How to Measure Radius for Curved Chute Liner Plates
A curved chute liner cannot be reproduced reliably from the word “radius” and one approximate number. The drawing must identify whether the dimension is inside, outside or mid-surface radius; which cross-section it belongs to; which datums locate the arc; and whether the worn sample still represents the original geometry.
This measurement guide is for replacement curved liners, hopper segments, cylindrical transitions and curved sidewall panels. It helps an RFQ become manufacturable, but the purchaser or responsible engineer must approve the final geometry.
Define which radius you are measuring
| Radius term | Meaning | Common risk |
|---|---|---|
| Inside radius | Radius of the concave surface. | Supplier assumes mid-surface or outside radius. |
| Outside radius | Radius of the convex surface. | Plate thickness is added in the wrong direction. |
| Mid-surface radius | Reference near the plate mid-thickness for geometry control. | It is treated as a forming neutral-axis calculation without agreement. |
| Local section radius | Radius at a specified station on a tapered or compound part. | One section is assumed to represent the complete panel. |
| Best-fit radius | Radius fitted to measured points. | Local dents or wear are hidden by the fitted value. |
Establish datums before taking arc measurements
Identify stable surfaces or features that locate the liner in the equipment: shell face, flange, centre plane, controlled straight edge, hole pattern or mounting surface. Do not reference a jagged worn edge merely because it is easy to reach. ISO 5459:2024 specifies terminology and rules for datums and datum systems in technical product documentation.
Put the datum scheme on the drawing so the site, fabricator and inspector reconstruct the same profile. If a temporary fixture or template establishes the datum, identify and preserve it.
Use chord and sagitta for a circular arc
For a true circular arc, measure a chord length c between two defined endpoints and the sagitta s, which is the perpendicular rise from the chord midpoint to the arc. The calculated radius is:
R = c² / (8s) + s / 2
Use consistent units and measure the sagitta perpendicular to the chord. A small sagitta error can create a large radius error on a shallow arc, so repeat the measurement and report instrument resolution. The formula does not prove that a worn or compound surface is circular.
Chord-and-sagitta measurement checklist
| Step | Action | Evidence to retain |
|---|---|---|
| 1 | Mark the exact cross-section and arc endpoints. | Photograph or sketch with station ID. |
| 2 | Measure chord directly between the defined points. | Actual value, unit and instrument. |
| 3 | Locate the chord midpoint and measure perpendicular sagitta. | Actual value and direction. |
| 4 | Repeat at nearby stations. | Table of results, not only an average. |
| 5 | Compare inside/outside profiles and thickness. | Evidence of wear, distortion or taper. |
| 6 | Verify calculated geometry with a template or point cloud. | Overlay, gap record or inspection report. |
Three-point and multi-point methods
A three-point gauge or coordinate set can fit a circle when the surface is accessible. Multiple points are better for detecting whether the profile departs from a circle. Record the point coordinates relative to the defined datums and state how outliers caused by dents, welds, scale or missing material are handled.
For large or compound surfaces, photogrammetry, a laser scan, CMM or a controlled template may be more reliable than tape measurements. Provide raw or processed data together with units, coordinate system, alignment method and fitting rule.
Do not measure only the worn liner
A removed liner may be thinned, flattened, cracked or bent during extraction. Compare it with the shell, adjacent panels, original drawing, bolt pattern and unworn margins. Mark each input as original, measured, calculated or proposed. Where the sample conflicts with stable equipment datums, escalate the difference for engineering approval.
Separate a smooth curve from a sequence of bends
A rolled circular panel, a press-brake faceted panel and a plate with one discrete bend can look similar from a distance but require different drawings and processes. State whether the requirement is a true radius, a profile tolerance, a set of bend lines or an approved segmented approximation.
For discrete bent parts, use the separate bend angle and radius measurement guide.
Inside, outside and mid-surface relationships
For a simple constant-thickness concentric circular plate, inside and outside radii differ by the finished thickness. Real liners may have taper, wear, machining, backing or compound curvature, so do not apply that relationship blindly. A forming neutral axis is a process concept and is not automatically the same as the dimensional mid-surface required for inspection.
Specify profile, not only nominal radius
A radius dimension alone may allow unacceptable local waves or twist. ISO 1101:2017 defines the symbol language for geometrical tolerancing, while ISO 1660:2017 provides rules for profile tolerancing. The purchaser should select the applicable drawing convention and acceptance method.
| Drawing control | What to state | Inspection approach |
|---|---|---|
| Nominal profile | Radius, arc endpoints, section and centre/location. | Template, coordinates, CMM or scan comparison. |
| Profile tolerance | Tolerance zone and datum relationship. | Agreed point density and evaluation method. |
| Arc length/angle | Included angle or developed arc limit where functional. | Endpoint and overall-profile verification. |
| Twist/straightness | Control of axial edges or generator lines. | Straightedge, fixture or coordinate report. |
| Thickness | Finished nominal and acceptance requirement. | Grid or specified measurement locations. |
Locate holes after the profile is controlled
Hole coordinates on a curved panel can be defined in 3D, by developed layout, by chord offsets or from local datums. State whether dimensions are on the inside, outside or developed surface. Avoid measuring every hole from a worn arc edge. Use the bolt-hole pattern checklist for datum and orientation controls.
Templates and trial fit
A rigid profile template can verify the intended section if its identity, material, datum contact and wear condition are controlled. Record the permitted gap and measurement locations. A flexible strip that conforms to the part cannot prove the original radius. For critical replacement sets, agree a fixture or trial-fit requirement before production.
RFQ package for curved liners
- general arrangement, installation position and flow direction;
- inside/outside/mid-surface definition and finished thickness;
- datum system, section locations and coordinate convention;
- chord, sagitta, arc endpoints and repeated measurements;
- scan, template or original drawing where available;
- hole coordinates, fixing details and left/right orientation;
- material, forming method, tolerances and inspection method;
- sample condition, wear notes and unresolved discrepancies.
Related pages: curved chute liner plates, tapered chute liner plates and pre-shipment inspection checklist.
Engineering and safety boundary
Do not enter, measure or remove liners from operating equipment. Follow the site’s isolation, stored-energy, access and lifting procedures. This article explains measurement documentation and does not approve a final profile. Updated by the EB China engineering-content team against the cited current ISO scopes; the purchaser-approved drawing governs production.