Group Presentation:

New Crossbar Design

The new crossbar has a triangular face with a base of 125mm and a height of 90mm

nb. it should be 5mm thick

New Leg Design

During the stress analysis we didn't realise how thin the walls were in the crane legs at just 1mm so a new design was made. The walls are now 10mm thick so during the new stress analysis a larger second moment of area was calculated.

I = 1.0479e-6 m^4
P(critical) = 55,862N
mass = 7.53kg

Crossbar Calculations

The crossbar was designed to be hollow instead of solid so the same equations were used in the stress analysis but a different second moment of area was used.

Crossbar ( initial )

The crossbar which was initially designed was similar to andy's , however, andy's design was picked as we developed it to suite the needs of the rest of the crane. Even so, the stress analysis which was initially done for the crossbar is given below.

Basic sketch for the crane design .

After all the stress analysis was done, a basic sketch was done of the design to see what it would look like after the open top legs and the 1.7m legs were decided. The CAD which will be done will show the dimensions to a greater degree of accuracy, however, this basic sketch allows the CAD to be formulated. Below is the sketch.

Calulations for the crossbars

The original idea which was posted up by romi was constructive , however, a sample of calculations for the crossbar have been given below . It was important to make sure the stresses were correct on the crossbar as it has a lot of loading. Below are a sample of calculations.

Calculations for the sidebars

The sidebars took quite some time to do as they were an important part of the structure , hence everything had to be moulded around the dimensions for the sidebars. Below are examples of the sidebars being calculated with a second image showing a more realistic result when doing

the calculations .

The bar was concluded as being 300 mm in depth and 50 mm wide.

initial calculations for the legs

When working out the level at which the legs will buckle at in accordance with the amount of stress induced, a force of 250 g is applied from top . Basic stress analysis was required for the legs. Eulers equations were used to determine buckling. A sample of calculations can be seen below.

Calulations for End bars

The end bars are the two bars at the back of the crane whilst the side sidebars are the bars running along the side of the beam Calculations needed to be made for the end bars so that they could fit inside the top part of the legs, and appropriate dimensions were calculated obtaining a suitable weight . The end bars were circular rather than rectangular like the rest of the bars. The calculations are shown below.



The calculations were fairly straight forward and simple , however, the only hardship was to get the right dimensions in accordance to the other bars in the crane.

Tender Proposal: Leg & endbar calculations

Leg

• Aluminium
• E = 70 E9 N/m ^2
• Stress = 40,000,000 N/m^2
• Stress (crane ) = P= ((Pie ^2) *E*I) / 4L ^2 = 8659 N ( Max load = 2453N)
• b = 15 mm
• d= 52 mm
• I = 1.4489e-7m^4
• 5 mm thick
• Volume = 9.69 e-4 m ^3 ( minus the cut away = 5.94e-4 m^3)
• Density = 2690 kg / m^3
• Mass = 2.61 kg

Endbar

• Aluminium
• E = 70 e 9 N / m^2
• stress = 40,000,000 N/ m^2
• Stress ( crane) = 6,688,013 N/m^2
• diameter = 50 mm
• I = 2.6704 e-7 m^4
• 10 mm thick
• Density = 2690 kg / m ^ 3
• Mass = 20 .282 kg
• Volume = 7.5398 e-3 m ^3

Tender Proposal - crossbar calculations

Crossbar

• mild steel
• E = 200x10^9 N/m^2
• σ yield = 220 MN/m^2
• σ = 71,038,835 N/m^2
• b = 40 mm
• d = 120 mm
• I = 2.4325x10^-6 m^4
• 5mm thick
• volume = 0.0018750 m^3
• density = 7825 kg/m^3
• mass = 14.672 kg

Sidebar

• aluminium
• E = 70x10^9 N/m^2
• σ yield = 40 MN/m^2
• σ = 10,006,703 N/m^2
• b = 50 mm
• d = 500 mm
• I = 1.2867x10^-4 m^4
• 5 mm thick
• volume 0.0054 m^3
• density = 2690 kg/m^3
• mass = 14.526 kg

Stress calculations for alluminium sliding beam

It would have been great to 'get the hopper' or just 'get that hopper in' , however, it seems as though destiny does not want it to be dear team mates . After doing some calculations for stress , the grasshopper design has some problems, hence why me and andy have decided that it would be a lot less problematic to do the second design we came up with as it fits the specification ; does its job and is a lot more simple to calculate the stresses and strain on the struts.



Moving on to the important stuff now, lets recall the second design which had 4 struts as legs with a sliding bar in the middle to move the load in the x direction. Calculations below have been done for this bar which will lift the load . Calculations for other struts ( e.g the legs) will be done during the course of this week. For the sliding strut, different combinations of length , materials ( steel or alluminium) , etc have been taken into consideration when doing the calculations. I have pointed these points out below. The first set of calculations were done to these specifications :

• material: alluminium




• length : 1.25m




• deflection : 1mm or 0.001
  
  


• weight of beam : 1020 g ( 1000kg of load plus 20 kg for winch ) = 10006.2 N
  

The calculations are below.

