Blog Purpose

The CTG Technical Blog is intended as a source of information on subjects related to industrial and precision cleaning technology. The writer of the blog, John Fuchs, has 40+ years of experience covering the entire gamut of cleaning. Mr. Fuchs has extensive knowledge of ultrasonic cleaning technology having been employed by Blackstone-Ney ultrasonics and its predecessors since 1968. The blog is also intended to serve as a forum for discussion of subjects related to cleaning technology. Questions directed to the blog will be responded to either in the blog (if the topic has general interest) or directly by email. Emails with questions about the current blog should be entered in the comments section below. Off-topic questions related to cleaning may be sent to jfuchs@ctgclean.com.

Rinse to Resistivity

August 8th, 2014

Previous blogs have discussed the importance of rinsing as part of the overall cleaning process.  Because of its importance, many schemes have been devised to assure that parts are adequately rinsed after cleaning.  Most of these schemes embrace the “more is better”  concept which often leads to overkill as there is often no practical way to verify the adequacy of rinsing.  Although rinsing is important, it is also expensive! Multiple rinses and high flow rates can add considerably to the cost of cleaning as equipment and utility costs escalate. Although it is not perfect solution in every case, “rinse to resistivity” may offer a means to assure (and verify) adequate rinsing while controlling cost, especially in highly critical applications.

Illustration showing rinse to resistivity concept

At the start, rinsing is removing ions from the part being rinsed. As rinsing progresses, fewer and fewer ions are present. Once the resistivity meter measures resistivity adequately low enough, the rinse process is considered complete.

The concept of rinse to resistivity is quite simple as shown above.  As I stated at the outset, however, it is not always viable in every cleaning application.  In order for rinse to resistivity to work, certain criteria must be met or considered.

  • The contaminant to be rinsed off must be conductive.  If the contaminant is not conductive (and there are some that are not) the resistivity meter will not be able to see any resistivity related to the contaminant and give a misleading result.  This is especially likely in the case of particles.  The resistivity may be below the target level even though a significant number of particles are still present in the rinse.
  • The rinse vessel volume should be as small as possible yet still accommodate the parts being rinsed.  If too large a vessel is used, one of two things can happen.  First, the contaminant on the part may not provide enough ions to register on the resistivity meter thereby indicating an adequate rinse when, indeed, little or no rinsing has taken place.  Second, the turnover time of a large vessel may significantly delay the detection of an adequate rinse condition.
  • Adequate flow rate must be supplied to provide a short turnover in the rinse vessel.  If the flow only provides one tank turnover every 20 minutes (for example) it may take 20 minutes to see the resistivity as measured at the outlet to increase enough to show a complete rinse.
  • The incoming rinse water must be significantly lower in resistivity than the target resistivity at the outlet.  If it is not, it may take longer to achieve the target resistivity.  Of course, if the resistivity of the incoming water is above the target resistivity level, complete rinsing will never be achieved.

Rinse to resistivity is a useful tool but must be applied correctly to be of benefit.  In many applications, rinse to resistivity is simply not applicable.  When considering rinse to resistivity, make sure that the quality people, the process people and the equipment manufacturer are on the same page.  Otherwise, the results may be less than expected.

-  FJF  -

Heat Alternatives for Cleaning

July 28th, 2014

I have stressed the importance of temperature to cleaning processes many times in previous blogs.  This blog will discuss the heat source options for achieving and maintaining the temperatures required for effective cleaning.

Electric -

Strip Heaters

Examples of electric strip heaters.

Electric heat is, without a doubt, the most prevalent heat alternative used for cleaning applications.  This is because electric heaters are relatively inexpensive, relatively small, easy to control, and because electricity is a utility readily available in most facilities.  Electric heaters are appropriate for applying heat to both liquids and gasses making them useful in heating liquids for cleaning and rinsing as well as air for drying.  The two basic types of heaters used to heat liquids are surface mount heaters (often called “strip” heaters) and immersion heaters.  Surface mount heaters have a flat profile which allows them to be affixed to the exterior of tanks using clamps, weld studs with bolts and other means.

Contoured Heaters

Examples of contoured electric heaters.

