ESKOM DISTRIBUTION, WESTERN OPERATING UNIT

Report Vacation Work

Arno Paul Scholtz

2013

A report on my four week vacation training period at Eskom Distribution division, Brackenfell. The purpose of this vacation work was to learn and gather information on the engineering industry and gain experience.

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Contents Organisational Structures ....................................................................................................................... 3

Government Partnership .................................................................................................................... 3

Divisional boundaries .......................................................................................................................... 3

Generation ...................................................................................................................................... 3

Transmission ................................................................................................................................... 3

Distribution ..................................................................................................................................... 3

Hierarchy ............................................................................................................................................. 3

CNCs ................................................................................................................................................ 4

TSG .................................................................................................................................................. 4

Labour relations .................................................................................................................................. 4

Production Methods ........................................................................................................................... 4

Work orders .................................................................................................................................... 5

Mechanical Workshop .................................................................................................................... 5

Workshop layout ............................................................................................................................. 6

Handling of materials ...................................................................................................................... 6

Contractors ..................................................................................................................................... 6

Logistics ............................................................................................................................................... 6

Financial .................................................................................................................................................. 7

Procurement ....................................................................................................................................... 7

Quality Assurance ................................................................................................................................... 7

Maintenance ....................................................................................................................................... 7

Live Work ............................................................................................................................................ 7

Application of OHS Act ............................................................................................................................ 8

Lifesaving Rules ................................................................................................................................... 8

Procedures .......................................................................................................................................... 8

Healthy Working Environment ........................................................................................................... 9

Training ............................................................................................................................................... 9

Equipment ............................................................................................................................................. 10

Insulator ............................................................................................................................................ 10

Bushing .............................................................................................................................................. 10

Transformer ...................................................................................................................................... 11

Conservator ....................................................................................................................................... 12

Tap changer ....................................................................................................................................... 12

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NECRT (Neutral Earth Compensator Resistor Transformer) ............................................................. 12

CT (Current Transformer) & VT (Voltage Transformer) .................................................................... 13

Cables & Lines ................................................................................................................................... 13

Circuit Breaker .................................................................................................................................. 13

In the Future ......................................................................................................................................... 14

Smart grid .......................................................................................................................................... 14

HVDC (High Voltage Direct Current) ................................................................................................. 14

Personal Project: Shaft design for cable roll ........................................................................................ 15

Problem statement ........................................................................................................................... 15

Distributed loading ........................................................................................................................... 16

Point loading ..................................................................................................................................... 17

The existing shaft analysis................................................................................................................. 19

Solid shaft .......................................................................................................................................... 20

Tubular shaft ..................................................................................................................................... 20

Conclusion: ........................................................................................................................................ 21

ACKNOWLEDGEMENTS ..................................................................................................................... 22

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Organisational Structures

Government Partnership Although Eskom is registered as a private company, it is 100% South Africa government owned.

Divisional boundaries Eskom is split up into three divisions, namely Generation, Transmission, and Distribution. Each of these divisions has their own responsibilities and specialties.

Generation

This division is responsible for electricity generation. They control and maintain each power station

no matter the type. Each of these power stations has its own substation, also controlled by the

generation division. These substations convert and step up the supply generated by the power

station. Normally power is generated at 12kV and stepped up to around 400-765kV and 50Hz.

Transmission

After the power supply has been stepped up at the power station, Transmission division takes over.

Transmission builds and maintains all power lines larger than 132kV. These high voltage lines form

the backbone of the electricity grid and cover great distances. On high voltage lines you will see sets

of four lines. Three of these are for each of the phases and the forth is a supporting line to help

carry fault currents (shield or earth wire).

Distribution

The distribution division ensures that the customer receives electricity. This division is responsible

for all lines up to 132kV. Contrary to belief, Eskom rarely supplies households with electricity, rather

Eskom supplies electricity to municipalities, industries, mines, refineries and farms. Municipalities

then distribute the electricity from there to small customers. On medium voltage (MV) lines, up to

33kV one will see a set of three lines for the three phases. Eskom’s MV lines are 3-wire lines fed

from transformers with delta secondary windings. Due to the delta winding an “artificial” neutral

point needs to be created in order to detect earth faults on the MV network. This is achieved by a

neutral earth compensator with a resistor in series with the neutral (NECR). The purpose of the

series resistor is to limit the earth fault current in order to minimise fault current damage to

equipment.

