Page 139 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Transmissions and driveline CHAPTER 5.1

from the gear friction losses and the oil churning losses transmission without allowing water in or oil out.
respectively. The pump losses must always be considered Considering that:
if the transmission has a high-pressure hydraulic circuit.
oil inside the transmission can be extremely aerated
In belt and toroidal CVT/IVT transmissions the slip
there is often no part of the gearcase that does not
losses can also be significant. As discussed in Section
have oil splashed on the inside
5.1.4 the speed ratio (indicating slip) is always consid- the gearbox is often partly or wholly submerged
ered in torque converters.
during wading through flooded roads. It can be
5.1.6.3 Other transmission components readily seen that the breather not only has an im-
portant task, but can prove difficult to engineer.
No review of transmissions would be complete without Sealing of the gearcase on the transmission is required to
at least a brief mention of some of the other components prevent oil leaking on to the ground and the subsequent
that can be found within the driveline system of the loss of oil. The breather system has been considered
vehicle: above and is one part of the whole system. The other
differentials areas are where the shafts enter and exit the unit, the
breather systems joints in the casing and the integrity of the casing itself.
the gearchange mechanism. Proprietary ‘rubbing’ lip oil seals are used on most shafts
(also called dynamic shaft seals). The temperature re-
Differentials are used in order to allow the left and right quirement of the application means that most are made
hand wheels on any one axle to rotate at different speeds. from a synthetic material which has a thin lip contacting
This is essential on road going vehicles as the two wheels the rotating part. Important design aspects of these parts
take different paths when the vehicle goes round include the surface hardness, surface roughness and ad-
a corner, hence travel different distances and the inner equate oil supply to the lip; the contact area needs to be
wheel will rotate slower than the outer. This effect can lubricated to avoid excessive wear. The manufacturers of
also be seen between the front and rear wheels of a four these seals provide good literature to allow them to be
wheel drive vehicle so a centre differential is required applied correctly. To complete the sealing of the casing,
between the front and rear axles. the joints between the housings need to be sealed care-
As a result of the above requirement, differentials fully, usually using silicone or RTV sealant applied at the
are part of the gearbox assembly on front-wheel-drive point of assembly. Gaskets can also be used. The casings
vehicles as the drive is taken directly to the wheels by themselves can be a source of leaks if the castings used
two separate, output driveshafts. On rear and four are allowed to be porous. This can occur in the casting
wheel drive vehicles the differential is invariably part process and needs to be avoided by both good design and
of a separate axle assembly. It should be noted that manufacturing practice.
axle assemblies also contain part of the gearing The gearchange on both manual and automatic
required to reduce the speed of the rotating parts transmissions require a mechanical connection between
from engine speed to wheel speed. This is often the driver’s controls and the transmission unit. For
referred to as the final drive and typically on a pas- manual gearboxes this is obviously used to shift the
senger car would be a reduction of around 4:1. Within gearchange mechanism within the gearbox. On auto-
an axle this would be achieved using a bevel gear pair matics it can sometimes do little more than engage the

which also turns the drive through 90 . The differ- parking mechanism within the ‘box’. With older ATs
ential assembly then rotates with the output gear of though, the shift mechanism can be mechanically
this pair and allows the wheels to rotate as described linked to the internal hydraulics and used to directly
above. It can be seen that the assembly often referred influence the gearchange. The requirement to
to as the ‘differential’ on a vehicle actually serves mechanically link the two is reducing because of the
other purposes as well. impact of electronic control. In practice, the difficulty
Breather systems are one part of the transmission of linking the controls (gearlever) with the transmission
system often ignored by many vehicle engineers, but unit is much to do with the relative position of the two
can prove to be difficult to design. As the transmission and the interface at the gearbox end. It goes without
warms up and cools down the air inside needs to be saying that rear or mid-engined installations can be
able to expand and contract. The breather allows this difficult, as the connections on the power unit often
to happen without the air inside becoming pressurized, face the rear of the vehicle. For manual transmissions,
as this could push oil out past the oil seals, joints, etc. the use of solid rod connections is often favoured so
The problem with developing these is finding a posi- the driver gets a more direct feel of the gearchange,
tion on the transmission and a design of breather, although cable systems can provide a very good solu-
which allows the air to move in and out of the tion. Either solution will have two parts to it; one to

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