Project Overview

Problem Overview:

The task at hand is to design a functioning manual transmission. The gearbox should be able to achieve high speeds and dish out large amounts of torque. This will be accomplished by manipulating the gear ratios. When a large gear, attached to the input rod, comes in contact with a gear one-fifth its size, this ratio is a 1:5 ratio and produces speed. If we reverse this ratio and make the smaller gear the input, the ratio becomes a 5:1 ratio, meaning for every five times the small gear spins, the large gear spins once. A 5:1 ratio produces torque. First gear would have a ratio similar to this one because torque is required to initialize the movement of the car, which would be at rest. Fourth gear would have a ratio around 1:1, as speed would have increased from first gear to fourth gear. Depending on the ratio of fourth gear, fifth gear would have a ratio around 1:2 or 1:3. At this gear the output would be the fastest with the least amount of torque. This being said, the problem is to find a set of ratios that give out enough torque to pull sufficient weight while being able to sustain high speeds.

Design Constraints:

The design constraints for the manual transmission vary upon size, materials, and types of gears. The entire transmission system must fit within an open Lego®-built transmission housing that measures 4.1in x 4.7in x 2.6in. All of the materials that will be used for the design will be fabricated using Pro-Engineer and printed using a rapid prototype machine. While this technology can create intricate gears and parts, its limitations lie in the dimensions of the part being fabricated and said part must have at least three layers of abs plastic (must be thicker than one hundredth of an inch). The transmission will be powered by a Lego® NXT RC servo motor and the output of the motor will be uniform throughout the operation of the transmission (testing, competition, etc.).

Figure 1: Lego NXT Housing

The size of the gears is one of the most important constraints. The design calls for the use of industry standard gear sizes. Additionally, the pitch angles and pressure angles of the gears being used must be uniform throughout all gears to ensure proper fitting. Also, the gears must fit inside the box and must be large enough to be fabricated using Pro-Engineer. The smallest gear also must have a minimum of 15 gear teeth. The design requirements also call for the transmission to have at least four speeds in order to show the intermediate steps between the lowest and highest gears.  

Pre-Existing Solutions:

Figure 2: Automated manual transmission

Background:

This past gearbox uses manual shift lever to preselect the next gear to be used and a foot pedal is used to engage this operation without needing a manual clutch.

Figure 3: Preselector gear box used in the 1930's-1950's

 Modern:

There are two main types of current manual transmissions, unsynchronized and synchronized. Unsynchronized the gears are spinning freely and require changes with road speed and engine revolutions. Synchronized transmissions the most common type; contain gear sets meshed together as well as dog clutches which provide non-slip coupling of two rotating members. The synchronizer rings make contact with the gears while spinning at different speeds. It then changes the gear ratio based on the use of the clutch and gear stick. 

Figure 4: Dog Clutches

Figure 5: Synchronizer Rings

Design Goal:

The goal in this module is to design a manual transmission that reaches high torque and has very high speed. After doing a myriad of research and adapting to the design constraints, designing a linear manual transmission will be the most reliable compared to other types of transmissions. Gear ratios are going to be the most important aspect in the transmission. The goal is to have a very low ratio for the most powerful torque and a very high ratio for the most speed. In the pre-existing solution, there are synchronizer rings. In this module, the rings will most likely not be incorporated. Our shifter shafts will be somewhat similar to Figure 4 because that is what will be used to connect the input and output shafts together in order to achieve certain gear ratios. After planning the placement and radius of each gear, the entire system will be drawn on the computer in order to print the prototype.

Project Deliverables:

The manual transmission project requires that we design and create a manual transmission that works with a motor on a much smaller scale. The motor that we will be testing our transmission on will be a Lego NXT motor. With that being said, we will have to produce a transmission that fits to the Lego NXT motor. We will be using Pro ENGINEER to design and simulate tests on our transmission before we use a Rapid Prototyping Machine (RPM) to print out the pieces that will be used to construct it. The RPM will be printing the parts out in ABS plastic. These transmissions will be put through a series of tests in a competition to see which group built the transmission that could operate the fastest and pull the most weight. By the end of the term each group is expected to have a full design on Pro ENGINEER, a working transmission made from the RPM, a final report explaining how our transmission operated and our the work required for us to complete it, a website that includes updates weekly throughout the whole design process and an understanding of what they created in order to do a presentation on it to their fellow classmates.

Project Schedule:

The time each group has to complete this project is about eight weeks because the competition is during the ninth week. So our group plans to have the plan concreted by the end of week three. By week five, we plan to have our gears designed in Pro ENGINEER and ready for a stress analysis. During weeks six and seven we will be completing the design and sending in the STL files for the printer. Week eight will be the time when we receive our parts.  If the parts are flawed we will be printing new ones before the competition. After every week we will be updating our website on our progress. As our transmission comes together we will be preparing for the competition during week nine. By the end of week nine we plan to have our final report and presentation completely finished. These will include our final results and conclusions to our whole project. 

Projected Budget:

The group’s transmission design will most likely consist of 15 gears in total. 5 gears on the input shaft, 5 gears on the shifting shaft, and 5 gears on the output shaft. According to McMaster-Carr.com, a six inch diameter steel- finished spur gear costs around $128 per gear. A transmission has different sized gears throughout; therefore, some gears will be smaller and cheaper than the appraised six inch gears. The price for 15 gears of varying size is around $1750. A transmission also needs shafts and transmission housing. Three gear rods, or shafts, cost about $86 depending on the actual dimensions. Assuming the transmission housing would be giving as it is in lab, the total cost for a real size transmission would be around $1836.