Report: Design and Selection of Bearing for Wave Energy Converter

This report focuses on the design and selection of a journal bearing for a wave energy converter, emphasizing minimal maintenance and resistance to oceanic conditions. It begins with an introduction to wave energy conversion and the role of the bearing in the system. The report provides detailed design data, including calculations for bearing dimensions, pressure, angular speed, wear rate, and frictional force. The selection of stainless steel for the bearing material is justified due to its corrosion resistance. Additionally, the report proposes a breakwater monitoring system to assess the bearing's operational condition, including temperature and clearance sensors. The conclusion summarizes the findings, highlighting the suitability of the selected bearing and material for the application and the importance of regular maintenance and monitoring.

Design and Selection of Bearing for the Wave Energy converter
Prepared By:
Dated:

1. INTRODUCTION
There is increased need for sustainable energy conversion. In this report, discussion is
centred on the design and selection of bearing. Additionally, the design also considers
minimum maintenance requirements. Referring to this new technology, it is at an
advanced stage of commercialization; nevertheless, it presents a robust harnessing
strategy of oceanic tidal energy. It comprises a float with an arm connecting to the
level that provides actuation to the piston pump. The power to be generated would
often depend on the magnitude and frequency of the tidal waves. Averagely, it
assumed, for the purpose of bearing design, that the wave height is 4m and it is
detected after every 5seconds. Once perceived by the floating mechanism, the
hydraulic power is transformed into mechanical power via a mechanical system
comprising the rigid arm, level and piston and of course the bearing, which provides
relative rotational motion for the pump to be actuated. The mechanical power is again
transformed into electrical power via generators. In this case, the focus is on the
design and selection of the bearing (Khurmi and Gupta, 2005). The aim is to design a
bearing that would withstand the oceanic conditions and facilitate minimum
maintenance as it is always costly to undertake regular overhaul for the bearing
replacement. The type of bearing selected in this case is the journal bearing that
exhibits the simplest working mechanism and fit to operate in such an environment
(Khurmi and Gupta, 2005).

1. DESIGN AND SELECTION
The following refers to the data that was provided for the purpose of design. It should be
noted that in cases where some parameters are not provided, a relevant assumption is made
with a justification for the same.
Table 1: Design data
PARAMETER VALUE (Given ) Calculated Value
Area around floater 4m2 -
Arm length 3m -
Lever length 1.5m -
Wave height 0.4m -
Intervals between waves 5sec -
Force on piston 4000N -
Force applied by float 2000N -
Mass of system 250kg -
Wear on bearing Not more than 1mm
Electrical Power 1500kW
2.2 Bearing Design
It should be noted that the total electrical power being produced is 1500kW
The system is composed of 10 floats, with uniform energy conversion capacity hence:
Each unit therefore must generate about 1500/10= 150kW
Let us assume hydraulic and mechanical efficiencies of 87% and 79% respectively given
most pumps and generator operate at this range.
Hence overall efficiency= 0.87 x 0.79= 0.6873
Hydraulic power = 150/0.6873= 218.245kW
Total Weight of components is given by W= mg= 250x10= 2500N
The resultant force Fr= 2500+2000+4000-2500/2= 7250N
Torque on bearing= Fv x L2= 7250 x 3 = 21.75kNm
Rocking speed is given by Wave height/time+ correction factor for reliability=
0.4/5+0.0045= 0.0845m/s

Based on the float area, the diameter of the float can be determined. Then by physical
estimation, the diameter of the bearing can be estimated by stating that: the diameter of the
float is 31 times bigger than the bearing diameter.
Hence Diameter of bearing is fixed at d = 2240/31 = 72.26mm
The ratio l/d would need to be determined using standard tables as the one given in table 26.3
(Khurmi and Gupta, 2005).
Now, bearing length is l = 72.26x 1.90= 137.294mm
And the bearing pressure will be given by P= W/ld= 8500 /(72.26mmx 137.294mm)=
0.8568N/mm2 but companies like SKF have their own formula for determining the bearing
pressure, they often call it static bearing load. In this case, for the purpose of design, let us
focus on the conventional methods then it can be compared with the standard bearing sizes
available commercially. Mostly selection of the right bearing that perfectly fits all
requirements is never one way trade-off optimization techniques like graphical work can be
used to select the best bearing that will mate with the shaft with just the right clearance
(Atmaca & Ates, 2017). In the case above, the tolerance level should be intermediate such
that it can accommodate expansion during high temperature condition before bearing can be
cooled automatically.
It should be noted that the maximum allowable bearing pressure from the same table is
1.75N/mm2. Comparing this with the calculated value, it can safely be stated that the design
is appropriate.
Now, angular speed is often given by P/T. Substituting: 218.245/25.5= 8.55rad/s
Or it can simply said that w= 2xπxN/60
Therefore N= 60x8.55/2x3.142= 81.646rpm
Next, maximum wear rate can be determined:
The standard formula for determining coefficient of static friction between the two surfaces is
given by:
u = 33x10-8(ZN/P)(d/c)+k
From table 26.3 of the same book, the following are retrieved:

