Table of Contents
This phase of the project will outline the methods and parts used in our prototype - the scale model of the Infinity Statue. As a result of the small nature of the drive train, the team has elected to build a full sized system with a scaled down statue.
Team Vision for Detailed Design PhaseGoals for this Phase
- Select parts for scaled down prototype
- Plastic parts in place if metal where possible
- Servos in place of motors
- Choose exact size of prototype and constituent
- Plexiglas windows for sight access to inner workings of prototype
- Will frame made from wood serve prototype purpose?
- Continue developing CAD model of full scale drive
- Including frame for system to rest on in chamber
- Look further into intricacies of coding full scale
- Will we, as a team of all mechanical engineers, be able to code this ourselves?
- Examine possibility of cheaper/smaller parts for
- Could a small rotary encoder work for our prototype?
- Part selection for scaled down model is complete -
bill of materials shown below
- Plastic parts wherever possible
- Cheap DC gear motor selected for our application
- Size of prototype chosen to be approx. 43% scale
(approx. 5ft tall)
- Acrylic sheets will be used as "windows"
- Wooden/acrylic frame will be easiest to assemble, will be used
- CAD model is complete
- Includes frame that is height adjustable
- System coding still being examined
- Coding ability still a concern
- Cheaper/smaller parts detailed in BOM
- Cherry sensor will be used for both setups
1) Gearbox Shell 2) Worm Wheel 3) One Way Clutch 4) Hall Effect Tachometer 5) Rare Earth Magnet 6) AC Gearmotor 7) Needle Bearing 8) Worm Gear Axle 9) Worm Gear 10) Thrust Bearing 11) To Statue Flange 12) Frictional Torque Limiter 13) Main Shaft 14) Support Framing 15) Hall Effect Tachometer
Constraint: L * W * H < 60 * 31 * 20 [in]
Actual: L * W * H = 18.5536 * 12.8176 * 20 [in]
Note that the H is set to max, however it is allowed to change by +/- .75" to accommodate for tolerance in the 20" constraint
Constraint: Cost < 6000 [US Dollars] Actual: Cost = 3159.67 [US Dollars]
Figure below shows the BOM for the final design. The cost is for raw materials only, and neglects installation and machining costs
Conducted Simplified ANSYS simulation to determine if support was sufficiently strong. Load was determined by taking the volume of our gearbox + Motor, multiplying my the density of steel and then multiplying by 2 to account for each gearbox and finally dividing by 4 to account for the four supports. This is a conservative load as the average density is < steel.
Since the deflection was not neglectibly low, it was determined that an additional analysis should be completed to see what kind of force would be put on the main shaft. If this force is too high, it could could increased part wear and seal failure.
At a worst case stress where only the gearbox cap would move the main axle, It can be seen that 3000+ psi of pressure would be applied to the seal, which would result in seal failure.
To avoid this scenario, slop will be machined into the support structure at location of 4 loading bars to insure that any deformation in the support does not negatively impact the main axle.
The figure below shows the assembly explosion for each gearbox. Gear is held rotary to the shaft by a key, and axially by 1 retaining ring. It is not necessary for a second retaining ring as the worm gear is always forced in the same axial direction. Bearings are held axially by the housing or axle on one side, and a retaining ring on the other.
To make sure that the flange was strong enough to avoid alignment problems, an ANSYS static structural analysis was run to demonstrate that low deformations would be seen
Worm Gear Interface
Conducted simplified ANSYS transient simulation of gear tooth stress, strain and total deformation analysis to ensure that the worm gear can perform adequately once it is built. The analysis concluded that the gears will not be under high stress. The other analysis (strain and total deformation) demonstrate there is a significantly small change in shape and size due to external applied forces which indicate that both gears will perform with no issues.
The friction torque limiter shown below has a maximum torque of 175 ft-lbs before it begins to slip. From our assumptions the system will have a maximum torque of approximately 162 ft-lbs.
Through the rough calculations shown a 200 lb person running and swinging from the statue will result in approximately 380 ft-lbs, and a backpack being thrown at the statue will result in approximately 75 ft-lbs of torque on the system.
Person grabs statue at 10mph, and completely decelerates to 0 mph in 1 second. Backpack is thrown at 20mph, and completely decelerates to 0 mph in 0.5 seconds
In the event of either of these scenarios the torque limiter will cause the statue to slip, allowing the motor & gears to remain undamaged for the duration of the external load.
This Torque Limiter is manufactured by Dalton Gear (and sold through McMaster-Carr). They have ensured us that this device will work well in the extremely damp environment, and can be used continuously without any issues given our expected loads. Since we are changing out the sprocket they use (to eliminate the need for a chain which would complicate our system) as long as we use a ferrous material with a 32 micro inch finish, the torque limiter will work, with some minor changes in its rated torque limit. This torque limiter is also significantly cheaper (at $163.19) and more reliable for long lasting use than coupling torque limiters which we also considered.
Prototype Design (Incomplete)
Extensive analysis was done to determine the viability of various materials and various motors we could use to construct the prototype. The calculations assume that the required torque to rotate the gears is negligible, but is conservative in that the friction in the system is exceedingly high, much higher than what we expect to see.
The prototype system will use a 15 RPM motor so the rotation of the statue can me more easily seen during presentation.
The Nextrox motor was selected for its higher torque and lower price when compared to other motors.
In order to save space on the design, we have elected to forgo the sprocket for the hall effect sensor. In its place, we will insert a set of rare earth magnets into the shaft (0.25" diameter, 0.0625" thickness). It will have the same effect as if the sensor were facing a moving gear. The sensor will be mounted into an aluminum block at a maximum distance of 0.06 inches from the magnet face.
As a result of the stainless steel makeup of the shafts, the magnets will not naturally adhere to them. To overcome this, we will use Devcon 20945 High Strength 5-Minute Fast Drying Epoxy (waterproof) to hold the magnets in their slots.
The air gap for this sensor is 0.06".
The prototype will make use of plastic gears. Their cheap and lightweight natures, while still possessing the ability to function as needed, make them ideal for this purpose. We will be purchasing plastic gears for our prototype.
NOTE: Not actual representation of this project.
Drawings, Schematics, Flow Charts, Simulations
Schematic of the electrical and sensor wiring. Each of the Hall Effect sensors will use a 1 k-ohm pull up resistor, while the thermocouple amplifier will use a 4.7 k-ohm pull up resistor.
Shown below is a schematic of the wiring that will be used for the prototype. A breadboard will be used for the prototype, while all of the wiring in the final product will be soldered together. Additionally, all of the wiring will be contained in a waterproof case.
A power relay will be used to switch the power between the two motors. Below is an image of the relay that will be used for the prototype.
Bill of Materials (BOM)
Based on the calculations and analysis done in the Motor Selection section, we have decided that it would be in our best interest to build the prototype from any of the listed plastics at a 35% scale of the actual Infinity Statue - creating a base at approximately four feet tall with a rotating piece at approximately one foot tall. The result will be a wooden or plastic base with acrylic sheets as "windows." The inner workings will be metal and plastic.
For a wooden structure: lengths of lumber will be used as supports in the corners, and cut plywood will be used as the walls, floor, and top of the base. The acrylic sheets will be cut into appropriate shapes and sizes and placed in the plywood. The piece on the base, representing the mobius, will be a worked piece of metal, wood, or plastic to achieve a shape that resembles the actual statue as closely as possible.