Technically Speaking

The following technical papers are full length versions of the original abbreviated articles that have appeared in our bi-monthly AKcelerator newsletters. Click here to receive our AKcelerator newsletter.

SELECT AN ARTICLE BELOW

1. The Effects of Screw and Barrel Wear on
Extruder Performance.

2.  Melt Temperature Measurement. How to do it
and what does it mean.

3. Feed Section Choking Your Profits?

4. Pressure Stability 101...

5. Live Long and Prosper... Let's Talk Gearboxes

TECH TIP No. 1
From the AKcelerator - March Edition

The Effects of Screw and Barrel Wear on Extruder Performance

Wear of screws and barrels can significantly effect throughput and quality of extrusion products. The level of negative impact depends on four key factors:

1. Amount of combined screw and barrel wear
2. Viscosity of the polymer(s)
3. Head pressure
4. Screw speed

Materials of Construction:

Wear can occur through both abrasion and/or corrosion. In our attempt to minimize wear from both of these sources, there are many choices to choose from including:

Common choices for abrasion and corrosion resistance on screws are:

	Colmonoy 56: 2% carbon/13% chromium/2.7% boron/4% silicon/4% iron/balance nickel
	Colmonoy 83: 2% carbon/20% chromium/1% boron/1.4% silicon/1.4% iron/34% tungsten/balance nickel

For abrasion resistance on barrels:

	Xaloy 102 (or equivalent): Boron/Iron alloy with 58-66 Rc hardness (Iron, Boron, Nickel, Chrome, and Silicone)*
	Xaloy 800 (or equivalent): Nickel alloy with 55% - 85% Tungsten carbide with 58 - 68 Rc hardness (11-17% Chrome, 4-4.5% Iron, 3.25-4.25% Silicone, 2.5-3.5% Boron, balance is Nickel)*

For corrosion resistance on barrels:

Xaloy 306 (or equivalent): Nickel based alloy with 50-59 Rc hardness with 0.5% iron maximum (3-4% Boron, 4.5-5.5% Silicone, 6.5-7.5% Chrome, balance is Nickel)*

History:

The "standard" pairing for good compatibility is Colmonoy 56 and X-102 barrel linings. Some have successfully mixed and matched Colmonoy 83 with 102 and Colmonoy 56 with X-800, but accelerated wear can sometimes occur in the "weaker" component. Best compatibility when tungsten carbide hardened screws are employed (Colmonoy 83) is a tungsten carbide barrel lining (X-800 or equivalent).

In an effort to improve the wear resistance of Colmonoy 56, American Kuhne specifies that it be applied in two passes on 4.5 inch screws and larger, reducing the dilution with the base 4140 steel and improving the depth of the wear resistance protection. Colmonoy 83 is fully effective when applied in a single pass.

It is also important to achieve "full width" application of the Colmonoy (56 or 83) to provide maximum protection to the vulnerable edges of the flight. Many suppliers do not apply Colmonoy to the flight edges because they are unable or unwilling to mill the hardened material at the edge of the flights.

As a general industry wide rule of thumb it is believed that typical barrel life is, on average, 2 to 3 times that of the screw life (with compatible wear resistant materials). Additionally, it is generally believed that 50% to 200% more life is typically realized with the Tungsten Carbide based materials compared to the "standard" materials. However, the processing "window" for every application differs. For example, is the material high or low viscosity, is there high or low head pressure, is the machine already being pushed for maximum output or is it running at 50% capacity, can the downstream cooling tolerate higher melt temperatures or is it at its limits already?

Wear is typically monitored on production lines and the life of various barrel linings and screw coatings are either determined to be adequate and repeated (screw rebuilds and barrel replacements with the same material), or improvements are desired and additional capital investment is justified. Screws are typically offered with Colmonoy 56 as a standard weldment and Colmonoy 83 given as an option. Barrels are commonly offered with Xaloy 102 (or equivalent) liners with Tungsten Carbide as an option. The additional cost can then be compared to the promise of added screw and barrel life.

Processing Issues:

The advent of higher performing resins with improved physical properties has often created greatly increased wear compared to more traditional "standard" resins. The following scenarios are considered "high wear" and worthy of serious consideration in favor of the Tungsten Carbide based materials:

	50% or greater blends involving fractional melt HDPE, LLDPE, RPVC w/TiO2, some Metallocenes, and/or fillers of various types.
	Screw speeds higher than 50 rpm
	Colder than "normal" barrel temperature set-points
	Die back pressure above 4,000 psi

As wear occurs, the natural reaction of process engineers and operators is to simply increase screw speed to compensate for the evolving loss in output. While this approach is successful in accomplishing the short term goal to regain throughput, the increase in screw speed can result in significant increases in melt temperature causing problems in cooling and sizing the product. These increased screw speeds can also result in product quality problems as well, especially dimensional instability due to surging.

