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CEMA 550 Material Class Codes for Wood Chips, Bark, Sawdust, and Minerals

ANSI/CEMA 550-2020 is the CEMA standard for classifying bulk materials by their physical characteristics and handling properties, and wood chips, bark, sawdust, and mineral solids are among the materials a conveyor designer has to classify before equipment is selected. UTEC Industrial designs, engineers, machines, fabricates, and installs custom material handling systems for aerospace and heavy industry from its Spokane Valley, WA facility, integrating Allen-Bradley PLC and motion control with in-house CNC machining, heat treating, and stress relief. This article sets out what the public record shows about the standard and its material code, what federal and peer-reviewed studies have measured for wood residues, why a bulk density or flowability figure means little without its test conditions, and how a material's class carries into sensing and controls. Classification is the first input at the design end of the build chain, design → engineering → parts machining → fabrication → assembly → weld fatigue → stress relief → drives → controls → tuning → monitoring, and the same properties come back at the monitoring end as the plugging, load, and moisture signals the controls watch.

What is ANSI/CEMA 550-2020, and is it the current edition?​

ANSI/CEMA 550-2020, Classification and Definitions of Bulk Materials, is a 67-page American National Standard from CEMA's Committee for Materials Classification & Definition. CEMA's store describes it as establishing a "systematic method for the classification and description of bulk materials based on their physical characteristics and handling properties", and says it "defines terminology, test procedures, and classification criteria that are fundamental to the proper selection and design of bulk material handling equipment", with "detailed definitions, material characterization methods, and classification tables".

The ANSI webstore labels the 2020 edition the most recent. It lists what the 2020 revision changed:

  • metric conversions were added;
  • "the bulk density in Table 7 in Chapter III was updated";
  • "a table about how to read the CEMA Material Code was added";
  • cosmetic changes and updates to the bibliographic references were made, with "a disclaimer in the foreword regarding material properties".

A revision is open. ANSI's Standards Action of February 21, 2025 carried a project notice for BSR/CEMA 550-202x, a revision of 550-2020. Its scope line for the project reads: "Provide a precise definition and accurate classification of materials according to their handling characteristics under a specific combination of conditions of temperature, humidity, sizes, distribution of lumps, friability, and so on, including all factors that influence the selection of proper types and sizes of conveyors, horizontal, inclined or vertical." Until a new edition is published, 550-2020 is the edition to cite. Where 550 sits among CEMA's other American National Standards is set out in the unit-load versus bulk-solids article (ANSI/CEMA 550-2020; ANSI Standards Action Vol. 56 No. 08, 2025, PINS p. 7; Conveyor Equipment Manufacturers Association, ANSI/CEMA Standards, accessed 2026).

What does a CEMA material code describe?​

A CEMA material code is a short designation that describes a bulk material for handling-equipment selection. The ANSI webstore says the standard presents "materials classifications with physical characteristics of each, hazards that affect conveyability, along with suggested test procedures". In 550-2020, the code combines, in order, a bulk density, a size class, a flowability class, an abrasiveness class, and letters for miscellaneous characteristics, and the material tables give the full code for each listed material, with wood chips, bark, sawdust, and the listed minerals among them.

CEMA's equipment books also code bulk materials. The public table of contents of CEMA No. 375-2017, the Bucket Elevator Book, lists for its Chapter 2 "material tables, material classification code chart, sample analysis, size system table and flowability/abrasiveness coding systems", and that chapter's code chart uses the 550 material code. ANSI's 2025 notice for the reaffirmation of ANSI/CEMA 350-2021 lists "Bulk Materials (Code, Size, Conveyor Speed, Component Groups)" among the screw conveyor standard's contents.

There is an international counterpart. ISO 3435:1977, Continuous mechanical handling equipment — Classification and symbolization of bulk materials, "Establishes a classification and symbolization of bulk materials in order to assist communication between manufacturers and users of handling equipment", and its stated object is "to secure from enquirers an accurate description of the material to be handled in respect of any specific project." ISO confirmed it as current in 2025. Neither publisher says the ISO symbols and the CEMA code match, and neither should be read as a translation of the other (ANSI/CEMA 550-2020; CEMA No. 375-2017, contents, Ch. 2 p. 11; ANSI Standards Action Vol. 56 No. 08, 2025, PINS; ISO 3435:1977).

