TTEC 4844 Transmission
Student: Yoey Chooi
Date: 1-9-2011
Reflections : Transmission / Transaxle
Introduction
This article will reflect my understanding of the theory and practical side of the automotive transmission in todays car.
What is a transmission? In layman’s term a transmission is basically a gear box that transmit the rotary motion of engine crankshaft to the wheels of the car. Besides transmitting the rotary power of the engine to the wheels it can also increase or decrease the amount of torque transferring to the wheels. The power is transfer in the following sequence :
Engine Crankshaft → Flywheel → Clutch → Transmission →Final Drive → Drive Shaft → Wheel.
Transmission Layout
There are different types of gearbox layout depending on the engine configurations. Diagram below show a Front Engine Rear Wheel Drive layout also designate as FR. Note that the Differential is a separate unit from the Transmission. They are connected by a long drive shaft.
Another engine configuration is Front Engine Front Wheel Drive also known as FF. (Diagram below).
For this layout the transmission is position transversely or east west of the vehicle. The differential is built into the transmission in this setting and it is called a Transaxle. The picture below shows a torque converter which means the vehicle uses an automatic transmission but for a manual transmission vehicle the layout is the same as well. For this article I will concentrate on the manual transmission drive only.
A few differences between an inline and a transverse transmission are ;
· An inline transmission uses a long drive shaft to propel the differential at the back. The rear drive shaft is equal length to both wheels.
· The drive shaft for transverse layout is normally 2 pieces but in different length. This is because the position of the differential is normally off centre from the vehicle centre line. This will create torque steer(front end move to one side) under hard acceleration.
· Weight distribution of FR (Front Engine Rear Wheel Drive) layout is much more balance than FF (Front Engine Front Wheel Drive). The FF layout is front heavy with all the engine and transaxle located in the front of the vehicle. (This will create less traction for the front wheel as weight is transfer to the back during hard acceleration)
Understanding Manual Transmission
The gearbox use in this article is a Toyota transaxle model C52. It is a 5 speed front wheel drive gearbox use in vehicles like Corolla and Starlet in the 1980’s model. The gear ratio which I have measured is similar to the specification derived from the internet. (Ref: Toyota C transmission; Wikipedia).
The picture below show a C52 transaxle. The rear cover has been removed and showing the fifth gears and selector fork.
The following diagram shows the internal gears.
As the service manual given by the tutor already explained the power flow of all the gears I will only briefly go through it here. The input shaft is install into the pilot bearing (or spigot bearing) of the crankshaft. In between the input shaft and the engine sit the flywheel and the clutch. The gearbox bell housing will cover the flywheel and the clutch. There is a gap in the bell housing so that the starter motor can engage the flywheel to crank the engine.
When the engine rotates the input shaft and output shaft rotates as well. If any of the gears is engaged then power is transmitted out to the output shaft. This in turn drive the final drive. The power from the final drive is transfer via the driveshaft to the vehicle wheels. See diagram below.
Picture below shows where the drive shaft is attached to. There is another hole opposite it.
The drive shaft in turn will drive the wheels. Picture of a drive shaft below.
How the gearbox works.
As the engine is started and the vehicle gear is in neutral the input shaft will start turning but it is not transferring any torque to the output shaft. But all the gears on the input and output shafts appear to be turning. This is because the gear is constantly meshing. Hence the term constant mesh gearbox.
Some of the gears are integrated to the shafts. It will rotate together with the shaft. Other gears are attached to it and will rotate freely via a needle bearing. A device call a synchronizer is use to engage the gear by sliding it either left or right to the gear. Picture below shows there are 3 synchronizers for this gearbox.
Notice the 5th synchronizer only move to the right as there is only one gear to engage on the right. The 3rd and 4th synchronizer can move left or right depending on which gear is selected. The reverse and 1st synchronizer can also move left or right.
All the gears are helical gear(angle cut) except the reverse gears which is a spur gear(straight cut). Helical gear are stronger and quieter to use.
The synchronizer is activate by a selector fork and its shaft. The selector fork and shaft is connected by linkage to the gear shift stick in the driver compartment.
There are grooves on the selector shaft. This allow a spring loaded ball to sit on the groove. This action hold the shaft in place and give a positive feel to gear shift. There are 3 selector forks for this gearbox.
