Non-Controllable Variables
There are a number of variables that affect
rates and patterns of wear which cannot be
controlled. These should be understood so
you can explain their effects.
Non-controllable variables include soil and
underfoot conditions (abrasion, impact,
packing, etc.), application conditions (what
the machine is doing) and terrain conditions.
Soil & Underfoot Conditions
Generally, soil and underfoot conditions
cannot be controlled. They include the
abrading, impacting, packing and even
corrosive and temperature effects of the
immediate environment.
Abrasiveness
Abrasiveness of underfoot conditions is
the most difficult to accurately measure
except by its effect. We use these descrip-
tions to identify relative abrasiveness in
terms of high, moderate and low:
Abrasiveness Ratings Descriptions
High — Saturated wet soils containing a
majority of hard, angular or sharp sand
particles.
Moderate — Slightly or intermittently
damp soils containing low proportion of
hard, angular or sharp particles.
Low — Dry soils or rock containing a
very low proportion of hard, angular sharp
sand or rock chip particles.
The amount of moisture plays a big role in
defining abrasiveness. For example, dry,
pure quartz sand may be only 1/10 as
abrasive as saturated wet, pure quartz sand
slurry and only 1/2 as abrasive as it is in a
damp condition. This is because moisture
affects the rate that particles are carried to
and stuck to the metal surface being worn.
Some abrasive combinations tend to attack
the bushing, others the grousers and still
others the links and rollers. These differences
are difficult to quantify except by actual
experience, but you should be aware of the
possibility.
Usually, the link is the best component to
use for comparing overall relative abrasive
effects because it is least subjected to other
variables at the same time. This is why we
use the link as the “base” or “barometer”
component when comparing wear lives to
the service limit of different components
in different abrasive conditions.
Impact
Impact is not affected by other variables
such as moisture or hardness of the parti-
cles that make up the soil. It can be
defined as high, moderate or low. Impact
is determined by weight of the machine
and speed is defined under APPLICA-
TION CONDITIONS later in this sub-
section.
Impact Ratings Description
High — Non-penetratable hard surfaces
with constant exposure to 6" (15 cm) or
higher “bumps.”
Moderate — Partially penetratable surfaces
with constant exposure to smaller bumps.
Low — Completely penetratable surfaces
(which provide full shoe plate support)
with low exposure to any height “bumps”.
The most measurable effect of impact is
on structural problems such as bending,
cracking, breaking, chipping, spalling,
roll-over and hardware and pin and
bushing retention. However, the degree of
impact when combined with abrasiveness
may affect component wear rates and
wear life by a factor of two or more. For
example, a D6 dozing at a given speed on
completely compacted or frozen sandy soil
with a deeply furrowed surface (high
impact) may get only one-half the wear
life on links as it would if the same soil
were loose, soft and smooth (low impact).
Generally, wear life of larger machines are
less affected by variations in impact than
smaller machines.
21
General Information
NON-CONTROLLABLE VARIABLES — UNDERFOOT CONDITIONS