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Eccentric gear train

An eccentric gear train consists of two or more spur gears of equal size that not only rotate, but also move together with a circular or orbital motion at the same speed. Of two meshing gears, one rotates and orbits in the same sense, while the other has opposite rotation and orbit. (For the sake of simplicity, parallel gears on the same axis are regarded here as a single gear). The composite motion of the gear with opposite rotation and orbit is referred to as elliptic(al) motion (not to be confused with the motion of planets), because every point on it follows an elliptical path. C. H. Dearing proposed a construction with an eccentric gear train for an ellipsograph in 1927.1 As a circle and a straight line are both special cases of an ellipse, an eccentric gear train can also be used to convert reciprocating motion to rotary motion and vice versa.

The elliptical motion places the eccentric gear train in the same category as the Archimedes trammel and Cardano's hypocyclic gears, whose origins reach back to antiquity, with the difference that, in mechanisms for the interconversion of reciprocating and rotary motion, it avoids the double-speed, double-load bearings of the latter two, is subject to fewer design constraints and can be made to be more robust. While the slider-crank mechanism requires a connecting rod that swings from side to side, producing second-order inertial forces and making it impossible to seal the cylinders of an engine from the crankcase, mechanisms that use elliptical motion avoid these problems.

If the elliptic gear of the eccentric gear train has eccentric pins, and all eccentricities are equal, an eccentric gear train can be constructed to convert rotary motion to reciprocating motion and vice versa that can be used in an internal combustion engine. A patent application for 4-cylinder cruciform engine with an eccentric gear train was filed by West Virginia University, giving Professors A. J. Stiller and J. E. Smith as the inventors, on 6 July 1984.2 Stiller claimed he got the idea from an old Appalachian do-nothing-machine.3 The “revolutionary” engine was said by Smith to be "really unique" and "really off the wall", and a "patent search had revealed nothing remotely similar". There was "nothing else like it in the real world".4 But the "invention" was not new: A patent application by M. P. Mayer for eccentric gear trains and their embodiments in different engines had already been published on 15 March 1984 5 and was available on microfilm.

The Stiller-Smith engine had a 3-gear inline eccentric train in which the outer gears were mounted eccentrically on fixed-axis shafts, one of which was an output shaft, the other an “idler shaft”6. The middle gear had only an eccentric pin on each side with an offset of 180°, and no axial shaft, and was referred to as a floating trammel gear. The pins engaged piston rods angled at 90° for alternate support of each side of the gear in the x- and y-directions respectively. The pins were not in the same plane, the floating trammel gear acted as a cantilever, the pistons and rods could rotate about their own axes, and the rods had a finite stiffness, which combined to allow the trammel gear to precess and the diameter of its orbit to grow, according to the operating conditions, providing variable port timing (in a two-stroke engine) and a variable compression ratio. The gears were subject to high stress concentrations.9, 10, 11 Linear bearings mitigated these irregular motions and loadings by reducing bending in the piston rods.12, 13

The distance between the crankpins and the distance between the three (parallel) axes of rotation and orbit also made balancing a consideration that received close attention.14

The engine never progressed beyond the experimental stage, but it did provide a rewarding model for computer simulations and analyses of complex loading scenarios, which spawned a series of research papers and presentations that can be found in the usual portals for academic publications. Some of the papers are available free of charge in the internet.

In an eccentric gear train consisting of two gears, the gear with elliptical motion is guided in a circle, if it has a pin that engages a crosshead that is guided to reciprocate along a line that crosses the axis of the orbit of the elliptical gear and the axis of rotation of the output shaft.15 The gear then needs no axial shaft. A single such unit is difficult to balance, but an engine with two or more units inline generates a moment that is readily balanced with counterweights on the output shaft.

Eccentric gear trains in which the middle of three gears is eccentrically mounted on a shaft and the outer two have elliptical motion and pins that engage a common slider make possible four-stroke“ H” and two-stroke double “A” engine configurations, the latter with uniflow scavenging for greater efficiency and lower emissions.16

References

1. UK patent specification 281,193

2. US patent application serial No. 628248, see US 4,641,611

3. Chemical Engineering at West Virginia University: A Living History

4. WVU professors design 'revolutionary' engine

5. Patent No. DE3232974

6. The Stiller-Smith Engine: Floating Gear Analysis

7. High-load, squeeze action linear bearings with beam deflection

8. Effective Modeling of Elastic Mechanical System Through Objective-Aimed Finite Element Strategies

9. The Stiller-Smith Engine: Floating Gear Analysis

10. Design loads of the Floating Gear System in the Stiller-Smith Mechanism

11. Unique Kinematic Features of the Gears in the Stiller-Smith Mechanism High-load, squeeze action linear bearings with beam deflection

12. High-load, squeeze action linear bearings with beam deflection

13. Effective Modeling of Elastic Mechanical System Through Objective-Aimed Finite Element Strategies

14. Complete Balancing of the Stiller-Smith Engine: 4,8,12,16 or More Cylinders

15. Patent No. DE 3,232,974

16. Patent Application No. DE 102,020,004,112