In the case of irregularly shaped tanks, the heaters can be formed to match the profile of the tank.  It is important that heaters of this type be firmly and permanently attached to assure efficient heat transfer from the heater through the tank wall and into the liquid.  In some cases, a heat conductive paste is used between the face of the heater and the tank wall to maximize heat transfer.  The surface of the heater not facing the tank wall can be covered with insulation to minimize heat radiation to the ambient and is especially important in enclosed spaces.

Immersion Heater

Example of an immersion heater.

Immersion heaters are placed directly in the liquid to be heated with electrical leads either looped over the top edge of the tank or exiting the tank through an appropriate fitting mounted on the tank wall.  In the case of immersion heaters, the efficiency is nearly 100% since the only path for heat is into the liquid.

In cases where air (or another gas) is to be heated, electric heaters may be provided as an open coil or with an enclosed element (similar to a strip heater) with fins to improve conduction and radiation.

Open Coil Heater

Example of an open coil heater for heating air.

Finned Heater

Example of a finned heater for heating air.

Steam -

The use of steam circulated through a heat exchanger immersed in a liquid is another means of heating liquids for cleaning and rinsing.  In most cases, steam is selected when the facility already has an existing source of steam making it economical.  The control of steam is usually accomplished using valves electrically actuated by a temperature control or by mechanical means using a capillary bulb or bi-metallic actuator.  Steam is an excellent choice in systems using flammable materials as, with steam, there is a greatly reduced chance of electrical discharge which might otherwise initiate combustion of flammable liquids or vapors.

Gas -

Gas heat utilizes a burner head attached to a combustion tube immersed in the liquid to be heated.  The application of gas heat is used primarily in very large tank systems because of the inherent size and configuration restrictions that apply to the combustion tubes which are several inches to a foot or more in diameter and must be of sufficient length to assure complete combustion.  The products of combustion of gas, of course, also need to be vented to the outside of the facility which is not always easy.  In general, tanks with a volume less than 800 to 1,000 gallons are not practical candidates for being heated by gas.

- FJF  -

Heat Capacity and Temperature Control

July 21st, 2014

How Much Heat? -

It is common for the design engineer to calculate the heat requirement for the tanks of a cleaning system based on the tank volume, the target operating temperature and the desired heat-up time from ambient temperature to operating temperature taking into account, of course, heat losses through the tank walls and from the surface of the liquid.  In most cases, this calculation is sufficient, but there are cases in which other factors must be taken into consideration.  The problem with heaters (especially electric heaters) is that they still continue to heat after being turned off.  In general, the larger the amount of heat, the more heat the heaters store in their mass as thermal inertia.  Even though the heat is turned off when the desired temperature is reached, the residual heat in the heaters may cause a “spike” in temperature that may be detrimental to the process.  Excess heater capacity can lead to temperature over-shooting and wide temperature variations due to higher thermal inertia.

Illustration showing the effect of thermal inertia.

In the above, the graph on the left shows the result of a heater with lower capacity while that on the right shows a heater of relatively higher capacity. Notice that although the heater with higher capacity achieves the temperature set point more quickly, there are greater temperature variations due to the greater thermal inertia of the higher capacity heater once the temperature target is achieved.

In another example, consider a case where cold parts introduced into the cleaning tank have significant mass or are being introduced in large numbers.  In such a case, since the parts are consuming heat as well, the heat required to maintain the desired temperature in the tank may be more than just that required to heat the tank up to the desired temperature in a given time.  Another consideration is in ultrasonic systems where ultrasonic energy provides heat to the tank.  As the majority of ultrasonic energy delivered to a cleaning tank ultimately turns into heat, a 2,000 watt ultrasonic transducer is very similar to a 2,000 watt heater in its effect on tank heating!  Depending on the heat load introduced into the tank by the parts, heat provided by the ultrasonic system may be adequate to maintain tank temperature without any additional heat at all.  In fact, there are rare cases where cooling must be employed to keep the tank temperature within the desired limits during ultrasonic operation.

Temperature Control -

Control of temperature in a cleaning tank may, at first, seem very simple – - a temperature sensor is used to turn the heat on when the temperature is below a set limit and off when the desired temperature is reached.  In many cases, a control of this type is adequate.  There are others, however, where due to thermal inertia, more sophisticated control schemes are required.  The two most common are the use of dual heaters and temperature controllers that anticipate the temperature target.  In the case of dual heaters, a higher capacity heater is used in combination with a lower capacity heater.  The two are used together until a temperature near the target temperature is reached.  At that point, the higher capacity heater turns off allowing its thermal inertia along with the lower capacity heater to achieve the temperature target.  In the case of anticipating temperature controls, a “smart” controller throttles the heat by cycling power on and off (time proportioning) or reducing voltage as the temperature starts to approach the target temperature.

