The technology of hurling steel or glass shot
at more than 100 mph is pretty simple. But in the world of shot
blasting machines, simple does not mean maintenance-free. The
essence of these machines, no matter what their design, is that
clean core parts come at the price of dramatic wear-down of media,
as well as wear to machine components that come in contact with
media.
Of course, you can load ’em and run ’em and not check the condition
of the shot media, filters and wear parts until there’s a real
problem. But that’s going to be done at the cost of efficient
operation, which can affect your bottom line in a hurry.
The debates about which type of machine is most effective and
easiest to use for specific parts, or which type cleans parts
at the lowest cost are beyond the scope of this article. Instead,
our purpose is to provide insight about how each type operates,
and how best to keep all designs running at peak efficiency. Suffice
it to say that each type of shot blaster: (a) airless centrifugal
"paddle wheel," (b) airless centrifugal "center-fed
wheel," and (c) "air blast" types have some maintenance
requirements in common and some that are unique to each design.
Dust is serious business
We mention dust collection first because it
is an often overlooked but tremendously important area of shot
blast maintenance. A high volume of dust is generated as the shot
knocks off dried up oil, paint and gasket materials from used
auto parts. Then spent shot or "fines" is added to become
a nasty mixture that has to be removed promptly, or efficiency
and cleaning quality problems soon follow.
"Both dust collection and shot mixture
maintenance are key, and the importance of both are grossly underestimated"
says Gus Enegren, president of Viking Corp. in Wichita, KS, which
builds both types of centrifugal airless blasters. "Most
users tend to focus on wear items, such as ‘how worn are my blades’
or ‘is my material handling system worn out’, and too often they
couldn’t care less about the cleanliness of the dust collector
filter media or the shot work mix," said Enegren. "But
those two issues, probably more than anything else, will determine
the success of a blast cleaning operation."
All shot blasters have designed-in (but not maintenance-free)
mechanisms to remove dust from the interior of the blast cabinet.
Usually they use a blower or vacuum air pump to create a negative
pressure within their cabinets that pulls the dust out and into
a bag or cartridge-type dust collector.
Although done in different ways, keeping the dust collecting system
working properly is a number one maintenance priority, with consequences
that can affect not only the cleaning efficiency and shot utilization,
but safety as well. "If the dust collector is clogged up
and not pulling off the right amount of dust, your parts come
out with a haze on them, not shining the way they should,"
comments Michael Wigart, sales manager at LS Industries, another
Wichita, KS-based manufacturer of airless paddle-type blasters.
"Plus, dust build-up inside a shot blast cabinet creates
the same type of explosive atmosphere you can have in a flour
mill," Wigart added.
The blast machine manufacturers we talked to all said that their
maintenance checklists have dust collector condition as a "check
daily" item. Most bag and cartridge filter systems come equipped
with either a basic "U-tube" manometer, or a mechanical
differential pressure gauge. Either instrument should be read
while the dust collector is in operation to get a reading on the
difference in pressure between the clean and dirty sides of filters.
If there isn’t such an instrument on the filter array, one should
be installed. They’re not expensive.
Blast machine manufacturers sometimes offer recommendations as
to what differential pressures are acceptable, but variations
in user conditions make it hard to establish firm pressure drop
guidelines. Users can log manometer readings at regular intervals,
concurrently noting the level of cleaning efficiency to establish
their own guidelines for pressure differential readings. If this
isn’t feasible, its a good idea to simply operate the bag shaking
mechanism on a regular basis; most manufacturers recommend at
least once a day.
Many filter arrays now use cartridge elements, which can be equipped
to "pulse clean" at regular intervals, or when a sensor
detects excessive pressure differentials across the filters. Of
course, regular inspection of ductwork and the filter media itself
for holes, fan belt slippage and bag shake-out mechanisms is important,
too.
Airless blasters are predominant in auto rebuilding shops, but
many of them also use air-driven machines where shot is propelled
through a hose and out of a nozzle, either directly with compressed
air, or else drawn through the hose by a venturi force. In such
machines, efficient dust collection is doubly important because
the operator has to be able to see the workpiece he or she is
trying to clean, and too much dust obscures the view.
