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Shot Blasting Equipment Maintenance: Simple Does Not Mean Maintenance-Free

By Tom Glover

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.