Download Report

Experiments on Journal Bearing Friction by McKee
Brothers
Fig. 5.5 is given as Fig. 5.29.
It shows the results of tests
on friction by McKee
brothers. A plot of variation
of coefficient of friction with
bearing characteristic
number.
Bearing characteristic
𝜇𝑁
number =
. It is a non𝑝
dimensional number, where
μ is the viscosity, N is the
speed of the bearing and p is
𝑊
the pressure 𝑝 = , d and L
𝑑𝐿
being diameter and length of
the journal respectively
Experiments on Journal Bearing Friction by McKee
Brothers (Contd.)
The plot shows that from B to D with the
increase in bearing characteristic number
the friction increases and from B to A with
reduction in bearing characteristic number
the friction again increases. So B is the
limit and the zone between A to B is
known as boundary lubrication or
sometimes
termed
as
imperfect
lubrication. Imperfect lubrication means
that metal – metal contact is possible and
some form of oiliness may be present.
Coefficient of friction is minimum at B. The
value of bearing characteristic number
corresponding to B, the point of minimum
friction is called Bearing Modulus.
The design point of a bearing should be
around 5(K) so as to ensure that operation
does not come near point B under any
circumstances.
Experiments on Journal Bearing Friction by McKee
Brothers: Stable Operation
Suppose a bearing is operating at some point
between C and D. If there is a change in the
value of any parameter, say rise in
temperature, the viscosity of the lubricant
will decrease, thereby, the bearing
characteristic number decreases. Hence, the
operating point will shift towards C, resulting
in lowering of the friction and the
temperature. As a consequence, the viscosity
will again increase and will pull the bearing
characteristic number towards the initial
operating point. Thus a self control
phenomenon always exists. For this reason
the design zone is considered stable or safe
between C and D. The lower limit of design
zone is roughly five times the value at B.
On the contrary, if the bearing characteristic
number decreases beyond B then friction
goes on increasing and temperature also
increases and the operation becomes
unstable.
Methods for journal bearing design
There are several methods of designing journal bearings.
We shall consider the following two methods:
First Method: developed by M. D. Hersey
Second Method: developed by A. A. Raimondi and J. Boyd
as already discussed
Method developed by M. D. Hersey
This method is based on dimensional analysis, applied to an
infinitely long bearing. Analysis incorporates a side-flow
correction factor obtained from the experiment of S. A.
McKee and T. R. McKee (McKee Brothers).
An important aspect of bearing design is friction and heat
balance and the same will be discussed next.
Friction in Journal Bearing - S. A. McKee and T. R.
McKee (McKee Brothers).
McKee equation for coefficient of friction, for full bearing is given
by,
𝐶𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛, 𝑓 = 𝐾𝑎
𝑧𝑛
𝑝
𝑟
𝑐
10−10 + ∆𝑓 (5.20)
Where,
p : pressure on bearing (projected area) =
𝑃
𝐿𝑑
L : length of bearing d : diameter of journal
N : speed of the journal
μ : absolute viscosity of the lubricant
c : difference between bush and journal diameter
f : side-flow factor = 0.002 for (L/d) = 0.75-2.8
Ka = 541.33 , where  is the circumferential length of the bearing
= 0.195 x 106 for full bearings.
Friction in Journal Bearing- A. A. Raimondi and J. Boyd
Dimensionless performance parameters include the following:
r
f from which the friction coefficient f is calculated
c
Q
from which the flow of lubricant is calculated
rcns L
𝑄𝑠
from 𝑤ℎ𝑖𝑐ℎ 𝑡ℎ𝑒 𝑠𝑖𝑑𝑒 𝑓𝑜𝑤 𝑜𝑟 𝑙𝑒𝑎𝑘𝑎𝑔𝑒 𝑄𝑠 𝑚𝑎𝑦 𝑏𝑒 𝑐𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑
Q
Temperature Rise
• Heat is generated in the bearing due to viscous friction, f
• Assuming that all the heat generated is carried away by the
total oil flow in the bearing.
• Heat carried away:
Hc = mCpt
m = mass of lubricant oil passing through the bearing, kg/s
t = temperature rise, 0C
Cp = specific heat of lubricating oil, kJ/kg0C
• Mass of lubricating oil is
m = ρQ(10-6) kg/s
Heat Balance
In operation, oil temperature rises till a balance is reached , i.e.
Heat generation = Heat dissipation
Two or three iteration may be needed to assume a temperature
rise, so that the above equation is satisfied.
Heat dissipation in the case of method developed by Hersey is
empirically calculated.
LUBRICANT SUPPLY
Lubricant present at the bearing surface gets
depleted due to side leakage and to maintain the
hydrodynamic lubrication continuous supply of
lubricant must be ensured. The principal methods
of supply of lubricant are:
1. Oil Ring lubrication
2. Oil collar lubrication
3. Splash lubrication
4. Oil bath lubrication
5. Oil pump lubrication
Oil Ring lubrication
Fig.5.30 shows an oil ring lubricated bearing. The ring of 1.5 to 2
times the diameter of the shaft hangs loosely on journal. As it
rotates with the journal, it lifts oil to the top. The bearing sleeve
is slotted to accommodate the ring and bear against the journal.
This method of lubrication has been found efficient in many
applications.
Fig. 5.30 Oil ring lubricated bearing with water cooling
3.1.2 Oil collar lubrication
This case a rigid collar integral with the journal as shown in Fig.
5.31 dips into the reservoir at the bottom. During rotation it
carries the oil to the top and throws off into a small sump on
either side of the collar. From there it flows by gravity through
the oil hole and groove to the bearing surface as shown in Fig.
5.32.
Fig. 5.31 Oil collar lubrication
Fig. 5.32 Bearing with oil hole and axial groove
Oil splash lubrication
In some machines, oil is splashed by rapidly moving parts can be
channeled to small sumps maintained above the bearings.
Besides this, small oil scoops on rotating parts can dip into the
main oil sump and thereby carry that flow into bearings. Typical
examples of this can be seen in automobile engine wrist pin
lubrication wherein the crank splashes oil when it dips into the
oil sump below. Another example is lubrication of the bearings of
gearboxes wherein the gears splash the oil into bearings.
Oil pump lubrication
This is a positive means of supplying oil. Fig. 5.33 shows the pressure fed
lubrication system of a piston engine or Compressor. Pumped oil fills the
circumferential grooves in the main bearings. Through the holes in crankshaft
oil is then carried to the connecting rod bearings. Circumferential groove in
them transmits the oil through riffle drilled holes to the wrist pin bearings. In
many automobiles to reduce the cost and also weakening the crankshaft,
riffle drilled holes is eliminated and the wrist pins are splash lubricated.
Fig. 5.33 Oil pump lubrication of an engine
crank shaft