(57) The pitch of the blades (22) of a fan is adjusted by a pneumatic piston and cylinder
combination (48, 50) which rotates with the hub (20) of the fan. The cylinder element
(50) is reciprocable axially with respect to the hub and is connected to the blades
by a cam groove (92) in the cylinder element, and a plurality of cam followers (98)
engaged in the groove for translating axial movement of the cylinder in one or other
direction into pivotal movement of the blades to adjust the pitch of the blades. Pneumatic
pressure is applied to the piston and cylinder combination to reciprocate the cylinder
element (50) via a rotary union (82) which accommodates rotation of the combination.
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