ERGONOMICS OF MANUAL MATERIALS HANDLING - Environmentally Conscious Materials Handling - page 56

cycle time. Assist devices should not be viewed as simply replacing human task
components, but should be viewed as devices that have unique capabilities and
limitations. For example, most hoist devices have load carrying and manipulation
capabilities that far exceed the capabilities of a worker. Where a worker may
have picked up a single part, transported it to the assembly line, and fixed it to
the assembly, a hoist may be capable of loading and transporting many parts at
once. Clever end effectors can be designed to load multiple parts, thus reducing
the number of transports involved in the job and more than compensating for the
extra time to attach and release the parts.
3.2.2 Horizontal Pushing and Pulling
Many materials handling assist devices replace lifting and carrying job com-
ponents with horizontal pushing and pulling job components. Although these
job components are usually less hazardous, as compared to lifting and lowering
tasks, they can involve significant force and effort. Forces up to 500 N, or 80
percent of the worker's maximum strength capability, have been recorded while
using a materials handling assist device (Resnick and Chaffin 1997; Woldstad and
Chaffin 1994). Similar to lifting, pushing and pulling can be hazardous activities
depending on the work configuration (see Chaffin et al. 2006 or Mital et al. 1997
for a more detailed treatment of the ergonomics of pushing and pulling).
Among the factors that increase the force required to push and pull a device
are the mass of the system and the internal joint friction (Woldstad and Chaffin
1994). In general, people are very poor judges of mass and inertia. As a result,
they accelerate very quickly and do not allow themselves enough time and space
to decelerate the load and come to a stop. High deceleration forces and a ten-
dency to overshoot the desired stopping location are common with high inertia
loads. To address this problem, materials handling devices should be
designed and configured to minimize the mass that is being moved or trans-
ported.
In addition to mass, the joint friction should be carefully considered within
each device. If the joint friction is too high, increased push and pull forces will
result. However, if the friction is too low, the inertial mass of the system will
be hard to control, as already described. An intermediate value that allows easy
control of the system and resists inadvertent movement, while minimizing push
and pull force, is desired. This is often the result of careful tuning and adjustment
of the device once it is installed.
Push and pull forces will be limited by the coefficient of friction between the
floor surface and the workers' shoes. Coefficient of friction is defined as the ratio
of the vertical force to the horizontal force acting on the surface. For pushing
and pulling, the vertical force is determined by the weight of the person. If the
person tries to exert a large horizontal force, the coefficient of friction of the
shoe-floor interface must be high enough relative to the weight of the person to
support the activity. If the coefficient of friction is not high enough, the person's