PV calculations. Load and speed
All plain bearing materials have limits in their ability to sustain a given loading at a particular speed. This is known as the pressure x velocity or PV limit. The heat buildup that limits the PV is influenced by a number of factors including:
-
Coefficient of friction between the bearing and the shaft
-
Loading of surface, expressed as P
-
Sliding speed, expressed as V
-
Ability to dissipate the heat
-
Lubrication and cooling available
-
Dry, wet or immersed application
Vesconite
and Vesconite Hilube are internally lubricated and have low friction
coefficients. These materials are also polymers, and as such cannot
dissipate the heat as easily as metals.
The
following equations apply:
|
Loading (P) = |
mass supported per bearing (kg) x 9.8 |
| shaft diameter (m) x bearing length (m) |
|
Loading (P) = |
mass supported per bearing (lbs) |
|
shaft diameter (inches) x bearing length (inches) |
The load
(P) is in Pascal (Pa)
Convert
Pa to MPa by dividing by 106 (1 000 000)
The
recommended design loading for general applications is 30 MPa (300 kg/cm²)
for negligible sliding speeds.
Surface speed (V) = RPM x 3.14 x shaft diameter (m)
|
Surface speed (V) = |
RPM x 3.14 x shaft diameter (inches) |
|
12 |
Surface
speed (V) is given as meters per minute.
(fpm)
If there
is negligible or no load, the surface speed is limited to 300 m/min
in dry conditions. Higher surface speeds are acceptable when
lubricated or immersed in water.
PV is
the combination of the load (P) in MPa (psi) and the speed (V) in m/min (fpm)
The
units of PV are therefore MPa.m/min
The
level of lubrication that is required is determined by the PV.
This lubrication is required to dissipate the heat build-up and to
reduce the friction coefficient.
| MPa.m/min | Psi.fpm | Lubrication requirements |
|
PV < 5 |
PV < 2500 |
No lubrication is required although lubrication aids a long life. |
|
PV < 10 |
PV < 5000 |
Initial lubrication on installation of the bearing |
|
PV < 20 |
PV < 10 000 |
Regular lubrication is required |
|
PV < 40 |
PV < 20 000 |
Circulating oil lubrication is required |
|
PV < 200 |
PV < 100 000 |
Bearing must be immersed in water |
Vesconite
can be easily lubricated by grease, oil, water and certain fluids.
Where
the application has a PV higher than 10 Mpa.m/min, please contact
VescoPlastics.
While
many applications do not require lubrication, lubrication is useful
as it promotes extra-long life. Even once off initial lubrication at
installation gives a better wear life.
Simple grooves
further enhance life substantially.
Updated Jun 02 2005
