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Unit-Load vs. Bulk-Solids Handling: The CEMA Split and Why It Drives Design

Unit-load handling moves discrete items that keep their integrity as one load, while bulk-solids handling moves loose material such as sand, cement, or coal as a continuous flow. 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 explains where the two families divide, how the Conveyor Equipment Manufacturers Association (CEMA) organizes its standards and reference books along the same line, and why the choice between them sets the conveyor type, the design load, the controls, and the questions a specifying engineer has to answer. A heavy handling system is built along one chain, design → engineering → parts machining → fabrication → assembly → weld fatigue → stress relief → drives → controls → tuning → monitoring, and deciding whether a material moves as units or in bulk is the first decision on that chain.

What separates unit-load handling from bulk-solids handling?​

The dividing line is the form in which material is presented for handling, not the industry. Kay's material-handling course notes at North Carolina State University sort materials into three categories in Table 1:

Material categorySolidLiquidGas
Individual unitsPart, subassembly——
Containerized itemsCarton, bag, tote, box, pallet, binBarrelCylinder
Bulk materialsSand, cement, coal, granular productsLiquid chemicals, solvents, gasolineOxygen, nitrogen, carbon dioxide

Kay then maps each category to an equipment family. Individual units and containerized items produce a discrete material flow, which is handled as unit loads with unit handling equipment. Bulk materials produce a continuous material flow, which is handled with bulk handling equipment. Kay's Figure 1 makes the point with a single dry bulk material shown two ways: bagged on pallets and moved by boxcar, pallet, and fork truck as unit loads, or moved loose by hopper car, pneumatic conveyor, and bulk storage bin.

The same notes define material handling itself, quoting Coyle and colleagues, as short-distance movement that usually takes place within the confines of a building such as a plant or a warehouse and between a building and a transportation agency. Material handling can create time and place utility, as distinct from the form utility created by manufacturing, that is, fabrication and assembly. In a process plant both families can run side by side: a sawmill moves lumber as packages but its chips and sawdust in bulk, and a steel mill moves slabs and coils one at a time while its raw materials arrive loose. The warehouse form of the unit load, the standard pallet, is covered in Industrial vs. Warehouse Material Handling for Heavy, Hot Loads and is not repeated here (Kay 2012, §1, Table 1 and Figure 1).

How is a unit load defined, and what counts as one in heavy industry?​

Kay defines a unit load as either a single unit of an item, or multiple units so arranged or restricted that they can be handled as a single unit and maintain their integrity. The Institute of Industrial Engineers' 2000 terminology gives a shorter form, "any load configuration handled as a single item," as quoted in the CIRRELT equipment-selection working paper by Ahmed Bouh and Riopel. Both definitions turn on handling the load as a single item, and Kay's adds that it maintains its integrity.

Kay lists these basic ways of restraining a unit load:

  • Self-restraining: one or more units that keep their integrity when handled as a single item, such as a single part or interlocking parts
  • Platforms and sheets: pallets, skids, and slipsheets
  • Containers: reusable tote pans, pallet boxes, bins, and bulk containers, or disposable cartons, bags, and crates
  • Racks
  • Load stabilization: strapping, shrink-wrapping, stretch-wrapping, glue, tape, and wire

Kay also names four ways of moving a unit load: a lifting device under the mass of the load, such as a pallet and fork truck; a lifting element inserted into the body of the load, such as a coil of steel; squeezing the load between two lifting surfaces; and suspending the load from a hoist and crane.

As engineering reasoning, many heavy-industry loads sit at the self-restraining end of that list. A hot-rolled coil, an aluminum ingot, a mining mill liner, a hydroelectric wicket gate, or an airframe section is its own unit load, with no pallet or container at all. Kay notes that self-restraining materials can be formed into a unit load with no equipment; for design, the handling device then has to engage the part itself, by a mandrel through a coil eye, by a lifting beam, by clamps, or by a cradle, so the part's geometry and center of gravity set the equipment (Kay 2012, §1, The Unit Load Concept; Ahmed Bouh and Riopel 2015, §II.B).

