Eccentric and concentric reducers: which one to use and how each one develops on plate

Every reducer joins two round openings of different diameters. On the concentric both sit on one axis; on the eccentric one opening is shifted until the two are tangent on one side, and that side comes out plumb. The first is a right cone and opens out as a trammel sector; the second is the frustum of an oblique cone, with its apex off the axis, and opens out by triangulation. This guide says when to use each and how the layout changes.

When to use the concentric and when to use the eccentric

The concentric is the standard reducer: funnel, hopper, chimney transition, reducer on a vertical pipe run. The eccentric exists for a fitting reason: with one side plumb, a horizontal run stays supported on the same bottom line (or top line) after the change in size. On a pump suction line, the eccentric with the flat side up avoids an air pocket; on a run resting on saddles, with the flat side down it keeps the support.

In hoppers and silos the eccentric turns up when the outlet has to sit against a wall or a beam: the bottom opening is shifted until that side is vertical, and the discharge comes out where the structure allows.

What changes in the layout

The concentric cone is a right circular cone: the generators converge on an apex, and the plate opens out as a sector of an annulus, struck with a trammel from one centre. Two radii and an angle settle everything.

On the oblique cone and the offset cone the generators still converge on an apex, but it sits off the axis and every generator has a different true length -- there is no trammel sector. The layout is by triangulation: each opening is divided into stations, each bottom station is joined to its two neighbours at the top, the true length of every generator is worked out and the triangles are rebuilt on the plate with compasses. The plate comes out asymmetric: long on the sloping side, short on the plumb side.

Offset of the eccentric

v = (D2 − D1) / 2 It is the shift between the axes that leaves one side vertical. With v = 0 the part is the concentric; with v between 0 and (D2 − D1)/2 it is an offset reducer; above that the small opening leaves the projection of the large one, the external eccentric.

True length of a generator

ℓ = √( h² + Δx² + Δy² ) Δx and Δy are the distances in plan between the bottom station and the top one, and h the height. It is Pythagoras in three dimensions, once per generator.

One part that does both

The offset cone takes the shift as a dimension: zero gives the concentric, (D2 − D1)/2 gives the classic eccentric with one side plumb, and any value between the two gives the offset reducer that a crooked pipe run sometimes calls for. It is the same page for all three, and it always triangulates, even at zero shift; for the concentric, the exact trammel sector is on the cone page.

Common mistakes

• Laying out the eccentric as a trammel sector. Only the right cone opens out as a sector; an eccentric laid out that way closes with the opening skewed and the seam does not meet.
• Entering the wrong offset. The eccentric with one side plumb has v = (D2 − D1)/2 exactly; more than that and the part becomes an external eccentric, less than that and it is not plumb.
• Too few divisions. Triangulation approximates the curve by chords; 16 divisions is the minimum on 800 mm openings, and the calculation works from the arc length so the opening does not come out short.
• Forgetting the thickness. Both openings take the thickness offset on the neutral axis, as on the concentric cone; a plate that does not comes out π × t over at the seam.
• Rolling the eccentric in one pass. It is not a surface of revolution: it is formed in sectors, against a template, which is why the template has to be right before cutting.

Frequently asked questions

What is the difference between a concentric and an eccentric reducer?

On the concentric both openings sit on one axis and the part is a right cone. On the eccentric one opening is shifted by (D2 − D1)/2, the two are tangent on one side and that side comes out plumb. The concentric opens out as a trammel sector; the eccentric, the frustum of an oblique cone, by triangulation.

When is an eccentric reducer used?

When a pipe run has to keep its bottom or top line while changing size: pump suction with the flat side up, a supported run with the flat side down, a hopper with the outlet against a wall.

How do I develop an eccentric reducer?

By triangulation: divide both openings into stations, work out the true length of every generator with √(h² + Δx² + Δy²) and rebuild the triangles on the plate. The Planichapa offset cone does that with the offset you enter.

Can I make an eccentric reducer with the oblique cone?

Yes: the oblique cone is the offset cone with the shift locked at (D2 − D1)/2, the value that leaves one generator vertical. In that case both give the same part and the same plate area, but with the seam on different generators, so the outline of the plate changes.