Large industries take MV supply, the so-called large power users. Farms generally take 3 phase low

voltage power (415V). Residential households are supplied at 230V single phase.

Hierarchy Each Eskom division is managed by a group executive. The group executive has a supporting

structure that plays an advisory role. Under the group executive there are several operating unit

managers. There are in fact nine, one for each province. The operating units consist of different

sections, controlled by sector managers. These sections are, to name a few, planning, protection, HR

(Human Resources), etc. The section managers are in charge of the employees, which consist of

engineers, office workers, labourers, etc.

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Group Executive

Distribution Transmission Generation

Supporting structure

- Advisory role

Operating Unit General Managers

- One for each province

Section managers

- Planning, protection, HR etc.

Employees Engineers, office workers, labourers, etc.

CNCs

The Western Cape operating unit has two zones, namely the Protea and Atlantic zones. These zones in turn consist of sectors (West coast, Tableview, Khayalietsha, Helderberg, Overberg, Boland, Garden Route) which are divided up into areas maintained by Customer Network Centres (CNCs). They respond to problems on the network and mainly work on poles and pole mounted transformers. They are not authorized to take oil samples, do certain work on transformers and do more specialised work. The specialised work is done by the Technical Services Group (TSG) in Brackenfell complex. For instance, all transformer maintenance is done by the transformer workshop.

TSG

The Technical Service Group consists of the following sections: MV switchgear (Medium Voltage), HV switchgear (High Voltage), Mechanical workshop, Live work, Transformer workshop and Cable section.

Labour relations The recruitment process follows BEE standards and procedures. New employees are sent on a 18 month “tour” of Eskom, where they receive training and experience in working in each division within Eskom. Thereafter Eskom gives employees the opportunity to attend in house training, as well as training outside of Eskom. Employees may be part of a union, which debates, mitigates and fights for the rights of employees.

Production Methods Eskom does not actually make anything. Most equipment is bought. If Eskom needs something, specifications are written up and the equipment is manufactured by someone else. There is a list of contractors that are approved and authorized to do work for Eskom. Eskom merely brings a lot of parts together that eventually deliver electricity to you and me.

Figure 1: Hierarchy structure

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Work orders

All work is assigned and monitored by the use of work orders. These are created by a program named Maximo. The program generates mostly work orders for planned maintenance.

Mechanical Workshop

While working at Eskom I had the chance to work on and learn about some of the machinery in the

workshop.

• Arc welding, uses a metal rod, forming an arc and in

doing so melting the rod to join the surfaces being

welded. Different rods are used on different

materials and different amperage settings are used

for these materials, while also taking the rod

thickness into account. This is the easiest method of

welding.

• MIG (Metal Inert Gas) has a wire feed that also uses

an arc to melt and join the metal. This method is

faster than arc welding. The inert gas prevent

oxidization while welding.

• TIG (Tungsten Inert Gas) uses a tungsten electrode to

produce the weld. This method is used on non-

ferrous metals and uses a non-reactive gas to shield

the weld from the atmosphere. This method is

better than MIG welding.

When welding there are a few items of personal protective equipment (PPE) that you will need, such

as appropriate gloves and a welding visor. Eskom uses Speedglas welding visors, which use an LCD

electronic shutter. When an arc is struck, the shutter automatically closes and darkens the screen. It

gives the welder the ability to see better just before and just after the arching.

A lathe is a machine for shaping materials by means of a rotating axle. Different bits are used for

different types of cuts. When cutting thread, timing and precision is key, this can be very nerve-

racking.

A milling machine can perform different cuts and works on three axes. Apart from using the normal

cuts for facing and extruding I used a fly cut to cut a groove into a piece of metal. It was interesting

to see the limitations and possibilities of this machinery and how CNC (Computer Numerical Control)

machines are an improvement on milling machines.

While working in the workshop there are several things to remember, like using the right PPE,

staying alert because mistakes are easily made, acceptable tolerances and securing a work piece

properly.

