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Control Systems In Shot Peening

Electrical and electronic control systems in shot peening equipment have greatly evolved in the last decade. Any peening machine manufactured now has, at the bare minimum, a Programmable Logic Controller (PLC) to monitor and control the programmable features of the process with an Human Machine Interface (HMI) so the machine operator can command the machine to perform desired tasks.

Shot peening machines of the past relied on relay logic controls, pushbuttons and other forms of controls/interfaces. A large part of this evolution was driven by users in aerospace who, with their familiarity of CNC controls from other equipment such as machining centers, raised the benchmark for shot peening equipment. Also, their desire to promote repeatability, accuracy and reliability along with process reporting requirements made it a compulsion for electrical controls in shot peening equipment to be upgraded to current levels. Conformance to specifications and audit criteria also assisted in this evolution.

These are steps in the right direction, but peening equipment may now be more complicated than it needs to be. Veteran experts in our industry often say, “Blast cleaning and shot peening as processes are not as complicated as the science of rocket propulsion!” There is a lot of truth to this statement, especially when compared to other machine tools such as multiple-axis machining centers and routers where precision is critical.

With this in mind, we should ask, “Are we over-complicating our peening machines?” Control sophistication comes at a cost, and it could easily be the single most expensive cost component in machines.

Process Controls in Shot Peening

In order to discuss electrical/electronic controls, we must understand the role played by process controls in shot peening. The prime variables that control the outcome of a peening cycle can be categorized into the following:

  1. Impact Energy - represented by velocity of the blast media and its type/size/hardness
  2. Exposure Time - this determines the percentage of coverage on the component being peened

Let us analyze the factors that determine impact energy:

  • In a centrifugal wheel machine, the velocity is determined by wheel diameter and its speed of rotation. Gradual wear of wheel parts also has a marginal effect on the impact energy. Media velocity and impact energy are directly proportional to wheel speed. Variable frequency drives for blast wheels, some with closed loop feedback, ensure maintenance of constant wheel rotational speed.
  • In an air blast machine, the velocity is determined by the air pressure/nozzle orifice size in direct proportion. Also, like with a centrifugal wheel, nozzle wear has an effect on the generated impact energy. Closed-loop feedback or air pressure monitoring will correct fluctuations in air pressure delivered to the blast nozzle.
  • In both cases, type and quality of media affects the end result. Cast steel shot, the most commonly used peening media, is susceptible to the inherent imperfections of a cast product. MIL-specified cast steel shot is typically used for shot peening applications and the cast media is pre-screened and imperfections are separated out to provide ideal media conditions for peening.
  • Size consistency of blast media is also very critical in peening applications. A mixture of blast media sizes will lead to difficulty in achieving saturation—the measure of process stability. Some of us have experienced the occurrence of the ‘double knee’ when plotting the saturation curve, signifying deterioration in the quality of abrasive in the machine, typically due to contamination of two or more sizes of media. Size consistency is kept in check by using a vibratory classifier. Some aerospace applications also require the use of a spiral separator to remove broken media from the mix (shape classification).

In comparison, the factors that determine exposure time are relatively simple. Peening coverage is always checked directly on the component being peened. Exposure time can be changed by changing the speed of the conveyor in an inline machine, or the speed of the rotary table in a table-type machine. Part exposure time is independent of the time taken to achieve time “T” on our saturation curve.

Simple Control Architecture

How is this discussion relevant to the use of a PLC in our shot peening machine? The PLC has digital and analog inputs and outputs that monitor the health of all the elements that have an effect on the impact energy. For example, an inbuilt digital timer in the PLC will trigger an alarm to shutdown the process if the air pressure doesn’t reach the pre-set/desired value within a specified time, or if it exceeds the pre-set value. Similarly, a feedback loop will attempt to correct the wheel speed in a centrifugal wheel-type machine through digital outputs from the motor and variable frequency drive. The diagram below is of a simple control system.

