Group 25 - Transmission
The demands placed on today's vehicles are immense. They must be able to travel at a wide range of speeds, to back up under their own power, to accelerate quickly, and to tow heavy loads, and they have to do all of these things without destroying the engine inside. It seems simple enough, but try to do every item on that list without changing gears and suddenly things get much more difficult. This is where a transmission comes in. A transmission alters the speed and direction of an output shaft, while allowing it's input shaft to rotate at a fairly constant speed, this allows the engine to remain at it's optimal speed, while allowing the vehicle to travel in the desired direction and speed, greatly reducing engine wear while improving performance and fuel economy. An automatic transmission is a step up from the manual or "standard" transmission. It allows the driver to focus on the road, without being distracted by the continual gear shifting required by a manual transmission. Furthermore, the manual transmission requires much less maintainence and is much harder to damage than a manual transmission. This is important as it provides the consumer with decreased maintainence costs, and increased vehicle reliability.
- Leo Christian-Tabak
- Group Leader (organizational duties)
- Oral Presentation (Presentation and Preparation)
- Wiki Page
- Andrew Roberts
- Wiki Page
- Oral Presentation Preparation
- Tom Svialia
- Wiki Page
- Oral Presentation Preparation
- Vinisha Patel
- Wiki Page
- Mike Allen
- CAD Drawings
- Wiki Page
- Type: Automatic Transmission
- Manufactured by: General Motors
- Type: Hydramatic
- Non Working
- Severe damage to housing and cover plate.
- Drained of all fluids and lubricants.
- Bent drive shaft couple.
- Missing output shaft.
- Missing bolts.
- Shattered disc.
- Rusting on several components.
- Detached control cable (no apparent way to re-attach).
The automatic transmission's function is to change the speed and direction of an output shaft, while allowing the input shaft to remain within a small speed range. Most commonly used in vehicles, this device protects against engine wear, allows for greater acceleration, allows the engine to disconnect from the wheels, allows the vehicle to stop and reverse direction, and provides increased performance and fuel economy. This is done by activating or deactivating sets of discs called Clutch Packs which change which gears in the transmission receive power, thereby changing the speed or direction of the output shaft.
The transmission was given to us in a non-working state. The transmission had sustained severe damage to the main body, in the form of a large crack encircling the point where it connected to the engine. There was a somewhat deep dent running the length of cover plate A. A disc shaped part inside the transmission had shattered beyond the point where we could determine it's function. The transmission had been drained of all working fluids. The transmission requires electrical power to operate and the corresponding power supply was not provided. There were several bolts missing from one of the cover plates.
The purpose of the product is to variate the speed and direction of the output shaft, while allowing the engine to remain in its optimal range of speeds.
The transmission is made up of the following materials:
- Cast Iron
- Filter Material
We broke the transmission down into four distinct sections, each having it's own purpose.
Click on any picture for full-size image
Purpose: Park and fluid filtration.
Purpose: Fluid control and distribution.
Purpose: Speed and direction control.
Purpose: Speed/Direction sensor providing feedback to vehicle.
|Step #||Procedure||Tools Used||Difficulty (1-5)||Picture|
|1||Remove cover plate and attached electronics||4.5mm wrench||2|
|2||Remove speed sensor housing||10mm socket wrench||1|
|3||Remove speed sensor shaft||No tools required||1|
Most of the components inside the transmission were made out of steel. There were also a few plastic parts (such as the gaskets) and one rubber hose on the external frame of the transmission. Steel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.14% by weight. Carbon is the most cost-effective alloying material for iron. Carbon acts as a hardening agent, preventing dislocations in the iron atom. The reason why steel was used for the majority of the parts is because steel can take the tremendous stresses present during operation. The fact that steel is competitively priced means it is easy to obtain and also cheaper if bought in bulk; reducing production costs.
Plastic was used for many of the non load-bearing parts because it is light, inexpensive, corrosion resistant, and easy to manufacture.
Aluminum was also used for a lot of the parts of the transmission. It is common in the automotive industry to make parts using the Aluminum die casting method, because making parts in this way is inexpensive. Also the properties of Aluminum make it lightweight and strong. The fact that it is one third the weight of steel makes it an ideal component for many cars today. Aluminum is corrosion resistant and also extremely durable. They are necessary in the production of cars today because it tremendously reduces the weight of the car, increasing performance and fuel economy.
