Using An Ergonomic Approach To Moving Material Safely

Over 1 million lower back injuries occur in the workplace each year, costing industry billions of dollars in workers’ compensation claims, legal fees, lost time and production, and medical costs. At the same time, workers injured in these accidents suffer physical, mental, and financial damage. These situations occur even though training, education, warnings, and equipment to prevent most of the problems are available.

Manual material handling operations generate about 35% of the workers’ compensation claims, and back injuries account for 20% of all claims. Expenses associated with an average back injury approach $50,000. Equipment to prevent the problem is a bargain compared to this number.

Surprisingly, over 50% of all workplace injuries occur while handling light loads (less than 50 lb) using the correct lifting technique. The problem is cumulative trauma disorder (CTD), which is the result of doing the same lifting and moving task repeatedly through the 8-hr work day. Strength or size of the employee are nonfactors in the risk of CTD. Overexertion and CTD injuries cost the U.S. economy about $30 billion/yr.

Enlightened management began to realize the tremendous expense associated with the problem and began looking for ways to handle loads safely and efficiently. At the same time, unions started negotiating clauses in their contracts dealing with the injury and safety issue, and OSHA and NIOSH moved strongly to address the problem.

Ergonomics — the science of designing work and working conditions to minimize worker fatigue and discomfort — was born. The basic tenet is that each job is designed with the proper tools, equipment, training, and procedures to match the employees’ abilities and limitations. The intent is to reduce fatigue, promote comfort, and prevent injuries. The end result for the company is more productivity, better efficiency, reduced costs, and a safer and happier workforce.

The task was simplified because a great deal of equipment was already available to handle the problems associated with CTD and light load lifting, but were never promoted as “ergonomic” until the past several years. The only “new” ergonomic features developed lately relate to areas such as controls and grips, rather than a different type of lifting aid.

This article takes a close look at three of the most commonly applied ergonomic material handling products: balancing hoists, manipulators, and lift tables. The section “Other options” provides brief descriptions of several different material handling products available to assist worker health and safety.

Balancing hoists

Pneumatic balancers provide the full range of human motion for assembly operations in work stations. Contrary to a traditional hoist, they do not travel very far or lift very high.

Compressed air supplies a cushion to attain precise alignment. A pendant control puts the part in a weightless fashion close to alignment, and hand movement floats the product into final positioning. The product moves up or down about 9 in. via hand control.

Wire rope and chain models are available. Rope provides a smoother operation, while chain is the logical choice if a more stable load positioning is required, since it won’t spin. A series of brakes and safety interlocks stop downward travel in case of air loss or upward movement exceeding prescribed limits in the case of a load loss.

End effectors of several basic configurations provide the actual holding of the part. Vacuum venturi types running off the same compressed air supply as the balancer use a sucking action to hold the part. Clamp-on effectors have moving jaws to grab the item on the outside. Probe-into models insert a device within a product opening. Cradles use a device to nestle lifted products. Since many end effectors are custom designed to handle a specific product, literally thousands of versions are available.

Standard balancing hoists use 50 to 100-psi air, travel up to 200 fpm, lift 10 ft, and provide precise positioning and repetitive transferring of standard and awkward loads of 2 to 500 lb. (Some models are capable of handling 2000-lb loads.) Balancers are effective for palletizing and stacking, picking and placing, and loading and unloading in tool balancing and assembly operations.

Manipulators

These products are rigid-arm-based systems, usually mounted on the ground with a pedestal, but may be overhead or attached to a wall. Manipulators contain linkages to reach in and out of spots where overhead systems cannot operate, and move up and down while supporting the weight of the part. They have the same type of end effectors (tooling) as the balancing hoist and handle similar loads.

Manipulators are generally regulated by the operator at or near the load control devices. The load is in a near weightless condition similar to the balancer. The product performs lifting, rotating, turning, tilting, reaching, and positioning tasks. Manipulators and balancing hoists are usually designed for specific applications.

Hydraulic manipulators handle loads up to 1500 lb, provide about 5 ft of vertical lift, and reach about 9 ft. Pneumatic models achieve vertical positioning via a lift cylinder operated by compressed air. Lift capacity approaches 1000 lb, vertical lift is about 8 ft, and reach extends up to 12 ft.

Most models are pneumatic and handle much lighter loads, typically 200 lb or less. Arm movement goes downward to 45 deg, upward to 30 deg, and around 360 deg. Weight distribution of the piece to be handled, center of gravity, orientation required, and working radius determine the correct manipulator.

Lift tables

Scissor lift tables raise, lower, convey, or transport material or products between elevations. The bottom-up lifting tables are available in a large variety of standard and custom platform sizes (2 ft 3 3 or 4 ft typical), capacities (2500 to 6000 lb most common), travels (2 to 4 ft most common), and configurations. They are surface or pit mounted, fixed or mobile, and can include a tilting and/or turning mechanism.