What does "the CEMA split" actually refer to?​

CEMA is a trade association. Its online store, CEMASTORE, sorts its publications into two subject categories, Unit Handling and Bulk Handling, alongside other categories such as Books & Handbooks, PDFs, and Standards, and each subject category has a major reference book:

  • Bulk handling: Belt Conveyors for Bulk Materials, known in the industry as "The Belt Book." The 7th edition was first issued in 2014; its second printing, published in August 2020, runs 815 pages and collects the errata. The first edition, in 1966, ran 331 pages. The book covers basic data, fundamentals of design, belt conveyor capacities, belt selection, general applications, and the Ki and Ai factors.
  • Unit handling: the CEMA Application Guide for Unit Handling Conveyors, 2nd edition, 2016, 680 pages. CEMA describes it as intended to help designers, engineers, building contractors, operations managers, and procurement managers design, develop, or purchase conveyor systems that integrate into customer-defined systems. Its product chapters cover definitions, design, application and selection, options and accessories, maintenance, and safety, and it discusses sub-component systems such as motors, gearing, belting, chain, bearings, rollers, and pulleys.

At least one document spans both families. ANSI/CEMA 102-2022, Conveyor Terms & Definitions, is the tenth edition of CEMA's glossary since the first in 1952; it runs 172 pages, lists and defines over 1,500 terms with 100 illustrations, illustrates many of the more than 150 types of conveyors, and CEMASTORE files it under both Unit Handling and Bulk Handling.

"The CEMA split" in this article means exactly that observable division of CEMA's store into two subject families. CEMA's standards page states no numbered rule that assigns every conveyor to one side, and none is implied here. As engineering reasoning, the practical value of the division is that it tells a project engineer which body of reference practice to open first: the Belt Book and the bulk-material standards for loose solids, or the unit-handling guide and the package-conveyor standards for discrete loads (Conveyor Equipment Manufacturers Association, Belt Conveyors for Bulk Materials, 2020; CEMA Application Guide for Unit Handling Conveyors, 2016; ANSI/CEMA 102-2022).

Which CEMA standards cover bulk materials, and which cover unit or package conveyors?​

CEMA's list of the American National Standards it currently publishes is a single list, but it can be sorted the same way. By subject, the bulk-material documents are:

  • ANSI/CEMA 300-2021, screw conveyor dimensions
  • ANSI/CEMA 350-2021, Screw Conveyors for Bulk Materials
  • ANSI/CEMA 550-2020, Classification and Definitions of Bulk Materials

The package and unit conveyor documents on the list, all numbered in the 400s, are:

  • ANSI/CEMA 401-2020, non-powered roller conveyors
  • ANSI/CEMA 402-2003 (R2020), belt conveyors
  • ANSI/CEMA 403, 404, and 405, belt-driven live roller, chain-driven live roller, and slat conveyors, each 2003 (R2020)
  • ANSI/CEMA 406-2020, lineshaft-driven live roller conveyors
  • ANSI/CEMA 407-2019, motor-driven live roller (MDR) conveyors

The list also carries the ANSI/CEMA 102-2022 glossary and two component standards for pulleys, B105.1-2021 for welded steel conveyor pulleys and 501.1-2021 for welded steel wing pulleys. CEMA's page states no numbering rule, so the grouping above describes the current list, not an official series convention.

For a bulk system, CEMA calls 550 a foundational reference: it describes the standard as a systematic method for classifying and describing bulk materials by their physical characteristics and handling properties, defining the terminology, test procedures, and classification criteria that are fundamental to the proper selection and design of bulk handling equipment, and it states that these classifications are essential for determining conveyor type, size, capacity, and operating parameters. CEMA 350 covers engineering and application practice for screw conveyors handling those materials. Both are paid standards, and this article cites them at title and scope level only (Conveyor Equipment Manufacturers Association, ANSI/CEMA Standards, accessed September 2026; ANSI/CEMA 550-2020; ANSI/CEMA 350-2021).