Figure 2: Speedglas welding visor

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Workshop layout

The workshops are laid out so that there are clear escape routes, ventilation is optimized and there is adequate working space for the employees. There is usually one wide channel, to provide an area for workers to move around and the overhead cranes to operate. These channels have several smaller walkways leading out of them for each worker to get to his or her work station. Some areas are enclosed and can be locked, have heaters and is equipped for a specific function.

Handling of materials

Work orders are aided by MRs (Material Requests). These allow employees to calculate the amount of material they will need to complete a work order and are then supplied with these materials accordingly. The use of appropriate PPE (Personal Protection Equipment) is very important when working in a workshop. For example, different types of gloves are needed for different conditions in the workshop. Thick leather gloved are used for welding, more comfortable material gloves are used for general work and rubber gloves are used when working with dangerous substances like acid.

Contractors

Eskom has a list of approved contractors named the vender list. These businesses are chosen by strict guidelines on the procedures they follow. Whenever Eskom needs something, these companies are asked for quotes and used accordingly.

Logistics Eskom has many of its own vehicles that can be used by authorized employees. These employees need to be in possession of a valid drivers license and a valid Eskom driving permit for the vehicle in question. Mostly pickup trucks are used by the CNCs and technical teams. There are trucks available, but can only transport a very limited load. The newest Mercedes trucks with a crane on the front and no load are already overloaded according to the national road laws. These trucks can be upgraded with crane rests and better axles or double wheels to be in compliance with the road laws. Employees that drive trucks are sent for training on how to load their trucks in order to stay within legal limits. The trucks can also be weighed at the Brackenfell complex and CNC’s to ensure correct weight distribution. There are also fleet vehicles available for business use.

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Financial All employees and contractors must have bank accounts in order to get paid. Bonuses are calculated mainly by the overall performance of Eskom, thereafter the performance of a division and section is taken into account. Your bonus is a combination of the above and you’re PA (Performance Appraisal). The managers are responsible for allocating a employees PA.

Procurement When it becomes necessary to buy equipment and/or services three quotes are needed.’

Unfortunately I was not exposed to the procurement department and its operation, and therefore

cannot provide any sensible detail.

Quality Assurance

Maintenance Routine maintenance is done on a three yearly basis. Regular tests and inspections are done during this three year period in order to detect any problems with equipment early on. The maintenance procedure also includes upgrades to the equipment and old equipment will be replaced with newer versions. This is essential to keep up with the ever growing demand for energy.

Live Work To improve customer service and as a result the rating of Eskom as a business, live work is done. Live work is when a person or team performs work on a power line, while it is in service, i.e. when a current is flowing through it at normal operating voltage. The live work section is responsible for inspecting, reinsulating, washing of insulators, joining lines and repairing circuit breakers. There are three methods used to protect the workers from being electrocuted and/or injured. The first is called the gloved method. This method insulates the worker by means of rubber gloves, blankets, etc. The gloved method is only used up to 44kV, thereafter the bare hand method is used. This method charges the worker to the same potential as the line, similar to when a bird sits on a power line. Work is performed from inside an insulated aerial bucket. A faraday suit is worn when using the bare hand method. The suit acts as a conductor and charges the whole body of the wearer to the same potential. The bare hand method is used when working on lines up to 66kV, thereafter the stick method is used. An insulated pole is used to reach the line and perform tasks on it. Equipment is attached to the end of the pole to perform the work. The pole ensures that the worker is sufficiently far away from the power line to be out of danger. All live work equipment is tested on a 6 monthly basis and all aerial vehicles are tested on a 12 monthly basis.

Figure 3: Faraday suit

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Application of OHS Act The prevention of incidents is much more favourable than providing protection for possible

incidents. That is why Eskom has five lifesaving rules in place, derived from the OHS Act

Lifesaving Rules 1. Open, Isolate, Test, Earth, Bond and/or Insulate before touch

a. Equipotential zone created – If you can touch it, insulate it 2. Hook up at heights

a. When working in an elevated position, above two meters, Eskom requires that its employees wear FAS (Fall Arrest System) equipment.

b. To be able to use a FAS, employees must be found competent after receiving training on how to use the equipment.

