1. Field of the Invention
The present invention relates to the combination of a variable fill fluid coupling and an improved control means therefor.
2. Incorporation by Reference
The entire disclosures of U.S. Pat. Nos. 3,210,940; 3,237,408 and 3,862,541, including the drawings thereof, are hereby incorporated by reference herein.
3. Description of the Prior Art
The aforementioned patents relate to combinations comprising a variable fill fluid coupling for coupling a constant speed drive motor to a driven centrifugal pump having a discharge conduit, the fluid coupling having a compression spring-loaded pivotal charging stream splitter assembly, and a control means for controlling the movement of the splitter assembly responsive to pressure change in the discharge conduit. The preferred construction of the mechanism which controls movement of the splitter assembly incorporates a form of diaphragm or bellows assembly within which a fluid under pressure may act. This fluid is generally gathered from the discharge conduit at a location near the discharge end of the pump connected to the fluid coupling and transmitted to the bellows assembly via tubing called a pressure control line. A plunger or rod connected to the bellows on one end and engaging the splitter assembly at the other end causes the splitter assembly to move about a fixed pivot in response to motion of the bellows created by increase or decrease of fluid pressure in the discharge conduit.
A further essential feature of the control mechanism disclosed in the aforementioned patents is the externally adjustable compression spring. The spring provides a force on the splitter arm of the splitter assembly in opposition to the force created by fluid pressure acting on the bellows and transmitted to the splitter arm via the connecting plunger or rod. The spring force is made adjustable to allow for manual selection of equilibrium positions of the splitter mechanism with respect to a desired fluid pressure acting on the bellows. When the opposing forces of the compression spring and the bellows assembly are established in equilibrium, the diverting portion or vane of the splitter assembly may be located at any position across the coupling oil charging stream or jet, such that the volume of oil entering the coupling will establish an output torque to drive the connected centrifugal pump at a speed which will yield a certain pump volumetric flow rate at the selected pressure. Changes in fluid pressure acting on the bellows alters the equilibrium forces on the splitter assembly such that the oil stream is intercepted in a greater or lesser degree which in turn affects the output speed inversely to the control pressure direction.
The preferred construction of the control mechanism utilizing fluid pressure feedback to a bellows assembly to create coupling response is useful only when the fluid pressure signal is taken from a source near the fluid coupling. Protection of long lengths of fluid carrying tubing from damage, leakage integrity and losses in pressure signal strength are some of the problems with remote pressure sensing via pressure control lines described in the aforementioned patents. To allow use of the variable speed device named with remote pressure sensing, a system has been commonly employed which utilizes a pressure transducer, a signal processing center, an air compressor and tank, and an electrically operated solenoid valve.
The commercial tank and air compressor supplies an independent fluid (air) pressure to apply at the coupling bellows assembly, the solenoid valve varies the air pressure in response to electrical signal changes from the controller which receives a functional signal from the transducer. The transducer signal is generated in varying strength in proportion to fluid pressure changes at the sensing location. This combination has had several problems.
1. The tank and air compressor take up valuable space as well as adding to the electrical energy use of the equipment.
2. Reliability and accuracy of the speed changing signal is impaired by too many translations, i.e., hydraulic to electrical at the transducer, electrical to mechanical at the solenoid valve, mechanical to fluid at the air compressor-solenoid combination and fluid back to mechanical at the coupling.
3. The commercial tank and air compressor used produce a usable air signal strength of 3-15 psi. The commercial compression springs used in the coupling control mechanism have a plus or minus 10% tolerance on the selected spring rate. With the low signal pressure value and low range available, the wide spring rate tolerance makes the control of speed between two or more parallel operating couplings very difficult due to mismatched control mechanism forces.
4. For the reasons stated in (3) above, a shift in the spring rate to a lower value, with the low end pressure value on the bellows, sometimes causes the control mechanism to flutter, resulting in unstable speed control.
5. The use of air as a pressurized working fluid has created special problems with leakage at connections and joints.