Design And Selection Of Bearing For Oceanic Tidal Energy Conversion

Design a bearing assembly that minimises power loss.
Select bearing materials and coatings for minimum friction and wear.
Design a complete conditioning monitoring system for a machine to ISO standards.

Design Data and Calculations

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

• 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.

Bearing Design and Selection

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.

Sustainability and Minimum Maintenance

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.

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.

References

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: https://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: https://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.