3. Buckle up

a. Some Eskom vehicles are equipped with drive cameras and black boxes that record the speed, acceleration, the driver/passengers and/or the road.

b. Most vehicles also have a buzzer that informs the driver that he/she is driving too fast. 4. Be sober

a. No employer shall permit any person who is or appears to be under the influence of liquor or drugs to enter or remain at a workplace. No person may be in possession of liquor or drugs at the workplace. In the case where a person is taking medicine, an employer shall not allow such person to perform duties at the workplace if side effects from the medicine constitute a threat to health and safety. To enforce these regulations Eskom performs random alcohol breath test on people entering their grounds. When tested positive, an employee is sent home without pay and may be suspended or dismissed thereafter.

5. Ensure that you have a permit to work. Violation of any of these lifesaving rules is grounds for dismissal.

Procedures To ensure that persons at work receive prompt first aid treatment in the case of injury or emergency there are first aid boxes available in the workshops and offices. The contents of these boxes are adjusted according to the possible injuries that may occur in that workspace. Before work can be started, a risk assessment form must be filled in. It is only necessary to complete this form when not working alone. Performing a risk assessment is done by visual inspection of the surroundings, identifying all possible dangers and then taking appropriate precautions. Everyone present must understand the risks involved with the task at hand and make the necessary provisions for safety equipment and PPE to perform the task. When the risks have been explained everyone must sign the form, stating that they were clearly informed and understand the risks involved. If you feel that the task will be unsafe you may refrain from agreeing to work, for instance, if a vehicle does not have a seatbelt installed, you may refuse to make use of that vehicle without any consequences.

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In the case of an incident, it is necessary to report the incident to a manager and/or health and safety official, and in the case of an environmental issue, the environmental department must be contacted as soon as possible. For instance, an oil spill has occurred, the spill must firstly be managed and contained. Proper PPE must be used to clean up the spill. The environmental office must be phoned and they will send an authorized person to inspect the area. In the case of a massive spill, the environmental office must be contacted immediately and they will respond accordingly.

Healthy Working Environment The duties of the employer: The OHS Act states that every employer shall provide and maintain, as far as is reasonably practical, a working environment that is safe and without risk to the health of his employees. The employer shall cause every employee to be conversant with the hazards to his health and the safety attached to any work that he/she might perform. Information, instructions and/or supervision shall be provided as far as reasonably practical to ensure safety at work. Effective protection must be provided and used, such as the necessary PPE (Personal Protection Equipment), effective ventilation, sufficient light, etc. All persons operating equipment must be competent and must be fully instructed in the safe operation and use of such equipment and in the hazards that may arise from its use. Every employer shall provide sanitary facilities at the workplace. Toilet paper and soap shall also be made available free of charge to the employees. Hot and cold water shall also be made available for washbasins and showers. The room where these facilities are located shall be clearly labeled with a sign to indicate the gender of the persons for whom the room is intended. Ventilation in such rooms must be in accordance with the National Building Regulations. The necessary screen doors or partitions shall be provided in order for privacy. The duties of the employee: Every employee shall take reasonable care for the health and safety of himself and other persons whom may be affected by his actions or omissions. The employee shall carry out any lawful order given to him and obey the health and safety rules and procedures given to him by his employer. If a situation is unsafe or unhealthy the situation is to be reported to his employer or the health and safety representative. Any incident or injury is to be reported as soon as practically possible.

Training When an engineer in training is appointed at Eskom he/she is entered into an 18 month induction period, where he/she will visit all sections and gain experience in various workspaces. Eskom employees receive training such as Fall Arrest System, first aid, workshop safety and driving courses. Refresher courses are done on regular intervals (usually 3 monthly) to update and ensure that employees remember the procedures set in place. All managers at Eskom are trained in first aid.

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Equipment The following section gives an overview of my understanding on how some of the equipment that I came into contact with at Eskom operates and what their uses are.

Insulator An insulator is most commonly found on poles and support structures. Their only function is to prevent an arc over between the line and the structure, i.e. to insulate a live conductor from ground. A jumper cable is connected over the insulators to conduct the electricity to the other side of the structure and to the next section of line at a strain tower. Insulators are made from glass, porcelain or most recently silicone rubber. They are now made from silicone rubber because the material is more resistant to vandalism than the other insulating materials previously used. The insulators consist of several sheds (“rings”) to increase the distance over which the arc will have to travel. The number of sheds is dependent on the voltage that the line is to carry.

