Gearboxes are widely used to reduce or increase the speed of prime movers to match them to driven machinery. Especially in critical operations where redundant equipment is not installed, the gearbox is as important to reliability as any other link in the power train. Yet many engineers tend to be more interested in the gas turbines, electric motors, compressors, and pumps than in the gearbox.
As the power and power density of rotating machines continue to increase, however, turbo gearboxes are approaching their limits in terms of safe and reliable operation. Some radical re-thinking in gearbox design is called for if future machinery trains are not to be compromised by gear unit capacity.
Traditional gear designs for turbomachinery fall into two types: Parallel shaft and epicyclic.
Parallel shaft gearboxes have two shafts, each carrying a single gear. They are available in power ratings up to 140 MW for small gear ratios
Epicyclic gearboxes split the power between several “planet” gears which move around a central “sun” gear while also meshing with a surrounding “ring” gear. Epicyclic gears are compact and provide co-axial input and output, but are limited to a power rating of 45 MW in the demand for a gear ratio >6.
To understand the limiting factors in gear design we need to translate external operating characteristics – power ratings and speeds – into gear-specific design parameters. The main factors controlling power and speed limits are:
- Pitchline velocity (PLV) – the linear velocity of the gear teeth
- Elastic deflection produced by torques and bending moments on all parts of the gearbox, especially as it applies to the pinion (the smaller of the two gears in a parallel shaft gearbox)
- Desired input and output speeds
- Operating limits of the bearings (loads and journal velocities)
- Factor of safety chosen for the application.
The job of the gear designer is to find the best balance between these limiting factors, which sometimes conflict. For instance, taking advantage of the increased speed capacity of a high-performance bearing implies an increase in the PLV, which in turn raises the circumferential forces within the gears and the tendency for oil flow to be interrupted. In a parallel shaft gearbox the solution is to increase the center line distance (and hence the overall gear size) to create a larger lever effect.
(More in the September/October, 2012 issue of Turbomachinery International magazine)