From the specification given, here is my explanation justifying the design of our model crane.
Key points from the specification:
- The system should make effective and efficient use of materials to provide good strength/weight characteristics and to withstand stresses during operation.
- The system should have an effective power transmission system.
- The system should be efficient in performance.
Good strength/weight ratio:
1. Cut-outs save on material cost
2. Cut-outs save on material weight
– this make the horizontal arm and other moving components easier to move under force.
3. Shape of the cut-out:
– The distance from the inner cut-out to the edge is uniform for all to make strength equal all over. This is because the structure is only as strong as its weakest point.
– Inside corners are weak and break easily, so they have all been replaced with curved circular corners for better strength.
Withstand stress during operation:
1. For strength, every component is attached at several points with snap fittings to part of a cuboid:
– The cuboid structure means twisting and linear motion is better absorbed
– There is less room for movement, meaning that the gear setup is more accurate during operation.
– There is more contact area and points with snap fittings, spreading the stress so that each individual joint takes less stress and force.
– The strength of each snap fitting is a lot more strong than other methods like glue for example many forces.
2. Snap Fittings
– Snap fittings also meant a more accurate joining method, there is too much inaccuracy in gluing for example.
– In a laser cutter the laser actually has a diameter which means a cutting width. However this changes depending on the power, frequency and speed settings which change depending on material and thickness, so to find the best snap fitting, we prototyped a range of test pieces using one fixed male part and many female parts with different lengths.
– We found that reducing the female length by 0.2mm each side, 0.4mm overall gave the strongest snap fitting.
– Snap fittings also meant that prototypes could be assembled and dissembled easily.
Effective power transmission system:
1. Uses a 5 to 6 gear set-up to transform the low torque, high speed electric motor, to higher torque and lower speed for use.
– The higher torque is needed to overcome the resistance in the cog setup and weight of moving components.
– The lower speed is needed be able to control the crane effectively, and is also necessary to get higher torque.
2. Motor Cog
– The cog attached to the motor is very small. This is because the motor has a very low torque and a smaller cog requires a lower torque to turn the next cog.
– To get the horizontal cog distancing accurate we used ‘spacers’. Made from 3mm like the cogs were used.
4. Teeth Distancing
– To the the cog teeth distancing accurate. Prototypes were produced to test the teeth at different distances to find the optimum distance with the least resistance and friction, but also with the best strength to stop teeth jumping under stress. Several distances were made, changing by 0.25mm to get a very accurate result.
More visible on the right prototype, but we put holes with varying distances from each other and tested each to find the best
5. Softer Teeth
– During prototyping, we found under close inspection that sharp teeth on both the small and larger cogs was causing a poor contact, so produced prototypes of both the smaller and larger cog with softer teeth and tested them with sharp opposites. We found that softer teeth on the small cog with sharp teeth on the larger cog was the most effective.
6. 75% reduction
– To save on material, and to reduce the weight of the gear setup to reduce torque, we shrank the whole gear setup by 50%. This proved to small, the quality of the teeth produced by the laser cut were too poor at that scale. So we tried 75% and found that it the quality was up to scratch while the weight and material was reduced substantially.
– Spoke like cutaways from the center of the cogs reduces weight to reduce torque, while keeping each wheel structurally strong.
– The reduce tilt on the gears positioned on the axle, we designed ‘joiners’. They incorporate 4 small rods of steel to connect the smaller and larger cog together to form one whole cog. The joiner is the smaller piece sandwiched between the two cog, without any teeth.
The result is a very solid cog now with a 9mm horizontal thickness which means a wider contact with the axle to reduce tilt.
– This design also meant that for the very long gear that moves along a shaft on the main horizontal arm (that is made up of 118 small cogs), the positioning of them together would be extremely accurate minimizing the likely-hood of it catching on the drive cog below.
