The technology of hurling steel or glass shotat more than 100 mph is pretty simple. But in the world of shotblasting machines, simple does not mean maintenance-free. Theessence of these machines, no matter what their design, is thatclean core parts come at the price of dramatic wear-down of media,as well as wear to machine components that come in contact withmedia.
Of course, you can load ‘em and run ‘em and not check the conditionof the shot media, filters and wear parts until there’s a realproblem. But that’s going to be done at the cost of efficientoperation, which can affect your bottom line in a hurry.
The debates about which type of machine is most effective andeasiest to use for specific parts, or which type cleans partsat 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. Sufficeit to say that each type of shot blaster: (a) airless centrifugal"paddle wheel," (b) airless centrifugal "center-fedwheel," and (c) "air blast" types have some maintenancerequirements in common and some that are unique to each design.
Dust is serious business
We mention dust collection first because itis an often overlooked but tremendously important area of shotblast maintenance. A high volume of dust is generated as the shotknocks off dried up oil, paint and gasket materials from usedauto parts. Then spent shot or "fines" is added to becomea nasty mixture that has to be removed promptly, or efficiencyand cleaning quality problems soon follow.
"Both dust collection and shot mixturemaintenance are key, and the importance of both are grossly underestimated"says Gus Enegren, president of Viking Corp. in Wichita, KS, whichbuilds both types of centrifugal airless blasters. "Mostusers tend to focus on wear items, such as ‘how worn are my blades’or ‘is my material handling system worn out’, and too often theycouldn’t care less about the cleanliness of the dust collectorfilter media or the shot work mix," said Enegren. "Butthose two issues, probably more than anything else, will determinethe 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 negativepressure within their cabinets that pulls the dust out and intoa bag or cartridge-type dust collector.
Although done in different ways, keeping the dust collecting systemworking properly is a number one maintenance priority, with consequencesthat can affect not only the cleaning efficiency and shot utilization,but safety as well. "If the dust collector is clogged upand not pulling off the right amount of dust, your parts comeout with a haze on them, not shining the way they should,"comments Michael Wigart, sales manager at LS Industries, anotherWichita, KS-based manufacturer of airless paddle-type blasters."Plus, dust build-up inside a shot blast cabinet createsthe same type of explosive atmosphere you can have in a flourmill," Wigart added.
The blast machine manufacturers we talked to all said that theirmaintenance checklists have dust collector condition as a "checkdaily" item. Most bag and cartridge filter systems come equippedwith either a basic "U-tube" manometer, or a mechanicaldifferential pressure gauge. Either instrument should be readwhile the dust collector is in operation to get a reading on thedifference in pressure between the clean and dirty sides of filters.If there isn’t such an instrument on the filter array, one shouldbe installed. They’re not expensive.
Blast machine manufacturers sometimes offer recommendations asto what differential pressures are acceptable, but variationsin user conditions make it hard to establish firm pressure dropguidelines. Users can log manometer readings at regular intervals,concurrently noting the level of cleaning efficiency to establishtheir own guidelines for pressure differential readings. If thisisn’t feasible, its a good idea to simply operate the bag shakingmechanism on a regular basis; most manufacturers recommend atleast once a day.
Many filter arrays now use cartridge elements, which can be equippedto "pulse clean" at regular intervals, or when a sensordetects excessive pressure differentials across the filters. Ofcourse, regular inspection of ductwork and the filter media itselffor holes, fan belt slippage and bag shake-out mechanisms is important,too.
Airless blasters are predominant in auto rebuilding shops, butmany of them also use air-driven machines where shot is propelledthrough a hose and out of a nozzle, either directly with compressedair, or else drawn through the hose by a venturi force. In suchmachines, efficient dust collection is doubly important becausethe operator has to be able to see the workpiece he or she istrying to clean, and too much dust obscures the view.
No matter what type of machine, the challenge in designing a dustcollection system is to make sure it removes the dust but notshot particles that are still large enough to be effective. There’sa 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 collectorssold under the trade name Trinco Dry Blasters. "But in reality,especially if you’re using fine glass or plastic beads, you’regoing to have some carry-out of good particles. "You tryto keep that at a minimum."
The visibility factor of air-driven machines can help the dustcollection effort too. "Operators of our machines know whena filter is clogged when they notice a drop in visibility,"comments Amann. As a rule of thumb, he recommends that bag filtersbe shaken out after four to eight hours of blasting.
A host of things can effect how well a dust collection systemworks. Not only the amount of dirt on the filter, but the typeand size of shot being used, not to mention the physical distancebetween the blast machine and its dust collection filters havean effect. Blast machines that have a way to control the pressurewhich pulls dust out of the cabinet can compensate for variousconditions to some degree.
The center-fed, overhead wheel units usually have what’s calledan air-wash separator, where used shot is cascaded over a ledgein the unit. Air is uniformly "pulled" from behind thiscurtain of shot and dust. At the right pressure setting, the dust,"fines" and contaminants are pulled off and sent tothe dust collector system while the heavier, still effective shotmedia falls back into the hopper.
