I have been researching ways to design a planetary gearbox. It seems that the main configuration of the gears is to have the ring gear fixed, output the rotation of the planet gear carrier, and use the sun gear as the input. This would produce a reduction.
I have found little information about using a planetary gearbox the other way around to multiply speed. Is it commonly done or is there some other alternative to this? The design would be to have the ring fixed, the gear carrier as the input, and the sun as the output. Is the ratio just the reciprocal of the normal ratio.
I did see that the Wikipedia article on planetary gearboxes said that the ratio for what I’m talking about is 1+ Nring/Nsun. But it said a citation is needed, so I just wanted to double check.
The most common example is an old-school pencil sharpener (although maybe not as common anymore). The handle is attached to the carrier and the planets spin relatively fast compared to the crank handle, which causes several rotations of the sharpening cylinders around the pencil. There is no sun gear in this case, as the desired output is shearing of pencil pieces, rather than shaft output speed.
Yes, the ratio with a fixed ring gear with the sun as the output is 1 : (1+Nring/Nsun). You can do any of the 6 combinations of sun, carrier, or ring fixed with the other 2 being input and output. The math gets kinda fun, and I’m sure I could dig up an old textbook to cite. Certain configurations even produce backwards rotation.
As Tom is showing, technically, all 3 can spin at a given rate. One of the more recent and famous versions of this is the drivetrain of a Prius:
In your scenario, you are used to seeing a PSR or Planetary speed reducer. This is usually because you have a high speed source (like an electric motor) and want high torque to do some useful work.
Windmills are the opposite and have PSI or planetary speed increasers as they blades produce a lot of torque, but the generator wants higher speeds.
While it can be done, in FRC it usually isn’t, because our motors are generally too fast and we need to slow the speed down (without losing power). All the info above is good.
To increase speed, you need to drive the carrier (with a stationary ring). The Sun gear would be output. This is the opposite of the normal FRC arrangement.
If you want to play with one to see how they work, look for the Tamiya Miniature Planetary Gearbox Kit with Motor. They are about $20 and come with 4 sections to give you multiple reductions / increases.
(General plug, Tamiya makes a series of motor/gears/transmissions/etc. that are easy to assemble and watch how they work. )
I did not check the article, but it is a fact that certain IC engine speeds are more efficient than others. Depends on the engine of course but efficiency generally peaks around 2500-3000 RPM. If you put a stupid gear ratio in you can force efficiency down.
*For a given required mechanical power output at the engine crankshaft, there is a continuum of engine_speed + fuel_flow_rate combinations which will produce that power. Only one of those pairs minimizes the fuel_flow_rate.
Here is a link to some BSFC maps: http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_(BSFC)_Maps
BSFC or Brake Specific fuel consumption shows how much fuel is consumed for each useable work unit produced at a given operating point.
If you look at the:
Mercedes Benz 300SD OM617 5 cylinder turbo diesel map,
You will see behaviour that Don R is talking about. where there is a sweet spot to run the engine from a (fuel in)/(work out). For that engine, the sweet spot is between about 1900 and 2600 rpm and the engine is about 60-90% fully loaded.
For gas and diesel engines, the general rule of thumb for peak efficiency is around 50% max rpm and at and above 50% load. This is a very general statement.
If you had a 1 speed gearbox, you would figure out the most important attribute,a nd size your gearbox to match that (acceleration, top speed, fuel economy). For Supermileage, we had a single speed drivetrain.
If you want it all, then you would go for a CVT or continuously variable transmission that would (in theory), give you the optimal ratio for whatever scenario you are driving. These gearboxes tend to have other issues (efficiency, cost, longevity, weight…).
The Toyota Hybrid system does a really good job of acting like a CVT without some of the draw-backs, and with some other benefits. Its not “perfect”, but it is a really good system for a car, and a pretty neat one to figure out.
BTW, you can replace the generator/electric motor with Hydrualic pump/Hydrualic motor and achieve a similar set up… Of pressurized air… Or… Check out this Dana HVT. It is a 3 range IVT (infinitely variable transmission) powersplit…
Nissan and CVTs have a pretty long history, so a little searching should give you some background. I believe they use a “push style” belt which is pretty neat. Check out “Van Doorne” for some history on the push belt.
Engine efficiency is not (necessarily/generally) a function of incomplete fuel combustion. Engines do not provide 100% of the energy produced by burning fuel as useful mechanical energy. The balance of the energy is released as heat.
A modern gasoline engine has an efficiency typically in the range of 25-30%.
Engines operate most efficiently within a certain rpm range. This provides the connection between gear ratios and engine efficiency.