9. Axle Resistance
– To get the cogs spinning as smoothly as possible, we needed to reduce as much resistance and friction as possible. Since many of the cogs shared an axle, they would have to spin on the axle. We also made the axle able to spin to reduce resistance overall in the cogs spinning even more.
10. Gear Box
– Since many transmissions would need to be used, it would be more efficient to concentrate on creating one accurate gear system and apply it to each motor. So we designed a gear box that used space efficiently, using only 2-3 axles and standard parts.
– We made it possible to add or remove gear cogs, so that even with the final prototype we could change and alter the transmission ratio without having to redesign and re manufacture components.
12. Standard Parts
– Standard parts were made for not just the gear box but all the components where there were needs for multiples of them. This made designing a lot easier, but more importantly it will make assembly, replacement, repairing, and redesigning a lot more efficient as easier.
– We used acrylic so that we could use the laser cutter for consistent and high quality components. 3mm sheet acrylic was optimum, 6mm and 5mm produced to much resistance as there was too much contact resulting in too much friction. 3mm also reduced weight and material cost, improving cost and performance.
– Acrylic is also very smooth and very hard making it a good material where the teeth interact with each other.
Efficient in performance:
Efficient in performance:
– The large, wide and heavy base
– Counter weight on horizontal arm for better average balance
– Use of rollers on the horizontal arm keep it well in place with a low friction/resistance solution
– The design is lightweight to keep unbalanced forces minimal
– Appropriate centre of gravity:
(1) Heaviest parts, transmission and motors have been designed to be as low and central as possible
(2) The whole design has be very extensively designed to be as compact and as low as possible above the base area
– The use of ball bearings in a ‘lazy susan’ reduces friction and resistance considerably. This reduces the torque needed to turn the base, making it suitable for the low torque motor used.
– Cheap materials
– Few materials making sourcing more efficient.
– Easy assembly with snap fit joints, standard parts and flat pack design (could be similar to hobby model plane construction). Barely any tools needed for assembly
– Theoretically, in mass production, flat pack packaging lowers cost with more efficient transportation.
– Choice of acrylic means cut-outs that reduce weight on design can be recycled, reducing cost.
– All acrylic used can be cut from only 2 cycles on university laser cutter. Efficient manufacture means cheaper manufacture.
– 3mm acrylic chosen for:
(1) Its compatibility with the laser cutter to produce high quality and consistent results.
(3) Hard, smooth, and strong, meaning suitable for gear cogs, snap fittings, structure.
(4) Lightweight, meaning good for balance, strength and performance.
(5) Structurally uniform on all axis, whereas wood grains effect strength in certain directions.
(6) Recyclable (cut-outs that reduce weight on design can be recycled)
– 3.18mm steel chosen for:
(1) Strength and rigidity as an axle
(3) Smooth, meaning easier to combine with acrylic for cogs.
– Marbles chosen for:
(1) ‘Lazy susan’ bearings
(3) Easily sourced
(4) Use of only 3 materials makes the design more simple, easy to replicate elsewhere, faster to locate materials.
5. Direct Drive
– One of our earliest concerns in the design process when we applied knowledge and principles of existing cranes to the brief was the use of a cable. It is not necessary with the brief given:
(1) We don’t need the large vertical distance as we only need to lift the matchsticks very far from the ground.
(2) With limited time to complete the challenge, the thought of a hook swinging around on some string sounded to wasteful.
So we designed a direct drive system at the end to eliminate the wobbly hook, and give more control to the operator.
6. Designing for the Brief
– To make the design as efficient as possible, we scrapped lots of the traditional crane components and methods in favor of better ideas that were more suitable for the brief given. Our outcome meant using simple straight arms powered directly from the transmission. Compared to the traditional crane this method:
(1) Reduced components
(2) Reduced friction/resistance
(3) Reduced materials
(4) Reduced weight
(5) Improved ease of operation
(6) Improved accuracy
(7) Improved balance and lowered center of gravity