Axle assembly having a ballast insert assembly and a method of control

1. An axle assembly comprising:

an axle housing that has a center portion that receives a differential; and

a ballast insert assembly disposed in the center portion below the differential, wherein the ballast insert assembly includes:

a body that at least partially defines a cavity and that has a lower wall, an upper wall disposed opposite the lower wall, first and second end walls that are disposed opposite each other, disposed in a nonparallel relationship, and that extend from the lower wall to the upper wall, and a lubricant port that extends through the first end wall, wherein lubricant enters and exits the cavity through the lubricant port; and

an inflatable bladder disposed in the cavity;

wherein a volume of lubricant in the cavity is greater when the inflatable bladder is deflated than when the inflatable bladder is inflated.

2. The axle assembly of claim 1 wherein the inflatable bladder forces lubricant out of the cavity through the lubricant port when the inflatable bladder is inflated.

3. The axle assembly of claim 2 wherein the inflatable bladder substantially fills the cavity when inflated.

4. The axle assembly of claim 1 wherein lubricant enters the cavity via the lubricant port when the inflatable bladder is deflated.

5. The axle assembly of claim 1 wherein the upper wall faces toward the differential and the inflatable bladder moves away from the upper wall when the inflatable bladder is deflated.

6. The axle assembly of claim 5 wherein an additional lubricant port extends through the upper wall.

7. The axle assembly of claim 5 wherein the inflatable bladder is disposed on the lower wall and is spaced apart from the upper wall when the inflatable bladder is deflated.

8. The axle assembly of claim 1 wherein two lubricant ports extend through the first end wall and a lubricant trough is disposed between the lubricant ports that is configured as a recess that extends from the first end wall to the upper wall.

9. The axle assembly of claim 1 wherein the axle housing has a first arm portion that extends from the center portion and that receives a first axle, wherein the lubricant port in the first end wall directs lubricant into the first arm portion when the inflatable bladder is inflated.

10. The axle assembly of claim 1 wherein the second end wall has a second lubricant port.

11. The axle assembly of claim 10 wherein the axle housing has a first arm portion that extends from the center portion and that receives a first axle, a second arm portion that is disposed opposite the first arm portion and that extends from the center portion and receives a second axle, wherein the lubricant port in the first end wall directs lubricant into the first arm portion and the second lubricant port directs lubricant into the second arm portion when the inflatable bladder is inflated.

12. The axle assembly of claim 10 wherein the body of the ballast insert assembly has a third lubricant port disposed in the upper wall between the lubricant port and the second lubricant port.

13. The axle assembly of claim 10 wherein the inflatable bladder is disposed inside the body between the lower wall and the upper wall and between the first end wall and the second end wall.

14. An axle assembly comprising:

an axle housing that has a center portion that receives a differential, wherein the center portion has a housing trench that is disposed directly below the differential and that has a first housing trench side wall, a second housing trench side wall disposed opposite the first housing trench side wall and disposed parallel to the first housing trench side wall, and a housing trench bottom wall that extends from the first housing trench side wall to the second housing trench side wall; and

a ballast insert assembly disposed in the center portion below the differential and that at least partially fills the housing trench, wherein the ballast insert assembly includes:

a body that at least partially defines a cavity and that has a lubricant port, wherein the body has a first side wall that engages the first housing trench side wall, a second side wall that engages the second housing trench side wall; and

an inflatable bladder disposed in the cavity;

wherein a volume of lubricant in the cavity is greater when the inflatable bladder is deflated than when the inflatable bladder is inflated.

15. The axle assembly of claim 14 wherein the body has a lower wall that extends from the first side wall to the second side wall and that is disposed on the housing trench bottom wall.

16. The axle assembly of claim 14 further comprising a differential carrier that rotatably supports the differential and that engages the first housing trench side wall.

17. An axle assembly comprising:

an axle housing that has a center portion that receives a differential; and

a ballast insert assembly disposed in the center portion below the differential, wherein the ballast insert assembly includes:

a body that at least partially defines a cavity and that has a lubricant port, a first side wall, a second side wall disposed opposite the first side wall, a lower wall that extends from the first side wall to the second side wall, an upper wall that is disposed opposite the lower wall and that extends from the first side wall to the second side wall, and first and second end walls that are disposed at opposite ends of the body and that extend from the first side wall to the second side wall and from the upper wall to the lower wall, wherein the first and second end walls are disposed in a non-parallel relationship; and

an inflatable bladder disposed in the cavity;

wherein a volume of lubricant in the cavity is greater when the inflatable bladder is deflated than when the inflatable bladder is inflated.

18. The axle assembly of claim 17 wherein the lubricant port is disposed in the first end wall.

19. The axle assembly of claim 17 wherein the upper wall extends along a continuous arc from the first end wall to the second end wall.