The strength of the air wash "pull" is normally adjustable."With the Wheelabrator air wash," says Wheelabratorsales manager Tom Warren, "we have a ‘compensated flow separator’that you can set to pull out the level of dust and fines thatyou need to." Warren emphasizes that without a separator,abrasives continue to be used beyond the point where they’re effective.Mostreaders will be familiar with the centrifugal airless "paddlewheel" blasters where parts are loaded into a cylindricalbasket, which is spun on a shaft while shot media is thrown atthe 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, plustheir ease of maintenance make them the shot blaster of choicefor cleaning large and small parts in many rebuilding shops, usuallyusing steel or aluminum oxide shot.But lest anyone think thatthe gravity-fed tumble machines have no way to control the degreeof dust removal, Viking’s Enegren is quick to point out that histumble units have an adjustable paddle baffle on their blowers."As you increase or decrease your abrasive size, you canincrease or decrease air flow through the system without pullingthe abrasives out.
"In other words, you can change the static pressure as youneed to," he said, adding that his paddle blast machinescan be designed to pull 20 air changes a minute through the cabinet.Other manufacturers of these types of machines control airflowthrough their cabinet with adjustable orifices to create or diminishthe negative pressures.
The magic is in the media mix
Selection of the maximum shot size for a particularapplication is also important. (See sidebar on page 48). However,directly related to dust collection, another overlooked but criticalshot blast maintenance issue has to do with maintaining a mixtureof shot sizes.
"You have to develop what’s called a good working mix ora blending of sizes," says sales manager Dennis Wolff ofNational Metal Abrasives, Inc., a Wadsworth, OH, supplier of shotmedia. "That’s always the way to get the most effective cleaning.You need the bigger sizes that have the most impact intensityto knock off the larger chunks of contaminants, and the smallersizes that can handle smaller pieces of contaminants and get intothe cracks and crevices."
It should be said that there is no universal agreement with thisphilosophy, and some blast machine manufacturers claim that thebest approach is to use a highly efficient machine, and concentrateon keeping it filled with the smallest media that will do thejob effectively.
Getting to the good working mix requires that the original "batch"of shot be used long enough to become "seasoned" bymother nature. It’s obvious that the shot itself is subject totremendous wearing forces, and as wear occurs, the shot supplynaturally becomes a mixture of new, full-size shot that may havebeen added recently, as well as a full spectrum of medium-sizeand small-size shot pellets that have become smaller through continueduse, down to non-functional, discard sizes.
Maintaining a stable mix of shot in the machine is significantlyinfluenced 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 bythe size of the new abrasive being added to the system, whilethe smallest size particle retained is determined by the separatorair setting.
How can the shot blast operator tell when his mix is no longereffective, and when new shot should be added? The most accurateway is to analyze a sample mix from the feed hopper with a screeningkit which can be used in conjunction with a scale to perform amixture analysis. For rebuilders who lack the time and personnelto 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 beproduced if replenishment is made frequently with small amountsof the coarsest abrasive used in the machine.
Cleveland Metal Abrasives suggests that the way to maintain aconsistent size distribution is to make small, perhaps daily additionsof new shot equal to the rate of abrasive consumption. Of course,consumption rates vary depending on usage, and its easy to getto a situation where there’s just not enough shot media in themachine to clean effectively, or the mixture is unbalanced withtoo much undersized media.
A check of the ammeter, which is normally supplied on airlessshot blasters, will quickly reveal this situation. The ammetermeasures the current draw on the wheel motor or motors that propelthe shot. When the correct amount of media is being thrown, themeter will read at the full load rating of the motor. This ratingwill be clearly stated on the motor’s nameplate. "You needto check your ammeter two or three times a day," said LSIndustries’ 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’reconstantly trying to destroy themselves as their component partsmove 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 mentionthe walls and doors of the centrifugal airless cabinets are allexposed to the abrasives, making frequent visual inspection andmaintenance an imperative.
Paddle wheel blasters
In the "paddle wheel" design thathurls the shot at parts inside a basket, the paddles are normallyrectangular in shape when they’re new, but the working surfacetends to become ‘scooped out,’ the corners becoming rounded withuse. At some point, the wear will begin to affect the cleaningcycle time, and it becomes necessary to replace the blades.
No manufacturer is going to make a hard and fast statement abouthow long their blades will last. But one suggested that undernormal circumstances, users can expect between six and 12 monthseffective use out of a set. As indicated in the chart on page51, blade wear is a "check daily" item.
The "paddle" blades used are usually made of cast steelor armor plate alloy, and are sold as a matched and balanced set.Some manufacturers suggest that worn blades be reversed, and makeit easy to do so. Another school of thought says that blades thathave become worn on one end can be seriously weakened, and couldfly apart and cause damage.
