About gearboxes and gear systems for cars, other vehicles and mechanical machines
In mechanics, a transmission or
gearbox is the system of gears and/or the hydraulic system (called variously "hydrodynamic",
"fluid" or "automatic" transmission) that transmits mechanical power from a
prime mover—such as an engine or electric motor—to a typically rotary output
device at a lower angular momentum but at a higher motive torque.
Generally
Transmissions provide a speed-power conversion known as gear reduction (in
speed) to a higher torque (rotational force) using gearsets. In motor vehicles,
the transmission provides different speed-power ratios known as "gears" or
"speeds", some of which may be in the reverse direction. Tractors and large
trucks especially may have a dozen or more forward "gears" which vary from a
crawling speed at high torque to high speed at low torque where the only torque
needed with a load coasting along at a given speed are that small additional
energy (force) needed to overcome ongoing friction and other road losses such as
climbing a grade. When the torque needed to surmount a grade is insufficient at
a higher rotational speed, the gearbox is shifted into a lower gear to provide
more power, as was needed when initially accelerating said vehicle to the
desired road speed. Gearing has much in common with the mechanics and mechanical
factors present in pulley systems. One trades distance (numbers of rotations)
for increased force.
Explanation
Early transmissions included the right-angle drives and other gearing in
windmills, horse-powered devices, and steam engines, in support of pumping,
milling, and hoisting. Most modern gearboxes either reduce an unsuitable high
speed and low torque of the prime mover output shaft to a more usable lower
speed with higher torque, or do the opposite and provide a mechanical advantage
(i.e increase in torque) to allow higher forces to be generated. Some of the
simplest gearboxes merely change the physical direction in which power is
transmitted.
Many typical automobile transmissions include the ability to select one of
several different gear ratios. In this case, most of the gear ratios (simply
called "gears") are used to slow down the output speed of the engine and
increase torque. However, the highest gears may be "overdrive" types that
increase the output speed.
Uses
Gearboxes have found use in a wide variety of different—often stationary—applications.
Transmissions are also used in agricultural, industrial, construction, mining
and vehicle equipment. In addition to ordinary transmission equipped with gears,
such equipment makes extensive use of the hydrostatic drive and electrical
adjustable-speed drives.
Simple
The simplest transmissions, often called gearboxes to reflect their simplicity (although
complex systems are also called gearboxes in the vernacular), provide gear
reduction (or, more rarely, an increase in speed), sometimes in conjunction with
a right-angle change in direction of the shaft (typically in helicopters, see
picture). These are often used on PTO-powered agricultural equipment, since the
axial PTO shaft is at odds with the usual need for the driven shaft, which is
either vertical (as with rotary mowers), or horizontally extending from one side
of the implement to another (as with manure spreaders, flail mowers, and forage
wagons). More complex equipment, such as silage choppers and snowblowers, have
drives with outputs in more than one direction.
Regardless of where they are used, these simple transmissions all share an
important feature: the gear ratio cannot be changed during use. It is fixed at
the time the transmission is constructed.
Multi-ratio systems
Many applications require the availability of multiple gear ratios. Often, this
is to ease the starting and stopping of a mechanical system, though another
important need is that of maintaining good fuel economy.
Gearboxes in cars and motorized
vehicles
Automotive basics
The need for a transmission in an automobile is a consequence of the
characteristics of the internal combustion engine. Engines typically operate
over a range of 600 to about 7000 revolutions per minute (though this varies
from design to design and is typically less for diesel engines), while the car's
wheels rotate between 0 rpm and around 2500 rpm.
Furthermore, the engine provides its highest torque outputs approximately in the
middle of its range, while often the greatest torque is required when the
vehicle is moving from rest or travelling slowly. Therefore, a system that
transforms the engine's output so that it can supply high torque at low speeds,
but also operate at highway speeds with the motor still operating within its
limits, is required. Transmissions perform this transformation.
Most transmissions and gears used in automotive and truck applications are
contained in a cast iron case, though sometimes aluminum is used for lower
weight. There are three shafts: a mainshaft, a countershaft, and an idler shaft.
The mainshaft extends outside the case in both directions: the input shaft
towards the engine, and the output shaft towards the rear axle (on rear wheel
drive cars). The shaft is suspended by the main bearings, and is split towards
the input end. At the point of the split, a pilot bearing holds the shafts
together. The gears and clutches ride on the mainshaft, the gears being free to
turn relative to the mainshaft except when engaged by the clutches.
