An undercarriage assembly comprising: a roller (32,132,232,332) having a bore formed therein defining a major axis; a roller shaft (38,138,238,338) extending through the bore to support the roller (32,132,232,332) for rotation about the major axis; a seal cavity (44,45,244) extending between the roller (32,132,232,332) and the roller shaft (38,138,238,338) and formed in part by a first thrust surface (46,47,246,346) that faces a first axial direction and is fixed relative to the roller (32,132,232,332), and a second thrust surface (48,49,248,348) that faces a second axial direction opposite to the first axial direction and is fixed relative to the roller shaft (38,138,238,338); a thrust washer stack (50,51,150,250,350,351) trapped between the first thrust surface (46,47,246,346) and the second thrust surface (48,49,248,348), to react thrust loads between the roller (32,132,232,332) and the roller shaft (38,138,238,338); and a dynamic seal (56,57,156,256,356) fluidly sealing the thrust washer stack (50,51,150,250,350,351) within the seal cavity (44,45,244), characterised in that the thrust washer stack (50,51,150,250,350,351) includes a first thrust washer (52,252,352) in contact with the first thrust surface (46,47,246,346), and a second thrust washer (54,253,353) in contact with the first thrust washer (52,252,352) and with the second thrust surface (48,49,248,348), and each of the first thrust washer (52,252,352) and the second thrust washer (54,253,353) is free to rotate about the major axis; wherein contact between the first thrust surface (46,47,246,346) and the first thrust washer (52,252,352) causes the first thrust washer (52,252,352) to rotate with the roller and contact between the second thrust washer (54,253,353) and the second thrust surface (48,49,248,348) cause the second thrust washer (54,253,353) to remain stationary, with the first thrust washer (52,252,352) rotating, respectively, relative to the second thrust washer (54,253,353). a roller (32,132,232,332) having a bore formed therein defining a major axis; a roller shaft (38,138,238,338) extending through the bore to support the roller (32,132,232,332) for rotation about the major axis; a seal cavity (44,45,244) extending between the roller (32,132,232,332) and the roller shaft (38,138,238,338) and formed in part by a first thrust surface (46,47,246,346) that faces a first axial direction and is fixed relative to the roller (32,132,232,332), and a second thrust surface (48,49,248,348) that faces a second axial direction opposite to the first axial direction and is fixed relative to the roller shaft (38,138,238,338); a thrust washer stack (50,51,150,250,350,351) trapped between the first thrust surface (46,47,246,346) and the second thrust surface (48,49,248,348), to react thrust loads between the roller (32,132,232,332) and the roller shaft (38,138,238,338); and a dynamic seal (56,57,156,256,356) fluidly sealing the thrust washer stack (50,51,150,250,350,351) within the seal cavity (44,45,244), characterised in that the thrust washer stack (50,51,150,250,350,351) includes a first thrust washer (52,252,352) in contact with the first thrust surface (46,47,246,346), and a second thrust washer (54,253,353) in contact with the first thrust washer (52,252,352) and with the second thrust surface (48,49,248,348), and each of the first thrust washer (52,252,352) and the second thrust washer (54,253,353) is free to rotate about the major axis; wherein contact between the first thrust surface (46,47,246,346) and the first thrust washer (52,252,352) causes the first thrust washer (52,252,352) to rotate with the roller and contact between the second thrust washer (54,253,353) and the second thrust surface (48,49,248,348) cause the second thrust washer (54,253,353) to remain stationary, with the first thrust washer (52,252,352) rotating, respectively, relative to the second thrust washer (54,253,353).