The system PLC also stores recipe/technique information and provides the data for downstream processing through an Ethernet (or similar) connection. The motors and associated variable frequency drives in the architecture could drive a centrifugal blast wheel or different axes of a multi-axis nozzle manipulator. The output is graphically represented in an HMI (touchscreen or otherwise) which also provides the ability to create recipes, store and retrieve when required.

The Role of Specifications

Specifications and their interpretation also had a role to play in the evolution of controls. For our purposes, let’s refer to two of the commonly used specifications: AMS 2430 (Rev. S, revised 2012-7) - (R) Shot Peening, Automatic (only relevant discussion points are cited from the specification)

  • The purpose (1.1) is identified as “specification covers the requirements for automatic shot peening of surfaces of parts by impingement of media, including metallic, glass or ceramic shot.”
  • 3.2.1.1 states, “the peening machine shall run automatically and may be computer controlled.”
  • Under 8. Notes, the specification defines Automatic (8.2.1)as “A class of peening machine that precludes use of manual movement or either the shot stream or the work part but relies upon mechanical means to provide these features”.
  • 8.4.5.3: Peening Equipment states as follows, “Robotic machines provide line of sight media impingement for a wide variety of geometries reducing multiple setups. Computer controlled and monitored machines offer the industry’s best practice for process control. Computer controlled shot peening equipment should be considered for use in man flight [sic] vehicle components, components where shot peening is used as part of the design strength of the component, and components that are considered critical to system success.”

AMS 2430 also elaborates on maintenance of media quality in the machine, measurement of results and other aspects for a thorough peening process set-up.

The terms “computer controlled and monitored” could be open to interpretation not only in terms of this specification, but also in general use of the terminology. However, our industry has taken the safe approach and automated its controls to use PLCs and PCs. Interestingly, the specification defines the process without forcing the user to employ a particular type of control system in the machine. In the simple architecture shown below, the enhancement to “computer controlled and monitored” will result in the use of an industrial PC to store a greater number of recipes/techniques and also provide the interface to transfer process information through an electronic data highway to the customer’s central controls system for further processing. Some industrial PCs are also available with a soft PLC integrated into the PC as a software PLC. This results in less hardware with a possible cost savings.

AMS 2432 (Rev. C, revised Sept 2007) - Shot Peening, Computer Monitored (only relevant discussion points are cited from the specification)

  • The purpose (1.1) is identified as “specification establishes the requirements for computer-monitored peening of parts surfaces”.
  • 3.2.4 states that “Peening machines shall be equipped with computers for continuously monitoring and recording the parameters shown in Table 1 within the tolerance indicated.” Table 1, paragraphs 3.2.4.1 to 3.2.4.12 lists all critical parameters such as media flow, air pressure, wheel speed, nozzle speed, and table speed. with their respective allowable process tolerance (shutdown limits).

AMS 2432 elaborates on process monitoring and the user could draw similar inferences about the use of computers/ industrial PCs when referring to this and AMS 2430. However, AMS 2432 provides background information on a much debated topic in our industry—motion control.

Motion Control in Shot Peening Equipment

To quote from AMS 2430S 3.2.1.1: “…The machine shall provide a means of propelling, at a controlled rate, media with air pressure against a part…The nozzles and the part shall be held and moved mechanically. The part shall not be subject to any random movement during the process. The machine shall be capable of consistently reproducing the required shot peening intensities.”

The goal of a peening process specification is repeatability and accuracy in a reliable machine. With regards to motion control related to shot peening, this means maintaining a constant stand-off distance from the component being peened, and repeating it when the same part is processed at a later date. This also means maintaining the same angle of impingement to all surfaces of the component, usually between 45 to 80 degrees, preferably towards the higher end of the range. AMS 2432C, 3.2.4.11 and 3.2.4.12 tabulate process tolerances for nozzle/wheel position and table/part indexing at 0.062" (1.57 mm)/5 degrees. My machine programmer colleagues in this industry will agree that these tolerances are a far cry from tolerances of 0.00004" to 0.004" that could be possible and even a requirement with other machine tools. In order to achieve such tolerances, the use of CNC machines is inevitable.