Bushing Bushings differ from insulators in the sense that they allow electricity to flow through them. They are used to insulate live conductors entering equipment. They look similar to insulators, but are much more complex in internal construction. The sheds are there to prevent an arc over on the outside of the bushing. Inside the bushing is a copper rod, acting as conductor. The rod is covered by an oil impregnated roll of paper. Within this roll is a series of different sized metal plates. These plates act as capacitors, reducing the magnetic field of the copper rod. The voltages over the plates reduce to zero further away from the conductor. The bushing is filled with oil, and together with the paper roll prevents an arc over on the inside of the bushing.

Figure 4: Bushing

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Transformer Transformers are an essential part of any AC electrical network. They are used to step up and down the voltages of lines in order to minimize losses over lines (high voltage) and provide usable power to customers. The equation for power is as follows; , as you can see, when reducing the current you will reduce the power absorbed by the line. Therefore transformers step up voltages for long distance transmissions. Transformers differ in size, but work on the same principal of magnetic induction. Insulated copper wire is wound around a core consisting of multiple insulated steel plates. The core increase flux density – the magnetic field strength and thus reduces losses in the transformer. Around the windings is a paper insulation and between separate phase windings is a plate that prevents crosstalk (magnetic fields of different phases should not affect each other.) Each phase has two windings; these windings are wound over each other, with the low voltage winding on the inside. The reason for this being that it carries the highest current and to reduce flux leakage (losses) it needs to be a close to the core as possible. Whenever a current flows a magnetic field is created around the conductor. This fact is used in transformers, so that when an alternating current flows through one winding it induces a current, directly proportional to the windings ratio, in the second winding. Voltage is transformed in the

following manner:

and current is transformed as follows:

A transformer is filled with oil for the purpose of insulating and cooling the transformer. Virgin oil, which sometimes looks like water, but usually has a light brown color, is used. Regular oil samples are taken from transformers. Cans are washed out twice with the oil that is to be tested, thereafter the can is filled to the brim with the oil. When the can is closed the can overflows and prevents air contamination. Samples are taken from the main transformer tank. Oil samples are tested at the transformer lab and if found that oil filtration is necessary, the transformer oil is filtered on site, or at the transformer workshop. FURAN analysis tests the amount of furanoid compounds in the oil, which is an indication of insulation degradation. The oil is filtered to 2 parts per million moisture content. While working at the transformer workshop I was overhauling an oil pump when I found a piece of welding flux stuck in the impeller of the pump. This highlighted to me the importance of filtering oil.

Figure 5: Simplified Transformer

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Figure 5: Simplified transformer

Conservator The conservator is an oil storage tank for the tap changer and transformer. It is connected to the

main tank by means of a Buchholz, it is installed in the pipe connecting the conservator tank with the

main transformer tank. Being the highest point, all gasses gather here, when gas levels reach a

critical point, an alarm will be set off. The Buchholz will also detect an oil surge which indicates a

faulty transformer. When this happens, the Buchholz will shut down the transformer and set off an

alarm.

Attached to the conservator is a breather, it facilitates for expansion of the oil and allows air to

move in and out of the main tank to the conservator. The breather contains silica to prevent

moisture contamination of the oil.

Tap changer A tap changer modifies the output voltage of the transformer to supply a constant output voltage to the consumer. The step between output voltages is standardized within Eskom at 1.25% of rated voltage. When switching between voltages (taps) an arc is created and therefore the oil storage in the conservator and main tank (transformer) is separate from the tap changer. The arc decreases the lifetime of the oil, by altering its composition and reducing its quality.

NECRT (Neutral Earth Compensator Resistor Transformer) The NECR is a smaller transformer that provides an artificial neutral earth point for a 3-phase, 3-wire network in order to detect earth faults. A current limiting resistor is connected in series with the NEC neutral internal to the tank, and the other side of the resistor is earthed.

Figure 6: Oil Pump Section

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A NECs has a cross configuration (zig-zag windings), preventing current from flowing to ground, but allowing current to flow from ground to the line. Earth faults are detected by a current flowing from earth to the line, when this happens a breaker will open and trip the line - preventing further damage to the network. An auxiliary transformer is built into the NECR, which is used to provide auxiliary supply to substation equipment, such as protection and battery chargers.