Single-leg models are the most common type, offering stability and heavy concentrated loading.

Multiple-width tables consist of two or more single legs side-by-side with a common top and base, and are used for heavy loads.

Multiple-length types are two or more single legs end-to-end with a common top and base, and accommodate very long loads.

Multiple-height versions are two or more single-leg lifts stacked on top of each other.

Multiple-stage lift tables are two or more single-leg sets directly connected without an intervening platform and base. These units offer high travel and relatively short platform length.

Hydraulic lift tables are the most common and most economical. Hydraulic rams or cylinders are mounted in the leg set, base, or remotely.

Pneumatic models use air bags or cylinders for up and down power. They are applicable when fixed intermediate elevations are not required.

Mechanical versions use linkage, screw, lever, or counterweights for movement. They serve jobs where precise control at specific elevations or when extended periods at specific heights are required.

Key concepts

Poor handling practices associated with light load lifting cost industry billions of dollars annually.

Combining common sense and relatively inexpensive equipment greatly reduces handling costs and the chances of employee injury.

A variety of products, including balancing hoists, manipulators, and lift tables, help create a safe, efficient material handling operation.

Ergonomic economics

Ergonomic material handling equipment not only protects the health, safety, and well-being of the worker, but also provides a number of very significant economic benefits to the company.

Efficiency increases as the employee is able to move products quicker and more often. The fatigue factor is practically eliminated from the performance equation.

Lost time accidents, medical costs, and insurance premiums are cut drastically. Estimates place annual lost time costs in the billions, with back, hand, and finger injuries accounting for nearly 50% of the claims. These expense numbers are rapidly rising.

OSHA fines for violation of the General Duty Clause, which requires manufacturers to provide a safe workplace, are reduced. Fines in excess of $1 million are not uncommon.

Typical tasks

Various types of equipment are able to rotate, lift from above or below, tilt or upend, transfer, and transport products. Three types of duties are commonly performed.

Work positioning is the most common application of ergonomic material handling equipment. The operator is able to spot the load exactly where needed for production steps or transfer without bending or stretching, and with minimal effort.

Material transfer moves products from one level to another, or from one location to another nearby.

Machine feeding or unloading allows employees to match the performance characteristics of production operations.

Other options

Besides the three types of equipment discussed in this article, a number of other material handing devices perform ergonomic duties.

Expandable conveyors adjust to employee height and are used for moving material, assembly, order picking, and loading/unloading trucks and trailers.

Pallet rotators/inverters transfer loads from one pallet to another by turning the stack upside down. The equipment is used to replace damaged bottom pallets, pull out crushed or broken goods from the base of stacked loads, and transfer loads from incoming/outgoing shipments.

Stackers are manually powered lift trucks with platforms or forks used for transporting and elevating wire containers, totes, crates, skids, and pallets up to 12 ft. Loads up to 3000 lb and 48-in. square are handled.

Tilters lift containers, boxes, and baskets of parts and products from the floor and angle them toward the worker for loading or unloading. Proper positioning eliminates bending, stretching, reaching, and unnecessary lifting. Portable and stationary versions are available.

Vacuum assists use regenerative blowers to lift loads up to 1000 lb, although usually less than 50 lb. A variety of interchangeable vacuum heads are available to match the shape and surface of sacks, cartons, drums, or sheets.

Workstation cranes lift loads from 150 to 4000 lb, with easy push or powered horizontal movement. The enclosed track crane is used for precise load positioning, and is available in single or double girder bridge, jib, and monorail designs. Hoists, balancers, vacuums, and manipulators with a variety of end effecters provide the vertical lift.

Workstations with vertical adjustment of the height and work surface match the employee size and product configuration. The workstation is designed for ergonomic operator comfort and performance, may include a mechanical assist for lifting, and is properly lighted.

Manual moving

A number of ergonomic factors should be considered when performing manual material handling tasks. The basic idea is to minimize body motions and reduce product weight and force.

Body

Reduce bending by providing equipment that matches the employee with the correct working height.

Reduce twisting by having all tools in front of the employee, and a layout with sufficient workspace for the whole body to turn.

Reduce reaching by keeping tools and materials close to the employee, reduce the size of objects handled, and provide equipment to raise, lower, or rotate material.

Product

Reduce lifting/lowering by providing equipment that matches the proper working level; cutting the size, capacity, or weight of containers; or handle mechanically.

Reduce pushing/pulling and carrying by using powered or nonpowered equipment, shortening the movement distance through improved layout or relocation, or cutting load weight.

Acknowledgements

Plant Engineering would like to thank Ingersoll-Rand Co. and Zimmerman Handling Systems for their special contributions to the development of this article.

We also extend our appreciation to the following companies for providing the illustrations used in this article: cover photo, Advance Lifts; balancing hoist, Zimmerman Handling Systems; manipulator, Dalmec; and lift table, Lee Engineering.

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