A survey of various peening applications over the years makes it abundantly clear that such tolerances in a shot peening machine have never been called for. The peening process is very forgiving in terms of tolerances. Accuracy of ± 0.005"and repeatability of ± 0.002" are well within compliance with all specifications drafted to date for peening processes. Such values can be easily achieved using servomotors and motion controllers without the need for CNCs and a knowledge of their programming codes.

This discussion is not to advocate the use of one system over the other, in this case the use of motion controllers over CNCs, but to evaluate the need and simplify our equipment for a relatively simple process (shot peening).

A shot peening machine with simpler controls will allow the operator and maintenance personnel to focus on the most important aspect—the peening process itself. The use of robots in shot peening machines has added a new dimension to our discussion where complete proven and packaged solutions have eliminated discussions of motion control and G codes. Although not applicable for all applications, robots are also commonly used with nozzle manipulators to increase the versatility of the shot peening machine to handle parts of varying geometry.

Summary

  • The success of your peening operations depends on more han just controls. When your machine specification lists a “CNC Peening Machine,” it is beneficial to evaluate your peening process and determine whether CNC is really a requirement. Motion controllers are usually less expensive than CNCs and don’t require a special programming language. There is no argument about the aerospace customer’s familiarity with CNC equipment, but it has to be made clear that shot peening cannot be placed in the same category as a CNC milling center when discussing the process.
  • The next generation of shot peening machines need to emphasize user-familiarity with the process and make the controls intuitive with less needless sophistication. This can be achieved only if the user takes ownership of the equipment and develops the process with established and documented procedures.
  • The peening process has been established with proper measures for process stability such as the plotting of saturation curves. It’s important that shot peening be treated as a special process and not an extension of an existing blast cleaning process.
  • Machines are secondary; your peening process design comes first.

This article was written by ERVIN's Kumar Balan appearing in the Spring 2014 issue of The Shot Peener

Air or Wheel Peening?

An Application-Based Analysis

Many Engineering Problems have more than one solution. Application Engineers, however, are asked to find the one optimal solution given a list of variables and possibilities. In our industry, we are often called upon to recommend and validate our choice for the best media propulsion system—airblast or wheelblast—for the customer’s application. Very often, either type will work for their application, and each has its own inherent advantages and shortcomings. While I’ve had the luxury of being able to offer both airblast and wheelblast solutions since the company I represented manufactured both systems, I still needed to recommend one or the other to my customers.

 

In this article, I’ll review the basics of each option, categorize the applications, and define and validate the criteria that leads to the best solution.

The Basics

Wheelblast machines use a centrifugal blast wheel to propel media and airblast machines use compressed air. All other factors being equal, the productivity of the blast operation is directly proportional to the amount of abrasive propelled onto the part. In quantifiable terms, an airblast nozzle (3/8"or 9.5 mm diameter at 60 PSI or 4.1 bar) propelling metallic abrasive typically discharges only 10% of the blast media that can be propelled by a centrifugal blast wheel (15" or 381 mm diameter wheel powered by a 20 HP or 15 KW motor).

To get an even clearer perspective on operating efficiency, a single centrifugal wheel provides the same cleaning efficiency as eight (8) ½" or 12.7 mm diameter blast nozzles.

Centrifugal blast wheels are either direct driven or belt driven through a bearing system. The resulting line speed of the associated blast machine is directly proportional to the total connected wheel horsepower. Applications in wheelblast are wide and varied, and given that the first blast wheel was patented over a hundred years ago, wheelblast machines are prevalent in most automated cleaning applications.

The blast nozzle in an airblast system is powered by compressed air. The peening intensity is directly proportional to the air pressure level. There are two types of airblast systems—suction and pressure.

Suction-style propulsion systems are used for relatively lower intensity peening requirements with small ferrous and non-ferrous abrasives of all sizes. Pressurization of the blast media takes place inside the blast gun, eliminating the need for a separate blast tank. A suction gun is identified by two hoses, a red hose for compressed air and a black hose for the abrasive. A venturi or airjet inside the suction gun creates the suction and mixes compressed air and media prior to discharge from the gun.