By using the formula y ( deflection) = wl cubed divided by 48 EY i could work out the second moment of area . This allowed me to work out the dimensions of the beam . The weight of the beam would be 18.71 kg and the level of stress the beam would withstand with such loading at a 1mm deflection would be 32.21 MN/m2. However, in reality, the properties of alluminium do indicate that alluminium is not as stiff as some other metals although it has good tensile strength. In reality, under such loading, alluminium would not deflect by 1mm and would rather deflect by up to 5 mm hence i did another set of calculations with the deflection value set at 4mm but with every other value as the same. What i wanted to gain from doing this was to see how much extra stress the beam would have to take if there was more deflection. Below are the calculations :

















































By changing the level of deflection of the beam to a more realistic estimate, the level of stress went up to 81.7 MN/m2 from 32.21 MN/m2. This shows that the level of deflection has a positive correlation with the amount of stress being applied to the beam . Also the yield stress of alluminium is 100 mPA . This shows that the alluminium strut at a deflection of 4 mm would be nearing the yield stress . In reality deflection could be even more than 4mm which is why the sliding beam should NOT be made out of alluminium. In the next post, the same calculations have been made for steel to see if it is more suitable for the sliding beam .

Original Design (from meeting 18/03/10)

Initial Design


After discussing each of our initial designs as a group, we came to the conclusion that the best type of crane to meet the specification would be a luffing crane with a counterbalance. A basic schematic of the layout was drawn in order to give a sense of the proportions and scale of the design.

With a 2.6 metre boom at 40 degrees to the horizontal, the total reach of the crane would be 2 metres about the centre of its rotational axis. This allows for an object to be lifted and moved a total of 4 metres from its initial point of pick up. Using a large base, and keeping the main body of the crane fairly low to the ground will lower its centre of gravity and increase stability.


Developed Initial Design #1


The initial design was then developed to include a cable spanning from the counterbalance to the tip of the boom, as well as a support up from the main body. The rotational axis of the crane was moved back slightly on the base to increase stability, thus reducing the required weight of the counterbalance.

It was also decided that a the crane could be rotated via a handle situated on the counterbalance, and also that a hand powered crank would be situated here to operate the winch.

Legs Of The Crane

After doing some research , I concluded that alluminium alloy would best suit the legs of the crane. There are two types, wrought and cast. Cast alluminium alloys are stronger but there are many types. On overage the properties are given below:

Yield strength = 250-450 Mpa
Density = 2600- 2790 kg/m3
Youngs Modulas = 70 - 74 Gpa
Tensile Strength = 300- 550 Mpa

Disadvantages of Alluminium Alloys

Alluminium has a lower tensile strength than steel hence the diameter of the pipe have to be greater in dimension than that of a steel pipe to deal with the same amount of stress.

Alluminium costs more than steel .

Adavantages of Alluminium Alloys

Has a high strength to weight ratio making the crane easier to carry .

It is immune to corrosion unlike iron or even steel to certain extents.

21/03 - Design 3

Design 3 ( The British Bulldog )



This design is a mixture of design 1 and 2 . It is interesting as it is something totally different to what is out in the market. It consists of a boom with supersonic legs and has the load sliding down the boom . The reason why we called it the british bulldog is because the front 2 legs are higher than the back two and the overall design looks like the shape of a dog. This also has marketability.

Advantages

As the struts or legs give more balance to the boom there may not be such a need for a counterweight.

The crane has a sliding system which is quite simple as there is no need for bearings.

The legs can be adjusted causing the boom to change in angle which can reach loads which are higher up with greater ease .

It can be de-constructed within seconds. The legs can be detached and the boom can retract into 2 causing very little space to be used up within the 4 x 4 rover .

bending moment and stress calculations would not be too complex .

Disadvantages

As the load is sliding down the boom, it may hit the ground before the intended point. It is important that the load is kept close to the crane or enough clearence is givin at the bottom.

There is no rotation which can lead to a bit of restriction.

21/03 - Design 2

Design 2





This design has a different approach all together to design 1 and some may consider it to be more 'simple' . It consists of 4 main legs which will be adjustable with 2 rollers on the upper struts which will roll backwards and forwards in the x direction. There will be 2 winches, one to move the rollers in the x direction and one to move the pulley ( load ) in the y -direction. Andy is working on a sliding system for this design.

Advantages

The bending moments and stress analysis is simple to work out.

The legs are adjustable allowing them to reach places which are not each to get to .

There are no bearings involved which means there is less chance of failure within the crane .

No counterweights are needed for steadyness as the crane will be steady.

Disadvantages

Motion is limited. The load can only be transorted in a linear direction rather than at an angle as there are not bearings for rotation.

The cable can get caught with the winch.

As some legs would be shorter than others in certain situations, there can be a danger of tipping or the load sliding down at a faster speed.

21/03 - Design 1 (simplified from original)

Design 1

This is a simplified version of the original design. The design consists of a hand winch placed above the trunk and has a counterweight to allow steadyness within the crane.

The advantages and disadvantages of this design are stated below.

Advantages

Bearings will allow the crane to rotate , therefore there is greater accessibility for different angles.

The base is steady with four adjustable ( anglular adjustment ) legs.

The base does not take up much space and can access areas which are difficult.

Disadvantages

The bending moments and stress analysis will more complex .

The crane would suffer from the possibility of tipping over.

Bearings make the system more complex.

Additional points for crane specification:

Whilst looking at the initial sketch of the crane it is very important to take into the account the following :

1) The weight of the crane :

A crane which is going to lift 1000kg in weight is not going to be easy to lift as the boom and base will be quite heavy. Therefore it is essential that hollow sections are used for the struts etc. This will decrease the weight of the crane and can still give a reasonable amount of strength. The materials associated would be alluminium or steel. Alluminium would be more desirable but may be more expensive.

2) Hand winch

If you can lift 1000kg then we can make this crane just for u, hoooooowever, it is likely that you can not lift such a weight. The winch in the design would be manual rather than any other option hence we need some kind of a gear system for the winch . Maybe at a 15 to 1 reduction ratio reducing the amount of load from 1000kg to 66 kg etc etc.

3) Standard sections

All diameter of holes need to fit standard sections/tolerences. This would give ease when manufacturing such parts.