Graphs showing the effect of time and voltage throttling

In the example on the left, the heater is turned off and on with decreasing on time as the set temperature is approached. In the example on the right, voltage is reduced as the temperature nears the set point. In both cases, the heater capacity is reduced by about 50% to maintain a stable temperature at the set point temperature.

Controllers of this type normally have adjustments to set the temperature differential at which the throttling process begins.  Some even track the rate of temperature increase to determine when to start the throttling process to achieve the desired temperature without over-shoot.

-  FJF  -

 

 

Temperature Sensor Selection and Placement

July 18th, 2014

Meaningful temperature measurements depend on the selection of the proper sensors and controls and the proper placement of sensors to accurately measure the targeted temperature.  For example, it was previously stated that bimetallic sensors, in most cases, ultimately sense the temperature of air around them.  This makes bimetallic sensors a good choice for measuring air temperature in a dryer.  To employ them in sensing the temperature of a liquid in a cleaning tank, however, requires that they be placed at a location where the air temperature is directly proportional to that of the temperature of the liquid to be measured.  This is not a simple task as heat must be transferred from the liquid through the wall of the vessel containing the liquid and then through the air to the sensor itself.  As we have noted before, air (or any gas) is not a good heat conductor.  As a result, a temperature measured in this way may either not reflect the actual temperature of the liquid in the tank or may result in a delayed response due to the time required for the heat to be transferred to the sensor through the tank wall and air.  Accurate measurements of liquid temperature require that the heat path from the liquid to the sensor be as short as possible giving thermocouples and resistance sensing devices an advantage when it comes to measuring liquid temperatures.  Capillary bulb type sensors can be used to measure the temperature of both liquid and air but, in the case of liquid, the capillary bulb itself should be either immersed in the liquid or bonded directly to the wall of the vessel using a suitable mechanical clamp and surrounded by a good conductor such as a highly heat-conductive epoxy.

Thermocouple Placement Illustration

In the above case of a tank with a side mounted heater and no circulation, a thermocouple at location “A” will read a higher temperature than those at “B” and “C.”

Although selection of the appropriate sensor is critical, even the best sensor will not provide the needed temperature indication if the sensor is not properly positioned.  In the case of a tank with heaters mounted on a single side, for example, liquid temperature may vary by tens of degrees depending on the position of the sensor in the tank relative to the placement of the heaters and the depth of the tank.  More heaters positioned around the tank will better distribute the heat but even distributed heat will not prevent temperature stratification in a large or deep tank.  A better solution is to distribute the heat using a pump loop or propeller type immersion agitator.

It is always a good practice to verify the accuracy of temperature measurements using a reference thermometer.  Although most temperature sensors are highly reliable, it is not uncommon for them to become detached or dislocated over time in which case, of course, they are not measuring the targeted temperature.  I prefer a laboratory grade mercury bulb thermometer encased in a metal sheath as a reference.  Hand-held thermocouple or RTD devices are also useful and especially so in cases where there is no direct access to the area where temperature is to be measured.

Note – Ultrasonic vibrations have been known to have an effect on the accuracy of some temperature sensing devices.  To prevent this possibility, reference temperatures should be measured with the ultrasonic energy off in ultrasonic tanks to assure accuracy.

-  FJF  -

More Temperature Sensors – Electrical

July 16th, 2014

Preceding blogs have described a number of mechanical means for measuring temperature based on the expansion and contraction of liquids and solids.  Another major classification of temperature sensors are based on electrical phenomenon.

Thermocouples -

A thermocouple sensor utilizes a junction of two dissimilar metals to measure temperature.  The principle is based on the fact that an electrical potential (voltage) is created in any conductor exposed to a thermal gradient along its length.  The two materials in a thermocouple are chosen such that an instrument (really a specialized voltmeter) can measure the difference in voltage produced by two conductors both subjected to the same thermal gradient.

Note – Contrary to popular belief, the voltage produced by a thermocouple is not actually created in the junction but rather along the length of the two conductors subjected to a thermal gradient.

