EUREKA AUGUST COVER FEATURE STORY

Planetary gears do away with side forces

Tom Shelley reports on a development that overcomes a problem that has bugged mechanical engineers since before the beginning of the steam age

Ingenious mechanisms based on planetary gears allow a reciprocating piston to turn a shaft through a connecting rod that remains permanently in line with the direction of piston motion.

The latest concept to make use of the idea is invented in Lithuania and eliminates all side forces on pistons, and spreads or balances out most other reciprocating forces.

It should thus greatly quieten and improve the lifetimes of internal combustion engines and also, if used in the reverse sense, improve the functioning of pumps or any device that requires the conversion of rotary motion to reciprocating motion.

The problem of converting a reciprocating to a rotary motion or vice versa has been around for a very long time. A method of converting rotary to reciprocating motion without use of a crank was invented by the great sixteenth century Italian mathematician, Girolamo Cardano. If a gear runs round the inside a ring gear whose diameter is exactly double that of the small gear, a point exists on the periphery of the small gear which follows a reciprocating path.

A number of internal combustion engines have been built based on this principle, converting reciprocating motion to rotary. Their great advantage is that they do not require the connecting rod to move out of alignment with the direction of reciprocation. This simplifies construction, and reduces overall space and weight. One described on the Internet may be found at www.wisemanengine.com. It is the claimed invention of one Randall Wiseman, who apparently lives in Mississippi. Doug Rees, a retired truck owner, who lives in Chatham, Kent, is building his Mark II diesel engine based on exactly the same principles. He makes no claim to have invented the principle, which he says was originally used in some steam engines in the nineteenth century, and in experimental petrol engines in the early part of this century. A unique facet of his particular design is that it allows for easy variation in compression ratios.

Of his mark 1 engine, he says, "Its main advantage was the complete lack of vibration. Unless an engine has six cylinders, it is impossible to balance it using orthodox cranks. I built a four cylinder engine which was also two stroke diesel. The prototype did not vibrate, and the oil lubrication was not involved in the combustion. The engine did not suffer from the environmental pollution problems normal with two strokes. Also, it has one injector per two cylinders. In service, it should be much more economical when not under load. But it gave twice the expected power when called on due to the two pulses per revolution. There are no small conventional diesel engines that do either of these things." (Doug may be contacted at doug@trukstar.com).

It might also be mentioned at this point that James Watt also invented a planetary drive for his 'Rotative Engine', a 1788-built working example of which may be seen still driving its flywheel in London's Science Museum. In this case, the sun and planet is not used to much to reduce point peak loads, although it does, but to turn the shaft at a higher speed than is possible with a simple crank mechanism.

An alternative method of converting rotary to reciprocating motion and vice versa is by using a gear with teeth on only one side, placed between two opposing racks.

Despite these viable alternatives, most engineers down the years have just accepted crankshaft side loads and associated problems as a necessary evil. Massive linear bearings were provided for the ends of piston rods on steam railway locomotives and connecting rods were made as long as possible.

An inventor in Lithuania, however, has now come up with a mechanism related to that devised by Cardano, which he believes, converts reciprocating to rotary motion or vice versa, with greater efficiency than any mechanism has achieved hitherto.

Starting at the output (or input) shaft end, the shaft has a gear attached to it which meshes with two further gears which are attached to the ends of what the inventor describes as, "Immovably fixed crankshafts." The free ends of these are attached to "Movable crankshafts". The latter are fitted with gears, which run round the insides of ring gears with internal and external teeth. The outsides of the ring gears mesh with each other. By choosing the point at which the ring gears mesh with each other, the motion of the free ends of the crankshafts and the positions of the pistons may be selected so that they can be up to anywhere from 0 to 180 degrees different from each other.

The radii of the movable crankshafts and the gears attached to them differ. However, the sums of the radius of each crankshaft plus gearwheel have to be the same in each case, and equal to the radius of the inside of the ring gear.

It is clear that by changing relative dimensions, it is possible to make the new mechanism produce complex cyclic motions from shaft rotation, but its main interest is in producing or using a rectilinear motion without producing sideways forces and all their associated noise and vibration. It does require the use of seven gears per two pistons but they would normally be spur gears, and their cost and weight should be offset by strength and weight reductions elsewhere. We have not been to Lithuania to see and test the mechanism, but calculations sent to us suggest it should be both viable and efficient.

The inventor, and agent Dr Arvydas Sutkus, in the Lithuanian Innovation Centre in Vilnius, consider the invention suitable for use in internal combustion engines, pumps, machine tools, and in fixing and raking mechanisms. It should run much more quietly than conventional mechanisms, although it should be noted that much of the noise produced by internal combustion engines arises from the detonations within the cylinders. It should, however, virtually eliminate piston wear as well as allowing a very significant reduction in the amount of space taken up by the crank case. Used in internal combustion engines, efficiency should be improved significantly, although it has to be remembered that research has shown that 42% of the energy generated in such engines is lost to exhaust, and 28% to the cooling system, and gears are not 100% efficient either. The new mechanism should be less expensive to manufacture that other solutions suggested for reducing the inefficiencies of simple crank mechanisms, particularly the idea of doing away with cranks by powering cars using small gas turbines.

All aspects of the design are covered by patents either granted or applied for.

http://www.lic.it Lithuanian Innovation Centre
mailto:a.sutkus@lic.lt Email Dr Arvydas Sutkus
http://www.wisemanengine.com Wiseman Technologies
mailto:doug@trukstar.com Email Doug Rees

Pointers

* By connecting a piston connecting rod to an internal gear within a ring gear, or to an internal gear plus crank within a ring gear, where the single or combined radii of the internal components are half that of the ring gear, it is possible to avoid any need for the connecting rod to move from side to side

* This does away with the traditional small end bearing, and reduces forces at the big end.

* Engines and reciprocating pumps made in this way have the potential to be more efficient, smaller and much quieter than traditional designs

For more technical developments see www.eurekamagazine.co.uk

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