Which conveyor types handle unit loads, and which handle bulk solids?​

Kay classifies conveyors three ways: by the type of product handled, unit load or bulk load; by location, in-floor, on-floor, or overhead; and by whether loads can accumulate on the conveyor. Applying the first of those to Kay's Table 7 gives a working sort:

HandlesConveyor types (Kay, Table 7)Notes from Kay
Unit loadsWheel, roller, chain, slat, flat beltRoller: loads need a rigid riding surface, and at least three rollers must support the smallest load at all times. Slat: used for heavy loads or loads that might damage a belt. Chain: no accumulation.
Bulk solidsTroughed belt, bucket, screw, vibrating, magnetic beltTroughed belt: the loaded belt conforms to the troughed idlers. Bucket: vertical or inclined paths. Screw: a helix pushes loose material along a trough or tube.
BothChute, pneumaticChute: inexpensive, but the position of items is difficult to control. Pneumatic: material is fully enclosed in tubes.

Two entries matter for heavy plants. Gravity roller conveyors are Kay's heavy-duty alternative to wheel conveyors, but their slope for gravity movement depends on load weight, so, as engineering reasoning, a line that carries both light and heavy parts may not run both correctly by gravity. And the oscillating conveyor, which Kay describes as similar in construction to the vibrating conveyor but running at a lower frequency and larger amplitude, is used to convey larger objects such as hot castings; it sits on the boundary, a bulk-style machine moving discrete parts (Kay 2012, §5, Table 7).

Why do unit-load and bulk systems end up with different equipment families?​

Largely because the flow pattern, not the material alone, decides which machine is feasible; that is engineering reasoning, and the Material Handling Institute's College-Industry Council guide to equipment selection reproduces a comparison after Dunning that sets the three big transport families side by side:

  • Conveyors move uniform loads continuously over fixed paths, at high volume, from fixed origin to fixed destination; the material may be an individual item, a unit load, or bulk.
  • Cranes and hoists move varying loads intermittently to any point within a fixed area; the weight is heavy, the shape irregular, and the size mixed.
  • Industrial trucks move mixed or uniform loads intermittently over various paths, with maneuvering as the primary function.

In the comparison's material-type row, only conveyors list bulk. As engineering reasoning, loose material has no shape to grip and has to be carried by a trough, belt, bucket, flight, or pipe, while unit loads appear in all three columns, and heavy unit loads in a plant can end up on the crane-and-hoist or fixed-path side because they are too heavy for a truck and too varied for a standard conveyor.

At least one equipment-selection study treats the two families as separate problems. Ahmed Bouh and Riopel reviewed 27 selection papers and found that the largest prior selection system used 50 equipment types. Their new classification lists 122 unit-load equipment types in nine categories, and it deliberately excludes bulk-load equipment such as pipes and bulk load conveyors because its scope is individual-unit-load plants and warehouses. Kay's selection chapter adds the working method: select first at the intermediate level of equipment types, which narrows the choice to about 15 to 50 types, then test technical feasibility before economic feasibility (Peters et al. 1998, General Considerations; Ahmed Bouh and Riopel 2015, §II.C–D; Kay 2012, §9).

How does the unit-or-bulk decision change the design load and duty?​

A unit-load machine is designed around the heaviest single piece. Its governing inputs are the piece weight, its footprint and contact points, its center of gravity, and how often a piece arrives. A bulk machine is designed around a flow: the material's classification and physical characteristics under CEMA 550, the rate it must carry, and the handling properties that affect how it conveys. As engineering reasoning, the two lead to different structures, different wear patterns, and different duty ratings.

In lifting equipment the difference shows in duty classes. The CICMHE's 1998 introductory guide to equipment selection, which describes itself as educational rather than a basis for detailed design, grouped electric hoists into five duty classes. The table reproduces that 1998 guide's classification; it is not a current hoist rating scheme:

Hoist duty classTime busyStarts per hourExample service in the guide
Infrequent or standby12–25%75–100Power houses and utilities with infrequent handling
Lightnot givennot givenLight machine shops and fabricating industries
Standard25–50%150–200General machine shops, fabrication shops, assembly, storage, and warehousing
Heavyabout 50%up to 300High-volume steel warehousing, some machine shops and fabricating plants, mills, and foundries
Severeclose to 100%up to 600Bulk handling with buckets, magnets, or other heavy attachments