Automobile includes manual, automatic or semi-automatic transmission.
Manual
Main article: manual transmission
Manual transmissions come in two basic types: a simple unsynchronized system
where gears are spinning freely and must be synchronized by the operator to
avoid noisy and damaging "gear clash", and synchronized systems that will
automatically "mesh" while changing gears. The former type is only used on some
rally cars and heavy-duty trucks nowadays.
Manual transmissions dominate the car market outside of North America. They are
cheaper, lighter, usually give better performance, and fuel efficiency (although
the latest sophisticated automatic transmissions may yield results slightly
closer to the ones yielded by manual transmissions), and it is customary for new
drivers to learn, and be tested, on a car with a manual gear change. In Japan,
Philippines, Germany, Austria, the UK[1] [2], Ireland[3], Sweden, France,
Australia, Finland and Israel, a test pass using an automatic car does not
entitle the driver to use a manual car on the public road unless a second manual
test is taken.[citation needed] Manual transmissions are much more common than
automatic transmissions in Asia, Africa, South America & Europe.
Automatic
Main article: automatic transmission
Epicyclic gearing or planetary gearing as used in an automatic transmission.Most
modern North American cars have an automatic transmission that will select an
appropriate gear ratio without any operator intervention. They primarily use
hydraulics to select gears, depending on pressure exerted by fluid within the
transmission assembly. Rather than using a clutch to engage the transmission, a
torque converter is put in between the engine and transmission. It is possible
for the driver to control the number of gears in use or select reverse, though
precise control of which gear is in use is usually not possible.
Automatic transmissions are easy to use. In the past, automatic transmissions of
this type have had a number of problems; they were complex and expensive,
sometimes had reliability problems (which sometimes caused more expenses in
repair), have often been less fuel-efficient than their manual counterparts and
their shift time was slower than a manual making them uncompetitive for racing.
With the advancement of modern automatic transmissions this has changed. With
computer technology, considerable effort has been put into designing gearboxes
based on the simpler manual systems that use electronically-controlled actuators
to shift gears and manipulate the clutch, resolving many of the drawbacks of a
hydraulic automatic transmission.
Automatic transmissions have always been extremely popular in the United States,
where perhaps 19 of 20 new cars are sold with them[citation needed](many
vehicles are not available with manual gearboxes anymore).
Attempts to improve the fuel efficiency of automatic transmissions include the
use of torque converters which lock-up beyond a certain speed eliminating power
loss, and overdrive gears which automatically actuate above certain speeds; in
older transmissions both technologies could sometimes become intrusive, when
conditions are such that they repeatedly cut in and out as speed and such load
factors as grade or wind vary slightly. Current computerized transmissions
possess very complex programming to both maximize fuel efficiency and eliminate
any intrusiveness.
For certain applications, the slippage inherent in automatic transmissions can
be advantageous; for instance, in drag racing, the automatic transmission allows
the car to be stopped with the engine at a high rpm (the "stall speed") to allow
for a very quick launch when the brakes are released; in fact, a common
modification is to increase the stall speed of the transmission. This is even
more advantageous for turbocharged engines, where the turbocharger needs to be
kept spinning at high rpm by a large flow of exhaust in order to keep the boost
pressure up and eliminate the turbo lag that occurs when the engine is idling
and the throttle is suddenly opened.
Semi-automatic
The creation of computer control also allowed for a sort of half-breed
transmission where the car handles manipulation of the clutch automatically, but
the driver can still select the gear manually if desired. This is sometimes
called "clutchless manual" or "robotized". Many of these transmissions allow the
driver to give full control to the computer.
There are some specific types of this transmission, including Tiptronic,
Geartronic, and Direct-Shift Gearbox.
There are also sequential transmissions which use the rotation of a drum to
switch gears.
Bicycle gearing
The gearing on a bicycle is the
selection of appropriate gear ratios for optimum efficiency or comfort.
Different gears and ranges of gears are appropriate for different people and
styles of cycling. Multi-speed bicycles allow gear selection to suit the
circumstances, e.g. it may be comfortable to use a high gear when cycling
downhill, a medium gear when cycling on a flat road, and a low gear when cycling
uphill.
