Historically linkages are more common than direct coupled actuators. Control actuators were usually foot-mounted. A foot-mounted actuator has the mounting surface on the bottom and the drive shaft coming out the side. This arrangement invites linkage-type control. An example is a four-bar linkage. If any of these bar lengths are changed, it will affect the range and characteristics of the control valve. There are usually six security points in this configuration: actuator to crank arm, crank arm to swivel head, swivel head to linkage rod, and then double all of those for the valve side.
The actuator secured to the valve is critical to safe operation. The actuator becoming disengaged from the control valve is a significant safety concern. There is confidence in the crank arms' security and is considered a positive connection. The swivel heads are less secure. Typically the swivel head is clamped or presses a set screw into the linkage rod. Under conditions of vibration or varied temperature causing expansion and contraction, these connections come loose. Because of this hazard with linkage, you'll occasionally see a position switch connected directly to the valve to verify the valve position.
Because the connections of a linkage are not rigid, there is a problem with hysteresis. The least rigid connections are the swivel connections. The actuator will push on the swivel head when driving open and pull on the swivel head when driving closed. There will be two different valve positions based on the direction hysteresis is created for a given control signal. Why is this important? Hysteresis hurts emissions efficiency and temperature control.
More recent control actuator designs are face-mounted. These newer designs make the mounting surface and the drive shaft on the same plane with the mounting face actuator shaft and valve shaft in line. Direct coupling the actuator to the valve is an improved design.
For direct-coupled control, the stroke of the actuator equals the stroke of the valve. To adjust the stroke of the actuator, change the cams located under the cover to limit the stroke. The concept of cam adjustments in the actuator is commonplace for a direct-coupled arrangement.
There are just two secure connection points with direct-coupled: actuator to coupling and coupling to the valve. When done correctly, both of these points are positive connections. The machining of the actuator side of the coupling is commonly an irregular shape that doesn't allow the actuator shaft to rotate inside the coupling. On the valve side, a pin bolt goes through the coupling and the valve shaft. You can be more confident in a secure connection with a direct-coupled arrangement.
The inline connection of the actuator driveshaft and the valve shaft with a positively connected coupling eliminates linkage hysteresis. The coupling is a rigid piece that directly transfers the actuator movement to the valve movement, providing precise control which assists emissions requirements, optimizes efficiency, and facilitates high-performance temperature control.
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