant mesh gears.
With the lay shaft being driven, all pairs of constant mesh gears are driven
but in a freewheeling state on the main shaft due to those needle roller bearings.
In 1st gear, the gearbox gives us the largest gear reduction.
When the driver selects first gear he will move the selector into 1st gear position,
moving the synchromesh which is splined to the main shaft into mesh with mainshaft 1st gear
Drive is now transmitted to the wheels.
On acceleration, the engine rpm will rise to the point where you need a higher gear.
Luckily for us the box is full of them.
When the driver wants to change up to 2nd,
he comes out of 1st gear and goes through neutral moving into 2nd gear.
The synchromesh unit is moved to neutral and then moved into second gear position,
locking the synchro unit to main shaft 2nd gear,
giving us a different ratio due to the higher engine speed.
What’s that evil Matt? Why aren’t the gears making crunching noises??
Well, let me tell you………… it’s all about the synchromesh.
Let’s look a bit closer at the gear change.
As we can see and hear,
it moves across with no hint of grinding.
What we have here is 1st gear,
and you can see its smooth inside and not splined to a shaft.
It also has an integral cone as you can see.
The baulk ring has an internal grooved cone,
which matches the cone on the gear.
When forced together the frictional properties of the baulk ring force the gear to rotate with it.
The inner hub of the synchromesh unit is splined to the main shaft,
and the outer sleeve is splined to the inner hub,
but allowed to move backwards and forwards over the inner hub.
When the baulk ring is placed in the unit,
you can see that the outer sleeve will only slide over the top of the baulk ring
when the shaft speeds are synchronised,
guided by the baulk ring dog teeth.
So, if we put it all together, you can see as I put some force against the cone clutch,
friction takes over synchronising the speed of the gears.
Once the speeds have been matched,
further force allows the outer sleeve via the baulk ring dog teeth
to be guided effortlessly onto the dog teeth of the 1st main shaft gear,
locking it all up as one unit. Ta da.
Let’s have a look at that again at normal speed, and then again in super slow mo.
So, we can see the inner member, the outer sleeve, the baulk ring,
and here the mainshaft 2nd gear.
As the driver applies pressure to the selector fork.
(This is the selector fork here).
The inner member presses the baulk ring cone up against the cone of the main gear.
The baulk ring grabs hold of the main gear and starts to turn at the same speed
as they are being pressed together.
With further force on the selector fork and with the speeds synchronised,
the outer sleeve is guided over the baulk ring and onto the main gear dog teeth.
So, back to the gear box.
Were still accelerating and at the top end of 2nd gear with the engine screaming at us,
we need to change up again.
When the driver wants to change up to 3rd ,he comes out of 2nd gear and goes via neutral into 3rd gear.
The synchromesh unit is moved to neutral
and then third gear synchromesh is moved into third gear position,
locking the synchro unit to main shaft 3rd gear,
giving us a different ratio due to the higher engine speed.
Once again, were still accelerating and at the top end of 3rd gear with the engine shouting at us,
we need to change up again.
The driver selects 4th gear, locking the synchro unit to 4th main shaft gear
which also happens to be on the primary shaft.
Locking the primary shaft to the main shaft, giving us straight through 1:1 drive with no reduction.
Now, sometimes in life, you like to see where you’ve been and go in reverse.
Don’t look over your shoulder,
like a pro with a stern look, just use your mirrors.
So, how do we get reverse out of this gear box?
It’s easier than you think,
all we need to do is put an idler gear in between the lay and main shaft to reverse its direction.
When the driver selects reverse,
he slides an idler gear in between the spur gear on the main shaft and a spur gear on the lay shaft.
As they are spur gears, there is no synchromesh unit for reverse.
So reverse must be selected when stationary.
I’m sure we’ve all done it while moving and played a nice tune on the gear box.
There we have it, synchro gear box in a nut shell
Easy as that.
BOOM!
Thanks for watching.