Machine Designers Reference. J. Marrs
the science of designing a workspace environment and machine interface to fit the user. Like safety, ergonomics is a broad topic that is the subject of many excellent texts. Poor ergonomics can result in either acute injury or long-term repetitive use injury. It is essential to consider not only the physical geometry and arrangement of workspaces, but also the forces involved and the frequency in which they are encountered during a given workday. Actions like bending, reaching, grasping, lifting, turning, positioning, applying force, and releasing all must be conducted with proper ergonomics to achieve efficiency and assure a safe workplace.
Ergonomics issues one must consider when designing equipment and work space are:
•Operator body and workspace dimensions
•Operator performance capabilities and limitations
This section is meant to be an introductory reference only. The reference books and industry standards cited at the end of this chapter provide detailed information for the reader on these subjects.
•A list of books and industry standards are provided in Section 2.5 of this chapter.
BODY AND WORKSPACES DIMENSIONS
The figures and tables in this section represent only one set of data; many sets are available that generally represent information that has been accumulated by various groups and individuals over the years. Care must be taken to choose and refer to the appropriate information source for any given design task at hand.
BODY DIMENSIONS
Anthropometry is the study of body dimensions for the purpose of understanding human variations. Figures 2-1, 2-2, and related information are examples of anthropometric information available from various sources. The information in Tables 2-1 and 2-2 provide basic body dimensions representative of the adult civilian population in the United States. This type of information is particularly useful when designing workstations and controls and when determining various task assignments.
WORKSPACE, CLEARANCES, ENCLOSURES AND ACCESS OPENINGS
Machine workspaces for routine operation as well as maintenance and repair must be designed to accommodate operators as well as maintenance personnel. Interior access openings must be sized appropriately for the various purposes for accessing the area, whether for normal machine operation or for maintenance or repair. Openings must not only accommodate those parts of the individual needing access, but must also be large enough to accept whatever equipment the individual must insert or remove as well. Ideally, access openings should provide not only room for reaching and working, but also additional space for an individual to be able to see.
Figure 2-1: Body Dimension Illustrations
Additional anthropometric and workspace information can be found in industry standards ISO 14738, EN 547-1, EN 547-2, ISO 15534-1, ISO 15534-2 and ISO 15534-3 (see details in Section 2.5), ISO 14738 being particularly useful for designers of machinery.
Table 2-1: Body Dimensions, Inch
Adapted from: Pheasant, 20021
OPERATOR PHYSICAL CAPABILITIES AND LIMTATIONS: REACHING / GRASPING / MOVING / LIFTING
Ergonomics relative to individuals’ physical capabilities and limitations also plays an important role in the design of machinery, not only to make the machine functional and productive, but also to assure the workspace is reasonably safe. Issues of physical movement strength and limitations, and exposure to surrounding environmental conditions must be understood during the design process.
Table 2-2: Body Dimensions, Metric
Adapted from: Pheasant, 20022
Equipment, tools, or work areas that require reaching, grasping, or moving things must be designed to accommodate the strength capabilities and limitations of ordinary operators — being mindful of gender differences. Gender differences must be considered and planned for.
The illustrations in Figure 2-3 are examples of ergonomic information that can be found in the available literature (see citations at the end of this chapter). These types of illustrations are often accompanied with specific force (weight) limits, often representing the 5th percentile adult male (the weakest fifth percent) for the cited population group. Industry standard EN 1005-3 recommends that force limits for professional users correspond to the 15th percentile of the whole adult population (males and females) between 20 years and 65 years of age. For machines intended for domestic users, forces should be limited to the 1st percentile (the weakest one percent) of the same total population. This EN 1005-3 standard also provides de-rating factors for movement velocities, movement frequencies, and application durations. For an understanding of capabilities/limits one might expect in a given situation, refer to and compare data from several sources, understanding what the information and chart numbers represent.
Figure 2-2: Clearances for Work Spaces
Source: Sanders and McCormick, 19933
Note: These clearances may qualify as confined spaces and require a safety assessment.
Among the types of movement and force limits to be considered include (but are not limited to):
•Pedal force limits
•Handwheel rotational force limits
•Hand-cranking force limits
•Finger-grip/squeeze and hand-grip/squeeze strength limits
•Hand-grasped item twisting force limits
•Weight carrying limits in various carrying modes
•Arm — up, down, in, out, pull, push — force limits
•Whole body pushing, pulling force limits
•Lifts above shoulder height weight limits
Table 2-3: Access Openings
Source: MIL-HDBK-759
Figure 2-3: Reaching — Grasping — Moving Illustrations
Lifting is often required of machine operators while operating equipment or moving material into or out from a machine. An operator may be required to perform many lifts during a normal shift. There are limits to how much total lifting a person can do in a given workday, and it is dependent on the geometry of the lift, the weight involved, and the frequency of performance. When determining if a lifting task is reasonable, the Applications Manual for the Revised NIOSH Lifting Equation should be consulted (available