INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 10, pp. 1697-1701
OCTOBER 2013 / 1697
DOI: 10.1007/s12541-013-0228-2
Monitoring of Bearing Wear in a Marine Engine by
Change of Piston Position – Effect of Wear and Inertia
Pradipta Vaskar Biswas1, Jung Hwan Ahn2, and Hyo Ryeol Lee1,#
1 Department of Mechanical and Intelligent Systems Engineering, Pusan National University, Jangjeon 2-dong, Geumjeong-Gu, Busan, South Korea, 609-735
2 School of Mechanical Engineering, Pusan National University and ERC/NSDM, Jangjeon 2-dong, Geumjeong-Gu, Busan, South Korea, 609-735
# Corresponding Author / E-mail: hong30140@pusan.ac.kr, TEL: +82-051-510-3087, FAX: +82-518-3087
KEYWORDS: Eddy current sensor, Journal bearing, Wear, Marine engine
Wear monitoring of the journal bearings is one of the biggest issue in a marine engine because it can cause failure of the system
without prior notice. BMW (Bearing Wear Monitoring), which is an indirect method to monitor the bearing wear using a change of
the bottom dead center level of a piston, seems to be an effective way to monitor the bearing wear in real engine operation but it
is important to separate the effect due to the wear from the other due to the inertia and the load.
In this paper the characteristics of wear effect and inertia effect are investigated theoretically and simulated to find their differences.
Several previous researches on bearing wear are used to calculate the wear depth with a given real engine specification. The inertia
effects are investigated in a real engine with speed varied. A motor driven crank-slider is used to measure the total effects instead
of a real engine. It proves that the two effects are separated so that the bearing wear can be monitored by measuring the change of
the bottom dead center level of the engine piston.
Manuscript received: December 3, 2012 / Accepted: July 14, 2013
1. Introduction
In a mechanical system one of the most significant criteria is that there
is relative motion between the two bodies, which is a source of wear.1
For a large marine engine, a lined plain bearing shell of crank-train
bearings - main bearing, crankpin bearing, crosshead bearing – plays an
important role to withstand the dynamic loading from combustion and
mass forces. With continuing operation to transmit linear power of a
piston into rotational power of a crankshaft under severe condition, the
bearing lining gradually gets worn through. Furthermore, wear causes
the clearance between shaft and bearing to increase, which will
gradually start to misguide the force component of the piston. If it gets
worse to an extent where contact between the shaft and the bearing
shell steel backing occurs, overheating will cause even catastrophic
damages in the mechanical power transmission system.2 The presence
of clearances in the joints of the mechanical systems is likely to retard
system performance. Often, vibration, noise and joint reaction forces
characterized by large instantaneous value are experienced as a
consequence of joint clearance. The problem is further compounded
when the clearance size is increased and its shape is altered by wear.3
So after a certain period of wear the bearings should be replaced. Thus
© KSPE and Springer 2013
the key issue is to know the wear condition of the bearings so that we
can replace the bearings before it causes any threat and significant
performance loss. And also it is not economical and effective to open
the engine and go through inspection at regular basis which will cost
time and money.
To avoid such damages due to bearing wear and even open-up
inspection, such monitoring methods as the BWM(Bearing Wear
Monitoring) and the MBTM(Main Bearing Temperature Monitoring)
have been developed and partly implemented to real engines.2,4
The main idea of the BWM is the fact that any change in bearing
wall thickness of those crank-train bearings due to wear will result in
a corresponding change of BDC(Bottom Dead Center) level of the
crosshead relative to the engine structure. However, even though the
distance deviation from a set value of BDC level is measured, it can’t
be regarded just as the compounded wear of the three major bearings
because there are included other factors like elastic deformation due to
external load and inertia of moving parts, which also affect the change
of BDC level. Such indirect method as BMW seems to be an effective
way to monitor the bearing wear in real engine operation but it is
important to separate effects due to load and inertia from the whole
distance deviation.
1698 / OCTOBER 2013
INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 10
P. Flores et al. also studied the dynamics of mechanical system
including joints with clearances and lubrication.5 K. Soong and B. S.
Thompson have gone through the dynamic response of a slider crank
mechanism with radial clearances.6 In this paper, on the assumption of
no load, the effects due to wear and inertia are investigated through
simulation using a wear model of journal bearing and a motor driven
crank-slider mechanism instead of a real engine. Moreover, the inertia
effect is examined on a real engine with no load, i.e., free run mode.
P. Flores has modeled and simulated wear in revolute clearance joint in
multi-body system.7
2. Indirect bearing wear measurement in a marine engine
2.1 Power transmission mechanism of a marine engine
In a marine engine, engine power is transferred to a crankshaft
through a power transmission mechanism consisting of a piston, a
piston rod and a connecting rod as shown in Fig. 1. In a sense of
geometrical movement of the mechanism a linear up-down motion of
the piston rod is converted to a linear and rotational motion of the
connecting rod through a Crosshead Bearing (CHB) and a Crankpin
Bearing (CPB), and finally then converted to a rotation of the
crankshaft through a CPB and a Main Bearing (MB). For precise
oscillation of the piston the piston rod and the crosshead bearing is
guided by a slider and slide way.
2.2 Indirect bearing wear measurement
A schematic of the power transmission mechanism is depicted in
Fig. 2. The moving stroke of the piston and the piston rod is between
the Top Dead Center (TDC) and the Bottom Dead Center (BDC) which
has a stroke length of y. If there are any changes in bearing wall
thickness of the three bearings due to wear, the BDC level gets lowered
by a summed amount of those wears because all the three bearings are
in line at the BDC, which corresponds to the wear gap, δw. The BDC
level can be measured by a gap sensor, which is placed on the slide way
and directed towards the slider end as shown in Fig. 2. To increase
reliability a pair of sensor can be mounted for each piston along both
slide ways. If the bearings wear out gradually during operation, the
deviation gap of the BDC level from the normal one gradually
Fig. 1 Construction of a marine engine
increases which can be detected as δw. It means that bearing wear can
be monitored by measuring changes of the BDC level.
However, the problem is that the BDC level is affected not only by
the accumulated wear but also by the inertia force of all the mechanical
components at the BDC, where all the mechanical movements are just
reversed from downward to upward.
3. Simulation of journal bearing wear
3.1 Archard wear model in a journal bearing9
The Archard’s wear equation (1), which deals with adhesive and
abrasive wear, says that the amount of wear (volume Vw) is generally
proportional to the average applied force (F) and the sliding distance
(S) and is inversely proportional to the hardness (H) of the surface
being worn away.
Vw KF
------ = ------S
H
(1)
Dividing Eq.(1) by the wear contact area which is assumed constant
leads to Eq. (2).
δ
KP
-----w = ---------bS
H
(2)
where δw is wear depth, Pb is the bearing pressure and K is the
dimensionless wear rate. The wear depth is considered to increase
infinitesimally over the sliding distance, ∆S, as expressed by Eq. (3).
δwj+1 = δwj + ∆δwj
(3)
where δwj+1 is total wear depth up to the (j + 1)th step, δwj is the total
wear up to the jth step and ∆δwj is the amount of wear for the current
time step.
In order to investigate the wear trend of a journal bearing, the
Archard wear equation is integrated into the Dufrane wear model
depicted in Fig. 3. In the Dufrane wear model Ob and Oj are the bearing
and journal centres, Rb and Rj are the bearing and the journal radius, e
Fig. 2 Schematic power transmission mechanism