As engineering reasoning, in the guide's severe class a hoist in effect becomes a bulk-handling machine, running a bucket or magnet almost continuously. The guide also notes that the primary reason cited for hoist failure is the failure to consider the duty environment of the hoist. For current practice, crane duty classes for multiple-girder top-running bridge and gantry cranes are set in CMAA Specification No. 70-2025, whose 2025 revision expands its commentary on duty classes and expected service life, and ASME BTH-1-2023 sets the minimum structural, mechanical, and electrical design criteria for below-the-hook lifting devices; how plant duty differs from warehouse duty is set out in Industrial vs. Warehouse Material Handling for Heavy, Hot Loads (Peters et al. 1998, Cranes and Electric Hoists; ANSI/CEMA 550-2020; CMAA 70-2025; ASME BTH-1-2023).

Where does the same material cross from bulk to unit handling?​

A plant may convert material from one family to the other, and the conversion point is its own piece of equipment. Kay lists bags and bulk load containers among unit load formation equipment, both used to unitize and protect bulk materials, and lists palletizers, including manual palletizing, robotic pick-and-place palletizers, and conventional stripper plate palletizers, as the machines that build unit loads. In the other direction, unloading bags or bulk containers into a hopper returns the material to a continuous flow.

Kay lists the disadvantages of unit loads, which, as engineering reasoning, bear on where the conversion point is placed:

  • Time is spent forming and breaking down the unit load.
  • Containers, pallets, and restraining materials cost money.
  • Empty containers or pallets may need to be returned to their point of origin.

Against those costs, Kay lists the gains: more items handled at once, fewer trips, potentially lower handling cost, loading time, and damage, and the use of standardized handling equipment. A biomass plant bagging finished pellets, a mineral plant filling bulk bags with concentrate, or a sawmill bundling lumber into packages each pays the forming cost once so that everything downstream can be handled as a unit. As engineering reasoning, the forming and breaking stations are also places where people work close to moving equipment, which makes them candidates for automation. UTEC Industrial integrates FANUC robotic cells, including vision, with a FANUC design and engineering partner (Kay 2012, §1, The Unit Load Concept, and §3, Table 4).

Which safety rules apply to unit-load and bulk conveyors?​

ASME's conveyor safety standard does not split the two families. ASME B20.1-2024, Safety Standard for Conveyors and Related Equipment, applies to the design, construction, installation, maintenance, inspection, and operation of conveyors and conveying systems in relation to hazards, and its publisher's scope states that the conveyors may be of the bulk material, package, or unit handling types, whether the installation is designed for permanent, temporary, or portable operation. It excludes conveyors designed for, installed for, or used primarily for moving people. It is a paid standard, and this article cites it at scope level.

OSHA's general-industry materials-handling rules in 29 CFR 1910 Subpart N reach the two families from different directions:

  • Unit loads in storage. Paragraph 1910.176(b) requires that bags, containers, bundles, and similar items stored in tiers be stacked, blocked, interlocked, and limited in height so that they are stable and secure against sliding or collapse.
  • Bulk spillage and dust. Paragraph 1910.176(c) requires storage areas to be kept free from accumulations of materials that constitute hazards from tripping, fire, explosion, or pest harborage, a housekeeping problem that spilled bulk solids can create.
  • Heavy unit loads lifted overhead. Subpart N also holds the overhead and gantry crane rule, 1910.179, and the sling rule, 1910.184.

Clearing a jammed bulk conveyor is a hazard in its own right. OSHA's lockout/tagout rule, 29 CFR 1910.147, defines servicing and maintenance to include unjamming and cleaning, so a chute or screw conveyor cleared by hand is servicing under that definition, not a production task (ASME B20.1-2024; OSHA 29 CFR 1910 Subpart N-2019, §1910.176; OSHA 29 CFR 1910.147-1989, §1910.147 paragraph b).

What sensing and controls does each handling family need?​

A unit-load system has to know where each load is. A bulk system has to know how much is flowing and whether the flow has stopped where it should not. That difference shapes the controls design.