How are particle size and flowability classified?​

ANSI/CEMA 550-2020 assigns size classes by letter, each letter covering a stated range of particle size, and it defines a set of flowability classes together with the property each class is judged by. Those letters, ranges, and the flowability basis are in the standard's tables and are not reproduced from secondary sources here.

Public sources show why size matters before a code is even assigned:

  • Test applicability. ISO 17828:2025, the solid-biofuel bulk density method, "is applicable to all pourable solid biofuels with a nominal top size of maximum 63 mm while the maximum particle length is 200 mm", and describes a different method for fuels with a nominal top size above 63 mm. As engineering reasoning, coarse hog fuel and long bark strips can fall outside the first method.
  • Fines govern strength. ASTM D6128-22 states that for bulk solids with a significant percentage of particles, "typically, one third or more", finer than about 6 mm, the cohesive strength is governed by the fines, and cohesive strength and wall friction tests may be performed on the fine fraction only.

Flowability scales from the shear-testing literature are not the CEMA classes. Stasiak and co-authors note that, for certain materials, flowability is used as a measure of product quality and that its magnitude "characterises granular materials as either free flowing, easy flowing, cohesive, or strong cohesive", a scale the paper applies to its shear-test results; as engineering reasoning, it should not be substituted for the 550 class (ANSI/CEMA 550-2020; ISO 17828:2025; ASTM D6128-22, 5.3; Stasiak et al. 2019, §1).

How is abrasiveness classified, and what does it mean for minerals?​

ANSI/CEMA 550-2020 defines abrasiveness classes, each with a definition and a test basis, and the class is one position of the material code. As engineering reasoning, for a mineral handling line that position is the one that drives wear allowance. In a test-stand study of belt-conveyor elements, Romek and co-authors note that conveyors transporting spoil in a quartz sand mine are exposed to accelerated wear due to the effect of quartz on metal elements, and that the main mechanism of sample wear was abrasion from the interaction between the steel surface and hard sand particles.

This article gives no mineral bulk density or abrasiveness values. Chapter III Table 7 of the standard, updated in 2020, carries bulk density values, including values for minerals such as limestone and sand, with the conditions behind each.

How an abrasive class changes screw conveyor trough loading and component series is set out in the abrasive mineral processing article, and the liner choice for an abrasive class in the wear liner article, including what a laboratory abrasion test on liner metals can and cannot show (ANSI/CEMA 550-2020; Romek et al. 2021, abstract).

Which miscellaneous characteristics flag hazards in wood residues?​

The miscellaneous position of the code is where 550 records the "hazards that affect conveyability" the ANSI webstore mentions. In 550-2020 it uses letters for characteristics such as fibrous or interlocking particles, dustiness, explosibility, stickiness, degradation, aeration, hygroscopic behavior, and elevated temperature, and the material tables assign the applicable letters to bark and hog fuel.

The research record names the same behaviors in wood biomass:

  • Handling concerns. Salehi and co-authors list the particular concerns for biomass handling systems, citing earlier work, as wide particle size distribution, high and varying moisture content, low bulk density, particle elasticity, and irregular and interlocking particles that hinder the material flow.
  • Self-heating in storage. A 1979 USDA Forest Products Laboratory note states that whole-tree chips "deteriorate more rapidly than do clean, debarked chips and present a greater hazard for spontaneous ignition when stored in outdoor piles", and reports Zoch and co-authors' finding, in a laboratory-scale storage study, that aspen whole-tree chips evolved much more heat and lost ovendry weight six times faster than clean, debarked chips. The note adds that drying the chips before storage will prevent deterioration and heating, provided the chips are stored under cover.
  • Combustible dust. The federal Chemical Safety Board's 2006 study counts at least 281 combustible dust fires and explosions in general industry from 1980 to 2005, and reports that wood, food-related products, and metals each account for over 20 percent of explosions. NFPA 660-2025 is the consensus standard for combustible dusts and particulate solids, cited here at standard level only.