Direct pressure systems are more commonly used than suction systems. As a rule of thumb, pressure blast systems are about two to three times as efficient as suction systems. Pressurization of the blast media takes place in a separate pressure vessel called the blast tank or blast pot. A single hose, carrying pressurized abrasive, is connected to the blast nozzle. Nozzles can be straight bore or venturi style, as suited to the application.

Suction blast uses less compressed air than direct pressure. A 3/8" or 9.5 mm suction nozzle with a 3/16" or 4.5 mm diameter air jet will consume 40 CFM or 68 cubic meter per hour at 80 PSI or 5.4 Bar. In comparison, a 3/8" direct pressure nozzle at 80 PSI will consume 175 CFM or 300 cubic meter per hour of compressed air.

CRITERIA FOR CHOOSING BETWEEN WHEELBLAST OR AIRBLAST EQUIPMENT

Uses

  • Component needs to be completely treated with abrasive, without the need for masking
  • Treatment area is large
  • Large production volumes with large runs of physically similar components
  • User has several wheelblast machines and has an acceptable process for similar components
  • Process specification that calls for high-intensity values on large surface areas with large-size abrasive (common in automotive and railway applications)

Typical contenders for wheelblast peening are auto transmission components such as gears and shafts, connecting rods, coil and leaf springs and axle beams. In the aerospace industry, landing gear, aircraft wheels/brakes get peened in a wheelblast machine.

Criteria for Choosing Airblast Equipment

  • Only specific areas of the component need to be peened,with the other areas requiring protection from the abrasive
  • Component is to be treated with non-ferrous abrasive
  • Areas such as main bore, slots and other intricacies are to be peened
  • Application requires significant manipulation of the blast stream in order to provide proper coverage
  • Availability of compressed air

To recap, the same automotive and aerospace components that are good candidates for wheelblast should be peened in an airblast machine when only specific areas need to be peened and/or if the process requires non-ferrous media.

Wheelblast versus Airblast for Automotive Transmission

Gears and shafts can be peened in either machine type. In both cases, the machines are rotary indexing tables with multiple satellite fixtures on top that expose individual stacks of gears or a single shaft to the blast wheel, multiple blast wheels or reciprocating nozzles.

Wheelblast machines are used in high production environments and when the gear tooth geometry permits unimpeded access to the abrasive. The root section of the gear is the most critical area to be shot peened since this is where cracks tend to originate. Therefore, blast wheel location and part fixturing are important design parameters in this machine type. Using a more sophisticated arrangement, some wheelblast machine types also offer vertical wheel movement/oscillation to ensure proper coverage is achieved when peening a tall stack of gears or a shaft.

Though arguments can be made about the efficacy of air-type machines over wheel, there are clear lines of demarcation. Though production specifics will depend on several other factors, the production rate of airblast peening machines for gears is lower than its wheelblast counterpart. However, airblast provides precision or targeted blasting, resulting in lower abrasive consumption and breakdown. The direct tangible benefit is a more efficient use of available power and the elimination of unnecessary wear on machine components.

Wheelblast versus Airblast for Landing Gear

When the entire landing gear needs to be peened, as is typical in new gear manufacturing, a spinner hanger wheelblast machine provides efficient coverage. The blast chamber is fitted with multiple wheels in strategic locations to access all areas of the landing gear. When the specification calls for spot peening, typically in MRO and refurbishing operations, the applications are best addressed with an airblast machine. Also, an airblast lance is the only means of peening the ID of the gear, whether in specific areas or along the entire length

PROCESS PARAMETERS FOR BOTH WHEEL AND AIR MACHINES

  • Media flow rate monitored with a MagnaValve, or comparable valve, and validated through actual drop tests.
  • Media classification size and shape)using a vibratory classifier to maintain a consistent size and spiral separator to separate rounds from non-rounds. Due to high flow rates in a wheelblast machine, it is acceptable to sample the flow by diverting a percentage (usually 20%) of the total flow through a vibratory classifier on a continuous basis.
  • Exposure time monitoring by controlling the speed of work handling arrangement such as rotary table, inline conveyor or speed of nozzle movement in an airblast machine.
  • Consistent pressure delivery in airblast machines. The air pressure in an airblast machine determines the peening intensity so it is critical to monitor and maintain consistent pressure delivery in order to get consistent peening intensity results.
  • Wheel speed in wheelblast machines, if all other parameters are stable, determines the intensity in a wheelblast machine. Blast wheels in a wheelblast peening machine are fitted with a variable frequency drive in order to monitor and alter the wheel speed as required.

