Mechanical Design 3bGroup 2 Egg Lifter Project

Ahmad Sheikh, Andrew McKenna, Campbell Simpson, Jack Lucas, Jonathan Smith, Max

Brown, Ross James

Introduction

The following report describes the changes made to the design created in Mechanical Design 3a, and why these changes were necessary. It also includes the construction methods along with difficulties overcome in order to reach the finished model.

A revised project plan including material details and costs is also included, followed up by a review of the final considerations from the last semester.

Finally, the report concludes with any current issues or improvements that could have been made.

Design Changes

A large number of design changes were made to improve upon the final design of last semester.

The first major change is in the base. Following feedback given after the previous presentation, the material of the base was chosen to be Perspex rather than wood. This allowed for a more accurate shaping process, and it was more appealing. The shape of the base was revised to a triangle and the number of wheels changed from 5 to 3, for increased stability with only 3 points of contact. The two driving wheels were moved to the front of the base and the third wheel changed to a ball “flying saucer” wheel at the rear of the base so allow for full movement in any direction.

The rack and pinion that was to be the lifting mechanism of the arm has been removed. In its place is instead a large threaded steel rod, as it was significantly cheaper and lighter, and it allows for the motor to be placed in the base as opposed to on the arm as the rod itself is the piece that is being rotated rather than a gear on the arm. The motor is attached to the rod using bevel gears, and the rod is held at both ends to ball bearings to allow for reduced friction in rotation.

In order for the arm to be forced up the threaded rod as it spins, a second rod was required. A hollow aluminium rod was placed behind the threaded rod to provide a counter force as the arm wants to rotate, so that the only direction it can move in is vertical.

The arm material was changed to glass reinforced plastic as it has a high strength to weight ratio and it was delivered in a square tubing shape which allowed for additional weight saving, and so it was more suitable than wood.

The pincer design was required to be adapted as the two previous considerations, foam and 3D printing, were unsuitable. The foam was too malleable and so did not offer the rigidity required to hold the egg with enough stability, and the quote given from the Design department was beyond the budget of this project. The pincers were crafted using excess material from the arm design, although the gear and motor assembly remained similar. However, the gear ratio was increased to reduce the speed at which the pincers move.

A counterweight on the arm was included in the initial design to reduce friction as it moves up and down, however this was altered slightly so that the weight is at a distance from the centre so that the same moment could be provided with a smaller weight.

A top was added to connect the two rods to help hold them in place, which again was created using additional glass reinforced plastic.

Construction Methods

BaseThe Perspex used to create the base was delivered as an 800x600mm sheet. To create the individual shapes, a CAD model was created so that a drawing file could be used for the sheet to get water jet cut. The first attempt to do so resulted in a fail, as a crack propagated along the edges of the shapes. It was thought that the reason for this was due to the drawing having the small pieces too close together, so the drawing was revised and the second attempt at cutting the shapes was a success.

The large holes for the rods were precisely laser cut from the CAD model, and smaller holes for mounting the motors for the driving wheels and rod and for mounting the rear wheel were drilled into the Perspex.

The individual shapes were fixed together using Araldite, as was the bottom of the ball bearing for the threaded rod attached to the surface of the Perspex. Additional layers of Perspex were also glued in place to provide a mount for the aluminium rod.

ArmThe arm was created using two sections of the glass reinforced plastic tubing, to allow for a step down so that the pincers could lower to ground level. The sections were cut using a hacksaw, and 3 holes were drilled in each one so that they could be securely bolted together. Two larger holes were drilled through two sides of one of the sections by a technician for the rods to go through. A suitable nut was mounted onto the topside of the arm section so that the threaded rod could apply a force to the arm as it rotates.

PincersThe pincers were also cut from the GRP tubing using a hacksaw. Holes were drilled to allow for the pincers to rotate around two bolts, and a POM gear was glued onto each pincer in the same axis at the bolts. Bolts were also inserted at the front of the pincers to create a front surface to prevent the egg from falling out of the pincers. The pincers were attached to an L-shaped piece of GRP to hold both in place along with the motor. This L-shaped piece was fixed to the arm using brackets. Foam was then inserted into the pincers to provide suitable support for the egg so that it would not be damaged.

TopThe top was also a section of the GRP tubing cut again with a hacksaw. The holes for the two rods were also drilled by a technician. Perspex was inserted inside the section to mount the other side of the aluminium rod, along with the second ball bearing for the threaded rod.

Threaded and Aluminium RodsThe rods themselves came as they are. The thread was already in place on the steel rod. However, the rods were too long so they were cut to size with a hacksaw then filed flat.

The threaded rod was attached to the bevel gear at its base by a bolt that was inserted through the central axis of the rod. This was drilled into place by a technician. The other side of both ball bearings was then glued to either end of the rod.