  • Unit tracking. Kay's fifth equipment category, identification and control equipment, covers bar codes, radio frequency identification tags, voice recognition, magnetic stripes, machine vision, and portable data terminals, all used to collect and communicate the information that coordinates material flow. On a heavy line the same job can be done by presence sensors and encoders that follow each coil, slab, or casting from station to station.
  • Accumulation logic. Kay notes that live roller conveyors can use a force-sensitive transmission to disengage rollers for accumulation, while chain and slat conveyors cannot accumulate at all. As engineering reasoning, a PLC controlling a heavy chain transfer then has to stop the upstream section before loads touch, rather than letting them queue.
  • Positioning and load sensing. A unit transfer that stops a heavy part at a machine tool or furnace door needs position feedback and, where overload is possible, load cells. A servo axis indexing parts can use drives such as the Allen-Bradley Kinetix 5700, which supports DSL and Hiperface encoder feedback and features a safe torque-off function.
  • Program structure. Logix 5000 controller tasks can be configured as continuous, periodic, or event, and a periodic task executes automatically at a preconfigured interval, so interlock and sequencing logic for a line of conveyors can be placed in a periodic task.
  • Safety functions and electrical basis. Emergency stops, guarded-zone entry, and jam-clearing access run on safety-related control parts designed to ISO 13849-1:2023. IEC 60204-1:2016 applies to the electrical, electronic, and programmable electronic equipment of machines not portable by hand while working, including a group of machines working together in a co-ordinated manner, which is how a line of bulk conveyors can be built.
  • Energy isolation for jam clearing. On a bulk line the controls design has to support lockout: OSHA 1910.147 states that push buttons, selector switches, and other control-circuit-type devices are not energy-isolating devices, so a stopped screw or belt is not an isolated one.

UTEC Industrial, a Rockwell Automation Recognized System Integrator, builds this sensing and Allen-Bradley ControlLogix and CompactLogix control into the handling systems it supplies (Kay 2012, §2 and §5; Rockwell Automation 2198-UM002E-EN-P, Kinetix 5700; Rockwell Automation 1756-RM094N-EN-P-2025; ISO 13849-1:2023; IEC 60204-1:2016; OSHA 29 CFR 1910.147-1989).

How does the unit-or-bulk choice carry through the design-to-monitoring chain?​

Every link of the chain, design → engineering → parts machining → fabrication → assembly → weld fatigue → stress relief → drives → controls → tuning → monitoring, is set differently for the two families:

  • Design and engineering: a unit system starts from the heaviest piece and its center of gravity; a bulk system starts from the material's CEMA 550 classification and the required rate.
  • Parts machining: unit conveyors depend on roller, sprocket, and shaft fits that carry concentrated piece loads; bulk conveyors depend on pulleys, screw flights, and shafts that run continuously under a distributed load.
  • Fabrication: unit decks, slats, and cradles are shaped to the part; bulk troughs, chutes, and hoppers are shaped to the flow.
  • Assembly, weld fatigue, and stress relief: a unit line sees a discrete load cycle each time a piece lands, while a bulk line sees near-continuous running, so the welded frame details are checked against very different cycle counts. Frames that carry machined bearing seats are stress-relieved before final machining to help those seats stay where the drawing puts them.
  • Drives, controls, and tuning: a unit axis is tuned to stop a known mass at a known point; a bulk drive is sized to start a loaded belt or screw and to run for hours.
  • Monitoring: unit systems trend cycle counts and positioning faults; bulk systems trend running hours, motor current, and stoppages.

Kay's selection method fits this sequence: technical feasibility first, then economic feasibility among the feasible types. UTEC Industrial stress-relieves and machines the welded frames of the conveyors and transfer equipment it builds before assembly (Kay 2012, §9; ASME B20.1-2024; ANSI/CEMA 550-2020).