On a bicycle, power is transmitted from the rider's legs to the rear wheel via
the pedals, crankset, chain, and rear hub. A cyclist's legs produce power
optimally within a narrow pedalling speed range. Gearing is optimized to use
this narrow range as best as possible. As in other types of transmissions, the
gear ratio is closely related to the mechanical advantage of the drivetrain of
the bicycle. On single-speed bicycles and multi-speed bicycles using derailleur
gears, the gear ratio is the ratio of the number of teeth on the chainring of
the crankset and the rear cog or sprocket. In the case of a derailleur-equipped
bicycle, this sprocket is one of several comprising the cassette. On hub gears
the ratio is determined by the internal planetary gears within the hub.
For the same speed, or more accurately, power output at the rear wheel, a lower
gear (larger mechanical advantage) will require the rider to pedal at a faster
cadence, but with less force. Conversely, a higher gear (smaller mechanical
advantage) allows a higher speed for a given cadence, but requiring greater
force. Different cyclists may have different preferences for cadence and
pedalling force. Prolonged exertion of too much force in too high a gear at too
low a speed can increase the chance of knee damage.
General considerations
The gearing supplied by the manufacturer on a new bicycle is selected to be
useful to the majority of people. Some cyclists choose to fine-tune the gearing
to better suit their strength, level of fitness, and expected usage. When buying
from specialist cycle shops, it may be less expensive to get the gears altered
before delivery rather than at some later date. Modern crankset chainrings can
be swapped out, as can cassettes.
As far as a cyclist's legs are concerned, when changing gears, the relative
difference between two gears is more important than the absolute difference
between gears. This relative change, from a lower gear to a higher gear, is
normally expressed as a percentage. This measure is independent of what system
is used to measure the gears. Cycling tends to feel more comfortable if nearly
all gear changes have more or less the same percentage difference; a larger
percentage difference may be acceptable for lower gears where the absolute
difference is not too large. Thus, the absolute gear ratios should be in
logarithmic progression.
Racing cyclists often have close-range gears with a difference of around 7%.
Many general-purpose gears have a difference of around 15%. Differences of 25%
or more require a very substantial change in cadence and often feel excessive. A
step of 7% corresponds to a 1-tooth change from a 14-tooth cog to a 15-tooth cog,
while a step of 15% corresponds to a 2-tooth change from a 13-tooth cog to a
15-tooth cog.
By contrast, car engines deliver power over a much larger range of speeds than
do cyclist's legs, so relative differences of 30% or more are common for car
gearboxes.
History
Bicycles and their predecessors were directly driven by the feet. The
penny-farthing used a large driven front wheel to increase the top speed, the
distance covered per revolution depending only on the wheel size. The safety
bicycle introduced driving the rear wheel via a chain, allowing faster travel on
a smaller wheel and introducing the concept of bicycle gearing. Later
improvements included the freewheel and shifter.
An early multi-speed bicycle used a double-sided rear wheel, with
different-sized sprockets on each side. To change gears, the rider would stop
and dismount, remove the rear wheel and reinstall it in the reverse direction.
One example of this type of bicycle is in the Science Museum (London, UK).
Derailleur systems were first developed in the late 1800s, but the modern
cable-operated parallelogram derailleur was invented in the 1950s.
External type
External gearing utilizes derailleurs, which can be placed on both the front
chainring and on the rear cluster or cassette, to push the chain to either side,
derailing it from one sprocket to a neighboring sprocket. The sides of the
sprockets may be sculpted to help catch the chain, pulling it up onto their
teeth to change gears. There may be 1 to 3 chainrings, and 5 to 10 sprockets on
the cassette or freewheel. Derailleur type mechanisms of a typical mid-range
product (of the sort used by serious amateurs) achieve between 88% and 99%
mechanical efficiency at 100W. In derailleur mechanisms the highest efficiency
is achieved by the larger cogs. Efficiency generally decreases with smaller cog
and chainwheel sizes.[1] Derailleur efficiency is also compromised with
cross-chaining, or running large-ring to large-cog or small-ring to small-cog.
This also results in increased wear because of the lateral deflection of the
chain.
