A fluid powered rotary actuator comprising a first member which is pivotally connected to a second member, the first and second members (23, 25) also being operatively connected by at least one hydraulic actuator, the or each hydraulic actuator having; a substantially toroidal shaped or part toroidal shaped spear or piston, a substantially toroidal shaped or part toroidal shaped cylinder configured to receive the toroidal shaped spear or piston, and a seal assembly configured to minimise or prevent leakage of fluid from the cylinder; wherein the seal assembly is configured to allow a range of movement of a sealing member of the seal assembly, the movement not being restricted to a direction that is substantially parallel to the direction of travel of the part of the spear or piston that is adjacent to the sealing member, the seal assembly further including a seal carrier configured to support the sealing member, the seal assembly being held within a seal assembly mounting groove by a mounting collar which forms a part of the seal assembly mounting groove, the seal assembly being configured such that the seal carrier can move in a plane substantially perpendicular to the direction of travel of the part of the spear or piston that is adjacent to the sealing member; the seal carrier including a first bearing surface configured to bear against a corresponding second bearing surface of the seal assembly mounting groove defined on the mounting collar, the first bearing surface is a surface on the seal carrier that is configured to be placed under bearing stress by the fluid pressure within the cylinder, characterised in that one of the first or second bearing surfaces (45, 47) is a convex bearing surface. a substantially toroidal shaped or part toroidal shaped spear or piston, a substantially toroidal shaped or part toroidal shaped cylinder configured to receive the toroidal shaped spear or piston, and a seal assembly configured to minimise or prevent leakage of fluid from the cylinder; wherein the seal assembly is configured to allow a range of movement of a sealing member of the seal assembly, the movement not being restricted to a direction that is substantially parallel to the direction of travel of the part of the spear or piston that is adjacent to the sealing member, the seal assembly further including a seal carrier configured to support the sealing member, the seal assembly being held within a seal assembly mounting groove by a mounting collar which forms a part of the seal assembly mounting groove, the seal assembly being configured such that the seal carrier can move in a plane substantially perpendicular to the direction of travel of the part of the spear or piston that is adjacent to the sealing member; the seal carrier including a first bearing surface configured to bear against a corresponding second bearing surface of the seal assembly mounting groove defined on the mounting collar, the first bearing surface is a surface on the seal carrier that is configured to be placed under bearing stress by the fluid pressure within the cylinder, characterised in that one of the first or second bearing surfaces (45, 47) is a convex bearing surface.