  Soft robotic fingers can safely grasp fragile or variable form objects, but
their force capacity is limited, especially with less contact area: precision
grasps and when objects are smaller or not spherical. Current research is
improving force capacity through mechanical design by increasing contact area
or stiffness, typically without models which explain soft finger force
limitations. To address this, this paper considers two types of soft grip
failure, slip and dynamic rotational stability. For slip, the validity of a
Coulomb model investigated, identifying the effect of contact area, pressure,
and relative pose. For rotational stability, bulk linear stiffness of the
fingers is used to develop conditions for dynamic stability and identify when
rotation leads to slip. Together, these models suggest contact area improves
force capacity by increasing transverse stiffness and normal force. The models
are validated on pneumatic fingers, both custom PneuNets-based and commercially
available. The models are used to find grip parameters which increase force
capacity without failure.
