middle of the crawler belt where the low-pressure area is located, even while climbing stairs,
as shown in Figure 21. The total friction force of the blocks is expressed as the sum of the
adhesive friction force, which depends on the face characteristics of the material and the
friction force due to deformation that occurs during motion. The adhesive friction force
depends only on the facing material, and the friction force due to deformation depends only
on the inner materials. For example, friction forces due to deformation are the same between
the urethane rubber block and the urethane rubber blocks inside the tube. The difference is
the adhesive friction force due to the face material of the tube. Moreover, the friction force
due to deformation of the inner powder can be calculated as the total friction force of the
powder-filled blocks minus the friction of the tube, which is adhesive friction. Thus, the
ratio of adhesive friction to the friction due to deformation for a specific loading can be
expressed as shown in Figure 22. Almost all of the friction of the powder-filled blocks is
attributed to the deformation. Therefore, it appears that a stable grip force can be always
obtained, despite the grounding state of the environment. However, the friction force of the
rubber blocks depends on the friction at the surface, and this is not desirable.
This result also shows that the crawler with the powder-filled belt has a relatively smaller
friction force on flat surfaces, such as asphalt or concrete. When the crawler moves over a
flat surface, the powder-filled blocks deform little because the ground presses equally
towards the powder-filled blocks; little energy is lost by rolling resistance which depends on
the hysteresis loss. Therefore, the crawler with powder-filled blocks also has better mobility
for tasks on flat surfaces such as curving or pivot turning (by relatively small surface
friction) and for climbing stairs (by large frictional force due to deformation).
Next, the same experiments were performed in order to compare the effects of the size of
particles and materials. The results are shown in Figure 22, which compares the 3 mm
diameter aluminum balls with 6 mm plastic balls. The large equivalent frictional coefficient
and hysteresis characteristics were approximately the same. Therefore, variations in the
inner material and size do not play a very important role in defining the friction force
generated by the block. Flour, however, becomes harder and stiff and does not change its
form once it has been subjected to loads greater than 2500 N. Thus, the size and the
materials used for the inner powder should be decided according to the intended
environments and the load carried. Otherwise, the particles can be destroyed and the block
will no longer be able to change its form.
After several experiments, the following results were obtained.