In order to accurately measure this difference in voltage, the leads from the “hot junction” of the thermocouple (subjected to the temperature to be measured) must be joined to other conductors at what is called the “cold junction.”  Although this secondary connection historically was made in an ice bath to assure that any voltage measured by the sensor was only the result of the thermal gradient produced by the temperature to be measured, instruments today usually incorporate a reference “cold junction” and do not require such measures.  Thermocouple leads are connected directly to the instrument which is then able to convert the measured voltage to a temperature readout in the appropriate units.  Thermocouple junctions are made using several different combinations of dissimilar metals based on the application.  It is important that thermocouples and instruments indicating temperature be compatible to assure the accuracy of any measurement.

Resistance Temperature Detectors -

The resistivity of many materials changes with temperature.  This change in resistivity can be used to measure temperature.  Common forms of resistivity temperature detectors (RTD’s) use pure metals either deposited on a substrate or as wire wrapped around a non-conductive core to measure temperature.  The measuring instrument senses the resistance of the device and converts it into temperature readings.  In general, RTD’s are preferred over thermocouples due to their accuracy, long life and interchangeability.

Thermistors -

Thermistors are made of semi-conductor materials which change resistivity with temperature.  Their most common use is in applications where the change in temperature of the thermistor due to a flow of current through it results in a temperature increase. The resulting change in resistance due to an increase in temperature is used to control the flow of current through the device directly.    Thermistors can be designed to either increase or decrease resistivity on heating.  This makes them valuable in limiting in-rush currents and providing controlled heat in many applications.

Calibration -

All of the above can accurately measure temperature change but must be calibrated to measure absolute temperature.  In the case of thermocouples, the reference or “cold junction” is produced within the measuring instrument but, generally, requires initial calibration to assure accuracy.  Further calibration of thermocouple devices is required as changes in insulation characteristics and other factors may compromise accuracy over time.  In the case of RTD’s, the calibration is performed in the manufacture of the sensing device and is permanent.  Calibration of RTD’s is generally not required except for verification.  The way a thermistor responds to heat is determined by the mixture of semiconductor materials used in the manufacturing process and may vary somewhat from one device to another.  Small variances are of no consequence in many thermistor applications but, if they are used to measure temperature directly, proper calibration is required to produce an accurate reading.

Although there are other ways to measure temperature including non-contact infrared sensing devices, the types described in the last few blogs are those most commonly used in industrial cleaning equipment.

-  FJF  -

Temperature Sensors Continued

July 11th, 2014

In the preceding blog we looked at the expansion and contraction of liquids as a means of measuring temperature.  The expansion and contraction of solids (usually metals) is another way to measure temperature.

Most solids expand and contract in a relatively linear way in response to temperature changes.  In most cases, solids expand as they are heated and contract as they are cooled.  The amount of expansion and contraction of solids (the coefficient of thermal expansion) is typically measured in inches per inch per degree Fahrenheit (or the metric equivalent) and, for common materials, is usually less than 100 times 10 to the minus 6th inches per inch per degree of temperature change.  Because of this very small change in dimension, using a direct observation of the expansion and contraction of a solid to measure even relatively large changes in temperature is quite difficult and impractical.  Solids, obviously, will not travel up a capillary tube as described for liquids in the preceding blog.  There are, however, ways to magnify these small changes in the dimension of solids that make them practical as a means of measuring temperature changes.

The elegant solution is to use two pieces of solid material (usually metals) with differing coefficients of thermal expansion that are fused together as shown in the illustration below.

Bimetallic sensor illustration

Metals with different coefficients of thermal expansion are fused together to make temperature sensors. As one metal expands (or contracts) at a different rate than the other, the result is deformation of the sensor.

As a bimetallic sensor is heated, the two solid materials expand at different rates which causes the composite strip or disc to deform in order to equalize internal forces.  Depending on the materials used, this deformation can be thousands of times greater than the actual growth of either of the materials the bimetallic is composed of by itself.  This larger motion makes measuring temperatures by using the expansion and contraction of solids practical.

In the case of a bimetallic strip, the motion of the bimetallic can be further amplified by forming the strip into a spiral.  With one end of the spiral fixed, the tightening and relaxation of the other end can be used to measure temperature changes directly.  If the outer end of the spiral is fixed, the center of the spiral will rotate with temperature change.  This rotation can be used directly to measure changes by adding a pointer.  An everyday example of this is a simple dial thermometer which can be purchased in any hardware store.  If the inner end of the spiral is fixed, the outer end will rotate.  Thermostats used to control home heating systems often use this motion to tilt a mercury switch which controls the heat source.

