Originally the robot was intended to compete in the heavyweight category, thus the chassis was
expected to support a robot with mass not exceeding 79.4 kg. The material of choice for the
chassis frame construction was steel. Steel is the most commonly used engineering material
because of its high yield strength and relative cheapness. Steel can also absorb a great deal of
impact energy before failure. Naturally the group was keen to produce the body panels, the main
frame and the arm of the robot from steel. It was envisaged that the frame would be
manufactured from steel "L" or "C" section bars, about 3 or 4mm thick. It was intended to
manufacture the frame by conventional welding processes.
However after most other components of the robot had been ordered it became clear that it would
be over-weight. The weight of the chassis therefore needed to be reduced. An immediate
suggestion was to manufacture the chassis frame from aluminium bar instead of steel. Aluminium
has a density about one-third of that of steel, but it is far more difficult to weld, has
inferior mechanical properties and is more expensive, as well as requiring the use of bolts to
join the pieces together.
After consultation with the EDMC a new and final approach to weight-saving was established. The
chassis framework is made from 1.5mm thick steel plate. The steel plate was cut into strips that
were bent along their length into L or C sections as appropriate. (See chapter 20, on the
bending of L and C sections). This negated the need for any form of conventional metal bars
in the chassis framework. The steel strips are thin enough to allow them to be spot-welded
together. Spot-welding is a quick and simple process that can be carried out by relatively
unskilled students. This decision saved a lot of time, weight and cost in the manufacturing of
the chassis framework.
After further weight concerns regarding the entire robot had been addressed, the decision was
taken to enter the robot in the super-heavyweight competition category. This required the
chassis to support about 50% more weight than originally intended. As a response to this
significant increase in loading, the front, rear and side panels of the chassis were constructed
from steel plate of greater thickness (2mm). These plates were spot-welded to the frame as
originally planned, and provide it with substantially greater rigidity in bending. The chassis
construction to this stage is shown below.
The critical issue of the chassis base plate was a far different concern. An important
consideration was the requirement of the base plate to be very rigid whilst supporting a great
load. The robot runs on pulleys and belts that allow approximately 10mm ground clearance at
the robot's centre, so the robot could not be allowed to sag. Furthermore, the bearings that
support the pulleys needed to be precisely aligned and easily mounted to the chassis base. In
addition, the internal components and the arm needed to be mounted easily and securely. Initial
material suggestions such as polycarbonate sheet and steel plate were totally inappropriate due
to poor bending performance. Medium Density Fibreboard (MDF) was identified as the most
appropriate material. An 18mm thick sample was chosen for use as the chassis base. The material
is both rigid and facilitates the use of screws and bolts. The base plate is attached to the
robot's chassis frame by 26 self-taping screws.
The chassis needed to hold the robot's arm rigidly in place. This was a very demanding
requirement as the fully set-up arm weighed over 20kg. All of the mass acts so as to tip the
front of the robot, although the centre of gravity is not very far in front of the base of the
A decision had to be made on how the arm would be securely attached to the chassis whilst
allowing the two items to be separated when desired. After consultation with the EDMC a
strategy was developed: An open-top steel box would be constructed and bolted to the chassis
base plate. The steel box has been made in the same manner as the chassis itself - i.e. 2mm
thick steel plates spot-welded onto an L section edge framework of 1.5mm thick steel. The base
of the arm (which was also made as a box-section) fits snugly into the steel box. A bolt, placed
through both the arm and the steel box holds the two together. When tightened the bolt prevents
the arm from moving within the box. Although the box was bolted to the chassis base,
reinforcement was needed to be sure that the arm would not buckle under its own forces. Thus
C section steel strips were spot-welded to the top of the edge framework behind, and in front
of, the arm's base. The box and C section strip are shown below.
Polycarbonate sheet (6mm thick) was selected for the top-plate of the chassis body. This material
was chosen because of its excellent resistance to heat and impact. It is essential to the
robot's maintenance that the top plate can be easily removed, the top plate was therefore
attached to the chassis frame by Velcro strips. It was seen in testing that Velcro is incredibly
strong, and the group was more concerned about the difficulty of removing the lid than the
possibility of the joins failing. A C section steel strip provides structural reinforcement to
the top of the frame, to help it withstand the forces transmitted to the base of the arm. The
polycarbonate top-plate rests on this steel strip as well as the top of the edge framework.
The drive train also needed to be accommodated within the chassis. Although the drive train is
dealt with in a separate chapter of the report, there was close liaison in the design of these
two parts. Their integration into the final robot needed to be smooth. Ample room was provided
within the chassis for both the motors and the gearboxes. The design process was held up
slightly by the late delivery of the motors, but this hurdle was overcome with relative ease.
The only significant problem with the integration of the chassis and the drive train involved
the robot's ground clearance. Due to the small diameter of the belt pulleys, the bearings that
support the freewheeling axles needed to be sunk into trenches milled into the MDF base-plate.
The problem is explained below.
The robot is potentially vulnerable to being flipped. Therefore it rides low on the ground and
has a ramp at the front, in order to prevent other robots getting underneath it. However, care
had to be taken to ensure that no part of the underside of the robot touched the ground. The
first measure taken was to ensure that the base of the chassis was as rigid as possible and
would not sag under loading. Hence, the selection of 18mm thick MDF for the base plate
construction. The problem then took the form outlined below:
The robot chassis is born on belt tracks. Each track held in tension by three pulleys, two of
which are in contact with the ground, each with a radius of 45mm. Given that the axles which
locate the pulleys had a radius of 8mm, there is 37mm height between the bottom of the axle and
The exact ground clearance was calculated by inserting dimensions into AutoCAD design software.
At its minimum value the ground clearance was found to be 5.5mm, as shown in the figure below. At the
centre of the robot, the ground clearance is around 10mm.
The mass of the fully constructed chassis is approximately 27kg, although this figure includes
both the arm mounting box and the unstressed lid. It also includes the robot's armour plating.
The chassis represents about 20% of the robot's mass.
The chassis was not intended to be as heavy as 27kg. However when it became clear that the robot
would be over its original weight limit, a decision had to be made i.e. Would the chassis be
reduced in weight in an attempt to come in under the original limit? Or would the chassis be
reinforced in an attempt to cope with the extra load upon it? The second option was taken.
Hence the front, back and side plates of the robot were made from 2mm thick steel rather than
1.5mm, and the weight of the chassis escalated accordingly.
The chassis proved to be a particularly challenging part of the robot. The design had to
accommodate all other components, and therefore was the last part of the robot design to be
finalised. This meant that any last-minute concerns over time, weight, and cost limits had
knock-on effects for the chassis development. However, because the chassis was the base of the
entire robot it was also the first part to be constructed. Hence there was little scope for
error in the chassis design or construction, as they could have held up the entire robot's
The chassis was designed and constructed in a manner that satisfies all the requirements placed
on it. In particular, it was very simple to construct and allows excellent access for
maintenance to internal components and the robot's arm. It was very cheap to produce because
most of the materials and manufacturing expertise were provided without cost to the project
group. The only failing of the chassis is its increase in mass, which was a necessity partly
due to mass increases in other parts of the robot.