Gear Ratio
Gear ratio is use to increase the torque of the engine. You will notice the gears at the input shaft gets bigger from 1st to 5th gear and vice versa at the output shaft. A smaller gear turning a bigger gear will produce a high torque but low speed revolution (Ratio 3.16:1 ). This is use to move the car from stationary where high torque is required. As the vehicle pick up speed less torque is require and higher gear (less torque but higher speed) is selected. When the car reach fourth gear the input and output gear is about the same size. (Ratio 1:1). For fifth gear the input gear is smaller then the output gear, this is called an overdrive gear (Ratio 0.815 : 1).
A reverse gear has about the same ratio as the first gear. It use an additional gear call Reverse Idler Gear to produce opposite rotation at the output shaft. In is interesting to note that only the Reverse Idler Gear will engage the Reverse Input Gear and Reverse Output Gear without the help of a synchronizer. The Reverse Idler Gear must engage the input and output gear at the same time. Therefore it is important to stop the car properly before engaging the Reverse gear or the gearbox will make a loud “KONK”. Picture of a damaged reverse idler gear is shown below.
Other components
· The gearbox also has a reverse switch. This activate the reverse light at the rear of the vehicle when it is reversing.
· A drain plug and a filler plug. The drain plug is for draining old gear oil and filler plug for filling in fresh gear oil. Normally gear oil is 80W to 90W depending on manufacturers specification. Always remove the filler plug before the drain plug if changing gear oil. This allow the oil to flow out faster.
· Magnet. The function of this magnet is to attract all the metal particles that accumulate in the gearbox.The metal particles will wear out the gears if it rub against the meshing gears.
· Speedometer gears. This gears are made of thermoplastic and use to indicate the speed of the vehicle. It is attached to the dashboard instrument with a cable.
· Oil Seals – prevent lubrication from leaking.
Servicing The Transaxle.
After prolong operation the gearbox may require servicing. This could be due to fluid leak cause by broken seals, unable to engage certain gears, hard shifting between gears and noisy operation. Although some of the problems associated with gear shifting could be cause by the clutch I will only go through the issues cause by the transaxle.
Pre - Dismantling Check
Selecting each gear from 1st to Reverse. This will indicate if any gears have difficulty in shifting. Check the spline of the input shaft and gearbox casing for crack.
After Dismantling :
Checking the Bearings.
Check the bearing for endplay and smoothness. As the bearing is holding the shaft and allow it to rotate, worn bearing will cause the shaft to vibrate. Pay particular attention to the 2 input bearing and 2 output bearing. Worn bearing must be replaced. Refer picture below.
The input shaft and output shaft rear bearing is attached to the shafts. (Picture Below)
Checking the Gears and Shafts.
Check the free rotation of the gears. If it is binding then the bearing holding it is no good. Some gears are part of the shaft and will not rotate independently. The teeth of the gear must not chipped or worn out. This will create uneven end thrust on the gear. The gear will have to be removed and replaced.
See picture below.
Checking the Synchronizers.
The synchronizer function is to engage the gear together. The baulk ring or synchronizer ring is made of brass and it is the part that slide into the main gear. The cone section of the baulk ring is exposed to friction during gear engagement. It can be damaged as shown in the picture below. It will caused very rough shifting as the area of the cone is insufficient to slow down the main gear during gear engagement.
Other Components.
Other components that may fail include:
Selector Fork – bend or worn out selector fork will not engage the synchronizer sleeve. This may cause the synchronizer to jump back to neutral.
Detent Ball and Spring – a broken spring will prevent the ball from sitting on the groove of the selector shaft. This may also cause the gear to jump back to neutral.
Circlip – broken circlip will not lock the gear in place and allow the gear to moves on its shaft. This prevent proper meshing of the gear. The broken piece may also grind on the gear damaging it further.
Insert Spring for synchronizer – broken insert spring will cause the inserts on the synchronizer to fly out of it. This prevent gear engagement as the synchronizer is no longer able engage.
Seals – replace all the seals just like in an engine service.
Conclusion.
Generally transmission dismantling is the last resort. First check all the external linkages, lubricant level , clutch adjustment link and broken seals before moving onto the gearbox. If the symptoms point to the inside mechanism of the gearbox then only dismantling is done.
References;
Picture : 3,6,8,9,10,12,13. Courtesy Gaza@unitec.
Picture :1,2; http:moodle.cnwl.ac.uk
Picture 4: http:corolland.com
Picture 5: http:sl113.org
Picture 7: http://www.partrequest.com/
Picture 14: http://www.noaanews.com/
Picture 15: http://dmitry.auto.com/
Picture 16: http://cj3page.com
