Lifting Mechanism
The lifting mechanism was considered the most critical design aspect of our project. To raise or drop a pallet was obviously a key aspect to the radio shuttle design and this had to be achieved in a consistent fashion.
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Considering the resources available, it was more than likely the lifting would be performed through the use of a motor. Pulley drums were also made available to us to transmit motor power should it be required. Our client had requested a 4-bar linkage with lifting plates fixed to it to actually make contact with the pallet. For the design of the lifting mechanism, we were tasked with designing a system which would raise two linkages (supporting a side of the pallet each) appropriately.
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During concept generation for the lifting mechanism, we came up with the following possibilities to be incorporated into the lifting mechanism design:
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scissor lift
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camshaft
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hydraulic actuator
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bike chain
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wire drum
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A selection matrix was devised to assist us in selecting the right design for the lifting mechanism. Among the design criteria, ease of manufacture, reliability and safety were of key importance. The matrix gave a clear indication that the camshaft provided the best overall design to lift the 4-bar linkages and once this was selected further design on the lifting mechanism could commence.
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The camshaft concept premise involved a motor rotating a shaft with two large bearings at either end of the shaft. The centre of rotation would not be directly through the middle of the bearing which would create a displacement as the shaft rotated and a definitive maximum and minimum heights achieved by the bearings would be noted. Two large bearings were sourced within the university and using a centre of rotation 15mm from the centre of the bearing would result in a maximum displacement of 30mm, perfect for raising the pallet above the shelf.
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In order for the shaft to rotate simultaneously with the bearing, a piece was designed and 3D printed which would fit inside the inner diameter of the bearing but with a section allowing the shaft to pass through (15mm from the centre). This would allow the shaft to rotate the bearing to the desired maximum height.
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To rotate the shaft, pulley drums acquired from the workshop would work in conjunction with a motor to rotate the shaft with one drum fixed to the motor shaft and one to the "lifting" shaft.
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For the shaft to rotate freely and be supported during operation, "shaft supports" were developed on Solidworks and 3D printed. These supports would hold the shaft at the correct height within the model and included a bearing which would encompass the shaft and allow stable rotation. The supports were designed with holes at the base so they could be secured to the base of the radio shuttle without any issues.
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To ensure the shaft rotated appropriately with the pulley above, a "key-way" piece was designed and printed due to its precise nature. The sleeve like component would encompass the lift shaft and slide over a key which had been placed on the shaft. An extrusion on the piece would fit into a cut section on the pulley drum and subsequently assist in the simultaneous rotation of both the lift shaft and the pulley drum.
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Regarding the design of the lifting mechanism, the 4-bar linkage was crucial to the success of it. The placement of the linkage and the height it should reach from the base of the prototype became more clear once the shelves were constructed and the distance between the shuttle base plate (which had been purchased) and the pallet shelf was known along with the height of the reinforcing frame the base of the linkage was to rest on. This frame was fixed to the base plate.
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It was key that the linkage should not lock as it was erected as once the bearing rotated past its maximum position it may not have collapsed properly. Therefore, a decision on the 4-bar linkage was made to attempt to design it at a low angle relative to the base of the shuttle. This would make the rods or bars quite long, but this was required anyway in order to lift the pallet sufficiently.
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Two key considerations regarding the design of the 4-bar linkage involved making sure the bearing on the lift shaft could rotate freely at all times and that the linkage did not interfere with any shafts or motors (made more difficult with the added length). A trial and error system was used to determine the minimum angle the linkage would rest at when the bearing was at its minimum displacement position.
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An initial model was developed on Solidwork and modified accordingly when certain material was sourced or design flaws were made aware (such as the contact between metal profiles and subsequent re-design of the joints). The joints of the linkages and bases were printed to ensure the appropriate dimensions were met and spare aluminium bars were to be utilised to make a large portion of the bars themselves.
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The Solidworks model was made to be compatible with other components within the radio shuttle assembly and once this was achieved and the appropriate height could be reached by the mechanism the design was considered complete and any adjustments necessary could be made during manufacture.
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Note that calculations concerning fatigue on the shaft and the maximum load possible our lifting shaft are presented in the final report.
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