I. Common Terminology for Transmissions
Basic terminology for transmissions:
(1) Driving gear and driven gear. The input shaft can be understood as being connected to the clutch and rotating under the drive of the engine. The gear fixed on the input shaft rotates synchronously with it. This gear is called the driving gear. The gear that is then connected to the output shaft is forced to rotate, so this gear is called the driven gear.
(2) Transmission ratio i. The transmission ratio is defined as the ratio of the number of teeth of the driven gear to the number of teeth of the driving gear.
When the relationship between the number of teeth on the driven gear and the number of teeth on the driving gear changes, the transmission ratio *i* changes. Under the condition that the engine speed remains constant, this will affect the output shaft speed, i.e., the wheel speed. For a pair of meshing gears, the number of teeth does not change during use, therefore its transmission ratio is constant. If several gears with unequal numbers of teeth are mounted on the input shaft and mesh with corresponding gears with varying numbers of teeth on the output shaft, a stepped transmission with different transmission ratios *i* can be obtained. Automotive transmissions achieve gear shifting based on this fundamental principle.
(3) Forward gear: The gear that allows the car to move forward. Reverse gear: The gear that allows the car to move backward. Neutral gear: All gears in the transmission are not in their working positions. At this time, the engine power is input to the input shaft and no longer transmitted to the output shaft.
(4) Direct Drive. A direct drive is a gear in which engine power is transmitted directly through the input shaft and the output shaft, which are directly connected to it, without passing through any gears in the transmission. The gear ratio of a direct drive is 1.
(5) Overspeed gear. This is the gear where the output shaft speed is higher than the input shaft speed.
(6) Number of gears. This refers to the number of gears in a stepped gear transmission. Commonly used gear transmissions have four to five gears, while three-speed transmissions are less common. The more gears a car has, the better it adapts to driving conditions and the lower its fuel consumption. However, the transmission and control mechanisms become more complex, making operation more difficult and increasing costs.
(7) Low gears and high gears. In a transmission, gears with smaller numbers are called low gears. The smaller the number, the larger the gear ratio, the greater the traction, and the lower the vehicle speed. For example, first gear has the largest gear ratio, the lowest vehicle speed, and the greatest traction among the forward gears. Gears with larger numbers are called high gears. The larger the number, the smaller the gear ratio, the lower the traction, but the higher the vehicle speed.
II. Transmission Fault Handling
1. Transmission skipping gears
When an electric vehicle is in a certain gear and the accelerator pedal is suddenly pressed, or when the vehicle is subjected to an impact, the gear lever automatically shifts back to neutral, the sliding gears disengage, and power transmission is interrupted—this is called "gear slippage." This phenomenon often occurs during sudden load changes at medium and high speeds, or when the electric vehicle vibrates; it is generally more common at high speeds. Although electric vehicle transmissions are equipped with self-locking devices to prevent gear slippage, after long-term use, the gears wear and become conical, generating axial force during engagement. Combined with vibrations and speed changes during operation, this forces the meshing gears to disengage axially along the transmission. Therefore, excessive wear can lead to malfunction and gear slippage. This is a relatively dangerous fault. For example, if an electric vehicle is heavily loaded and going uphill, and a gear slippage occurs, the vehicle may decelerate. When the driver applies the brakes, the vehicle may slide, easily sliding to the roadside and falling into a ditch, causing a rollover.
The specific manifestations of transmission skipping gears include: excessive wear of transmission gears or bushings, resulting in a tapered shape along the tooth length; wear of the shift fork shaft groove and locating ball, as well as a weak or broken locating spring, causing the self-locking device to fail; wear and looseness of the transmission shaft and bearings, or excessive axial clearance, causing poor gear meshing and skipping and axial movement when the shaft rotates; deformation and looseness of the operating mechanism, resulting in insufficient gear meshing at the tooth length position, etc. In electric vehicles, severe wear and looseness of bearings, gears, or bushings within the transmission; excessive wear and looseness of the splines on the second shaft and sliding gears; breakage or loosening of the retaining ring on the second shaft and intermediate shaft, causing gear back-and-forth movement; bending of the electric vehicle's shift fork or excessive wear of the fork end working surface; wear of the locating slot seat on the fork shaft, wear of the guide block groove, weakness or breakage of the shift fork shaft locating spring; loose or disassembled synchronizer locking pins, or severe wear of the sliding bushing length; and misalignment of the center lines of the transmission housing bearing holes, all of which can cause automatic shifting back to neutral.
2. Handling gear shifting issues
When a gear is found to have shifted out of gear, push the gear lever back into that gear, then remove the transmission cover and check the gear engagement. If the gears engage well, the problem lies in the shifting mechanism. Manually push the shift fork that has shifted out to test its positioning device. If the positioning is faulty, remove the shift fork shaft and inspect the positioning ball and spring. If the gears are not fully engaged, manually push the shift fork or gear sleeve; if it engages correctly, check if the shift fork is bent or worn, if the shift fork fixing screws are loose, and if the clearance between the fork end and the gear groove is too large. If the shifting is good and the gears or gear sleeves can fully engage, check if the gears are worn into a conical shape, if the bearings are loose, and if the transmission shaft moves back and forth.
Based on the problems identified in the above inspection, repairs should be made according to standards, and new parts should be replaced if necessary, to completely eliminate the transmission's gear shifting and slippage issues. Furthermore, during transmission operation, the bearings are subjected to varying torsional and bending moments, and the geared parts are also subjected to compression, impact, and sliding friction loads, which can cause damage to the shafts.
