The next most common bearing system used in
computing peripherals is the sleeve bearing. These bearings are made from materials like bronze,
beryllium copper and similarly hard metals that are typically pressed into porous forms
by powder metallurgy. The bearings are generally thick cylinders impregnated with oil,
with a highly toleranced internal shaft space. The oil is carried within the
sleeve by interconnecting pores which can account for up to 35% of the
The sleeve bearing operates on
the principle that the rod fitting within the shaft rests upon a very thin layer
of lubricant as it rotates which is drawn up by capillary action.
In an ideal situation the rod wouldn't make any contact with the
sleeve and operate essentially silently. Without that thin layer of fluid between the
rotating shaft and the sleeve, the bearing material will wear away from the
|In this image of a sleeve bearing (a. viewed from the top)
we can see the effects of the lubricant on the rod as it rotates within
the exaggerated internal space of the sleeve. While the image describes a
sleeve bearing slightly different from those used in many computers,
the important aspect is really the force diagram.
a) P denotes the force of the rod as it
rotates within the sleeve (caused by an unbalanced fan blade for example).
The oblong shape p denotes the reactive force applied to the
rod by the lubricant as it is compressed into a thin-film barrier between
the rod and the inner walls of the sleeve.
b) This diagram of the side simply illustrates how the
lubricants reactive force is distributed along the length of the
What does the mean? Well is illustrates the dependency
upon adequate lubrication all sleeve bearings rely upon. Remove the
lubricant and there is no barrier to prevent metal-to-metal contact as the
rod rotates within the sleeve. Conversely, change the viscosity of the
lubricant towards the viscous end, and the rotational speed of the rod is
Known sleeve bearing problems.
When sleeve bearings fail,
one of three things typically happens. Almost all failures will occur
because of issues with the lubrication.
Failure one, worn bearing- This occurs when the lubrication is insufficient or the rotating
component too unbalanced,. The rod begins to wear away the sleeve material to a
point when the cylindrical space is no longer round. The rod will begin to
vibrate producing an audible noise as it rotates. In instances such as this
there is nothing that can be done, but to replace the bearing or more typically
the entire fan.
Failure two, reduced fan speed - A situation can sometimes develop where the lubricant partially evaporates,
or becomes more viscous then its operational tolerances will allow. It
then resists the magnetic forces rotating the fan blades causing the
entire assembly to rotate at a lower then normal RPM. The results of this type
of soft failure
can be serious if the fan is cooling an essential device like a power supply
Failure three - Complete failure. Occurs when there is an absence of lubricant, or when highly viscous
lubricant overcomes the rotational force from the fans' motor. In these
cases the fan will not rotate at all, and devices which need cooling may overheat.
This is one of the more common failures among power supply fans because of
the continual amount of dust they suck in and their elevated operating temperatures. This
type of failure can also occur when improper lubricants are used which become
highly viscous through age or as a reaction to operational