Preliminary Detailed Design
Table of Contents
For this project the Preliminary Detailed Design phase will be focused on demonstrating the feasibility of the design. The parts and processes detailed below describe the setup for the full sized statue, with the scale model being explored in the Final Detailed Design phase.
Team Vision for Preliminary Detailed Design PhaseGoals for this Phase:
- Finalize part selection for possible eventual
installation in statue
- Sensor, gears, motor, bearings, torque limiter, etc.
- Create plan that details location of parts in system
- Continue to explore feasibility of selected parts
- How Hall effect sensor will work, be placed
- Why we chose worm gears
- Create CAD model of drive train
- Include all parts for gearbox, shaft, sensors, etc.
- Develop electronic schematics
- Begin to develop code for microcontroller to run from
- Examine possible connection from sensor to microcontroller to ensure proper functionality
- Perform external stress analysis on statue
- Attempt to account for loads caused by obstruction in mobius travel path due to human or object
- All parts for statue selected
- Each part detailed specifically for our purpose
- Sensors will be placed facing ferrous sprockets
- Worm gears are efficient enough for our purpose, cheap
- CAD model made in cloud-based program known as OnShape with all parts and mechanical devices present
- Schematics for electronic system created in full detail
- General stress analysis run on system, contingencies for possible failure developed
Feasibility: Prototyping, Analysis, Simulation
Motor SelectionMotor Selection was a huge portion of this project. We had to pick a motor that could not only meet torque and load requirements, but also one that provided long life at minimal power.
Hall Effect Gear Tooth Sensor and SprocketOne of the primary requirements of this project is that it possesses the ability to detect issues and send a notification to those in charge of maintenance. Hall effect gear tooth sensors do precisely that - observe a ferrous target such as a gear tooth as it passes the sensor and creates a waveform from the magnetic field created by the movement. By positioning such a sensor facing a gear or sprocket, we can monitor the rotational speed of said feature in order to determine the status of our system.
The Cherry GS100701 is the selected sensor for our design. It is designed to handle wet, dirty environments while maintaining a compact frame. We will place a sprocket on a shaft in our system and point the sensor at the teeth of the sprocket.
The lower end measuring abilities of the sensor and therefore minimum requirements for our sprocket are detailed below:
The sensing gap for this sensor is 0.006".
This output will be fed to the microcontroller, which will determine if the achieved amplitude of each pulse is sufficient to continue operation. If the pulses repeatedly fall below the target, an email will be sent to maintenance.
Our selected sprocket is a Finished-Bore Sprocket for ANSI Roller Chain for #40 Chain, 1/2" Pitch, 15 Teeth sprocket sold by McMaster-Carr. It meets all minimum requirements for our sensor.
In general worm gears have low efficiency compared to other types of gears. However, our mechanical system is operating at 10 RPM which means we do not need high levels of efficiency.
There are three types of worm gear that were taken into consideration for our design: non-enveloping, single-envelop, and Double-envelop. Non-enveloping worm gears do no concave features, therefore, the straight plane of contact between the gears creates a high level of stress on the gear teeth. Single-envelop worm gears have a concave tooth width as shown in Figure 1. This allows the worm drive to nestle into the gear which decreases stress. Double-envelop worm gears have both concave tooth width which maximizes efficiency and stress reduction.
Although double-envelop worm gears are ideal, it will be difficult to manufacture to our needs and an extremely expensive process. Single-envelop worm gears increase the gear ratio between the worm and worm wheel tooth. This means that there is a higher number of teeth meshing, sharing the load at all times.
Lubrication Needed for Worm Gears:
We will need a compounded gear oil to lubricate the worm gears. This oil type has high viscosity which is necessary for our system because the sliding friction of worm gears. Therefore, as it slides across the tooth of the wheel, it slowly rubs off the lubricant until there is no film of lubrication left.
Types of Lubricants for Worm Gears:
Compounded Gear Oils: Mineral based with rust and oxidation inhibitors blended with four to six synthetic fatty acid that provides excellent lubricity and prevents sliding wear. Typical commercial oils: Mobil 600W Cylinder and Super Cylinder Oil, Texaco Vanguard 460 and 680, Exxon Cylesstic 460 and 680, and Chevron Cylinder Oils 460 and 680.
Extreme Pressure Gear Oils: Best option for high pressure and temperature conditions. This oil reacts with the metal to form a soft, slippery chemical layer which prevents severe wear and welding. Typical commercial oils: Shell Omala, Texaco Meropa, Exxon Spartan EP, Mobilgear 634 and 636, and Chevron EP Gear Oil.
The friction torque limiter shown below has a maximum torque of 190 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.
Motor Life Expectancy
Electrical SystemThe electrical system is controlled by a microprocessor connected to various sensors and power switches to collect running data, run functionality checks, switch between motors, and alert maintenance if there is a problem.
In the image below, each sensor is connected to the microprocessor by input voltage, ground, and signal output wires. Based on the data collected from these sensors, the microprocessor will control the power to each of the motors using two power switches (relays), which will allow it switch which motor is getting power.