A NOTE ON HYBRID BLAST MACHINES

A hybrid blast machine capitalizes on the advantage of the wheelblast propulsion technique to peen the majority of the part surface and relies on blast nozzles to complete the process by targeting specific areas insufficiently covered by the blast wheels —all in a common enclosure and sharing reclaim and control system components.

he decision between airblast and wheelblast takes a different dimension with hybrid machines. These machines offer distinct advantages over dedicated air or wheel machines:

  • Cycle time savings due to reduced handling
  • Commonality of fixtures
  • Labor savings (operator required for one machine only)
  • Machine certification simplified because only one machine needs to be certified

Hybrid machines are ideally suited to ‘complete’ the process in a high-production environment with the majority of the area already processed by a single or multiple blast wheels. (For more information on hybrids, download “Hybrid Cleaning/Peening Machines” by Mr. Balan, The Shot Peener,Fall 2007 from the Library at www.shotpeener.com.)

CONCLUSION

As we have read, there are multiple equipment solutions for applications such as transmission components and landing gear. Application engineers must assess each individual process and the site constraints, keeping in view that the final goal remains unaltered—an accurate and repeatable peening operation.

This article was written by ERVIN's Kumar Balan appearing in the Fall 2013 issue of The Shot Peener

Blast Cleaning - Techniques for Structural Steel in Metal Buildings

Our discussion in the April issue focused on blast cleaning basics including initial conditions of steel and possible final finish quality. We also discussed the two main types of media propulsion systems and their individual applications. The article also elaborated on a particular application for blasting and painting plate steel.

The popularity of this mechanical surface preparation technique for processing plate and structural steel is widespread. This article will focus on different structural steel applications – both for cleaning pre- formed and fabricated structures commonly used in metal building construction.

Types of Structural Steel and Machine Variations

Formed structural steel consisting of standard shapes such as beams, channels, angles, hollow steel sections such as tubes and pipes is commonly referred to as pre-blast steel.

Fabricated beams, usually fitted with gussets, base plates, cleats etc. get categorized as post-blast structural steel.

Geometries of both types of structures vary significantly. Correspondingly, blast machines are designed to suit the geometry being cleaned.

Pre-Blast Structural Descaling Machine

When processing standard formed structural steel, the part is conveyed through the machine on rollers. Four (4) blast wheels, mounted at an angle on the cabinet wall ensure 90-degree impact on the part surface. Quantity of wheels varies based on the size of the structure being cleaned. This cabinet arrangement and wheel orientation provide appreciable versatility for the cabinet to process other smaller sections such as channels, tubes and angles in addition to beams.

Roller conveyors extend out at both ends of the machine to support the entire or majority of the length of part being processed. Rollers in the blast cabinet are manufactured from wear resistant material (cast manganese or such) to withstand the impact of abrasive from multiple blast wheels. Rollers are driven by a common motor except in case of very heavy structural steel where each roller is driven by its independent motor.

Line speeds are dependent on several factors such as:

  • Initial condition of steel including type of contaminant (rust, paint, etc.)
  • Desired final condition (Sa2, Sa2.5., Sa3)
  • Blast wheel HP/KW

For this purpose, most roller conveyors are provided with variable frequency drives (inverters) that allow speed alteration as may be required by the part being processed.

