# MED PRESENTATION SCRIPT

| Order of the Owls | Owls |
| --------------------- | ------ |
| Intro (1-6): | Sean |
| Airsoft (7-11): | Yan Le |
| Traverse (12-15): | Sean |
| Transmission (16-21): | Yan Le |
| Tank Treads (22-27): | Kris |
| Clutch (28-34): | Kevin |

### Slide 01 Sean
Good morning Prof GS, TAs and classmates. Our team's selected reverse engineering product was an RC model of the German tiger tank. The tiger tank was the main heavy tank of the German army in WW2. The real version weighed 54 tons and was equipped with an 88mm anti-tank cannon, making it a formidable opponent against the Allies.

### Slide 02 Sean
Our tank is a 1:16 scale model, with functioning BB launcher and can travel at a max speed of 2m/s with 760mm of threads on each side. The turret can move from side to side and up and down, which poses an interesting mechanical function for our reverse engineering project. Here is a video of the tanks working before deconstruction

### Slide 04 Sean
From the video, you can see the main mechanical capabilities of the model tank.

Upon de-construction, the various mechanical components that allowed for the movements are revealed.

### Slide 05 Sean
We have divided this into 4 main subsystems. Here you can see our CAD showing the inner workings of the various mechanical movements.

### Slide 06 Sean
The turret traverse gear - does things
The main drivetrain gearbox - does things
The bb launcher and elevation cam - does things
The tank treads - does things

### Slide 08 Yanle
Our first subsystem is the airsoft mechanism. It consists of 2 working parts, namely the launcher which is powered by spring potential energy and a CAM system to rotate (or aim) the barrel.

The working principle of the airsoft mechanism is as such.

Firstly.... secondly..... lastly

### Slide 09 Yan Le
The airsoft mechanism is rigidly mated to the barrel. However, to allow for freedom of movement in pitch, this subassembly features an integrated trunnion mount, pivoting on the end of the airsoft mechanism as shown here. Another motor based crank pin mated to a guide rail on the airsoft outer plastic shell.

### Slide 10 Yan Le
Here is our selection of the 3 weakest components in the subsystem. The first, a spring mating shaft of the extension spring, was due to the extremely small amount material. after calculations we ended up with a SF of 2.80 due to fatigue and 5.7 in static loading. This was appropriate. What was concerning were the other output gear towards the pinion and the crank pin. These components were selected due to high levels of stress due to cantilever geometry and high torque loadings. The safety factors for these are as shown.

### Slide 09
Here you can see the simplified gear diagrams for both the pin and the rack and pinion. Both employed relatively low torque 6V DC motors. However, at stall condition, we observed that the DC motors were actually over-specced for the plastic gears chosen due to the hgih gear ratios shown here. This was an oversight of the design. A stall condtion could easily be achieved by rigidly holding onto the barrel. V bad china designer.

### Slide 11
Other than the selection of plastic gears, we observed that this subsystem was very well designed. The use of springs ensured multiple mechanical "states" and ensured that it would always reset to the correct state. The gear reduction was also well chosen, as the output torques were well above the required spring compression forces. We also observed good care in the design to ensure that the springs and shooting mechanisms are sufficiently lubricated and constrained to only their required range of movement.

### Slide 13 Sean
The next subsytem is the traverse mechanism of the top turret. The design employed a 6V DC motor that drove a spur gear geartrain. The gear train was required to reduce the angular velocity and increase torque to drive the large turret mass. Our analysis was centered on the projected failure scenario, where the turret is in a stall condition. However, we observed that there is more than sufficient torque in normal operation. The geartrain in this design was more to do with the reduction in angular velocity rather than increasing the amount of torque.

Here you see the high gear ratio of about 1:450. allowed the turret to move at a sufficiently slow rate. Notably, this gearbox has been designed to be extremely compact, mounted on the top of the chassis housing.

### Slide 14 Sean
We selected the 3 weakest components to be the 6v Motor shaft, the gearbox output gear and the large turret traverse spur gear.
Firstly, we selected the motor shaft because it drives a worm gear bidirectionally and thus is subceptible to fatigue due to cyclical loading in both axial and radial directions. Our analysis yielded a SF of 10.6.
### Slide 15 Sean
Secondly, both the gearbox output gear and turret traverse spur gear both experience a large amount of gear teeth loading due to being the pair of gears that experiences the highest amount of torque in the geartrain. Thirdly, the large traverse spur gear is attached to the turret at 3 points with 3 fasteners and thus the 3 points experience shear stress concentration. Our calculations yielded these safety factors for the respective components that we have identified. Notably, the gearbox output gear has an extremely low safety factor, owing to the stall condition in which excessive torque is generated by the 6v motor. The large traverse spur gear has noticeably low factor of safety, likely due to the large amount of cutouts in the structure to reduce the volume of plastic used, leading to a tradeoff in the loading capability of the gear to support the turret as well as stiffness.

