Chain precession drive mechanisms are known in the art and are used to achieve higher gear ratios than are possible with conventional gear trains. A typical chain precession drive mechanism is described with reference to FIG. 1 and FIG. 2. A first sprocket is connected via an output shaft to a driven device which may be, for example, a pitch change arrangement for a set of rotor blades. Thus rotation of the first sprocket causes rotation of the driven device. There may be a gear train (not shown) to adjust the gear ratio between the first sprocket and the driven device. Encircling the first sprocket is a tensioned chain that is longer than the circumference of the first sprocket. The tensioned chain is held in tension by a tensioning arrangement which exerts a force, downward as illustrated, to take up the slack in the chain. The tensioning arrangement may purely rely on gravity but is more preferably a spring or similar that is constrained to move in a straight line towards or away from the first sprocket, for example in a slide track.
An orbit sprocket is arranged to be coplanar with the first sprocket and to be located within the length of the tensioned chain. A motor drives an input shaft that is coaxial with the output shaft. There is an arm extending perpendicularly from the input shaft and connected by one of its ends thereto. The orbit sprocket is connected to the other end of the arm so that rotation of the input shaft causes the orbit sprocket to orbit the first sprocket at a radius greater than the radius of the first sprocket. The track of the orbit sprocket around the first sprocket is shown in FIG. 2. There may be static structures to give rigidity to the mechanism and for mounting the shafts and sprockets.
The driven device is held stationary when the input shaft is not rotating. When the motor drives the input shaft in response to a resynchronisation signal, the orbit sprocket orbits the first sprocket which displaces the tensioned chain and causes the tensioning arrangement to release at least some of the slack to accommodate encircling the orbit sprocket as well as the first sprocket. Precession of the orbit sprocket first decouples a given link of the tensioned chain from the first sprocket and then permits it to recouple to a different tooth of the first sprocket. Once the orbit sprocket has completed one revolution around the first sprocket and returned to the position shown in FIG. 1 and FIG. 2, the tensioned chain has been displaced relative to the first sprocket by a known amount. Since the tensioned chain does not rotate, but only moves radially, the first sprocket has therefore been rotated by that known amount. Hence the driven device is stepped by this amount, or a scaled amount if a gear train providing a gear ratio is used. For example, for a mechanism in which the tensioned chain has N chain links and the first sprocket has M teeth, the output shaft rotates in the opposite direction at (N−M)/M times the input shaft speed.
The motor may drive the input shaft continuously to effect a high ratio gearing between the input and output shafts. The output shaft can be held at any position by arresting movement of the orbit sprocket but least wear is offered by arresting the movement where the orbit sprocket is decoupled from the tensioned chain 36, as shown in FIG. 2.
One disadvantage of this mechanism is that in order to drive a device located in a rotating frame of reference, for example to control the pitch of a set of rotating rotor blades, it is necessary to rotate the input motor. This may be complex to implement and may require electrical and hydraulic signals to traverse the stationary-rotating boundary in order to provide the resynchronisation signal. Where the mechanism is used to control the pitch of a rear set of rotor blades in a dual row rotor system, for example in a gas turbine engine having contra-rotating propeller stages in a pusher configuration, the complexity is significantly increased because the control signals are typically generated in the stationary frame of reference and must pass through the front rotating frame of reference to control the rotor blades in the rear rotating frame of reference. The same problem occurs where the mechanism is used to control the pitch of a front set of rotor blades in a dual rotor system in a puller configuration.
A further disadvantage is that the chain precession drive mechanism can be back-driven by the load if the motor is not able to hold the variable load of the driven device. This is the case if the mechanism is used to control the pitch of rotor blades in a rotor stage where the aerodynamic loading on the rotor blades may drive the pitch change mechanism against the motor. Therefore a “no-back” device is required, adding weight and complexity to the mechanism. The present invention seeks to provide a drive mechanism that seeks to address the aforementioned problems.