What should a specifying engineer state about the material before requesting equipment?​

The first line of a handling specification should say which family the material belongs to, because it decides which data the supplier needs. Kay lists the characteristics of materials that affect handling: size (width, depth, height), weight (per item or per unit volume), shape (round, square, long, rectangular, irregular), and other properties (slippery, fragile, sticky, explosive, frozen). For unit loads, Kay's unit load design adds three questions: the type, size, weight, and configuration of the load; the equipment and method used to handle it; and the methods of forming and breaking it down. For in-process unit loads, Kay gives a sizing rule: single part ≤ unit load size ≤ transfer batch size ≤ production batch size.

A request for a heavy unit-load system should therefore state:

  • the heaviest and lightest piece, its dimensions, contact points, and center-of-gravity range;
  • whether the piece is self-restraining or carried on a pallet, fixture, or container;
  • arrival rate, and whether pieces may accumulate;
  • temperature and surface condition at pickup.

A request for a bulk system should state:

  • the material's CEMA 550 classification, or the data needed to classify it;
  • the required rate, the path, and the transfer points;
  • the conversion points where the material is bagged, bundled, or palletized.

The CIRRELT classification of attributes for unit-load equipment selection runs to 81 attributes, of which 15 describe the unit load, 36 the movement, 23 the equipment, and 7 the environment. UTEC Industrial performs factory acceptance testing and on-site commissioning, so the stated rates and load limits can be demonstrated before handover (Kay 2012, §1, Characteristics of Materials and Unit Load Design; Ahmed Bouh and Riopel 2015, §III.B; ANSI/CEMA 550-2020).

Related Articles

References​

  • Kay MG. Material Handling Equipment (course notes). Fitts Department of Industrial and Systems Engineering, North Carolina State University, 2012.
  • Ahmed Bouh M, Riopel D. Material Handling Equipment Selection: New Classifications of Equipments and Attributes, CIRRELT-2015-63. CIRRELT, 2015.
  • Peters BA (ed.), Malmborg C, Petrina G, Pratt D, Taylor D. An Introduction to Material Handling Equipment Selection. College-Industry Council on Material Handling Education (CICMHE), Material Handling Institute, 1998.
  • ANSI/CEMA 102-2022: Conveyor Terms & Definitions. Conveyor Equipment Manufacturers Association, 2022.
  • Conveyor Equipment Manufacturers Association. Belt Conveyors for Bulk Materials, 7th ed. (second printing). CEMA, 2020.
  • Conveyor Equipment Manufacturers Association. CEMA Application Guide for Unit Handling Conveyors, 2nd ed. CEMA, 2016.
  • Conveyor Equipment Manufacturers Association. ANSI/CEMA Standards. CEMA (undated web documentation, accessed September 2026).
  • ANSI/CEMA 350-2021: Screw Conveyors for Bulk Materials. Conveyor Equipment Manufacturers Association, 2021.
  • ANSI/CEMA 550-2020: Classification and Definitions of Bulk Materials. Conveyor Equipment Manufacturers Association, 2020.
  • CMAA Specification No. 70-2025: Specifications for Top Running Bridge and Gantry Type Multiple Girder Electric Overhead Traveling Cranes. CMAA, 2025.
  • ASME BTH-1-2023: Design of Below-the-Hook Lifting Devices. ASME, 2023.
  • ASME B20.1-2024: Safety Standard for Conveyors and Related Equipment. ASME, 2024.
  • OSHA 29 CFR 1910 Subpart N-2019: Materials Handling and Storage. U.S. Department of Labor, 2019.
  • OSHA 29 CFR 1910.147-1989: The Control of Hazardous Energy (Lockout/Tagout). Occupational Safety and Health Administration, 1989.
  • Rockwell Automation 2198-UM002E-EN-P (2018): Kinetix 5700 Servo Drives User Manual. Rockwell Automation, 2018.
  • Rockwell Automation 1756-RM094N-EN-P-2025: Logix 5000 Controllers Design Considerations. Rockwell Automation, 2025.
  • ISO 13849-1:2023: Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design. International Organization for Standardization, 2023.
  • IEC 60204-1:2016 (Ed. 6.0): Safety of Machinery -- Electrical Equipment of Machines -- Part 1: General Requirements. International Electrotechnical Commission, 2016.

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