The most obvious answer to this question is that gear oil is generally
intended for use in manual gear boxes and transmission fluid is for automatic
transmissions. However, as is always the case with automotive lubricants, there
are exceptions to this rule. In order to fully understand what the differences
are between the fluids, we first have to take a look at the key differences
between gearboxes and transmissions.
Both a gearbox and a transmission essentially do the same job in that
they allow change of gears to control the speed, force and direction of
travel, but they achieve this in different ways and so require
different support fluids in order to operate properly.
Manual Gearbox
Manual gears operate based on a system of two shafts with gears which
mesh together after user input via the clutch and the gear stick. This means that the manual gearbox creates a
lot of heat, a lot of force and a lot of friction as the two moving objects
come together...
More after the jump...
As such, gear oil must be able to make these gear transitions as
smooth as possible to prevent damage to the components as they shift and to
achieve this gear oil has to have the following properties:
High Viscosity
The main thing you'll notice about gear oil is its viscosity. It's much
thicker than motor oil, with an average motor oil being 5W/30 and the average
gear oil being 75/90 (see this article for in depth
explanation of oil viscosity index).
The high viscosity ensures that the entire gear train is lubricated thoroughly and, most importantly, that the gears are well cushioned from shock damage when they come together.
The high viscosity ensures that the entire gear train is lubricated thoroughly and, most importantly, that the gears are well cushioned from shock damage when they come together.
Resistance to Heat
The working of a manual gearbox creates a lot of friction and therefore
a lot of heat. Gear oil is able to withstand high temperatures, transferring heat
away from the gear train whilst not boiling off too rapidly and not depleting
too fast to be of use for long.
Able to Function Under Extreme Pressure
Gear oils often come with extreme pressure additives in order for them to withstand the high pressures generated during the running of the vehicle, particularly where hypoid gears are involved. They help to keep the oil stable and functioning consistently.
Automatic Transmission
Automatic transmissions operate on in a planetary system where gears
switch automatically depending on the demands of the engine. The gears in an
automatic tend to be smaller and there are many more moving parts than a manual
gearbox.
As such, the lubrication required is on a different level to that of a manual gearbox. Not only does the transmission fluid need to provide good lubrication but it also needs to be able to transfer power from the oil pump to the clutches which control the movement of the gears. To achieve this transmission fluid must have the following properties:
As such, the lubrication required is on a different level to that of a manual gearbox. Not only does the transmission fluid need to provide good lubrication but it also needs to be able to transfer power from the oil pump to the clutches which control the movement of the gears. To achieve this transmission fluid must have the following properties:
Low Viscosity
Transmission Fluid is essentially thin hydraulic oil. In order to
effectively lubricate the delicate parts of the system, the viscosity of the
oil is kept low - generally around 0W/5 or 5W/10. Most importantly it needs to
be relatively free flowing in order to transmit power from the engine to the
transmission. The challenge presented to transmission fluid to maintain
lubrication, whilst keeping clutch engagement consistent.
Keep the Channels Between Components Clean
Detergent is added to transmission fluid to make sure that build-up in
the channels is kept under control.
Resistance to Heat
Transmission Fluid acts as a coolant in a similar way to gear oil in
that it transfers heat away from the mechanisms caused by friction and high
pressure, however its boiling point is lower than gear oil and as a result requires
additives in order to improve its life expectancy.
It is important that air is kept out of the fluid as this would
interfere with the transference of force through the liquid to the
transmission. As such, transmission fluid has anti-foaming properties to help
combat this.
What does this mean for my vehicle?
Whilst some manual gearboxes do use transmission fluids in place of gear
oil, the same cannot be said for putting gear oil into an automatic
transmission. Gear oil is much too viscous for these systems and will cause
clogging between components
The golden rule when it comes to automotive fluids is to always use what
is recommended by your manufacturer in your vehicle. Check your Owner's Manual
for details on what is safe to put in your system if you are going to conduct
maintenance work on your vehicle.
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This article is for general information only and is not intended as a diagnostic tool. Always consult your user manual before attempting any maintenance on your vehicle.
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