As engineering reasoning, a miscellaneous letter is a design instruction: an interlocking material needs an outlet and feeder sized for bridging, and a dusty, explosible one needs enclosure and isolation (ANSI/CEMA 550-2020; Salehi et al. 2024, §1; Springer 1979, abstract and Background; CSB 2006, Report No. 2006-H-1, §1.0 and §5.2.6; NFPA 660-2025).

What does a bulk density figure mean for chips, bark, and sawdust?​

A bulk density is a weight per volume measured under stated conditions, and without the conditions the number cannot be compared. Three public sources say so in their own words:

  • USDA Forest Service glossary. Wartluft defines bulk density as weight per unit volume of an aggregate material "at a specific moisture content and compaction ratio", and the compaction ratio as the original volume divided by the volume after compaction.
  • Briggs's handbook chapter. Briggs's summary concludes that "bulk density is ambiguous without clear statement of the species (wood specific gravity), type of residue, degree of compaction, and the moisture content of both the numerator and denominator." It also explains that because wood shrinks as it dries, residues from dry wood occupy less space, and that their oven-dry bulk densities are higher and their wet bulk densities lower, from shrinkage and lower moisture content.
  • ISO 17828:2025. "Bulk density is not an absolute value; therefore, conditions for its determination have to be standardized in order to gain comparative measuring results." Its note adds that the bulk density of solid biofuels varies with vibration, shock, pressure, biodegradation, drying, and wetting, and that measured bulk density "can therefore deviate from actual conditions during transportation, storage, or transhipment".

ANSI/CEMA 550-2020's Chapter III Table 7 gives bulk density values for listed materials, wood chips, bark, sawdust, shavings, and hog fuel among them, with the condition behind each value, loose or settled, stated in the standard. As engineering reasoning, a designer reading any bulk density, from 550 or from a study, should record whether it is wet or oven-dry, loose or compacted, and at what moisture, before using it to size a conveyor or bin (Wartluft 1976, glossary; Briggs 1994, Ch. 7, Effect of Drying and Summary; ISO 17828:2025; ANSI/CEMA 550-2020).

What bulk densities have studies measured for chips, bark, sawdust, and shavings?​

Published values exist, but none of them are CEMA 550 values, and each carries its own basis. Briggs's Table 7-3 compiles two earlier studies. Among the means it gives from Harris and Phillips (1989), hardwood sawdust and bark at 45% moisture on a wet basis is 25.8 lb/ft³ green and 14.2 lb/ft³ oven-dry, pine whole-tree chips at 40% are 18.8 and 11.3 lb/ft³, and hardwood shavings at 8% are 10.8 and 9.8 lb/ft³. From Risbrut and Ellis (1981), the same table gives an oven-dry range of 8.5 to 12.6 lb/ft³ for bark.

Two recent peer-reviewed studies measured metric values under laboratory conditions:

  • Stemwood chips. Salehi and co-authors report bulk densities for aspen, birch, Scots pine, and Norway spruce chips at two chipper settings in a narrow range, which the text gives as between 130 and 150 kg/m³; its Table 1 means run from 126.3 to 149.4 kg/m³ across the eight assortments.
  • Sawdust and shavings. Stasiak and co-authors report a bulk density of 143 kg/m³ and a tapped density of 203 kg/m³ for pine sawdust, and the lowest densities, 56 and 69 kg/m³, for one shavings fraction. Their materials were pine sawdust, shavings, and pellets, not chips.

Vidrine and Woodson found the bulk density of pine-site hardwood materials "greatly affected by the form of the product, method of packaging, state of dryness, and species", and caution that estimating a value between green and air-dry by linear interpolation from their tables "could be in error" (Briggs 1994, Ch. 7, Table 7-3; Salehi et al. 2024, §3.2 and Table 1; Stasiak et al. 2019, §4; Vidrine and Woodson 1982).

