How to make a divided-flow or "Y" fitting to branch ductwork.

Figure 1
Divided-flow or "Y" fittings are typically installed in line with the main-branch ductwork and used when some of the air is needed to go off in a different direction. In fact, many years ago, much of the higher quality main-branch ductwork was designed with divided-flow fittings.

You can save material and time by using divided-flow fittings, since using two separate elbows would require larger ductwork. You can get the same results using one "Y" fitting, as shown in Figure 1.

Figure 2

Time and space

Many contractors still use divided-flow fittings, shown in Figure 2 and Figure 3. Divided-flow fittings save time and space. One of the more popular fittings is a Y fitting, Figure 2.

The basic Y fitting is made by combining two elbows. This article will deal with the horizontal and transitional Y. To begin laying out the basic horizontal Y, start by extending a line from Point A to Point B, as shown in Figure 2. On Line AB, create two reference marks, 20 inches apart, shown as points 1 and 1a.

The pattern should appear like you are making two separate elbows and are going to merge them into one fitting. These will be 12 inches wide and will be developed with a 1-to-1 radius-to-width ratio. The lower corners, points 1 and 1a, will be the starting points for the elbows' throat. The radius to width is always measured from the center line of the elbow.

To make this line, set your trammel points to 6 inches (which would be the center of a 12-inch elbow), and using points 1a and 1 as center, draw an arc from Point 2a to Point 2b and Point 2 to Point 2'.

Reset your trammels to 12 inches (this is the 1-1 radius-to-width ratio). Place one end of the trammels on Point 2a and draw a reference mark at Point 3a. Repeat this step for the opposite side of the Y. Now keeping the trammels set to 12 inches, place one end on Point 2 and make a reference mark at Point 3, as shown. Points 3 and 3a are your centers for the throat and heel.

Now set the trammel points the distance from Point 3a to Point 1a and using Point 3a as center, create an arc from Point 1a to Point 1b. Keep the trammels set to the same distance, and using Point 3 as center, draw an arc from Point 2 to Point 2'. Keeping one end of the trammels on Point 3, reset the distance of the trammels to extend to Point 4. Then using Point 3 as center, make an arc at Point 5.

You are just trying to establish the intersection, so you only need to strike a partial arc. Repeat this by placing one end of the trammels on Point 3a. Now using 3a as center, strike an arc from Point 4a to Point 5a. Now you may complete the pattern of the cheek by adding the necessary allowance for the flange by using the established points used in creating the heel and throat, with the exception of setting the trammel points approximately one-quarter larger and smaller to complete the necessary arcs.

Figure 3


To make a transitional Y branch, begin by developing the cheek of the pattern as if it were going to be flat. A top view is necessary to develop a true-length triangle. Figure 4 shows the top view. There are several lines, including the bend lines, which you need to use to develop the pattern. Some of the lines are already seen in true length, such as lines 1-1, 1-2 and 3-4. The other lines not seen in their true lengths, such as 2-4, 2-3 and 1-3, are transferred to the true-length triangle.

Figure 4

True-length triangle

Figure 4 also shows a true-length triangle; the vertical line in the true-length triangle represents the different lengths as they are in the flat pattern. The base line in the true-length triangle represents the distance of the offset. For instance, if the offset from Line 1-1 to Line 3-4 were 2 inches, the base line of the triangle would be 2 inches. The points from the vertical line to the base are the established true-length lines.

Figure 5

Lay out the pattern

In Figure 5, you'll begin to lay out the pattern by establishing points 1-1 and 1-2. This section of the fitting is the same as in Figure 4, so make a line from Point 1 to Point 1. Set the trammel points from 1 to 2 (from Figure 4) and using 1 as center, make an arc near Point 2, then move your trammels to the other Point 1 and using it as center, draw the other arc intersecting the first arc near Point 2. Because this particular Y fitting is symmetrical - the pattern for one half will be the same for the opposite side - you can do both sides at the same time, and this will save you repeated steps.

Figure 6

Partial arcs

From the true-length triangle, set your trammels to the distance of Point 1 to Point 3, as shown in Figure 5. Using Point 1 as center, make a partial arc near Point 3. Again, from the true-length triangle, set your trammels to the distance of Point 2 to Point 3 and using 2 as center, strike an arc intersecting the previous arc created from Point 1. This intersection establishes Point 3.

The distance from Point 3 to Point 4 is shown in true length in the flat pattern in Figure 4. Take the true length of Point 3 to Point 4 and using Point 3 as center, draw a reference mark near Point 4 as shown in Figure 5.

Now that points 1, 2, 3 and 4 have been established, you need to develop the pattern for the heel and throat on the cheek. To draw the arc for the heel, as shown in Figure 6, the center point of the radius (7) needs to be established. This is accomplished by extending Line 3-4 for several inches, bisecting Line 2-4 and extending a line perpendicular until it intersects the line extended from Line 3-4. This intersection is the center point for the heel. Set the trammels from point 7 to 2, using 7 as center, and make an arc from Point 2 to Point 4. Extending the trammels to the allowance necessary for the flange (about one-quarter inch) and using Point 7 as center, strike the additional arc for the flange. This flange area is shown more clearly in Figure 8.

Figure 7

Points for the throat

Points for the throat are established at 1 and 3 as shown in Figure 7, in developing the throat, take the true length of 1-5 and 5- 3 using 1 and 3 as center, establish Point 5. Now bisect lines 1-5 and 5-3. Extend perpendicular lines as shown, where these lines intersect at Point 7 becomes the established center for the radius.

Figure 8

Trammel points

Placing one end of the trammels on center (7) and the other end of the trammels on Point 1, make an arc from Point 1 to Point 3. As you did earlier for the heel, make the necessary adjustment and draw an additional arc for the flange. The shaded area in Figure 8 shows the additional allowance for the flanges and the allowance for "S" slips and drives on all three sides.

Complete the fitting by laying out the heel and throat. The length of both the heel and throat will be the distance of Point 1 to Point 3 for the throat and Point 2 to Point 4 for the heel.

This must be measured from only the flat plane in Figure 4, not the developed pattern. As you created the cheek, the distance from points 1 to 3 and points 2 to 4 represent the true-length of the angles of the top and bottom of the heel and throat. The heel and throat are part of the transition; this means that when you lay them out, you'll need to take into consideration the offset.

The first thing you'll do is to mark off two lines perpendicular to the base at the true length of the heel and throat from Figure 4. From the base of your layout, measure the distance of the offset and mark it. From that point, measure and mark the width of the heel or throat. Once this is completed, you can add on the material needed for the flange.

Figure 9

The throat

The throat is developed in two pieces. When the fitting is symmetrical, one layout can be used as a pattern for the other. The heel must be developed in two pieces. The center of these separate pieces, as shown in Figure 9, have 2 inches of additional material on one and 1 inch on the other. This will allow you to bend a lap joint on one half of the heel and connect the two pieces.

All four of these pieces in Figure 9 must be rolled. The cheeks will be required to make two additional bends for the transition. From the inside of the cheek, make a slight bend up on Line 1 to 2. From the outside of the fitting, make a slight bend from 2 to 3. This is somewhat flexible as you attach the heel and throat. While you should be close, you will not have to have the exact angle on these bends.

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