CT (Current Transformer) & VT (Voltage Transformer) A CT transforms a high current to a low current, and a VT transforms a high voltage to a low voltage. They are required to monitor currents and voltages of high voltage lines. They look similar to massive insulators, but because Eskom has stopped working on these, I did not get to learn much about them

Cables & Lines Cables differ from lines in the sense that cables are insulated conductors and lines are bare

conductors. Cables are chosen based on the voltage level and load current, as well as the operating

conditions and the properties of their surroundings. These properties consist of the medium

surrounding them, the temperature in which they will normally operate and the moisture content of

its surroundings. The nominal load that the cable will carry, along with the allowable fault current in

the allowed time for a fault, plays a big role.

Circuit Breaker Breakers are switching devices that allow load and fault currents to be switched safely. The breaker

opens when a fault occurs on the network, acting as an automatic switch. Whilst opening an arc

forms between the two contacts, if the arc is not cleared quick enough it may cause the breaker to

fail. There are different methods of arc quenching and they are as follows:

• Oil breakers, a jet of oil is blasted through the arc, quenching it.

• Gas breakers; stretch the arc with a magnetic field. This type of breaker relies on the dielectric

strength of the gas to stop the arc. The gas used in the breaker is named Sulfur hexafluoride,

which forms a white powder after an arch has been quenched. This powder is named disulfur

decafluoride which tends to quickly reform into sulfur hexafluoride. This is called self-healing.

• Vacuum breakers have minimal arcing, because there is no air to ionize.

• Air breakers use compressed air to blow out the arc.

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In the Future

Smart grid A smart grid is an electricity network that can detect and manage faults on the network automatically. The network can isolate a fault and still provide unaffected sections of line with power. At the moment, if a fault is detected on a line, the whole line will be out of service, unless control manually adjusts the feeds to that part of the network.

HVDC (High Voltage Direct Current) High voltage direct current lines have very little losses over long distances and are thus favorable for transmission purposes. A direct current source also provides constant power and is desired for industrial purposes such as precision machinery or wherever a constant source of power is favorable. Both of these developments in technology are currently in the pipeline for the Eskom grid.

Figure 7: Air breaker

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Personal Project: Shaft design for cable roll

Problem statement Initially mentioned by Adeyemi was the issue concerning the largest cable roll (8 tonnes). Using

material strengths it is necessary to calculate the dimensions of a shaft able to lift the cable roll in

order for transport. Jan van Bosch then informed me that there are more problems concerning

these large rolls. Eskom trucks are only rated up to 6 tonnes and thus these larger cable rolls are

difficult and dangerous to transport. Although Eskom has the equipment to lift these rolls, they do

not have an effective and safe way of transport. These rolls make the trucks top heavy and thus very

unstable in hazardous operating conditions. These trucks frequently operate in rural and dangerous

areas in terms of the terrain they have to cross in order to get on site. Therefore a means of

transport is needed for a 8000kg cable roll.

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Distributed loading The load is distributed over the length of the shaft in contact with the cable roll.

Shear distribution starts at a maximum of the reaction force P, where the shaft is supported. This

force is constant over the distance, d, (distance between where P is applied and where the

distributed loading from the weight of the cable roll starts). The shear force distribution decreases

linearly over the distributed loading and becomes zero on the neutral axis (in this case due to

symmetry in the middle of the shaft)

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Moment distribution is zero at the point where P is applied, it increases linearly over the distance d

and thereafter the distributed loading takes on a parabolic form, reaching a maximum on the neutral

axis.

Point loading The load rests on the two sides of the roll, which can be simplified to two point loadings.

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The reaction force P is again the maximum shear force starting at the point where it is applied and stays constant over the distance, d, where after it becomes zero when the point loading is applied.

The moment distribution is zero at the point where P is applied and increases linearly over distance d. Then the point loading is applied and the moment stays constant over the entire length of the roll.

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The difference between these two loadings is immense. The normal forces created within the

material is much larger for the distributed loading, because a moment is created over a distance and

this has a larger effect than that of a point loading, but the shear forces in the point loaded member

is larger than that of the distributed loading shear. Because of the small distance between the two

point loadings the displacement of the shaft can be neglected for most designs in this case. In the

point loading scenario, the shear stress becomes the limiting factor for design.

