Gearbox calculation

Gearbox calculation

The main purpose of this project (Gearbox Calculation) is determine and choosing the main parameters of a car and the design gearbox unit. You must learn choose the specific materials to produce any details of assembly unit and choose right method for producing.

Gearbox Calculation

For Gearbox Calculations we need next parameters: type and class of car, load-carrying capacity or passenger capacity, maximum speed and
coefficient of total road resistance at maximum speed, coefficient characterizing the maximum resistance of the road in the first gear transmission. These calculations can be used as in project for modernization of a vehicle as in a new produce project.

To start with learn information about modern cars analogic to your project. It will be better if you choose the modern car. Then analyze the performance and technical qualities of similar cars and determine: car layout, curb weight, the distribution of mass along the axes and the height of the center of mass above the reference surface, base, gauge and overall dimensions of the car, car frontal area, tire type and dimension, number of axles, number of drive axles, number of gears, main drive type, engine type. It is important to consider the main trends in the development of car design such as a reduce fuel consumption and a curb weight reduction.

Design and calculation of gearboxes

  1. Gearbox scheme selection;
  2. Determining the basic parameters of Gearbox;
    • Center distance
    • Working width of the ring gear
    • District dividing module;
    • Determining the number of gear teeth gears;
    • Angle of inclination and direction of the gear tooth line;
    • Gear geometry selection;
    • Calculation of gears for strength;
    • Calculation of transmission shafts;
    • Synchronizer calculation;
    • Selection of bearings.
  3. Basics of  gearbox design;
  4. Materials and heat treatment;

Gearbox scheme selection

Gearboxes base cars are two-shaft and three-shaft types, depending on the layout of the transmission. The most commonly used schemes are three-shaft gearboxes.

The benefits of three-shaft gearboxes:

1.     The first gear is connecting first and second shafts without teeth gearing.
This increases transmission efficiency and reduces vibration noise of gearbox.

2.     The possibility of creating a relatively large gear ratio downshift transmission

, since the transfer of power is carried out by two parameters of gear wheels.

The Deficiencies of three-shaft Gearboxes

Reducing the efficiency of the intermediate gears to a value of 0.96-0.97 and the presence of an intermediate shaft.

The benefits of two-shaft Gearboxes:

  1. Convenience and simplicity of the layout of the transmission by the location of the power unit under the driving axle.
  2. High efficiency by intermediate gears 0,985.

The Deficiencies of two-shaft Gearboxes

1.     No direct transmission (transmission from shaft to shaft occurs through gearing, which causes a constant loading of gears and bearings in all gears.The possibility of increasing the gear ratio in the lower gear is limited .

2. The possibility of increasing the gear ratio in the lower gear is limited

Most schemes of three-shaft coaxial gearboxes have an identical arrangement of gears and synchronizers along the length of the box. It is considered rational to position the driven gears of higher gears closer to the front support of the output shaft, and the lower gears closer to the middle bearing.

For vehicles used schemes coaxial multistage gearboxes with an additional gearbox. There are three main options for schemes of multi-stage gearboxes: with an additional gearbox in front of the main gearbox and with an additional gearbox behind the main gearbox.

This ensures the manufacturability of the assembly and uniform load of the secondary and intermediate shafts, as the rows of pairs of gears with increasing circumferential force approach the rear bearings of the shafts. In a number of cases, in order to avoid overloading the rear shaft supports, as a last resort, put not the first gear row, but the reverse gear row.

Synchronizers are located on the secondary shaft, which is structurally simpler. Another arrangement of synchronizers is associated with the desire to reduce either the length of the gearbox or the reduced moment of inertia.

Gearbox scheme selection

For vehicles used schemes coaxial multistage gearboxes with an additional gearbox. There are two main options for schemes of multi-stage gearboxes: with an additional gearbox in front of the main gearbox and with an additional gearbox behind the main gearbox. In the first embodiment, the front gear is performed with a small range:

Where qcpo  – is the average pitch of the gear ratio of the main gearbox). The version with a front gear-divider provides a high degree of unification of the main gearbox with a multi-stage gearbox specified on its base. The disadvantage of this option is the need to have a relatively large intercenter distance, since a large torque acts at the output of the base gearbox. The variant is acceptable if, according to the layout conditions, the value of the center-to-center distance may exceed 160 mm.

In the second version, the rear additional gearbox should be performed with the range:

Where n – the number of gears forward stroke of the main gearbox.

The main advantages of the option with an additional rear gear are:

1) The ability to have a relatively small axial distance of the main gearbox, which is part of a multi-stage gearbox, since its output has a relatively small torque;

2) The ability to carry a large range of boxes

The main disadvantages of this option are:

1) Loss of a significant proportion of unification, since the main gearbox has a small range and cannot be used separately without changing gears and expanding the range;

2) Increase the loss on lower gears 5-6% when the lower stage of the additional gear is enabled;

3) A large number of gears in the gearbox with respect to the variant with the front arrangement of the gearbox – divider.
This option is recommended for large input torque values

, and, if necessary, to carry out a relatively large range of gear ratios

Center distance

There is a rational number of axle distances for trucks, depending on the maximum engine torque which you can see the table below.

Max. engine torque, Nm 170 260 340-420 700-850 900-1150
Center distance aw, mm 85 105 125 140 160
Number of gears, n 4 5 5 5 10

Center distance  is 60-80 mm for light cars.
For the preliminary determination of the center distance of the projected gearbox is determined by the empirical dependence.


Working width of the ring gear

The width of the ring gear is selected depending on the size of the center-to-center distance, where larger values ​​are selected for more loaded gears.

Working width of the ring gear

Mt module

The district dividing module is selected within the following limits for manual transmission and after geometric and strength calculations it is specified.

Mt module of gearbox

Author: delfi

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