Internal type (hub)
Internal hub gearing works by planetary, or epicyclic, gearing, in which the
outer case of the hub gear unit turns at a different speed relative to the rear
axle depending on which gear is selected. Rear hub gears may offer 2, 3, 4, 5,
6, 7, 8, 9, 12, or 14 speeds. Bottom bracket fittings offer a choice of 2
speeds, and are generally foot-operated. Internal hub gears are immune to
adverse weather conditions that affect derailleurs, and often last longer and
require less maintenance. However, they may be heavier and/or more expensive,
and often do not offer the same range or number of gears. Internal hub gearing
still predominates in some regions, particularly on utility bikes, whereas in
other regions, such as the USA, external derailleur systems predominate. In a
typical hub gear mechanism the mechanical efficiency will be between 82% and 92%
depending on the ratio selected. Which ratios are best and worst depends on the
specific model of hub gear.
Fixed gear
Fixed-gear track racing bikes can achieve transmission efficiencies of over 99%
(nearly all the energy put in at the pedals ends up at the wheel). Biomechanical
factors however determine that a human can deliver maximum power only over a
narrow range of crank rotational speed or cadence. To match the power source
with the load under varying conditions, a variable gear ratio is needed, and
they work very well, though at the expense of mechanical efficiency. The
efficiency varies considerably with the gear ratio being used.
Others
There have been, and still are, drivetrains that are quite different from those
above:
Retro-Direct drivetrains used on some early 20th century bicycles have been
resurrected by bicycle hobbyists. These have two gears but no gear lever; the
operator simply pedals forward for one gear and backward for the other.
Other bicycles of that era dispensed with the chain entirely and used an
enclosed driveshaft and bevel gears. These shaft-driven bicycle were strongly
built but were not mechanically efficient. They were primarily marketed to women,
as the enclosed gears would not entangle clothing. In recent years, a small
number of shaft-drive systems have reappeared on the market as a specialty
item[citation needed].
In recent years, Steve Christini and Mike Dunn added a two-wheel drive option to
bicyclists. Their AWD system, aimed at mountain bikers, comprises an adapted
differential that sends power to the front wheel once the rear begins to slip.
Automatic transmissions have been demonstrated and marketed for both derailleur
and hub gear mechanisms, often accompanied by a warning to disengage
auto-shifting if standing on the pedals. These have met with limited market
success.
Measuring gears
With a derailleur-based multi-speed bicycle, the gears can be denoted by the
number of teeth on the front chainring and rear sprocket, for example the
highest gear on a racing bicycle might be 53x11. For a road-racing cyclist, this
is useful because of the standard size of the wheel. However, this measure is
limited because it does not specify other aspects of the system. Gear inches and
metres of development are related measures that include the diameter of the rear
wheel. Gain ratio is a measure which also takes the length of the crankarms into
account.
With a hub gear, gear ratios are given directly.
Gear inches and meters of development of a gear combination are defined:
Gear inches = Diameter of drive wheel in inches × number of teeth in front
chainring / number of teeth in rear cog.
Metres of development = Circumference of drive wheel in meters × number of teeth
in front chainring / number of teeth in rear cog.
Metres of development corresponds to the distance (in metres) traveled by the
bicycle for one rotation of the pedals. Gear inches has no current physical
significance; it corresponds to the diameter of the main wheel of an
old-fashioned penny-farthing bicycle with equivalent gearing.
Infinitely
Variable Transmission (IVT)
A specific type of CVT is the
infinitely variable transmission (IVT), which has an infinite range of
input/output ratios in addition to its infinite number of possible ratios; this
qualification for the IVT implies that its range of ratios includes a zero
output/input ratio that can be continuously approached from a defined "higher"
ratio. A zero output implies an infinite input, which can be continuously
approached from a given finite input value with an IVT. Low gears are a
reference to low ratios of output/input which have high input/output ratios that
are taken to the extreme with IVTs, resulting in a "neutral", or non-driving "low"
gear limit. Most continuously variable transmissions are not infinitely
variable.
Most (if not all) IVTs result from the combination of a CVT with an epicyclic
gear system (which is also known as a planetary gear system) that facilitates
the subtraction of one speed from another speed within the set of input and
planetary gear rotations. This subtraction only needs to result in a continuous
range of values that includes a zero output; the maximum output/input ratio can
be arbitrarily chosen from infinite practical possibilities through selection of
extraneous input or output gear, pulley or sprocket sizes without affecting the
zero output or the continuity of the whole system. Importantly, the IVT is
distinguished as being "infinite" in its ratio of high gear to low gear within
its range; high gear is infinite times higher than low gear. The IVT is always
engaged, even during its zero output adjustment.
