The Wheel (Abrasive Throwing Device)
(Fig. 2) :
Abrasives (A) fed through opening into Impeller
(B), pass through Stationary Control Cage (C), and are
immediately picked up by the Rotary Blad (D). Blade repeats
these operations approximately 300 times in a second.
The impact coverage of the steel abrasive is governed by its
mass and velocity in accordance with the equation
of kinetic energy
KE = 1/2 MV2
where, KE = Kinetic Energy, M = Mass, V = Velocity
and rotation RPM govern the velocity in
centrifugal blast unit.
Standard 19.5” diameter wheel
at 2250 RPM develops abrasive velocity of approximately
245 ft/sec (or 167 miles per hour). Thus the impact force
delivered to the work piece will change only if the mass
factor (i.e. the abrasive size) is altered.
The relationship of abrasive size to both impact power and
coverage is shown in Figure 3 (i.e.a Table) here.
Effect of Shot Size on Impact & Coverage
Shot Impact Value varies as the cube of the diameter (2:1
Size = 8:1 Impact Value, and 1:8 number of shots/lb)
*Considering Impact value of 70 size shot as 1.
The key to understand the effect of the “Mass” via ‘size’
choice is that the mass of a shpere varies in proportion to
the cube of its diameter. Doubling the shot size increases
the mass or impact power per grain eight times. Conversely,
doubling the shot size reduces the number of shots per kg
When asked, “When did you last check your Blast Pattern ?”, and
“How frequently is it done ?”, most users did not
know the answer or they never had seen it done. Others thought
it was unnecessary.
Blast Pattern : Figure 4
shows Blast Pattern located at
8” to the right of the vertical
centerline on a clockwise wheel.
The pattern is called Hot Spot and
will feel quite warm to the hand,
if touched immediately after shot
blasting. The 1/2: movement of
control cage opening will shift the Blast Pattern by several inches.
We have found that out of 10 machines, five have off blast,
causing 30% loss in cleaning efficiency. Wear tolerance
has been built into blast equipment, but when wear goes
beyond that tolerance, components like Blade, Impeller,
Control Cage etc. cannot perform properly and Blast Pattern
shifts from the set target.
When wear on the leading edge of the impeller segments
exceeds 3 to 4 mm, the abrasives will hit the back of the
Blade rather then being delivered to the throwing face (Fig. 5).
As a result the Hot Spot and overall blast pattern
becomes badly diffused and wears the Bare Wheel and the
Blade (Fig. 6).
|Elevator (Fig. 7) : The elevator bucket lifts spent
abrasives from the elevator boot to the head for
discharge into abrasive hopper. Elevators are
supplied in various sizes, based on the volume of
the abrasives and contaimnants being handled
by the system,. The elevator belt should be kept
tight and should be in the center.
All elevator belts have adjustable bolts to keep
belt properly adjusted in center. If the buckets
are hitting the elevator casings, it is an indication
that the belt tension needs to be adjusted.
|Elevator will also jam
if blasting operation is started without elevator running or belt
slipping because of improper tension.
The wear on bucket should not exceed 6 mm as this will allow
abrasives to accumulate in the bottom section of
the elevator, causing elevator to jam and increase wear on boot