|
This article was first published in 2006.
|
If you’re a fan of turbocharged performance, you’ve probably heard of sleeve
bearing turbos and ball bearing turbos. What’s the difference, you ask? Well, in
the first of this two-part series we’ll examine the design of both types of
turbo and, in the second part, reveal a new Australian-built breathrough in turbo bearings....
Turbocharger Layout
Regardless of the type of bearings employed, turbochargers share the same
design elements – there’s the turbine (aka exhaust) side, compressor (aka inlet)
side and the centre cartridge which incorporates the lubrication and cooling
systems.
The turbine side of the turbocharger comprises a finned wheel that spins
inside a cast iron housing. The turbine wheel is driven by the engine’s exhaust
gasses and a wastegate valve regulates the shaft speed in order to limit boost
pressure.
The turbine wheel is connected to a turbine shaft that has a compressor wheel
attached to the opposite end. The compressor wheel spins inside an aluminium
housing and generates the boost pressure that gives your engine more power.
As shaft speeds can easily exceed 100,000 rpm there’s an obvious need for a
heavy-duty bearing assembly. The bearings typically support the shaft in two
locations along its length and, in a sleeve bearing turbo, a thrust bearing is
required to control axial load.
Most modern turbochargers also incorporate water cooling to remove heat from
the cartridge assembly and stabilise operating temperatures.
|
Sleeve Bearing Turbos
Sleeve bearings were the bread-and-butter of mass produced turbochargers
until Nissan introduced ball-bearing technology to the market in the late ‘80s.
So what exactly is a sleeve bearing (aka journal bearing) turbo?
As its name implies, a sleeve bearing turbo employs a pair of bearings that
fit around the turbine shaft (like sleeves) and allow it to rotate within the
cartridge. The sleeve bearings, which are usually made from bronze, are inserted
into both sides of the cartridge where the turbine shaft passes through - as
seen in this photo. A small retaining clip is used to hold each bearing in
place.
The turbo sleeve bearings have a series of small diameter holes
around their circumference – and these are vital. These small holes enable a
high pressure supply of engine oil to form a ‘cushion’ between the turbine shaft
and the bearing surfaces. In other words, the turbine shaft is suspended in oil
and should not have any metal-to-metal contact.
While the sleeve bearings let the turbine shaft spin and control radial
loads, a thrust bearing is necessary to control axial load. The thrust bearing
is fitted on the outside of the centre cartridge and, similar to sleeve
bearings, there’s a high pressure cushion of oil that’s formed to prevent
metal-to-metal contact. This photo shows the thrust bearing fitted around its
associated thrust collar – the tiny oil supply passages can also be seen. Note
that the thrust bearing is fitted to the compressor side of the turbo only.
While the thrust bearing is typically made from bronze, the thrust collar
(seen here) is made from tool steel. The thrust bearing assembly (the thrust
bearing together with the thrust bearing collar) is fitted to the cartridge
using dowels to ensure the oil passages are aligned.
Finally, a backing plate is fitted over the thrust bearing assembly. The
backing plate is typically held on by a large retaining clip. The turbine and
compressor housings can now be installed and the turbocharger starts to take
shape.
So that’s a sleeve bearing turbo – what are its
advantages and disadvantages?
Well, as evident by its widespread use, the biggest advantage of a sleeve
bearing turbocharger is relatively low production cost. They can also tolerate turbocharger harmonics and vibration because the turbine shaft rides on
a cushion of oil. The relatively high flow/pressure of lubricating oil also
helps remove turbocharger heat which means water cooling is not essential
(although it is preferable).
The bearing materials in a sleeve bearing turbo are quite
soft, are heavily reliant on oil pressure and are susceptible to damage from
contaminated oil - this photo shows a ‘coked up’ centre cartridge. They are
particularly sensitive to oil related problems in the thrust bearing area where
clearances are only a few hundredths of a millimetre. Risk of thrust bearing
damage is significantly increased whenever turbo boost is increased above
standard.
Note that many sleeve bearing turbochargers can be upgraded with a heavy duty
thrust bearing. A typical thrust bearing provides only 270 degrees of load carrying area. An alternative is a full-circle
360 degree thrust bearing which gives the maximum possible load carrying area.
This photo shows a conventional 270 degree thrust bearing alongside a 360 degree
version. Fitment of a 360 degree thrust bearing requires the use of a two-piece
thrust collar.
Ball Bearing Turbos
Ball bearing turbochargers are fundamentally the same as their sleeve bearing
counterparts except they use a pair of angular contact ball bearings instead of
the combination of sleeve/thrust bearings. Note that this type of ball bearing
turbo is sometimes referred to as a dual ball bearing turbo – this
differentiates it from some ‘hybrid’ ball bearing turbos which use a combination
of ball bearings (on the compressor side) and a sleeve bearing/thrust bearing
(on the turbine side). Turbonetics is one manufacturer of hybrid ball bearing
turbos.
So let’s look inside.
Similar to a sleeve bearing turbo, the turbine shaft in a ball bearing turbo
is suspended at two points along its length. However, instead of the shaft
spinning in a cushion of oil with bronze sleeve bearings, it spins the
inner race of two ball bearing assemblies. Each of the angular contact bearings
is designed to control axial loads as well as radial load – but only in one
direction. One of the angular contact bearings is oriented to receive axial load
toward the compressor and the other bearing receives load toward the turbine.
Axial shaft movement is eliminated by the centrifugal force of the angular
contact ball bearings (when the turbo is spinning).
The ball bearings, which are typically made from stainless steel, are caged
and fitted inside a bearing carrier tube which slides into the cartridge. The
carrier tube is usually held in place by a series of clips, a small bolt and a
locking pin inside the oil supply hole. This pin also acts as a restrictor to
reduce the oil pressure fed to the turbocharger – ball bearing turbochargers
survive on significantly less oil pressure/flow compared to a sleeve bearing
turbo.
Once passing through the oil feed restrictor (seen here), oil enters the
centre cartridge and lubricates the twin ball bearings. Oil also falls directly
onto the turbine shaft to further aid lubrication and improve cooling. An
internal void serves to reduce oil pressure so oil can be gravity drained into
the engine’s sump.
Perhaps one of the most overlooked aspects of ball bearing turbochargers is
their relatively small lubrication requirements. Put simply, ball bearing
turboss require only lubrication of the steel balls while sleeve bearing turbos
rely on substantial oil pressure to maintain proper bearing surface interaction.
So what are the advantages and disadvantages of ball bearing turbos?
Well, with much reduced shaft drag, they’re faster to spool up, are
more efficient across the operating range, can sustain more boost due to their
massive thrust load capacity (there’s no thrust bearing to cause problems) and
can survive with modest oil pressure/flow. Their relatively low dependency on
oil pressure also means they suffer less wear during engine switch-on and
switch-off.
The only disadvantages of a ball bearing turbo are increased production cost and a greater dependence on water cooling
(because there’s less heat conducted away by oil). Most ball-bearing assemblies
are also designed to be replaced as a complete unit rather than rebuilt.
In the second and final part of this series we’ll introduce an innovative
Australian-built turbo bearing system...
Thanks to: Mr Turbo +61 75596 1079
Did you enjoy this article?
Please consider supporting AutoSpeed with a small contribution. More Info...
|
More of our most popular articles.
|
|
|
Email a friend
Print article
|