How are wood chips, sawdust, bark, and hog fuel told apart before they are classified?​

A classification is only as good as the material name it is attached to. A USDA Forest Service research note's suggested glossary and an ISO fuel standard define these residues, most of them by how they were made:

  • Chips: "Wood particles of various small sizes produced by processing solid wood through chipping machines" (Wartluft).
  • Sawdust: "Wood particles resulting from the cutting and breaking action of saw teeth, finer than chips but coarser than wood flour" (Wartluft).
  • Bark: all peripheral tissues of tree stems, branches, and roots outside the vascular cambium (Wartluft).
  • Hogged residue: wood and bark residues from manufacturing operations, prepared by processing through an industrial-type hog or hammermill (Wartluft); hogged fuel is hogged residue for use as fuel.
  • Graded chips versus hog fuel: ISO 17225-4:2021 covers only wood chips "produced with sharp tools" and does not cover hog fuel, "produced with blunt tools". It covers chips from forest, plantation, and other virgin wood, by-products and residues from the wood processing industry, and chemically untreated used wood. ISO lists the standard as under review.

The source process matters within each name. Vidrine and Woodson measured bark separately from ring, rosser-head, and drum debarkers, and Briggs's compiled table separates clean pulp chips from whole-tree chips, which the 1979 Forest Products Laboratory note describes as chips produced from the entire above-ground portion of trees: bole, bark, branches, and foliage. As engineering reasoning, a conveyor request that says "bark" without the debarker or "chips" without stating clean or whole-tree leaves the classifier guessing (Wartluft 1976, glossary; ISO 17225-4:2021; Vidrine and Woodson 1982; Briggs 1994, Ch. 7, Table 7-3; Springer 1979, Introduction).

How is flowability measured, and why do testers disagree on wood particles?​

Flowability is measured with shear testers. Jenike's 1964 University of Utah bulletin covers the concepts of flowability of bulk solids and of flow channels, the flow-no flow postulate, equipment and procedures for testing the flow of bulk solids, and bulk flow equipment design. Two ASTM methods are current:

  • ASTM D6128-22 (Jenike shear tester) measures cohesive strength during continuous flow and after storage at rest, plus internal friction, bulk density, and wall friction. Its scope states it "is not applicable to testing bulk solids that do not reach the steady state requirement within the travel limit of the shear cell", and that one example "may be those consisting of highly elastic particles". Its scope gives the most common use of the results as the design of storage bins and hoppers to prevent flow stoppages due to arching and ratholing, stoppages its significance statement calls "common".
  • ASTM D6773-22 (ring shear tester) measures the unconfined yield strength of bulk solids during continuous flow and after storage at rest, plus internal friction, bulk density, and wall friction, with a ring shear cell, and adds that another application is "the measurement of the flowability of bulk solids, for example, for comparison of different products or optimization".

On wood particles the two testers can disagree. For pine sawdust, shavings, and pellets, Stasiak and co-authors found that the Jenike tester characterized sawdust as free flowing, pellets and shavings as free flowing or cohesive, and one shavings fraction as cohesive, while the ring shear tester characterized sawdust and shavings as cohesive and agreed on pellets. Salehi and co-authors summarize the literature as showing that Jenike's arching theory "works well with sawdust but is largely conservative for biomass with larger particle sizes", and they offer one possible explanation, which they put as an argument one could make: available shear testers were explicitly designed for powders and relatively fine granular materials and may not suit coarse, irregularly shaped particles. Their Jenike design openings were larger than the openings they measured, and their "safety factor of the Jenike approach" of 0.323 is their own comparison for those chips and that hopper, not a design factor. Schulze's textbook Powders and Bulk Solids (2nd ed., 2021) covers the testers and their use in silo design at book level (Jenike 1964; ASTM D6128-22, 1.1-1.3 and 5.1; ASTM D6773-22, 1.1 and 1.3; Stasiak et al. 2019, §4; Salehi et al. 2024, §1 and §5; Schulze 2021).

How does moisture change a wood residue's classification?​

Moisture enters a wood residue's classification twice: in its weight and in its flow. The USDA Wood Handbook defines moisture content as the mass of water divided by the mass of the ovendry wood, times 100% (Eq. 4-1), and states that the moisture content of green wood can range from about 30% to more than 200%. Briggs's Table 7-3 states moisture on a wet basis instead, and his worked log at 80% moisture on an oven-dry basis is 44.4% on a wet basis. As engineering reasoning, a moisture figure without its basis is a second source of error alongside an unqualified bulk density.