Angles are processed with their legs resting on the rollers. The sloped surface of the legs facilitates drainage of blast media into the lower reclaim hopper. However, this isn’t normally the case when processing structures that present a flat surface such as flange portion of beams, top face of channels and HSS. In order to minimize media carryout, machines are equipped with automatic high-pressure blow-off arrangements that blow the part clean before it exits the machine. The blower(s) is mounted either on the roof or at floor level near the machine and connected by galvanized steel ducts to the air cannons inside the cabinet. The reclaim floor is usually extended out at the exit end to drain and transfer abrasive removed by the automated blow-off or any manual means back to the machine reclaim system.

Traditional plate descaling machines are equipped with a brush-off (rotary or transverse brush) in combination to a blow-off. However, in a structural machine, due to the geometry, it is not physically possible to employ a brush-off arrangement. Machine ventilation arrangements are designed to handle the additional volume generated by the blow-off arrangement. This minimizes the possibility of media and dust leakage from the cabinet and offers a clean environment when the machine is in operation.

Post-Blast Structural Machine

Fabricated structural steel and weldments with complex geometries are processed in a post-blast structural machine. Given the variation in geometry, size and line speed requirement, a simple post-blast machine can be fitted with atleast four (4) wheels and extend as high as sixteen (16) wheels. Closer to home, post-blast structural steel would typically be fabricated beams with cleats, end plates and gussets welded on them. Such extensions / projects make it a challenge to handle them on a roller conveyor. In this section, we will discuss various options available for work handling of such parts.

Blast Pattern in a post-blast environment

The following aspects are taken into consideration when locating the blast wheels:

  • Since the parts are mounted with fittings on them, it is imperative that wheels are angled appropriately to ensure a near 90-degree blast on surfaces perpendicular to part travel.
  • Wheels are always mounted in a symmetrical fashion. Additionally, the application might on occasion also require an additional wheel to target specific areas on the part.
  • Wheels are also located staggered so that their patterns don’t interfere when the part has passed through and no longer available to shield the pattern.
  • In an application where cleaning is restricted to flat surfaces without geometrical complexities, it is relatively easier to estimate wheel HP/KW requirement. However, in case of a post-blast structural application, greater line speeds will have to be addressed with additional wheels and not just greater HP/KW of the wheels. Wheel locations are critical when faced with high speed requirements(particularlygreaterthan3MPM).
  • Proper location of the top tier wheels is very important from the standpoint of coverage on the part (since this is the farthest from the center of the part), and also to minimize media leakage from the roof slot seal and subsequent seal wear.
  • Lastly, the size of blast wheel determines the media velocity. Care should be taken that the available velocity is adequate for cleaning the contaminants at the desired line speed. In some sophisticated machines, the blast wheels are provided with variable frequency drives (inverters) to vary the wheel (and media) velocity as required.

Work Handling in Post-Blast Orientation

The two most common work handling arrangements for post-blast structural steel are (a) overhead monorail with a power and free conveyor, and (b) roller conveyor. Selection of one over the other is a matter of preference, and commonality with the work handling in other areas of the production process. Handling is an expense, and duplicate handling doubles the expense. This plays a major role in deciding on the appropriate work handling arrangement.

Some Points to Consider

  • Overhead monorails are the most versatile, but they also occupy more space than rollers on a floor mounted frame. An overhead conveyor passing above the machine cabinet requires a roof slot for the chain to pass through the machine. The roof slot is protected with brush seals which require periodic inspection, maintenance and replacement.
  • Rollers are completely enclosed and don’t require a roof slot. However, passage of parts without a flat surface poses a challenge on rollers. This can be addressed by fabricating frames with flat surfaces that can carry the part through the cabinet. The downside to this is that this frame is also subject to blast and needs periodic replacement.  An example is shown in the picture.
  • Larger structures such as bridge girders are located on independent work cars, one at each end, that transport the structure through the blast chamber.

Summary

This discussion revolved around the application specifics of structural steel commonly encountered in metal buildings. In the months that follow, our discussion will continue to other salient aspects of this application.

Blast cleaning machines for structural applications have evolved over the years. Some of the evolutionary features include:

  • Photocells at the entrance end that stop the blast wheels if no part is sensed for a pre-programmed duration of time
  • Abrasive adder that receives ‘low level’ signal from the main storage hopper and triggers a valve to replenish abrasive in the system
  • Mechanical sensors and switches that ensure conveyor shutdown if there’s the possibility of a part overshooting the conveyor
  • Safety features such as interlocks that shut the system down in the event accidental work door / access door openings.