No matter what type of machine, the challenge in designing a dust
collection system is to make sure it removes the dust but not
shot particles that are still large enough to be effective. There’s
a fine line between pulling out too much material, and not enough.
"In a perfect system, you wouldn’t lose good media,",
says Mike Amann, sales manager for Trinity Tool Co., a Fraser,
MI, manufacturer of air-driven shot blasters and dust collectors
sold under the trade name Trinco Dry Blasters. "But in reality,
especially if you’re using fine glass or plastic beads, you’re
going to have some carry-out of good particles. "You try
to keep that at a minimum."
The visibility factor of air-driven machines can help the dust
collection effort too. "Operators of our machines know when
a filter is clogged when they notice a drop in visibility,"
comments Amann. As a rule of thumb, he recommends that bag filters
be shaken out after four to eight hours of blasting.
A host of things can effect how well a dust collection system
works. Not only the amount of dirt on the filter, but the type
and size of shot being used, not to mention the physical distance
between the blast machine and its dust collection filters have
an effect. Blast machines that have a way to control the pressure
which pulls dust out of the cabinet can compensate for various
conditions to some degree.
The center-fed, overhead wheel units usually have what’s called
an air-wash separator, where used shot is cascaded over a ledge
in the unit. Air is uniformly "pulled" from behind this
curtain of shot and dust. At the right pressure setting, the dust,
"fines" and contaminants are pulled off and sent to
the dust collector system while the heavier, still effective shot
media falls back into the hopper.
The strength of the air wash "pull" is normally adjustable.
"With the Wheelabrator air wash," says Wheelabrator
sales manager Tom Warren, "we have a ‘compensated flow separator’
that you can set to pull out the level of dust and fines that
you need to." Warren emphasizes that without a separator,
abrasives continue to be used beyond the point where they’re effective.Most
readers will be familiar with the centrifugal airless "paddle
wheel" blasters where parts are loaded into a cylindrical
basket, which is spun on a shaft while shot media is thrown at
the basket and its contents with one or more "paddles"
located beneath the basket. These designs do have their limitations,
but their relatively high efficiency at reasonable cost, plus
their ease of maintenance make them the shot blaster of choice
for cleaning large and small parts in many rebuilding shops, usually
using steel or aluminum oxide shot.But lest anyone think that
the gravity-fed tumble machines have no way to control the degree
of dust removal, Viking’s Enegren is quick to point out that his
tumble units have an adjustable paddle baffle on their blowers.
"As you increase or decrease your abrasive size, you can
increase or decrease air flow through the system without pulling
the abrasives out.
"In other words, you can change the static pressure as you
need to," he said, adding that his paddle blast machines
can be designed to pull 20 air changes a minute through the cabinet.
Other manufacturers of these types of machines control airflow
through their cabinet with adjustable orifices to create or diminish
the negative pressures.
The magic is in the media mix
Selection of the maximum shot size for a particular
application is also important. (See sidebar on page 48). However,
directly related to dust collection, another overlooked but critical
shot blast maintenance issue has to do with maintaining a mixture
of shot sizes.
"You have to develop what’s called a good working mix or
a blending of sizes," says sales manager Dennis Wolff of
National Metal Abrasives, Inc., a Wadsworth, OH, supplier of shot
media. "That’s always the way to get the most effective cleaning.
You need the bigger sizes that have the most impact intensity
to knock off the larger chunks of contaminants, and the smaller
sizes that can handle smaller pieces of contaminants and get into
the cracks and crevices."
It should be said that there is no universal agreement with this
philosophy, and some blast machine manufacturers claim that the
best approach is to use a highly efficient machine, and concentrate
on keeping it filled with the smallest media that will do the
job effectively.
Getting to the good working mix requires that the original "batch"
of shot be used long enough to become "seasoned" by
mother nature. It’s obvious that the shot itself is subject to
tremendous wearing forces, and as wear occurs, the shot supply
naturally becomes a mixture of new, full-size shot that may have
been added recently, as well as a full spectrum of medium-size
and small-size shot pellets that have become smaller through continued
use, down to non-functional, discard sizes.
