Civil Engineering Reference
In-Depth Information
• Avoid sharp edges on the work table and part bins that may irritate the wrists when the parts are
procured; keep reaches within 50 cm (20 in) from the work surface so that the elbow is not fully
extended
Guidelines for process engineering
• Allow machinery to do repetitive tasks and leave variable tasks to human operators
• Provide fixtures that hold parts together during assembly, and which can present the assembly
task at a convenient angle to the operator
• Minimize time pressure or pacing pressure by allowing operators to work at their own paces
Guidelines for product design
• Minimize the number of screws and fasteners used in the assembly
• Minimize the torque required for screws
• Locate fasteners and screws at “natural” angles so they are easy for the operator to insert
• Design a product with large parts to permit gripping with fingers and palm instead of pinching
11.5
HAND TOOL DESIGN
Hand tool design affects the incidence of musculoskeletal disorders. Below we will
explain some of the issues in designing and selecting good hand tools.
Hand tools have been used since the beginning of humankind, and ergonomics was
always a concern. Tools concentrate and deliver power, and aid the human in tasks such
as cutting, smashing, scraping, and piercing. Various hand tools have been developed
since the Stone Age, and the interest in ergonomic design can be traced back in history
(Childe, 1944; Braidwood, 1951).
During the last century there has been one important modification: many hand tools
are now powered. The forces are greater, and thus the opportunities for injuries are also
greater. In this chapter we give some guidelines for designing hand tools. There are
several issues. A hand tool must
• Fit the task
• Fit the user and hand
• Not create injuries
There are two basic grips: the power grip and the precision grip (Figure 11.3). In the
power grip, the hand makes a fist with the forefingers on one side and the thumb reaching
around. There are three different categories of power grip that are differentiated by the
direction of the force: (1) force parallel to the forearm, e.g., a saw; (2) force at an angle to
the forearm, e.g., a hammer; and (3) torque about the forearm, e.g., a screwdriver (Konz,
and Johnson, 2004).
For precision grips there are two subcategories: (1) the internal precision grip where
the tool is held inside the hand, e.g., a table knife; and (2) the external precision grip
where the tool is pinched by the thumb against the index finger and middle finger, e.g., a
pen.
A hand tool can often be designed in different ways, since there are different ways of
exerting power on the tool and the task. An electric screwdriver can have a pistol grip or
an inline grip (Figure 11.4), and a surgical knife can be handled with an internal precision
grip or an external precision grip. The option chosen should depend on how the task is
organized and what is convenient for the operator.