The assembly of the product was almost the exact reverse order of dis-assembly, since there is some flexibility in the order in which some parts can be re-attached. During re-assembly, we discovered that it was easier to assemble the section between the bell shaped housing, and the outer clutch pack first, and then slide the entire assembly into the transmission. This is different from dis-assembly, where each piece came out independently.
We came across a few difficulties while trying to assemble the transmission. The difficulty came with placing the retaining rings back in the transmission body, and with replacing the spring loaded disk. We struggled getting the retaining rings out during dis-assembly and again during re-assembly. The problem was that we did not have the correct tools to push back the compression rings into their correct slots and so we had to be innovative and use pliers and screw drivers to get the rings in. This proved to be quite a challenge but it gave us a better understanding as to why they need to be fitted in so tightly. It explained the complexity of this transmission and put the job at hand into context.
As mentioned before, the tools that were needed, such as a ring compression tool, were not available to us so instead what we ended up using was a pair of needle nose pliers and several different different screw drivers. What we did was forced the compression rings into place while one or two people compressed the rings using these needle nose pliers. It was a great group achievement once the rings fitted into place.
The automatic transmission is probably one of the least understood transmission designs, as its design is very different from that of a manual transmission. Much of the confusion likely stems from the fact that automatic transmissions are thought by many to have "gears" in the same way that a manual transmission does. While an automatic transmission does have the standard settings, Drive, Reverse, 3, 2, 1, Neutral, and Park, these settings do not engage gears in the way one might think. The two main differences between an automatic and manual transmission is that the automatic has no clutch pedal and most importantly, no gear shift. With an automatic transmission, once it is placed into drive mode, nothing else has to be done for the car to move. An automatic transmission varies it's speed and direction with clutch packs.
A clutch pack
When transmission fluid pressure is routed to a clutch pack, it compresses the clutch rings, preventing them from sliding past each other. You may notice that the clutch rings alternate, with teeth pointing out or in. When the rings lock, the inner section and the outer section turn together, sending power to a different sun, planetary, or ring gear. In our transmission, there were three orbital gear sets (Sun, Planetary, Ring), although this number varies depending on transmission design. By engaging or disengaging clutch packs, the transmission can control the speed and direction of the ring and planetary gears connected to the output shaft. This is how the automatic transmission "shifts gears". Since an orbital gear set is essentially a mechanical speed comparator, by varying the speed and direction of the planetary gear, the sun gear, or both, the transmission can create different gear "ratios" with fewer moving parts than a manual transmission. The following video shows the operation and manufacture of an automatic transmission.
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The following applet shows the engagement and disengagement of clutch packs for different gears.
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When researching for the oral report, we discovered that the transmission was actually missing a critical component. The torque converter, which acts as the clutch for an automatic transmission was not given to us with the transmission. Without it, the transmission has no way to disengage from the engine when shifting. The torque converter is essentially two propellers facing each other. The propellers are surrounded in a fluid to keep mechanical loss to a minimum. When one propeller spins, it pushes the fluid through the second, providing power to the transmission. When the transmission changes gears, the increased resistance simply slows the second propeller, while allowing the first to continue spinning, eliminating the need for a movable clutch plate.
It became clear to us during dis-assembly that the transmission was meant for a front-wheel-drive vehicle. This is evident by the presence of holes for two output shafts, although the product only came with one output shaft. Also, the transmission housing is designed to mount an engine directly on top of it, rather than on the end.
Probably the most significant design change we would recommend is the increased use of aluminum or lightweight parts. The transmission is extremely heavy, even with sections 1 and 2 completely removed. A lighter transmission would provide increased performance and fuel economy, as the engine would not have to move as much weight. The second recommendation we would make would be to replace the set pin connecting the parking lock actuator components with a set screw, or keyed shaft. During dis-assembly, we spent nearly two hours trying to remove the pin ourselves, before bringing the transmission to the machine shop to have the pin removed for us. During re-assembly we encountered problems as well, since the pin is set facing into the transmission body (when assembled) making it very difficult to remove or re-attach. A set screw or keyed shaft would eliminate this problem with no loss of joint strength. The third recommendation we would suggest would be to replace the bolts holding down the speed sensor cover plate with a larger size. During dis-assembly, we nearly stripped the bolt heads trying to get them off. A larger size would eliminate the possibility of stripping with only a marginal increase in weight.
Karem, Nice. "How Automatic Transmissions Work"