Note: I also neglected the weight of the shaft itself in all calculations as it is small in comparison with

the weight it needs to support.

When you compare the point loading results with that of the distributed load you see that a point

load is favourable, due to the large difference in internal stresses between the two.

The existing shaft analysis In the analysis of the existing shaft I assumed that the loading is distributed as this seems to be the

more realistic and limiting case. Firstly I wanted to evaluate the existing shaft that the manufacturer

uses. It is very difficult to determine what type of steel the rod is made from and thus difficult to say

with certainty how the current shaft handles the forces exercised on it.

The shaft has a radius of 45mm and is 4m long. This length of the shaft is unnecessary and therefore

a waste of material, also making it very heavy and cumbersome. The shaft weighs +/- 200kg. By

using strength and material formulas I determined that the existing rod is sufficient to lift the roll, if

made from high tensile steel.

The largest stress acting on the shaft is 252.5 MPa and for high tensile steel the shaft will deform

about 5mm at the centre which is very small compared to the length of the shaft and therefore an

acceptable amount.

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Solid shaft This is the most simple shaft design and thus simpler equations. For the solid shaft with a distributed load the feasible range of radii lies between 0.027-0.056m (0.056m being the maximum radius that can fit thru the roll). For radii smaller than 0.027m the forces within the shaft become absurd and thus irrational. I calculated the stresses at the critical points on the shaft and determined a few maximum values, namely the maximum normal stress and transverse shear. I then used these values to calculate the principal stresses (maximum tensile and compressive stresses) and the absolute maximum shear stress. One of the two principal stresses was the largest of these 3 stresses. The shear stress is the smallest and compared to the principal stress it plays a small role in the failure of the shaft. The difference between the maximum principal stress and the maximum normal stress is +/- 1 % and can thus be ignored. The following graph shows the maximum stress in a solid shaft plotted against its radius and also the

yield stress of some common types of steel. (The reaction forces are located 30mm from where

loading starts)

Tubular shaft The tubular shaft design was a bit more difficult to analyse. In order to calculate the thickness of the tube it was necessary to determine the moment of inertia needed in order for the material to not exceed its yield stress. Instead of determining the stresses in the member I used the yield stress as a variable and calculated the dimensions needed to support such a load. Because the maximum stress is now an input value the only graph needed is the displacement graph. It is now important to remember that the shaft is designed for a maximum stress, and the material used is kept at an absolute minimum, thus this can only be used as an indication of where a feasible solution lies, and further calculations must be made to minimise the deformation and increase strength by using different combinations of outer radii and shaft thickness.

0

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Dis tributed load

Maraging S teel

Tool L 2

C arbon s teel 1090

A36 S teel

P oint load

R adius (m)

Sig

ma

(P

a)

P rinc ipal S tres s 1

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The following graph shows the displacement of the shaft’s centre point, where the maximum

displacement occurs, plotted against its radius for a high tensile steel. Note that the tubular shaft

uses the absolute minimum material to support the roll and thus if the thickness of the tube is

increased the tube will deform less.

The graph for a composite beam looks similar to the above one. This is due to the conditions I used to determine the minimum material needed to support the load.

Conclusion: When taking into account all of the calculations involved in the above piece report, the weight and

practicality of the shaft and the cost, I find it best to use a tubular shaft, with an appropriate safety

factor, i.e. increasing the wall thickness of the shaft.

Max Dis plac ement

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

0.090

0.100

0.00

10.

003

0.00

50.

007

0.00

90.

011

0.01

30.

015

0.01

70.

019

0.02

10.

023

0.02

50.

027

0.02

90.

031

0.03

30.

035

0.03

70.

039

0.04

10.

043

0.04

50.

047

0.04

90.

051

0.05

30.

055

R a dius

v (

m) Tubular S haft

S olid S haft

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ACKNOWLEDGEMENTS Herewith I would like to thank the following people for their assistance in giving me the necessary exposure to the Eskom business in order to fulfill my practical training requirements: Jan van Bosch Rustum Emjedi Adeyemi Ishekwene Willie Jooste Bianca Boesak James Dyssel