Design Development - Gantry Crane

Following discussion with the group, we have decided to divide the main guide rail into three sections in order to reduce focused stresses on the joints of the beams. Each section will now have a reduced mass, meaning it will be easier to transport.

Key issues with the gantry crane will be the bending moments about the ends of the guide rail and how the legs will be assembled to the rail in order to cope with these forces.

The legs will have to be adjustable in order to ensure that the I-beam is level. As previously discussed, an I-beam is the most efficient at coping with vertical bending forces, yet weaker when horizontal forces are applied.

After discussing this issue with the group, a hollow square cross-section is to be considered as this is a more effiecient shape at coping with bending in both the vertical and horizontal plane.

A solution for the legs could be the use of a tripod at each end. As discussed in our inital ideas, a form of tripod was thought of as an immediate solution for dealing with uneven ground whichever crane design was decided upon. Initially, the idea was to have a tripod on the end of two main legs, similar to this design:

After looking at the use of a tripod crane where the load is lifted throught the centre axis between the three legs (as shown below)


An idea could be to combine the two characteristics and design the legs assembly as below:


The larger the legs, the larger the support in the horizontal plane. This will help ensure the I-beam is as level as possible, thereby reducing the chance that the beam will be subject to horizontal forces.

Analysing The Possibility of an 8m Guide Rail

Until material properties have been researched, mild steel will be used as a basis for simple design calculations.

Density of Mild Steel: 7800 kg/m3

If the maximum weight is to be 30kg per section, then the volume available for the beam is:
V = m / ρ = 30 / 7800 = 3.85e-3 m3

If the length of the beam is 8m, that gives an available cross sectional area of:
3.85e-3 / 8 = 4.8e-4 m2 ( = 480mm2)

This is equivalent to 22 x 22mm square. Clearly this is inadequate to support a 1000kg load.

If the guide rail is constructed of 2 sections each 4m long, then the available cross-sectional area will be:
(30 / 7800) / 4 = 961.5mm2

Which would give us a 31 x 31mm solid square cross section of mild steel. Again, this suggests that it may not be possible to construct such a beam that will support a 1000kg load. However, until further calculations have taken place, the possibility of joining separate sections will be considered an issue that is to be addressed.

If two separate sections are used to construct the guide rail, we have to ensure that the join will be able to withstand the stresses and bending moments.

A possible solution to joining the sections could be to use two smaller I-beams that have smaller cross-sectional area. These would slot between the flanges of the guide rail.


In order to decrease the weight without sacrificing the properties of the I-beam, we can remove diamond cut-outs from the vertical face.


This also provides us with a mechanism for attaching the smaller joining I-beams to the guide rail via the use of a couple of pins.


As yet, the dimensions for the I-beam are yet to be decided until stress analysis takes place. However, from the properties from the second moment of area of an I-beam, we know that if the forces are assumed to be vertical then we can design the I-beam to be as narrow as possible, focusing the length in the vertical plane.

Initial Design Specifications - Gantry Crane

Following group discussion of various types of cranes already on the market, the possibility of using a gantry crane will be developed for the use outlined in the project brief.

The initial stages in designing the gantry crane will involve looking at the main beam used to support the 1000kg load.

As the load is to be transported 4m from the point of lift, in order to deal with the worst case scenario (where the load has most of its length in one dimension), a length of 8m for the guide rail has been decided upon. This will have implications for the cross-sectional area of the beam as each section is to be designed to weigh no more than 30kg in order to ease transportation.

The following structural shapes are commonly used in industry and may be relevant to our design:

• I-Beam
• Hollow Structural Sections (such as a square, or circular tube)
• Tee shapes
• Rail Profiles (asymmetrical I-beams)

A Universal I-Beam is the most efficient cross-sectional shape when vertical bending moments are applied. It is relatively weak when horizontal forces are applied (i.e. when acting as an H) or when torsional forces are applied.

For both horizontal and vertical forces, a hollow box section is the most suitable. The most efficient shape for bending in any direction is a hollow tube with a circular cross-section.

As the crane is designed to deal primarily with vertical forces, initial designs will focus on the use of an I-beam to support the 1000kg load.