### Slide 18 Yan Le
The next subsystem is the transmission gearbox. This is the main gearbox that drives tank movement. The tank, deviates from tank movement systems by using 2 sets of 12V DC motors to achieve differential steering. It is rigidly mounted to the tank chassis by use of a rigid sheet metal structure. We were able get the datsheets for the motors, and saw the hgih output power of 19.2Nmm at 20,0000 rpm. This power is further stepped down through a complex spur gear reduction to a large D shaft that powers the tank tread in rotation.

### Slide 19 Yan Le
We select these 3 components to analyse as the weakest in the system. The motor was mounted through another set of steel plates, which we observed would likely lead to static failure due to high toruqe. In addition, the whole subsasmbly experienced high torque in a stall conditon, and would transfer that toruqe to the chassis through only 2 m4 fasteners. We seeked to understand if 2 fasteners were appriately sized for this in compound shear. Finally, we selected the main gear as it experiened high output torque and was made out of plastic.
### Slide 20 Yan Le
Once again, we see the failure of the plastic geartrain as the stall condition leads to high torque transfered directly to the spur gear teeth.
The safety factor for the gear was very low at 0.16
#### Slide 21 Yan Le
In general, we observed that despite the high torque and large power delivered by the tank, the steel plate and m4 fasteners were extremely over-specced and more than able to deal with any loading that the tank would experience.

### Slide 24 Kris
The final subsystem is the tank treads. The tank treads work by creating maximium friction across the ground, leading to a reaction force propelling the tank forward. Hence, a large amount of torque is required to drive the tank treads. This is why the main drive train had such a large gear reduction.

### Slide 25 Kris
The three weakest components we chose to analyse are the sprocket shaft, teeth and the idler wheel axles. Our analysis yielded the following safety factors: 0.7 for the sprocket shaft, 1.87 for the sprocket teeth, and 10.57 for the idler wheel axles. The tank treads are driven forward by the rotational motion of the large sprocket, while the idler wheels support the load of the tank.

The sprocket shaft transmits the high output torque from the transmission gearbox to the sprocket, and is hence under high torque and radial force from the sprocket-chain interface. The high cyclic loading leads us to consider the shaft as a weaker component. The safety factor of 0.7 is concerning as the tank's movement is key to the tank's functionality.

### Slide 26 Kris
We selected the gear teeth for analysis because, like the shaft, the teeth transmits high output torque from the transmission gearbox to the tank treads, and the cyclic loading makes it prone to failure.

We also analysed the shoulder bolts that act as axles for the idler wheels, because they support the load of the entire tank and are subject to significant impact. The loads have long moment arms that result in high bending moments in the bolts, so we consider it to be a weak component.

### Slide 27 Kris
We observed that the sprocket shaft has a low safety factor due to the high torque output from the gearbox and stress concentration. The sprocket gear teeth may also deflect easily under load, due to its thin geometry. We noticed that the idler wheel axles are well designed to be able to support extra loads in the case of falling or overloading of the tank.

### Slide 28 Kevin
Here is a summary of all our analyzed components and their safety factors. We have expected high safety factors for the transmission gearbox mounting, which is very important due to its high cyclic loading. However, we see significantly low safety factors for the plastic gears, and we think that this is mainly due to our assumption of stall condition in the calculation of gears. We will also address this issue shortly in the redesign we propose. We also noticed that there is an unexpected low safety factor in the sprocket shaft due to stress concentration. However this is also under the stall condition, and the following improvement we propose can effectively improve the safety factor.

### Slide 30
Considering the low safety factor of the gears at stall condition, we need to install protective mechanisms to prevent over stressing of the gears. We noticed that all the gearboxes have output limits, and once the limit is hit, the gears will be stuck, and maximum torque from the motor will be transmitted through the gear train, causing gear failure. Therefore, we propose to install disc clutches between compound gears to prevent over-stressing of the gears.

### Slide 31
For sample calculation, we select the airsoft gearbox which has the lowest safety factor. We calculated the safety factor of the other gears in the geartrain, and decided that the second compound gear has a proper safety factor of 1.4 without the clutch. Hence we decide to install the clutch at the second compound gear to protect the following gears.

### Slide 32
We then did the calculation to determine the proper specs for the clutch and compression spring assuming that the end gear will also have a safety factor of 1.4.

### Slide 33
Here are the specs of the clutch and spring we design. We have also verified that the spring will not fail or buckle under the static load compressing the clutches.

### Slide 35
By installing the clutch at the second compound gear, we limit the torque output to all the following gears, hence increasing the safety factor of the weakest gear in the gear train to 1.4. The disk clutch is retrofitted into the gearbox. We only require an extra spring to apply normal force to compress the clutch, which will also be guided by the gear shaft. Hence, there is minimal addition to the BOM cost.
This clutch design can be extended to other gearboxes in the system. It would significantly increase the safety factors of the gears in conditions of gear stall. This would overcome the difficulties of using plastic gears.