Moisture also changes flow, and the studies disagree in a way that depends on the material and the measure:

  • Pine chips in hoppers. Lu and co-authors, at Georgia Institute of Technology and Idaho National Laboratory, found a fiber saturation point of 30% for loblolly pine chips and found compressibility and shear resistance rising with moisture content in meso-scale tests. Their hopper flow simulations suggest the effective discharge rate falls by 50% as moisture content rises up to the fiber saturation point, beyond which moisture has no apparent influence.
  • Pine sawdust and shavings in shear testers. Stasiak and co-authors, with materials prepared at 10 to 50% moisture content, found the moisture content did not significantly influence the flowability index of their materials.

As engineering reasoning, the two results are not in conflict: one is a simulated discharge rate for chips in a hopper, the other a flowability index for finer materials in a shear cell. For design, the classification should be stated at the moisture the plant will actually deliver, on a named basis (USDA Forest Products Laboratory 2021, Ch. 4, Eq. 4-1; Briggs 1994, Ch. 7; Lu et al. 2023, abstract; Stasiak et al. 2019, Conclusion).

What does 550 say about testing the actual material?​

CEMA's own wording favors testing over its tables. The 2020 edition added a foreword disclaimer on material properties, which states that the listed properties are guidelines and that testing of the specific bulk material is recommended. ANSI's 2025 notice of the revision gives the same position in the revision's project need: "Since many materials are handled in their natural state, the material properties in this publication should be considered guidelines, and testing of the specific bulk material being handled is strongly recommended."

The standard's test procedures chapter gives CEMA-defined methods for properties in the code, bulk density among them. Other published methods cover the properties wood residues raise:

  • Bulk density of solid biofuels: ISO 17828:2025 uses a standardized measuring container, which is filled and subjected to a defined shock before weighing, and gives a separate method for material above a 63 mm nominal top size.
  • Graded wood chips: ISO 17225-4:2021 includes informative guidance on the bulk density of graded wood chips in an annex.
  • Flow properties: the ASTM shear methods above, whose significance statements say that by measuring the flow properties of bulk solids and designing bins and hoppers on them, "most flow problems can be prevented or eliminated".

As engineering reasoning, a sample of the plant's own material, tested at its delivered moisture, is the way to replace a guideline value with a design value (ANSI/CEMA 550-2020; ANSI Standards Action Vol. 56 No. 08, 2025, PINS; ISO 17828:2025; ISO 17225-4:2021; ASTM D6128-22, 5.2).

How does a material class carry into sensing and controls?​

The material class predicts which failure the controls will see first. For pine chips in a wedge-shaped hopper, Lu and co-authors report that the hopper outlet width linearly controls the mass flow rate, that the initial packing determines whether flow is smooth or surging, and that surcharge-induced compaction creates flow impedance. The federal Feedstock-Conversion Interface Consortium's failure analysis of research-scale biomass equipment found clogging the highest-risk failure of a closed drag chain conveyor, detected by a trained observer monitoring feed rates and conveyor motor consumption and by level sensors. It named in-line moisture sensors as one of the primary mitigation strategies for downtime events caused by variable moisture and particle sizes.

As engineering reasoning, each part of the code maps to a signal:

  • Bulk density and moisture: in-line moisture measurement at intake and motor-current trending, since a wetter, denser feed raises load before it plugs.
  • Flowability and interlocking letters: level sensing above and below each bin outlet and feeder, with a plug alarm when the level above rises while the feeder runs.
  • Elevated temperature and self-heating: temperature measurement in surge storage and at hot discharges.
  • Dust and explosibility letters: interlocks between the conveyor and its dust collection and isolation devices.

Clearing a plug or entering a bin can fall under two federal general-industry rules. Under 29 CFR 1910.146, a confined space is one large enough and so configured that an employee can bodily enter and perform assigned work, with limited or restricted means for entry or exit, and not designed for continuous employee occupancy, and the rule names silos, storage bins, and hoppers among spaces that may have limited means of entry. A confined space is permit-required if it has one or more listed characteristics, among them that it "Contains a material that has the potential for engulfing an entrant" or "Contains or has a potential to contain a hazardous atmosphere". Under 29 CFR 1910.147, servicing and maintenance include unjamming where the employee may be exposed to the unexpected energization or startup of the equipment or release of hazardous energy, and (d)(5)(i) requires that, after lockout or tagout devices are applied to the energy isolating devices, all potentially hazardous stored or residual energy be relieved, disconnected, restrained, and otherwise rendered safe. UTEC Industrial, a Rockwell Automation Recognized System Integrator, builds UL 508A control panels and integrates Allen-Bradley ControlLogix and CompactLogix control with the sensing listed above (Lu et al. 2021, abstract; Emerson et al. 2022, p. 29 and Appendix G Table 15; OSHA 29 CFR 1910.146-1993, paragraph b; OSHA 29 CFR 1910.147-1989, paragraph d.5.i).