Selection of the appropriate blast machine for your application is important. However, also critical is to check the manner in which the part geometry and line speed are addressed by wheel design and location.

This article was written by ERVIN's Kumar Balan appearing in the May 2012 issue of Steel Fabrication Process

Blast Cleaning - Techniques for Industrial Structures

Issues & Solutions in Blasting the Post-Blast Variety

This current discussion is an extension of "Blast Cleaning - Techniques for Structural Steel in Metal Buildings." We will continue with blasting structural steel, introducing specific issues and appropriate solutions relating to blasting the post-blast variety. Fabricated structures could potentially have pockets that may not be completely cleaned in an automated machine. Such pockets have a tendency to carry-out entrained abrasive after the blast process. Such issues have to be addressed either in the machine design or through supplemental measures. Here, we will also discuss the relevant solutions.

Work Handling Arrangements

Before we get into the discussion of the issues during blasting, it’s important to determine the manner in which we are going to handle such structures through the shot blasting machine. In an automated machine environment, commonly used work handling arrangements include roller conveyors, monorails and work cars, with the first two being more popular than the last. When conveying post-blast structures fitted with cleats, gussets and other projections, if overhead conveyors are not an option, such structures will have to be located on a fabricated ‘sled’ or framework that will present a flat surface for conveyance on rollers. Such a framework will need to be deep enough to ensure that the longest projection stays within the frame depth and doesn’t cause a hindrance to smooth flow of the frame on the rollers Of course in cases where overhead conveyors, power and free, or otherwise, are available to process the structures, parts are simply hung on trolley hooks that carry them through the machine.

Overhead conveyors with a load bar to carry the structure at different sections of its length offer more versatility, as the same conveyor could carry the part through downstream processes such as washing, drying, painting and curing. Expensive handling is minimized with a simple transfer of the overhead conveyor as compared to more elaborate transfer arrangements required on a floor mounted roller conveyor. Also, the sacrificial sled / frame to carry the structure are not required with this arrangement.

However, the overhead conveyor arrangement is not bereft of areas of concern. Whenever the blast process is exposed by a slot or opening, the possibilities of leakage are inevitable. An overhead conveyor with a roof slot for the chain to pass through has to be properly protected with replaceable seals (brushes, rubber etc.) to minimize leakage of abrasive. Blast media traveling at 70 to 80 meters per second doesn’t offer much predictability about its travel path. Therefore, this aspect of maintenance has to be accounted for when considering an overhead work handling arrangement.

Given that the blast machine is not the only equipment your structural steel component is going through in your manufacturing operation, other processes or an existing layout may very well dictate the type of handling arrangement. Can complex geometries be cleaned in an automated machine?

When working with pre-blast structural steel, the only variable to consider is the part size. Geometries are clearly defined here. This makes the task of the design engineer relatively simple. As far as machine design is concerned, the variation is limited to the quantity of blast wheels and wheel KW. Handling systems are also standardized for such machines as they’re generally located downstream to a beam punch or drill line. Roller conveyors are the commonly used handling system. In a post-blast structural blasting application, wheel quantity and orientation take on special significance. Complex geometries make it important for the designer to properly lay out the blast pattern and select the correct wheel diameter to ensure part coverage.

Let’s consider an application where we are required to clean structural steel of size 1.5M width x 2.0M height, fabricated with attachments. With some knowledge of suitable blast equipment, the initial reaction would be to consider an eight-wheel machine, with wheels located at four identical cabinet corner wall plates. This solution will be best suited for a simple structural steel component. However, if your current part has a complex geometry, or such complexity is anticipated in the future, it may be prudent to choose a twelve-wheel machine instead. The additional four wheels though not required to cover the work envelope will go a long way in providing additional blast coverage to certain features on the part that require a different blast angle of attack. The obvious question is the resulting operating cost of the machine. In general, for post-blast structural cleaning applications, if X kilograms of blast media is required to clean a particular component at a fixed line speed, it is beneficial to spread these X kilograms over greater quantity of blast wheels. In the bargain, the operating costs of an eight-wheel machine and a twelve wheel machine remain very close in comparison.