Z= 0.03, c/d= 0.001
And k= 0.002 since l/d is between 0.75 and 2.8
Hence substituting:
U = 33x10-8(0.03x81.646/0.8568)(0.001)-1+0.002= 0.0029433
Next, the wear volume V is given by: 2/3πr3L/2r= 2/3 x3.142x (72.26/2)3x (72.26/2x
137.294)
= 0.489m3
Consequently, the specific wear rate= V/T= 0.489/25.5x1000= 1.9176x10-5
Looking at the resulting wear rate, it can be said that since the value obtained is extremely
small, the design is okay as far as wear is concerned. But there is a possibility of it increasing
given the operating conditions of the bearing under various dynamic loads and stresses.
The frictional force Fu= uN
= 0.0029433 x 8500
= 25.018N
However, it can never be guaranteed that the bearing will operate within this wear rate. The
maintenance engineer will have to monitor the performance of the bearing over a given
period to ascertain the operational characteristics. This information can then be used to make
critical improvements to the design and selection process.
Now, back to the design process; after determining the frictional force, the expected power
loss due to bearing friction can then be determined as follows:
The Linear velocity is given by v= wr= 8.55x 0.07226/2= 0.309m/s
Therefore power loss Pl= Fux v= 0.309x 25.018= 7.7306W
Remember that the mechanical power Pm is essential and can be determined as well:
Hence this is given as: Pe/η= 150 /0.79= 189.87kW
Power loss is: 7.7306/189.87x100= 4.075%

This can be said to be so low that the bearing operates at maximum efficiency. However,
again in this case, it can never be said with certainty that the bearing will not overheat.
Therefore, to cushion against catastrophic failure, artificial cooling will be essential and the
amount of artificial cooling can be determined using thermodynamic and heat transfer
theories.
Therefore, based on the environment in which the bearing is to operate and the fact that the
design requirements must be met, bearing material would be stainless steel which is often
very tough and resistant to corrosion. However expensive it may be but in the long run, it will
certainly prove economical. The shaft which is to mate with the journal can be made of mild
steel (hot tempered). As noted earlier, SKF has a range of bearing sizes and types. With the
output values, proper selection can be done (Yasui et al, 2013).
2. BREAKWATER MONITORING SYSTEM
The system of monitoring the bearing operational condition in real time is proposed. It should
be noted that the intention is to establish hydrodynamic state in the performance of the
journal bearing otherwise due to thermal stresses and frequent loading and wear, the bearing
life is drastically reduced as failure is inevitable (Machinerylubrication.com, 2018).
Therefore, two critical parameters must be checked at all times and this includes: temperature
and clearance ( as a functional of oil level in the bearing surface.
Figure 1: Block Diagram of the bearing monitoring system
Noise filtering
Oil temperature
Controller
Oil Clearence
Controller
Comparator
Feedback

Nevertheless, the system is made to operate such that there are two sensors; one is for the
temperature and the other is for the clearance between shaft and journal. The temperature
sensor will control the bearing temperature such that it sounds an alert to the main processor
to make a decision of switching on artificial cooler. At the same time, there are normally
leakages in the system, these are to be detected in a continuous fashion such that the system
automatically calculates the clearance and relay the same information to the display so that an
operator can check this every day and undertake corrective action. Notably, the type of
lubricating oil is needed to be selected such that its viscosity matches the minimum bearing
pressure. System oil leakage is often avoided during design and installation. However the
system is to be integrated with an oil reservoir such that the flow valves are activated when
the amount falls below minimum. The hot oil from the bearing is cooled in the reservoir and
the cold oil from the reservoir is supplied to the bearing for continuous operation (Patriot
Technologies, 2018).
CONCLUSION
From the findings above, the bearing size determined and the material selected can be said to
perfectly fit in the environment in which the bearing will operate. Due to the saline
environment, corrosion is expected to be at peak. Stainless steel provides extreme protection
against corrosion. However, for the other components, regular painting may be needed to
maintain the structural integrity of the components like the frame otherwise rust would
destroy the structural capability of these critical elements.

REFERENCE
Atmaca, B. and Ates, S. (2017). Determination of bearing type effect on elastomeric bearing
selection with SREI-CAD. Advances in Computational Design, 2(1), pp.43-56.
Khurmi, R.S and Gupta, J.K. (2005). Machine Design. Eurasia, New Delhi. pp 996-1020
Machinerylubrication.com. (2018). Journal Bearings and Their Lubrication. [online]
Available at: http://www.machinerylubrication.com/Read/779/journal-bearing-lubrication
[Accessed 19 Mar. 2018].
Patriot Technologies, Inc. (2018). Critical Infrastructure/Industrial Controls Security |
Patriot. [online] Available at: http://patriot-tech.com/solutions/industrial-control-systems-
security-monitoring/ [Accessed 19 Mar. 2018].
Yasui, K., Kosaka, R., Nishida, M., Maruyama, O., Kawaguchi, Y. and Yamane, T. (2013).
Optimal Design of the Hydrodynamic Multi-Arc Bearing in a Centrifugal Blood Pump for the
Improvement of Bearing Stiffness and Hemolysis Level. Artificial Organs, p.n/a-n/a.