Bimetallic discs deform with changes in temperature causing the center of the disk to be displaced perpendicular to the plane of the disc.  This is the principle behind the operation of common “click disc” thermostats.  Although commonly used to indicate that a target temperature has be achieved in an on/off control system, when properly designed, the disc will remain deformed even when the heat source is removed until it is mechanically returned to the initial position or “reset” making them useful in “safety” applications.  In most cases these click devices are pre-calibrated to actuate or “click” at a single temperature.  Devices using a bimetallic strip, however, are usually adjustable and can be calibrated to indicate a range of temperatures.

In general, bimetallic sensors are best used in applications requiring the measurement of the temperature of air.  Occasionally, the disc type sensors are applied to surfaces but, in fact, are measuring the temperature of the air near the surface and not the temperature of the surface itself.

Upcoming blogs will discuss the use of changes of electrical properties of materials to measure temperature.

-  FJF  -

Temperature Sensors

June 25th, 2014

In preceding blogs I have identified temperature as probably the single most important variable in a cleaning process.  Devices that sense and control temperature, therefore, are a critical part of a cleaning system.  Let’s take a quick look at a few of the types of temperature sensors that are available and suggest where each is applicable in cleaning.

Temperature sensors can be divided into two basic categories.  Those which are mechanical and those which are electrical in function.

First, let’s look at mechanical type sensors.

Thermometer

In a thermometer of this type, expansion and contraction of the liquid in the bulb is amplified as the liquid is pushed up into the attached small diameter tube.

Most gasses, liquids and solids expand in volume as their temperature is increased and shrink in volume as their temperature is decreased (with the notable exception of water which grows a little after it freezes).  Mechanical temperature sensors rely on the expansion and contraction of a sensing element (usually a metal, or a confined liquid or gas) to measure  changes in temperature.  A mercury thermometer is probably the best known example of a temperature sensor that uses the expansion and contraction of a liquid (mercury) to indicate temperature.  Since the expansion and contraction of the mercury is relatively small, an arrangement is used which consists of a large volume of contained mercury (the bulb) which is directly connected to a very small diameter tube.  In this arrangement, the small diameter tube amplifies the result of expansion and contraction of the mercury in the bulb as the mercury is pushed up into the tube.  The result is that a very small amount of expansion of the mercury in the bulb creates a relatively larger change in how far the mercury is pushed up into the small diameter tube.  This makes the device more accurate.  With all the sensitivity about the health concerns of mercury in today’s environment, mercury is often replaced with another liquid, colored alcohol for example, for safety’s sake.

Thermometers of this type are commonly used to measure the temperature of a liquid or gas in laboratory or domestic settings.  They are simple to use and allow precise, permanent calibration making them ideal for reference use.  In this simple form, however, thermometers of this type do not offer the ability to provide feedback to control temperature.

Industrial temperature sensors utilizing the expansion of a liquid to control temperature are often called capillary thermostats.  A large bulb, usually containing a liquid, connects via a small diameter (capillary) tube to a remotely located instrument with a diaphragm which flexes in response to the amount of liquid displaced from the bulb caused by temperature changes.  Since the volume of liquid in the capillary tube is insignificant compared to the volume of liquid in the bulb, the displacement of the diaphragm is primarily the result of the expansion of liquid in the bulb and not that in the capillary.  Movement of the diaphragm or bellows can be used to close or open an electrical circuit through a switch or directly control the flow of a liquid or gas (refrigerant or steam, for example) through the use of a valve.

Illustration of a capillary bulb thermometer.

In a capillary type thermostat, expansion of liquid in the sensor bulb results in physical displacement of a diaphragm or bellows connected to it by a small diameter (capillary) tube.  This motion can be used to actuate switches, valves, etc.

