Gearboxes for aircraft gas turbine engines
Gearboxes for aircraft gas turbine engines, in particular to arrangements for journal bearings such gearboxes, and to related methods of operating such gearboxes and gas turbine engines. Example embodiments include a gearbox for an aircraft gas turbine engine, the gearbox including: a sun gear; a plurality of planet gears surrounding and engaged with the sun gear; and a ring gear surrounding and engaged with the plurality of planet gears, each of the plurality of planet gears being rotatably mounted around a pin of a planet gear carrier with a journal bearing having an internal sliding surface on the planet gear and an external sliding surface on the pin.
1. A gas turbine engine for an aircraft, comprising:
an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;
a fan located upstream of the engine core, the fan comprising a plurality of fan blades; and
a gearbox configured to receive an input from the core shaft and provide an output drive to the fan so as to drive the fan at a lower rotational speed than the core shaft, the gearbox comprising:
a sun gear;
a plurality of planet gears surrounding and engaged with the sun gear; and
a ring gear surrounding and engaged with the plurality of planet gears, each of the plurality of planet gears being rotatably mounted around a pin of a planet gear carrier with a journal bearing having an internal sliding surface on the planet gear and an external sliding surface on the pin,
wherein:
a ratio of a length, L, of the internal and external sliding surfaces to a diameter, D, of each journal bearing is greater than 0.5;
a pitch circle diameter of the ring gear is no greater than 1200 mm; and
a diametral clearance of each journal bearing, defined by the difference between the diameter of the internal sliding on the planet gear and the diameter of the external sliding surface on the pin, is between 120 um and 600 um.
2. The gas turbine engine according to claim 1 , wherein a diameter of each journal bearing is between 120 mm and 200 mm.
3. The gas turbine engine according to claim 1 , wherein the internal or external surface of the journal bearing has a surface coating.
4. The gas turbine engine according to claim 3 , wherein a thickness of the surface coating is in the range of between 40 micrometers to 200 micrometers.
5. The gas turbine engine according to claim 3 , wherein the surface coating comprises a first layer and a second layer, with the first layer being positioned between an underlying material and the second layer.
6. The gas turbine engine according to claim 5 , wherein the first layer has a thermal expansion coefficient between that of the underlying material and the second layer.
7. The gas turbine engine according to claim 5 , wherein the second layer has a thickness between 50% and 95% of a total thickness of the surface coating.
8. The gas turbine engine according to claim 5 , wherein the thickness of the second layer is between 40 and 100 micrometers.
9. The gas turbine engine according to 5 , wherein the surface coating comprises a third layer positioned such that the second layer is between the first layer and the third layer.
10. The gas turbine engine according to claim 9 , wherein the third layer is composed of a material that has a lower hardness than the second layer.
11. The gas turbine engine according to claim 9 , wherein:
the first layer is between 10 and 20 micrometres in thickness;
the second layer between 40 and 100 micrometres in thickness; and
the third layer between around 1 and 15 micrometres in thickness.
12. The gas turbine engine according to claim 9 , wherein:
an underlying material on which the surface coating is provided is steel; and
the first layer is a copper-based alloy; and
the second layer is copper-or aluminium-based alloy; and
the third layer is a lead-based alloy.
13. The gas turbine engine of claim 3 , wherein:
the ratio of the length, L, of the internal and external sliding surfaces to the diameter, D, of each journal bearing is less than 1.4; and
the ring gear has a pitch circle diameter of 550 mm or greater.
14. The gas turbine engine of claim 1 , wherein a diameter of the fan is less than 220 cm.
15. A gas turbine engine for an aircraft, comprising:
an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;
a fan located upstream of the engine core, the fan comprising a plurality of fan blades; and
a gearbox configured to receive an input from the core shaft and provide an output drive to the fan so as to drive the fan at a lower rotational speed than the core shaft, the gearbox comprising:
a sun gear;
a plurality of planet gears surrounding and engaged with the sun gear; and
a ring gear surrounding and engaged with the plurality of planet gears, each of the plurality of planet gears being rotatably mounted around a pin of a planet gear carrier with a journal bearing having an internal sliding surface on the planet gear and an external sliding surface on the pin,
wherein:
a ratio of a length, L, of the internal and external sliding surfaces to a diameter, D, of each journal bearing is greater than 0.5;
a pitch circle diameter of the ring gear is no greater than 1200 mm; and
a diameter of each journal bearing is between 120 mm and 200 mm.
16. The gas turbine engine of claim 15 , wherein the internal or external surface of the journal bearing has a surface coating.
17. The gas turbine engine according to claim 16 , wherein:
a thickness of the surface coating is in the range of between 40 micrometers to 200 micrometers;
the surface coating comprises a first layer and a second layer, with the first layer being positioned between an underlying material and the second layer; and
the second layer has a thickness between 50% and 95% of the total thickness of the surface coating.
18. A gas turbine engine for an aircraft, comprising:
an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;
a fan located upstream of the engine core, the fan comprising a plurality of fan blades; and
a gearbox configured to receive an input from the core shaft and provide an output drive to the fan so as to drive the fan at a lower rotational speed than the core shaft, the gearbox comprising:
a sun gear;
a plurality of planet gears surrounding and engaged with the sun gear; and
a ring gear surrounding and engaged with the plurality of planet gears, each of the plurality of planet gears being rotatably mounted around a pin of a planet gear carrier with a journal bearing having an internal sliding surface on the planet gear and an external sliding surface on the pin,
wherein:
a ratio of a length, L, of the internal and external sliding surfaces to a diameter, D, of each journal bearing is greater than 0.5;
a pitch circle diameter of the ring gear is no greater than 1200 mm; and
the internal or external surface of the journal bearing has a surface coating.
19. The gas turbine engine of claim 18 , wherein:
a diameter of the fan is less than 230 cm;
the ratio of a radius of one of the fan blades at a hub to the radius of the fan blade at a tip is in a range of from 0.26 to 0.31; and
the gearbox has a gear ratio of 3.2.
20. The gas turbine engine according to claim 18 , wherein:
the ratio of the length, L, of the internal and external sliding surfaces to the diameter, D, of each journal bearing is less than 1.4;
the ring gear has a pitch circle diameter of 550 mm or greater;
the ratio of a radius of one of the fan blades at a hub to the radius of the fan blade at a tip is in a range of from 0.26 to 0.31; and
the gearbox has a gear ratio of 3.5 to 4.2.