Note: This tech tip is for when you are mounting a VAC rotary positioner on a linear valve.
The V100 and V200 are by nature rotary positioners. To get a rotary positioner such as a V100 or V200 to operate on a linear valve the linear motion of the valve has to be converted to a rotary motion. This is accomplished with the mounting. The mounting uses a turnbuckle and a linear arm whose positions are determined using the geometrical equations detailed below. In order to mount a V100, V200, D400 or a D500 to a linear stroke actuator we must determine a couple of things.
1. What is the total stroke of the actuator?
This is helpful for several reasons. For the V100 and V200 we must choose the correct cam in order for the positioner to work correctly on the linear actuator. A very general rule that VAC uses is anything smaller than 2” uses a 30° cam and any stroke 2” or larger uses a 90° cam. Below are several diagrams that show the different rotations 90°, 60° and 30° and how the formula is used to determine the distance between the positioner and valve stem centerlines, this is called the C-C dimension.
2. Where is the mid-stroke of the valve?
This is where we want our lever arm to be horizontal to the valve stem. The lever arm is what is connected to the back of the positioner. The lever arm is then connected to a turnbuckle, which is then connected to takeoff arm that is attached to the valve stem, either directly or with a stem clamp.
Examples of Formulas
90° cam with 2” of valve stroke:
.5 x valve stroke (2”) = C–C dimension of 1”
60° cam with 2” of valve stroke:
.87 x valve stroke (2”) = C-C dimension of 1.74”
30° cam with 2” of valve stroke:
1.87 x valve stroke (2”) = C-C dimension of 3.74”
Find out about the Linear Mounting Rules of Thumb and Linear Mounting Principle below.