We have been working to modify our engine in order to obtain a better efficiency and doing the required modifications to run the gasoline engine on ethanol. We are interested in changing the gear train material of our engine by some lighter material so that the losses are reduced. What alternate light material can be used by us? How much would we be able to increase the efficiency of our engine (need help with the process to proceed with the calculations)?
While suggesting keep a note that our engine has to run on low speed (below 40 km/hr) .
And it is not required for the new gear train to have an extremely long life.
A gear train is designed for a combination of torque and speed. Torque determines the teeth material via the bending stress limit and the flanks contact limit (Hertz stress). Speed is limited by geometrical errors of flanks and sliding speed in combination with used lubricant (elastohydrodynamic behavior).
Teeth are loaded by the tangential force which is related to the torque via the wheels diameters.
The trends being to reduce overall dimensions, thus weight, gears are designed for high stress and higher loads in order to reduce gears diameters and gearboxes weight.
Material used are steels with carburized (or other surface enhancing treatments for a high hardness layer) flanks and hardened core in order to satisfy both constrains.
Lower specific weight materials have NOT same properties thus you should increase dimensions and consequently weight and this is what you want to avoid.
If you consider the ration teeth length to module as constant the transmitted torque will be
T= k*sigma*n*m^4*z
where m= module;n= relative width and sigma the allowed bending stress for the gear material.
The Weight of the gear will be
G=gamma*PI/4*z^2*n*m^3.
As I presume that you want to have same T and same z (same transmission) it results:
G =K*gamma/sigma^(3/4)
You have thus to compare, in a first approximation, the ratios "gamma/sigma^(3/4)" if the new one is lower than for the original material you get a lower weight if not your choice will not fulfill your request.
If life expectancy can be low then you can modify the module and reduce gears diameters and weight but you should look at the fatigue behavior of chosen material for the treatment conditions you will decide to have. But if you look at the T equation you see that the product sigma*m^4 should be constant which means that even important changes of "sigma" will not affect "m" in an important way so that reductions will be very small.
Carbon Fiber for a wide variety of components.
Look at http://www.popularmechanics.com/cars/news/a15246/plastic-carbon-fiber-gears-alternative-to-metal/ for references to the commercial application of carbon fiber in gear sets.
If you are looking to improve performance on this bike, changing steel components (including body work) to carbon fiber or other composites should not be difficult due to the low power produced by the engine. While you did not mention exact requirements for the bike other than high life not required, an assumption was made that some form of 'high mileage' testing might be your ultimate goal. The use of carbon fiber pulleys and belt drive could also be ideally suited for your needs.
It is true that going from "gears" to pulleys could reduce the weight under circumstances BUT the overall dimensions could be increased since the distance between centers could increase and pulleys cannot have too small diameters because of friction angle or number of teeth involved and bending strains.
Nick has an accurate observation on pulley diameter. While the observation is correct, it should be relatively easy to overcome. The progression from chain to belt in the motorcycle industry has been accomplished on some models without any change to engine or transmission cases. Within the scope of a 'one off' test bed vehicle like this project seems to be, modifications for proper sized pulley(s) would be expected to be within the scope of design.
There is also the potential for improved efficiency by changing from chain to belt drive. The migration from chains to belts in automotive applications is well documented. From a practical standpoint, a look at Kart Racing shows belts overtaking chain drive in many classes because of improved speed. This could be significant to this project because of the relatively low power, high weight ratios in Karting and similarity to the motorcycle platform being used in this situation. A higher percentage of power getting to the rear wheel would be a good result.
There is another Research Gate project specifically for analysis of oscillation in chain drive systems. It would be nice if that project was expanded to belt drive, providing more objective comparison of drive train losses in both. But a subjective look at the practical applications would be expected to show the superior power transfer of belt.
I agree that belts have a better efficiency than chains easily explained by the fact that belts only bend and chains do slide under load between elements.at every entry or leave from the wheel, There are several studies I met over the years about belt oscillations.
My comments were related to the question which mentioned "gears" not "transmission". For me gears are gears. If the question would have been about the transmission my comment would have been different.
The dynamics of belts is -on my opinion - not as critical (if the center distance is not extremely big) as it is for chains. There are several reasons for it, first the specific mass of a belt is by far less than the same for a chain, the belt presents a bending stiffness which is not present in chains which have free rotations thus no "bending" stiffness. So that a combination of higher mass and less stiffness leads to lower frequencies and higher risks.
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