The term "Infinitely Variable Transmission" does not imply reverse direction,
disengagement, automatic operation, or any other quality except ratio
selectabilty within a continuous range of input/output ratios from a defined
minimum to an undefined, "infinite" maximum. This means continuous range from a
defined output/input to zero output/input ratio.
Ratcheting CVT
The Ratcheting CVT is a transmission that relies on static friction and is based
on a set of elements that successively become engaged and then disengaged
between the driving system and the driven system, often using oscillating or
indexing motion in conjunction with one-way clutches or ratchets that rectify
and sum only "forward" motion. The transmission ratio is adjusted by changing
linkage geometry within the oscillating elements, so that the summed maximum
linkage speed is adjusted, even when the average linkage speed remains constant.
Power is transferred from input to output only when the clutch or ratchet is
engaged, and therefore when it is locked into a static friction mode where the
driving & driven rotating surfaces momentarily rotate together without slippage.
These CVTs can transfer substantial torque because their static friction
actually increases relative to torque throughput, so slippage is impossible in
properly designed systems. Efficiency is generally high because most of the
dynamic friction is caused by very slight transitional clutch speed changes. The
drawback to ratcheting CVTs is vibration caused by the successive transition in
speed required to accelerate the element which must supplant the previously
operating & decelerating, power transmitting element. An Infinitely Variable
Transmission (IVT) that is based on a Ratcheting CVT and subtraction of one
speed from another will greatly amplify the vibration as the IVT output/input
ratio approaches zero.
Ratcheting CVTs are distinguished from Variable Diameter Pulleys (VDPs) and
Roller-based CVTs by being static friction-based devices, as opposed to being
dynamic friction-based devices that waste significant energy through slippage of
twisting surfaces.
Variable-diameter pulley (VDP) or Reeves Drive
In this system, there are two v-belt pulleys that are split perpendicular to
their axes of rotation, with a v-belt running between them. The gear ratio is
changed by moving the two sections of one pulley closer together and the two
sections of the other pulley farther apart. Due to the v-shaped cross section of
the belt, this causes the belt to ride higher on one pulley and lower on the
other. Doing this changes the effective diameters of the pulleys, which changes
the overall gear ratio. The distance between the pulleys does not change, and
neither does the length of the belt, so changing the gear ratio means both
pulleys must be adjusted (one bigger, the other smaller) simultaneously to
maintain the proper amount of tension on the belt.
Roller-based CVT (marketed as the Traction CVT,
Extroid CVT, Nuvinci CVP, or IVT)
Consider two almost-conical parts, point to point, with the sides dished such
that the two parts could fill the central hole of a torus. One part is the
input, and the other part is the output (they do not quite touch). Power is
transferred from one side to the other by one or more rollers. When the roller's
axis is perpendicular to the axis of the almost-conical parts, it contacts the
almost-conical parts at same-diameter locations and thus gives a 1:1 gear ratio.
The roller can be moved along the axis of the almost-conical parts, changing
angle as needed to maintain contact. This will cause the roller to contact the
almost-conical parts at varying and distinct diameters, giving a gear ratio of
something other than 1:1. Systems may be partial or full toroidal. Full toroidal
systems are the most efficient design while partial toroidals may still require
a torque converter (e.g., Jatco "Extroid"), and hence lose efficiency.
Hydrostatic CVTs
Hydrostatic transmissions use a variable displacement pump and a hydraulic
motor. All power is transmitted by hydraulic fluid. These types can generally
transmit more torque, but can be sensitive to contamination. Some designs are
also very expensive. However, they have the advantage that the hydraulic motor
can be mounted directly to the wheel hub, allowing a more flexible suspension
system and eliminating efficiency losses from friction in the drive shaft and
differential components. This type of transmission is relatively easy to use
because all forward and reverse speeds can be accessed using a single lever.
This type of transmission has been effectively applied to a variety of
inexpensive and expensive versions of ridden lawn mowers and garden tractors.
Many versions of riding lawn mowers and garden tractors propelled by a
hydrostatic transmission are capable of pulling a reverse tine tiller and even a
single bladed plow.