In any case, its important to replace (or reverse) all of theblades at one time. When installing new blades, be sure the mountingsurface is smooth and has no stray shot on it, which could crackthe hardened blade as you tighten down the mounting bolts. Also,inspect the shaft and mounting hub for bends and cracks as you’rereplacing blades, particularly if you’ve experienced a brokenblade situation.
Other parts of the paddle assembly that need frequent visual inspectioninclude the bolt heads on the blades. They should be replacedwhen they begin to look rounded over, if for no other reason thanthey’re easier to remove at that point. At a minimum, use Grade8 hardened bolts, heat treated if possible. Also check the integrityof the safety screen on the gravity feed-back.
The paddle wheel normally sits within a lined enclosure, consistingof an outer housing lined with hardened wear plates. These wearplates should be inspected monthly for excessive wear, which willbe indicated by a wavy or rippled pattern. Neglecting any wearplate wear can result in the shot eroding through, and gettingall over the floor when the machine is turned on.
Center-fed wheel blasters
In these units, the shot is usually fed throughsome sort of a gate arrangement that controls the flow of mediato the wheel, located above the parts to be cleaned. In most cases,there is an impeller and impeller hub, which can be adjusted toalter 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, amongothers. "The ammeter is your eye to what’s happening in themachine," says Wheelabrator’s Tom Warren. "Typically,you want to operate all your wheels at maximum amperage, and ifyou’re not getting a full amperage reading, it tells you there’ssomething wrong in the system."
Warren goes on to say that if the abrasive load is adequate, themost common cause of low amperage readings is wear to the wheelcomponents. "What happens is that the worn components arenot 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 blockageof media to the wheel, or it can indicate that the wheel is beingoverloaded or ‘choked,’" he said. Shot blast machine manufacturerspublish guides as to how ammeters can be read to indicate eachof these situations.
Wheel system components usually include a wheel hub with fourto eight blades, plus an impeller and impeller housing or hub.All of them are typically replaceable, and several manufacturersfeature quick-change arrangements where blades can be snappedon or off with ease. As in the paddle wheel designs, blades mustalways be installed in matched, balanced sets.
On the subject of the center-fed wheel blasters, it has to besaid that with their multiple-part wheel design, air wash separatorsand 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 inspectionof flights, belts and rollers, as well as alignments and sprocketdrives as "check weekly" items on their maintenancecharts.
In any shot blast machine design, the walls and doors of the cabinetsthemselves get hit with the same ricocheting shot that the workpiecesdo. All manufactures build in some form of internal protectionfor the inside surfaces of the cabinets. Some users even add theirown protective devices.
"I’ve seen blasters with truck mud flaps bolted into them,"comments Mike Wigart of LS Industries. The material used by variousmanufacturers is all over the map. One manufacturer supplies a11-14% manganese steel cabinet, with a one-half inch thick high-chromiumsteel liner.
Other cabinets are lined with a cast steel product. Whether ornot any of these liners are replaceable is often an option. Rubberor urethane liners are also popular, sometimes on all sides andsometimes just on doors and surfaces that take an indirect hitby the shot media. As with metallic liners, rubber liners aresometimes replaceable and sometimes not.
As a user of shot blast machines, the important thing is to knowthe indicators of excessive lining wear, and to have an idea ofwhen service replacement items are needed. LS Industries Wigartsuggests that as soon as the user notices a wavy or rippled lookto the inside cabinet surfaces, they should begin to monitor thecondition weekly, and get replacement liners on order. As a stopgap,a hardened steel plate, or perhaps a urethane pad can be put overan area that’s wearing too quickly. When the liner is very thick,such as one-half inch, a visual inspection will reveal excessivewear as well.
Unless they are sealed, the bearings for either the paddle orcenter-fed blade wheels need to be lubricated from time to time,but Wigart of LS offers this note of caution. "Bearings canbe over-greased easily which blows the seal out and allows shotto get into the bearings and ruin them." For the same reason,its important to inspect bearing seal plates for integrity. "Isuggest that the user grease the bearings every six months, withone or two pumps on the grease gun," advises Wigart.
In the world of air-driven shot blasters, a critical maintenanceprocedure is the frequent inspection of the shot blast gun. "Youneed to rotate both the air jet assembly and the nozzle on a regularbasis, and put new parts into the guns when those wear out,"says Trinco’s Amann. "Also, the siphon hose needs to be lookedat frequently for holes or thin areas. If you’ve got a pressuremachine, the pressure tank and the plunger and gaskets are subjectto wear from the swirling media, and they should be inspectedfor wear periodically, too."
In many ways the maintenance requirements of shot blasting equipmentgoes beyond what’s required by many machines in a rebuilder’sshop. However, by being aware of their special requirements andimplementing a regular service and inspection policy, rebuilderscan increase the output of these machines and extend their usefullife.
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