Maintaining a stable mix of shot in the machine is significantly
influenced by the ventilation and separation system. However,
the most important factor is the procedure for adding new abrasive.
The maximum particle size in the working mix is determined by
the size of the new abrasive being added to the system, while
the smallest size particle retained is determined by the separator
air setting.
How can the shot blast operator tell when his mix is no longer
effective, and when new shot should be added? The most accurate
way is to analyze a sample mix from the feed hopper with a screening
kit which can be used in conjunction with a scale to perform a
mixture analysis. For rebuilders who lack the time and personnel
to devote to such a procedure, information from Pangborn Corp.,
a Maryland-based manufacturer of center-fed wheel shot blasters,
suggests that a desirable operating mix will automatically be
produced if replenishment is made frequently with small amounts
of the coarsest abrasive used in the machine.
Cleveland Metal Abrasives suggests that the way to maintain a
consistent size distribution is to make small, perhaps daily additions
of new shot equal to the rate of abrasive consumption. Of course,
consumption rates vary depending on usage, and its easy to get
to a situation where there’s just not enough shot media in the
machine to clean effectively, or the mixture is unbalanced with
too much undersized media.
A check of the ammeter, which is normally supplied on airless
shot blasters, will quickly reveal this situation. The ammeter
measures the current draw on the wheel motor or motors that propel
the shot. When the correct amount of media is being thrown, the
meter will read at the full load rating of the motor. This rating
will be clearly stated on the motor’s nameplate. "You need
to check your ammeter two or three times a day," said LS
Industries’ Wigart. "When it drops below the full load point,
new shot needs to be added to bring the amperage draw back up."
Getting beat to death
It can be said of many machines that they’re
constantly trying to destroy themselves as their component parts
move against each other. This is especially true of shot blasters,
where even non-moving parts are subject to wear. The wheels,
impellers, paddles, baskets and baffle plates, not to mention
the walls and doors of the centrifugal airless cabinets are all
exposed to the abrasives, making frequent visual inspection and
maintenance an imperative.
Paddle wheel blasters
In the "paddle wheel" design that
hurls the shot at parts inside a basket, the paddles are normally
rectangular in shape when they’re new, but the working surface
tends to become ‘scooped out,’ the corners becoming rounded with
use. At some point, the wear will begin to affect the cleaning
cycle time, and it becomes necessary to replace the blades.
No manufacturer is going to make a hard and fast statement about
how long their blades will last. But one suggested that under
normal circumstances, users can expect between six and 12 months
effective use out of a set. As indicated in the chart on page
51, blade wear is a "check daily" item.
The "paddle" blades used are usually made of cast steel
or armor plate alloy, and are sold as a matched and balanced set.
Some manufacturers suggest that worn blades be reversed, and make
it easy to do so. Another school of thought says that blades that
have become worn on one end can be seriously weakened, and could
fly apart and cause damage.
In any case, its important to replace (or reverse) all of the
blades at one time. When installing new blades, be sure the mounting
surface is smooth and has no stray shot on it, which could crack
the hardened blade as you tighten down the mounting bolts. Also,
inspect the shaft and mounting hub for bends and cracks as you’re
replacing blades, particularly if you’ve experienced a broken
blade situation.
Other parts of the paddle assembly that need frequent visual inspection
include the bolt heads on the blades. They should be replaced
when they begin to look rounded over, if for no other reason than
they’re easier to remove at that point. At a minimum, use Grade
8 hardened bolts, heat treated if possible. Also check the integrity
of the safety screen on the gravity feed-back.
The paddle wheel normally sits within a lined enclosure, consisting
of an outer housing lined with hardened wear plates. These wear
plates should be inspected monthly for excessive wear, which will
be indicated by a wavy or rippled pattern. Neglecting any wear
plate wear can result in the shot eroding through, and getting
all over the floor when the machine is turned on.
Center-fed wheel blasters
In these units, the shot is usually fed through
some sort of a gate arrangement that controls the flow of media
to the wheel, located above the parts to be cleaned. In most cases,
there is an impeller and impeller hub, which can be adjusted to
alter the pattern of the shot as it hits the parts being cleaned.