In order to accurately sense temperature, the bulb of a capillary thermostat needs to be positioned in such a way that the entire bulb is heated or cooled as the temperature being sensed changes.  Immersion of the bulb in a liquid or gas, for example, will give the most accurate temperature reading.  Clamping a bulb to a tank containing a liquid, on the other hand, does not always result in an accurate temperature measurement as only one side of the bulb is seeing the heat source while the other side remains at ambient temperature.  In some cases, an insulating material is placed over the sensing bulb on the side opposite to the surface being measured to minimize this potential inaccuracy.

to be continued

-  FJF  -

Automation – Suspended Conveyors and Conveyor Belts

June 20th, 2014

A third classification of automation is a constantly moving or indexing device that conveys parts through the cleaning process.  This can either be in the form of a conveyor that suspends parts on hooks or other fixtures that hang from an overhead track or in the form of a conveyor belt on which the parts rest as they are moved through the cleaning process.  This type of automation is particularly attractive in cases where parts are already handled using a conveyor in manufacturing steps preceding or following cleaning and is also ideal in many “lean” applications.  Good examples are sheet metal parts being cleaned prior to plating or painting.  The use of a conveyor for cleaning often eliminates one or more handling steps which would otherwise be required if parts were cleaned in baskets or individual racks.

A clear benefit of using a conveyor system is that the cleaning process is continuous.  Parts can enter the system at random intervals and still receive consistent processing.  Parts progress through the cleaning process in order which means that batch integrity can be maintained if required.

Conveyors are probably best employed in applications where parts are not immersed for cleaning.  Conveying parts through a spray washer, for example, is a relatively simple matter.  Processing steps are aligned horizontally with the parts being moved by the conveyor.  In cases where changing of the elevation of parts is required (as in an immersion process), the attraction of using a conveyor may be somewhat diluted.

One excellent application for a conveyor is in the processing of parts in “near field” ultrasonic applications.  It is relatively simple to spin a part about its vertical axis using a simple sprocket arrangement similar to a rack and pinion mounted at the top of each suspension device.

  • In order to immerse a part for cleaning, a conveyor must ramp vertically into and out of the immersion tank since most conveyors can not accommodate a drastic change in elevation within a short horizontal distance.  Instead, the conveyor must gently slope to effect elevation changes.  These gentle slopes can add considerable length to a cleaning system especially in cases where the vertical profile of the part being cleaned requires a considerable change in elevation for total immersion.
  • In the case of a conveyor belt, immersion requires that the conveyor belt itself must be immersed in the cleaning process.  In essence, the conveyor belt adds to the overall load on the cleaning system as, although it may not be dirty, it still adds to tank – to – tank carryover and requires drying on each pass.

Considerations -

As is the case with walking beams and pushers (which are really just intermittently indexing conveyors with a vertical motion component), a conveyorized process is relatively inflexible.  The time in each processing step is determined by the relative horizontal length of each process step as the conveyor moves at a single speed.  Slowing down or speeding up the conveyor will result in the duration of all process steps being increased or decreased accordingly and will, of course, result in a associated change in the total throughput capability of the system.

Although not applicable in all situations, conveyors are a viable option for automation for cleaning and are worthy of consideration especially in cases where parts are already being moved through the manufacturing process by a conveyor.

-  FJF  -

Automation – Automated Hoist

June 16th, 2014

An automated hoist is probably the most commonly used means of moving parts through the steps of an automated immersion cleaning process.  These hoists come in almost unlimited variations but have in common that they can move a basket or rack of parts in two dimensions along the axis of a multi-station cleaning system.  Although the hoist suspension can be located directly over the tanks, the preferred configuration is to have a cantilevered load bar supported by a structure offset from the cleaning tanks to minimize contamination that might be introduced by the moving parts of the hoist.

Schematic illustration of an automated hoist transfer system.

An automated hoist can move baskets or racks of parts through an automated cleaning system by moving vertically and horizontally. This type of system is more versatile than any other means of automation commonly used in the automation of cleaning processes.

The above illustration shows the basic motion or an automated hoist.  Baskets or racks of parts are sequentially moved one step forward in the process.  In the illustration, the red arrows show the path of the hoist.  First, the hoist engages the carrier in process station 4 and moves it to the unload position.  The hoist then disengages that carrier and moves back to station 3 and moves the carrier in process station 3 to process station 4.  This sequence continues until all carriers have been moved forward one position.  The hoist then goes to process station 4 and repeats the process described above.

Advantages -

  • Process stations do not need to be spaced equally distant from each other nor do they have to be at the same elevation as the hoist can be programmed to account for such variations.
  • An automated hoist can be programmed to sequence carriers through the process stations in any order and with different dwell times at each process station.
  • Because the hoist only lifts one carrier at a time, the mechanical drives do not need to be as powerful as those for a walking beam transfer system.

