Non-Collinear Valve Actuator
- Tuesday, 01 July 2014
This device separates the actions of the pneumatic actuator and the spring.
Marshall Space Flight Center, Alabama
Typical large aerospace valves use pneumatic actuators with large return springs to define a normal state. These springs are exclusively in line with the pneumatic actuator, and therefore are forced to have the same stroke and forces. These typical systems use either a large helical spring or a stack of Bellville springs. Each is long to ensure that the forces at the end of stroke are large enough to move the valve to the normal position with some margin. This invention reconfigures the actuator through the use of either a drag-link four-bar system or a cam to separate these two motions. The spring is allowed to have larger loads with significantly short spring stack length. This eliminates the need for long housings, heavy springs, and thus reduces the mass of the flight system. This configuration can be used for commercial valve actuators. Although commercial actuators generally do not have weight limitations, the reduction of the massive spring could reduce the cost of the product.
Existing and typical aerospace linearly arranged valve actuators have limitations imposed by their layout. These large valves require large torques to overcome a variety of valve loads. Hydrodynamic, pressure, friction, bearing, and other loads can drive the torque requirements to thousands of inch-pounds of moment. Pneumatic actuators use pinion gears, drag links, or other mechanisms to overcome these gear train designs, and other factors have limitations that increase the distance the actuator acts from the center of rotation of the valve element.
This invention separates the actions of the pneumatic actuator and the spring. Using either a drag-link four-bar system or a cam, the actuator is free to be optimized for its function and the spring for its function. One is no longer a compromise to the other. For example, if a 4:1 distance separation is achieved, the actuator could use the 1.5 stroke and the spring would only stroke 3/8, although the spring would have to have four times the force. With conventional coil springs, the force is a function of the wire size of the diameter, but the stroke is a function of the number of turns. So a coil spring at this ratio would be about 1/3 the length of the typical linear system and about half the mass. For Belleville springs, the savings could be even greater, but generally are because of the packing efficiency. Belleville springs require a large serial stack of springs to achieve long strokes. For increased load, the springs are stacked in parallel. Therefore, parallel stacks are more efficient from a packaging point of view. So to increase the stroke by four times, four times the number of springs is required.
This idea of separation of these to load-producing elements is novel. The use of a drag link is one approach to perform this separation. This utilizes a stroke multiplier link with the spring attached near the pivot point to move the valve to the normal position. The actuator can act either on this link, on the drag link, or through a separate link to optimize its design. For a cam-based system, the actuator uses a standard rack and pinion gear optimized for the pneumatics. The gear has a cam attached and the spring acts on that cam, reducing the stroke and increasing the force through similar kinematics. Each has advantages, and either could be used.