Gearbox Design

FUNCTION OF A CAR TRANSMISSION GEARBOX

Your vehicle transmission system will include a gearbox for the following reasons:

• The gearbox will help the engine drive the car.
• The gearbox will enable the vehicle to be reversed
• The gearbox will connect the engine to the transmission system

The first point is obvious if you try to calculate the cylinder size you would need if you had to generate the required wheel torque at the engine shaft. You obviously cannot have a locomotive size engine driving your racing car. If you think a bit further along these lines, you would realise that the "required wheel torque" is not constant. You need a high torque to accelerate the car but the cruising torque does not need to be as high. This means your gearbox needs to be capable of different speed ratios.

The second point does not need any elaboration.

The third point is really a statement indicating a need for easily connecting/disconnecting engine from the transmission system. This is usually done by including a clutch between the engine and the gearbox.

MULTI-SPEED GEARBOXES

Automotive gearboxes must be capable of operating more than one speed reduction to accommodate differen torque requirements under different conditions. In this project you are asked to design a five-speed gearbox (four forward and one reverse).

Sliding Mesh

The first cars (and we are talking about 1890s) used to have gears sliding into and out of mesh as seen in the following figure.

You can imagine the difficulty of finding the right moment to slide the output gear into the mesh without destroying the gearbox. It would be virtually impossible to achieve a mesh without the teeth grinding against each other first. The gears would not survive too long with this arrangement. In the words of its inventor, Levassor, "it was brutal but it worked".

Constant-mesh Gear

The output gear is permanently in mesh but rotates freely on the output shaft until it is engaged by using a sliding dog clutch. A SLIDING DOG CLUTCH is a positive locking device to connect the gears to the shaft. You may want to check the following web page for schematic description of a four-speed (plus reverse) gearbox with constant-mesh gears and sliding dog clutches: John Kelly Technology College

 

IMPORTANT NOTE: In this Assignment, you are asked to use a sliding mesh.

HOW TO CHANGE GEARS

Designing a mechanism to let the driver select the desired gear ration can be quite complicated. For the purpose of this project, it is acceptable if you use of one of the earliest designs, a multi-rail gear selector. No car is manufactured with a multi-rail gear selector but is relatively easier to understand and to design. Here is an example of a multi-rail gear selector (Automotive Technology by M J Nunney, SAE Publications). You are free to explore your own version.

The above is a schematic drawing. Some features are not clearly defined. For example, it is not clear hw the gear lever and selector finger work to select and push different selector rods. Your drawings should not have any ambiguity and all components must be clearly defined.

DESIGNING A GEARBOX FOR DURABILITY AND STRENGTH

In addition to having the right number of gears and capability to reduce the speed at desired ratios, your gearbox must be designed to provide adequate strength and durability.

The following components require special attention:

The Gears

First decide on what type of gear you want to use. Since the shafts are typically parallel in this type of gearbox, the choice is between helical gears and spur gears. Helical gears are generally the preferred choice for the two simple reasons:

• they are typically quiter and smoother
• for the same width, they have a higher rating, which means a more compact gearbox.

Unfortunately, helical gears will not easily slide into and out of meshes. Therefore, in this Assignment, you may have to use spur gears.

It is assumed that you already know how to design gears. I expect that you will calculate the load bearing capacity for the gear meshes and demonstrate that it is lower than the loads you expect for this project. You must consider static as well as dynamic loads and static strength as well as fatigue life considerations.

You are reminded that the load bearing capacity for gear meshes is typically based on the following failure modes:

• Surface Pitting: pitting is failure of the gear surface by production of small holes. All gear meshes produce some initial pitting but the holes produced during this stage usually disappear with usage. When the surface is loaded beyond its rated capacity, the holes grow with usage, eventually leading to tooth breakage. The pitting rating for your gears must be high enough to avoid this.
• Breakage : Sometimes teeth break due to the bending stresses experienced at the root area. This failure may occur due to overload or due to fatigue. In either case, it is a dangerous form of failure and should be avoided. The resistance against tooth breakage in this form is expressed in terms of a Bending Rating, which should be higher than the operating loads.

The Shafts

The shafts must be designed against failure by overload as well as fatigue. There are well-established to design shafts and you are referred to appropriate standards or design textbooks.

Mounting the gears onto the shafts

You must design the method by which the gears are fitted onto the shafts. Some choices are splines, shrink fits or keys.

You have a free choice but should demonstrate the fitness of your design choice in terms of torque rating, ease of assembly/disassembly, and any other factors that are applicable.

You must make sure that your shaft (and hub diameters if applicable) have tolerances. You cannot produce shafts and hubs to precise dimensions. The higher the precision, the higher the manufacturing cost of delivering that precision. There are standard shaft/hub tolerances that you need to comply with. Again you are referred to appropriate standards or design textbooks.

Bearings

The shafts are supported by bearings. Select approriate type and size of bearing based on the following considerations:

• Load magnitudes
• Load directions
• Space limitations

The tolerancing between the bearing seat and the shaft needs to be specified as it is in the gear/shaft interface.

The bearings need to be mounted on your gearbox casing. Therefore, your gearbox casing needs to be strong enough to resist the shaft support loads.

Gearbox Housing

The gearbox housing or casing serves the following functions:

• Support the bearing housings as noted above
• Contain the lubricant
• Keep the gears in contact

The first two functions are obvious. The last one refers to the requirement of the gearbox to resist the torque. This is either done by designing a flange-mounted gearbox or by fixing the gearbox on the "floor", ie chassis. In this project, there are obvious reasons against using a floor-mounted gearbox. You must be able to figure out what they are.

Lubrication

Lubricants in a gearbox serve the following functions:

• reduce the coefficient friction between mating surfaces
• remove the dissipation heat
• protect bearings and gears against external contaminants

The heat is caused by friction and this corresponds to a loss from your useful power. The gearbox efficiencies typically range from 90% to 98%.

The last function needs gaskets and seals at the input and output shafts.

You must consider lubrication in your design. This consideration must include choice of lubricant and mode of lubrication. We will cover some aspects of lubrication in the lecture on Tribology.

MAIN CLUTCH

It is difficult to change gears when the gear train is transmitting torque. Therefore, you need a clutch between the gearbox and the engine to disengage the gearbox while switching gears.

There are three functions that the clutch should accomplish:

• enable changinggears while in motion
• connect a running engine smoothly to the transmission
• enable temporary stoppage without bringing the gear to neutral or without stopping the engine

The following figure gives different clutch configurations:

Source : Automotive Technology, M J Nunney. SAE Publications.

One important design constraint for the clutch is to have as small an inertia as possible. Why?

The plate clutch is replacing the old friction clutch mainly because of this reason.

The following describes the operation of a spring-loaded friction clutch.

A clutch coupling consist of plates squeezed between two shafts.  The compressive force may be provided hydraulically or by a mechanical spring.  In the latter case, the force may be adjusted by electro-magnetic action.  The magnitude of the force determines the torque rating for the coupling:

where

F    Spring Force, N
    Friction coefficient
N    Number of contact surfaces (one less than the number of plates)
R    Outer radius for the contact area, m
r    Inner radius for the contact area, m

Some representative friction factors:
 

Steel on dry steel	0.3 - 0.4
Steel on lubricated steel	0.06 - 0.1
Steel on cast iron	0.1 - 0.2
Steel on rubber	0.2 - 0.4
Rubber on rubber	0.4 - 0.6