Illustration

A 444 mm diameter blast wheel powered by a 15 KW motor at 3600 RPM propels 106 kilograms of abrasive per minute. Eight such wheels will flow a total of 848 kilograms per minute and twelve wheels will flow 1272 kilograms per minute of abrasive. Let’s assume that in order to clean your structural steel at 1 meter per minute, you require 800 kilograms per minute of abrasive (this calculation is the function of the blast machine designer).

Therefore, theoretically, eight-wheels will do the job effectively. However, due to complexity of geometry, you’re being recommended a twelve-wheel machine in order to take advantage of additional blast angles provided by the four extra wheels. Based on the above discussion, it’ll be prudent to spread out the 800 kilograms of abrasive to discharge over twelve-wheels, which brings down the flow per wheel to 800 / 12 = 67 kilograms. In other words, you can manually adjust the flow control valves for each individual wheel and restrict the flow to around 67 kilograms per minute per wheel. This in turn draws less wheel amps than the motor in an eight-wheel machine.

The overall flow rate of the machine remains unchanged and neither is there a change to the sizing of the other reclaim system components. The benefit you have derived is the additional blast angles provided by the four wheels. Reduced amperage of the blast wheels will result in the same or marginally higher power consumption in the twelve wheel machine when compared to an eight wheeler. Given the cleaning edge provided by the twelve wheels, the increased capital investment for a twelve wheeler is well justified.

In spite of additional wheels, it is not uncommon to encounter incompletely clean areas in complex structures. The only remedy to this situation is a manual touch-up station downstream to the automated blast cabinet. The manual touch-up station can be built integral to the main machine so that it shares the same reclaim system.

  • A blast tank with a single or double outlet can be used for manual touch-up operations within the confines of the manual touch-up station
  • A blow-off nozzle (or the same blast nozzle, with the abrasive turned off) can be used to remove abrasive entrained in the pockets of the structural steel being cleaned.
  • Entrained abrasive can also be sucked up using an industrial vacuum hose if the content isn’t very high.

It is important to consider this option prior to deciding on a machine so that adequate space is reserved in your layout for the whole system. In certain cases, pockets could have significant depth, resulting in a high rate of media carryout. Without a blow-off arrangement to remove this carryout, your machine could easily run-out of abrasive in a matter of minutes. Therefore, it is critical to return this entrained abrasive back into the reclaim system and to the blast process.

Automated Blow-off Arrangements

In applications where the structural steel has a relatively simple geometry, but still showing the presence of shallow pockets, you could consider an automated blow-off arrangement eliminating the need for manual intervention. Blow-off cannons could be located all around the work envelope of your part to blow-off the abrasive prior to the part exiting the machine, very similar to the air knives seen in an automated car wash. Air for the blow-off is provided by a single or multiple blowers. These air canons can be adjusted for specific target angles and elevations if the location of abrasive entrapment is predictable for every part through the machine. All blow- off arrangements add air to the system that needs to be ventilated to the dust collector. The dust collector has to be sized appropriately to handle the additional air. Properly thought-out inline blast systems that are immediately upstream to a paint system would have provision for a manual blow-off station in addition to the automated version.

Summary

In this article, we discussed the importance of proper machine selection based on the type of structural steel being processed. Selecting the most suitable work handling arrangement is very important to avoid excessive handling. However, your basic plant layout might be a deciding/limiting factor.

Wheel layout and quantity of wheel units play a major role in the cleaning quality that can be achieved. As illustrated in the example earlier, increased quantity of wheels doesn’t necessarily have to result in higher operating costs per kilogram of steel processed through the blast machine. A judicious audit of requirement and machine capability should be carried out to determine process parameters such as media flow rate and production line speed.

This article was written by ERVIN's Kumar Balan appearing in the June 2012 issue of Steel Fabrication Process

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