Considerations -

  • Carriers can not be processed through the system any faster than the longest process time will allow.  This, in essence, limits an automated hoist system to the same situation of having all process steps of equal duration just as in the case of a walking beam or pusher system unless multiple stations are added where additional time is required.
  • Due to inherent limitations in the speed of movement of the hoist, a process consisting of a number of relatively short duration steps may exceed the capability of the hoist thereby limiting the throughput of the system.  This is especially true if a large number of process steps are involved.
  • As the number of process steps is increased, the demand on the automated hoist grows.  Should the total time required for all transfers to occur exceed the time required for the longest process step, throughput will drop as the hoist can not return quickly enough to make the required transfer.

Increasing Capacity -

In many cases design engineers resort to a scheme using several moving hoists mounted on a single track in an effort to meet throughput requirements.  Although one would intuitively expect to double throughput using multiple hoists, the actual benefit is usually considerably short of that goal.  Throughput rates are notoriously difficult to predict, especially in the cases of “smart” controls which have decision making capability (which carrier must be moved next to satisfy process requirements).  In a case where one or more process steps are confined by a maximum dwell time (unless it is the maximum time of all process steps), special care should be taken to assure that process requirements can be met with the proposed hoist configuration.

In summary, automated hoists, when properly designed, are a great way to move parts through a cleaning system.  Automated hoist transfer systems, despite their capability and flexibility, do have limitations which should be considered before committing to such a system.

-  FJF  -

Automation – Walking Beams and Pushers

June 13th, 2014

As in nearly any decision making process, one must consider the options available and the pros and cons of each for industrial parts cleaning automation.  The next few blogs will describe some of the automation options that are available for industrial parts cleaning along with their benefits and potential shortfalls.

Walking beams and “pushers” have been around for a long time and are relatively simple in concept.  They are both implementations of the same basic idea of simultaneously moving a number of racks or baskets of parts from one process step to the next through the use of an indexing system.

Walking Beam -

Illustration of a walking beam transfer system

A walking beam transfer system lifts all baskets or racks simultaneously and indexes them to the next position.

A walking beam is usually positioned above the process path but is offset to the side in some cases.  This beam has several hooks that can engage the items to be moved.  The beam lifts the items being processed and indexes them to the next process position.  Once the items have been moved, the beam dis-engages and returns in preparation for the next indexing event.

Illustration of a pusher sysetem

In pusher automation, parts are lifted to a height above the process tanks on roller or slide tables and are engaged by a device that pushes them to the next process station. Once moved, they are lowered by the lift tables and the “pusher” retracts in preparation for the next indexing event.

In a pusher system, baskets or fixtures are lifted by a lifting means in each tank to a height above the top of the process tanks.  They are then pushed from one process station to the next by a sliding device which engages each basket.  Rollers or sliders are usually provided to bridge the gap between process tanks.

There are many variations of walking beam and pusher systems but, in general, any device that indexes baskets or racks from one station to the next simultaneously falls into this category.  The benefits and limitations are similar in all cases.

Advantages -

Walking beam and pusher systems are quite simple and are “clockwork” in nature.  The same linear motions are repeated each time parts are indexed.  They are both very efficient and able to fully utilize process resources as there is a basket or fixture in all tanks except for the short time while the transfer takes place.

Considerations -

I have used the header of “considerations” here as not all of the below are necessarily disadvantages but, rather, are things that must be considered when thinking of a walking beam or pusher system.

Flexibility – Once a system has been installed, the only real variable is the time between indexes.  It is not possible, for example, to change the time in each individual process step making it different from the others.  The only option is to use duplicate process stations to multiply process times if needed.

Spacing – Because all indexes must be the same distance, spacing between stations must all be the same.

Structure – In the case of the walking beam, the beam must be able to lift the entire weight of all baskets or fixtures at once.  The result is that air cylinders or other mechanical drives must be of higher capacity than those of a system that lifts one basket or fixture at a time.

Walking beam and pusher automation systems have served industrial parts cleaning well for decades and continue to be popular in simple applications where processes are well-established and not likely to change over time.  In general, they are economical, require little maintenance and are easy to understand and control.

-  FJF  -