All of the above parts are in continual contact with flowing media,
and subject to wear. Frequent visual inspection is important,
but the ammeter on this type of machine is also a valuable tool.
It indicates not only low abrasive load, as discussed earlier,
but excessive wear to critical blast machine components, among
others. "The ammeter is your eye to what’s happening in the
machine," says Wheelabrator’s Tom Warren. "Typically,
you want to operate all your wheels at maximum amperage, and if
you’re not getting a full amperage reading, it tells you there’s
something wrong in the system."
Warren goes on to say that if the abrasive load is adequate, the
most common cause of low amperage readings is wear to the wheel
components. "What happens is that the worn components are
not able to handle the prescribed volume of abrasives any more,
which is reflected in the ammeter readings," said Warren.
"Abnormal ammeter readings can also be the result of blockage
of media to the wheel, or it can indicate that the wheel is being
overloaded or ‘choked,’" he said. Shot blast machine manufacturers
publish guides as to how ammeters can be read to indicate each
of these situations.
Wheel system components usually include a wheel hub with four
to eight blades, plus an impeller and impeller housing or hub.
All of them are typically replaceable, and several manufacturers
feature quick-change arrangements where blades can be snapped
on or off with ease. As in the paddle wheel designs, blades must
always be installed in matched, balanced sets.
On the subject of the center-fed wheel blasters, it has to be
said that with their multiple-part wheel design, air wash separators
and elevators to transport the shot media back up to the wheel,
they are more complex than the other types of blasting machines,
and require extra maintenance checks. Manufacturers show the inspection
of flights, belts and rollers, as well as alignments and sprocket
drives as "check weekly" items on their maintenance
charts.
In any shot blast machine design, the walls and doors of the cabinets
themselves get hit with the same ricocheting shot that the workpieces
do. All manufactures build in some form of internal protection
for the inside surfaces of the cabinets. Some users even add their
own protective devices.
"I’ve seen blasters with truck mud flaps bolted into them,"
comments Mike Wigart of LS Industries. The material used by various
manufacturers is all over the map. One manufacturer supplies a
11-14% manganese steel cabinet, with a one-half inch thick high-chromium
steel liner.
Other cabinets are lined with a cast steel product. Whether or
not any of these liners are replaceable is often an option. Rubber
or urethane liners are also popular, sometimes on all sides and
sometimes just on doors and surfaces that take an indirect hit
by the shot media. As with metallic liners, rubber liners are
sometimes replaceable and sometimes not.
As a user of shot blast machines, the important thing is to know
the indicators of excessive lining wear, and to have an idea of
when service replacement items are needed. LS Industries Wigart
suggests that as soon as the user notices a wavy or rippled look
to the inside cabinet surfaces, they should begin to monitor the
condition weekly, and get replacement liners on order. As a stopgap,
a hardened steel plate, or perhaps a urethane pad can be put over
an area that’s wearing too quickly. When the liner is very thick,
such as one-half inch, a visual inspection will reveal excessive
wear as well.
Unless they are sealed, the bearings for either the paddle or
center-fed blade wheels need to be lubricated from time to time,
but Wigart of LS offers this note of caution. "Bearings can
be over-greased easily which blows the seal out and allows shot
to get into the bearings and ruin them." For the same reason,
its important to inspect bearing seal plates for integrity. "I
suggest that the user grease the bearings every six months, with
one or two pumps on the grease gun," advises Wigart.
In the world of air-driven shot blasters, a critical maintenance
procedure is the frequent inspection of the shot blast gun. "You
need to rotate both the air jet assembly and the nozzle on a regular
basis, and put new parts into the guns when those wear out,"
says Trinco’s Amann. "Also, the siphon hose needs to be looked
at frequently for holes or thin areas. If you’ve got a pressure
machine, the pressure tank and the plunger and gaskets are subject
to wear from the swirling media, and they should be inspected
for wear periodically, too."
In many ways the maintenance requirements of shot blasting equipment
goes beyond what’s required by many machines in a rebuilder’s
shop. However, by being aware of their special requirements and
implementing a regular service and inspection policy, rebuilders
can increase the output of these machines and extend their useful
life.