One class of riding lawn mower that has recently gained in popularity with
consumers is zero turning radius mowers. These mowers have traditionally been
powered with wheel hub mounted hydraulic motors driven by continuously variable
pumps, but this design is relatively expensive. A company call Hydro-Gear, a
joint venture between Sauer-Danfoss and Agri-Fab, Inc., of Sullivan, Illinois,
created the first cost-effective integrated hydrostatic transaxle suitable for
propelling consumer zero turning radius mowers. An integrated hydrostatic
transaxle (IHT) uses a single housing for both hydraulic elements and
gear-reducing elements. As of May 9, 2007, Hydro-Gear remains the only company
producing integrated hydrostatic transaxles for consumer zero turning radius
mowers in North America.
Some heavy equipment may also be propelled by a hydrostatic transmission; e.g.
agricultural machinery including foragers and combines, but not anything that
works the ground because the transmission cannot transmit enough torque.
Hydristor IVT
The Hydristor torque converter is a true IVT in that the front unit
connected to the engine can displace from zero to 27 cubic inches per revolution
forward and zero to -10 cubic inches per revolution reverse. The rear unit is
capable of zero to 75 cubic inches per revolution. The common "kidney port"
plate between the two sections communicates the hydraulic fluid under pressure
and suction return in a "serpentine-torodial" flow path between the two
Hydristor internal units. The IVT ratio is determined by the ratio of input
displacement to output displacement. Therefore, the theoretical range of
Hydristor IVT ratios is 1/infinity to +-infinity/1 but real-world ratios are
constrained by physics.
Simkins' Ratcheting CVT
This transmission is an example of a Ratcheting CVT, prototyped as a bicycle
transmission, protected under U.S. Patent #5516132. The input is the crank with
a round hub integrated with it, and an array of twelve arms that are pivotally
mounted to pins in the hub circle. Each arm has a pinion gear mounted on a one
way clutch that allows only clockwise rotation of the pinion relative to the
arm. All of these pinions are engaged with a large ring gear that is integrated
with the chainwheel as the output, and the ring gear/chainwheel assembly is
mounted to a mechanism that enables it to be adjusted from a position of
concentricity with the crank hub to various amounts of eccentricity with the
crank hub. Adjustment of this eccentricity variably changes the output/input
ratio from 1:1 to 2.6:1 as the ring gear/sprocket assembly is moved from a
position concentric with the crank hub to an eccentric position.
The eccentricity control mechanism is connected to a spring that pushes the
transmission into its eccentric high gear position. The largest spread of the
arms is indicative of the gear ratio because the spreading arms are the only
arms whose pinions (and one-way clutches) are locked and driving the ring gear/chainwheel
assembly. Strong pedaling torque causes this mechanism to react against the
spring, moving the ring gear/chainwheel assembly toward a concentric, lower gear
position. When the pedaling torque relaxes to lower levels, the transmission
self-adjusts toward higher gears, accompanied by an increase in transmission
vibration. This transmission behaves according to the definition of a Ratcheting
CVT.
Advantages and
drawbacks compared to hydraulic automatic transmissions
CVTs can smoothly compensate for changing vehicle speeds, allowing the engine
speed to remain at its level of peak efficiency. They might also avoid torque
converter losses. This improves both fuel economy and exhaust emissions. However,
some units (e.g., Jatco "Extroid") also employ a torque converter. Fuel
efficiency advantages as high as 20% over four-speed automatics can be obtained.
CVTs have much smoother operation. This can give a perception of low power,
because many drivers expect a jerk when they begin to move the vehicle. The
satisfying jerk of a non-CVT can be emulated by CVT control software though,
eliminating this marketing problem.
Since the CVT keeps the engine turning at constant RPMs over a wide range of
vehicle speeds, pressing on the accelerator pedal will make the car move faster
but doesn't change the sound coming from the engine as much as a conventional
automatic transmission gear-shift. This confuses some drivers and, again, leads
to a mistaken impression of a lack of power.
Most CVTs are simpler to build and repair.
CVT torque handling capability is limited by the strength of their belt or chain,
and by their ability to withstand friction wear between torque source and
transmission medium for friction-driven CVTs. CVTs in production prior to 2005
are predominantly belt or chain driven and therefore typically limited to low
powered cars and other light duty applications. More advanced IVT units using
advanced lubricants, however, have been proven to support any amount of torque
in production vehicles, including that used for buses, heavy trucks, and earth
moving equipment.
(The information in these articles i mostly
fetched from wikipedia.org, and is therefore free to reuse)