What should a material data sheet state, and where does it enter the build chain?​

The material data sheet is the first document in the build chain, and each item on it is used at a later link:

  • Name and source: chips (clean or whole-tree), sawdust, bark (and debarker type), shavings, hog fuel, or the mineral, used at design to select the conveyor family.
  • CEMA 550 code, or a sample for classification, used at engineering to size flights, buckets, belts, and trough loading.
  • Bulk density, loose and compacted, wet or oven-dry, with moisture on a named basis, used to size drives, chains, and structure.
  • Largest lump and size distribution, including the fraction finer than about 6 mm, used for outlet, feeder, and test-method choice.
  • Abrasiveness and temperature, used at engineering and fabrication for liner, plate, and component choice.
  • Hazard characteristics: dust, explosibility, self-heating, and stickiness, used at controls for interlocks, isolation, and monitoring.

As engineering reasoning, a data sheet missing any of these leaves the designer to assume a tabulated guideline value where CEMA recommends testing. UTEC Industrial performs factory acceptance testing and on-site commissioning, which is where the feeder rates, level trips, and interlocks set from the data sheet are demonstrated on the equipment (ANSI/CEMA 550-2020; ISO 17828:2025; ASTM D6128-22, 5.3; Briggs 1994, Ch. 7).

Related Articles

References​

  • ANSI/CEMA 550-2020: Classification and Definitions of Bulk Materials. Conveyor Equipment Manufacturers Association, 2020.
  • American National Standards Institute. ANSI Standards Action, Vol. 56, No. 08. ANSI, February 21, 2025.
  • Conveyor Equipment Manufacturers Association. ANSI/CEMA Standards. CEMA (undated web documentation, accessed September 2026).
  • CEMA No. 375-2017: Bucket Elevator Book: Best Practices in Design, 1st ed. Conveyor Equipment Manufacturers Association, 2017.
  • ISO 3435:1977: Continuous mechanical handling equipment — Classification and symbolization of bulk materials. International Organization for Standardization, 1977.
  • ISO 17828:2025: Solid biofuels — Determination of bulk density. International Organization for Standardization, 2025.
  • ASTM D6128-22: Standard Test Method for Shear Testing of Bulk Solids Using the Jenike Shear Tester. ASTM International, 2022.
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  • NFPA 660-2025: Standard for Combustible Dusts and Particulate Solids. National Fire Protection Association, 2025.
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  • Schulze, Dietmar. Powders and Bulk Solids: Behavior, Characterization, Storage and Flow, 2nd ed. Springer, 2021.
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  • Lu Y, Jin W, Klinger JL, Dai S (2023). "Effects of the Moisture Content on the Flow Behavior of Milled Woody Biomass." ACS Sustainable Chemistry & Engineering, 11(31), 11482-11489.
  • Lu Y, Jin W, Klinger J, Dai S (2021). "Flow and Arching of Biomass Particles in Wedge-Shaped Hoppers." ACS Sustainable Chemistry & Engineering, 9(45). DOI 10.1021/acssuschemeng.1c05628
  • Emerson, R., P. Burli, L. Vega-Montoto, and T. Bhattacharjee. Failure Mode and Effects Analysis Summary Report (FY22). DOE/EE-2734. U.S. Department of Energy, Feedstock-Conversion Interface Consortium, 2022.
  • OSHA 29 CFR 1910.146-1993: Permit-Required Confined Spaces. U.S. Department of Labor, 1993.
  • OSHA 29 CFR 1910.147-1989: The Control of Hazardous Energy (Lockout/Tagout). Occupational Safety and Health Administration, 1989.

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