Electrical Control SystemThe control of the motors is done through wires being connected to four switches and a PWM controller. The wires were wrapped neatly in a plastic casing leading to a control box created using excess Perspex which was cut and glued together, with holes drilled to mount the switches, PWM and power supply.

Problems Overcome

When the two driving wheels arrived, they were discovered to have a free wheel insert. When this was removed so that they could be powered by the motors, the shafts of the motors were no longer the same diameter as the wheel bore. This was corrected by a plastic bush insert created by a technician, with a hole drilled into the centre for the motor shaft. Grub screws were inserted to hold the motor shaft firmly within the bush, and the bush within the wheel to prevent any slipping.

Upon initial trials of turning the threaded rod, it was apparent that the edges of the Perspex hole and the hole at the top were catching on the thread and the size of the holes were allowing for too much movement of the rod. Two metal sheaths were created by a technician to enclose the rod with a larger, low friction surface to prevent these two problems.

Revised Timeline

The above Gantt chart shows our proposed timeline which we decided on in semester 1 however as soon as we began, the plans changed. While looking into actually purchasing components, we decided to change the design as described previously in this report, meaning the stages of “Finalise Design and Drawings” along with “Planning” and “Ordering” lasted longer than anticipated and dragged on until more like week 3. Ordering was fairly on schedule, following the design having been finalised however one of our main components, the Perspex took far longer than expected to arrive and this delayed things further. The Ordering stage also continued on into as late as week 10 when components broke or were deemed insufficient and replacements were ordered.

Further delays were caused by the base being shattered by the water jet cutter so manufacture continued on into and after the Easter break. The lost time with the base was made up for with individual parts being assembled early and left to one side as well as the wiring, switches and motors being soldered, all in preparation for assembly.

Once the base was complete, production could begin in earnest and progress snowballed. All motors were mounted very quickly and other pieces fell into place. Testing began as the semester finished and some minor modifications were added in week 13 such as the cable tidies, extra counterweight and steadying wires.

Final Costs

Shown previous is a table with a summation of our material and part costs. The total cost was £239.93, which is lower than the budget of £300. It is different than our estimate from last semester, however this was expected as there was significant design changes. Whilst the threaded rod was cheaper than the rack and pinion, other design changes resulted in an increase in cost, such as using Perspex in place of wood.

The items in red are those which resulted in a waste of money. The Polymorph mouldable plastic was ordered as a method to mount motors and switches. However, upon changes to the design it was suitable to use bolts instead. The first PWM we ordered was damaged in testing and so we were required to buy a second one. Two of the POM gears had an unsuitable pitch to be used in conjunction with the larger gears on the pincers, and so were replaced with the brass gears.

One of the ways in which the full budget could have been utilised is through a wireless control system. However, during the process of altering the design from last semester to the current one, we were quite conscious of going over budget so the expense of a wireless motor controller was disregarded. It also could have led to further expense due to problems or complications with our selected motors and we felt a wireless system in general would be unnecessary for the task in hand.

Semester 1 Considerations

From the design report from last semester, there were several issues that we recognised would have to be considered when moving forward with the design.

The first was the possibility of locking vertical movement to prevent the arm slipping back down. However, this was no longer necessary due to the change from a rack and pinion to a threaded rod.

The second was the possibility of a braking system being required on the wheels to provide more accurate driving. This was not needed as the driving motion is very slow and as the wheels have a rubber coating they provide a lot of friction and so come to a halt quickly.

There were concerns with the operation of the pincers last semester. Taking these into account, the motor used for moving the pincers has a very low RPM, and the gear ratio was increased for an even lower speed of movement. The foam was also inserted into the pincers in a concertina shape to provide adequate grip and support on the egg.

A runner on the arm was suggested to provide a reduction of friction as it moved up and down. However, as the lifting mechanism was changed from a rack and pinion this was no longer suitable. As the counterweight ensures that the arm will be balanced around the threaded rod, we felt that lubrication was all that was necessary.

Another point noted was whether it would be necessary to have an extension on the arm to allow the pincers to move towards the egg slowly and precisely. However, the driving motion is at a suitable pace so we decided that it was not necessary.

Further considerations were for the rack and pinion and so were no longer applicable.

Current Issues and Improvements

Upon testing the final model, it was discovered that the bottom of the threaded rod had a tendency to slip out of position and cause failure. We implemented a fix to this by gluing Perspex pieces around the bevel gear and ball bearing to ensure that they could not slip out of position.

Another issue is the vibration and movement of the arm and threaded rod. Despite having the ball bearings and sheaths, there was still a fairly high amount of movement. To help prevent this steel wires were added to the design. These wires were attached in high tension between the top and the base, to provide greater structural stiffness.

Further attempts at creating a smooth lift included balancing the arm, adjusting the length and weight of the counterweight and drilling weight saving holes in structurally redundant sections of the arm and pincers.