Discussion

Total wear is the combination of screw and barrel variations from new dimensions. In addition to the potential for lower output and higher melt temperatures, melt quality is often negatively effected as well. In high output and low melt temperature applications, any wear will reduce usable rate, since the extruder is operated in a maximized condition. Certainly wear of 0.020" on the screw diameter will reduce rate measurably (perhaps 10%), and rebuilding would be easily justified. Most manufacturers will allow screws to wear to .050-.070" or more, living with reduced rates or higher than desired melt temperatures (and the resulting decrease in productivity and quality). In co-ex applications, certain products will dictate that, at least some of the extruders will not run at maximized rates, so those machines have the versatility to allow some wear, which is overcome by simply increasing screw speed. The higher the viscosity of the polymers being extruded, the greater the wear can be expected. However, more wear can be allowed before a significant reduction in output and/or quality is experienced. The higher the screw tip pressure, the less wear can be tolerated (maximum pumping efficiency is required).

Conclusion

The advent of higher viscosity modern engineering resins, including Metallocenes has lead to added wear being realized over time and even more pressure to improve the wear resistance on screws and barrels. The cost "adders" to upgrade from Colmonoy 56 to 83 have dropped significantly in recent years and, as a result, many extrusion processors have decided on the Tungsten Carbide pairing of the screws and barrel linings to maximize life. The added initial investment is usually more than offset by the higher performance and/or quality, as well as the delayed maintenance of the barrel/screw changes.

Given the variety of blends, dies, and running conditions that a typical line experiences over an extended period of time, it may be difficult to accurately retroactively ascertain the wear rate and resulting effect on productivity. However, it's never too late to inspect screws and barrels as any measurement of significant wear (over .020" to .040") will certainly indicate a measurable loss in output and/or quality.

Fortunately, with a new barrel and screw, base line data can be documented to be compared at regular intervals for evaluation of output loss and melt temperature increase. Given production pressures and/or requirements for maximum profitability, new/rebuilt screws and new barrels are often easily justified with fast pay back, often in just a few months.

Ed Steward

Director of Process Technology
American Kuhne

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TECH TIP No. 2
From the AKcelerator - May Edition

MELT TEMPERATURE MEASUREMENT

How to do it and what does it mean

The majority of extrusion applications require the achievement of two primary process goals: maximum output and the lowest possible melt temperature. The exceptions would include extrusion coating, cast film and melt blown fiber, where melt temperature is often deliberately maximized for those processes. While there are many other important secondary objectives - pressure stability (no surging), good mixing, surface finish, and physical properties - the relationship between high output and low melt temperature is critical to most profitable extrusion operations. In fact, it is often impossible to talk meaningfully about extruder output without a simultaneous discussion of melt temperature level. An extruder and screw combination can be defined to pump a given output/RPM, so why can't the machine be operated at 200 rpm or higher to achieve greater output at given size extruder? The melt temperature rise with screw speed usually limits operation at these high screw speeds due to melt viscosity that gets too low to hold product dimensions. Furthermore, polymer degradation, and the associated decline of physical properties may occur at the elevated melt temperatures rendering the product useless. There are often additional extruder operational problems that would limit the maximum screw speed, including a decline in pumping stability (surging) due to the screw design, excessive gearbox temperatures, and insufficient horsepower necessary to attain the higher speeds.

What are the three basic methods to measure melt temperature?

Of the three most common methods used to measure melt temperature, two of them are used regularly in production and one is typically used during process development or between production runs.

The most common flush mount melt thermocouple is most often installed in the extruder barrel, but occasionally mounted in the die adapter. While it is the least "obtrusive" to the process, it is also the least accurate. Industry studies have shown that the indicated temperatures of flush mount sensors are more significantly influenced by the temperature of the metal they are mounted in, than the temperature of the plastic passing by them. Change the temperature of the zone in which the sensor is mounted and watch the indicated "melt temperature" change. This is a fairly popular approach, as a flush mount sensor can even be mounted inside a pressure transducer body. Since most extruders have a mounting hole for a pressure transducer, installing a combination transducer with built-in flush mount thermocouple seems advantageous. However, the resulting reading is not a good indication of true melt temperature. This approach appears to at least give a temperature reading without interrupting the melt stream, but the result is not accurate and repeatable enough to be useful as a process parameter.

A less common in-process approach is the immersed probe, typically mounted in the die adapter, but occasionally mounted in the die body itself. The immersed probe has the distinct advantage of minimizing the effect of the die adapter/body temperature as the sensor is positioned directly in the melt stream. In our experience, we have found the "exposed junction" immersed probe to be the most sensitive and accurate. An adjustable position (variable depth) immersed probe is shown below.

The immersed thermocouple method is not without its inherent inaccuracies. The sensor can be subject to high levels of shear (depending on resin viscosity and velocity in the die system) that significantly increases the indicated melt temperature compared to the actual melt temperature. The high sensitivity of the exposed junction feature allows melt temperature variation in the machine direction to be accurately measured. As a result, detailed conclusions regarding melting and mixing efficiency of the screw performance can be made. Melt temperature variation of more than 2 degrees or 3 degrees Fahrenheit usually indicates less than optimal melt quality/homogeneity. If one desires to determine the additive effect of the "in-process" shear, stop the extruder abruptly and watch the reading change. After a few minutes, the actual melt temperature will be seen as the reading "decays" from the operational reading. Stopping the flow that is passing by the thermocouple tip removes the shear, but the true temperature of the melt in the die/adapter will not change for some time. So, how do we accurately measure "actual" melt temperature?

The third approach is the most accurate, but may also be the most disruptive to the process. After over 30 years of study, we have determined that a hand held contact pyrometer (needle probe) gives us the most accurate melt temperature readings. A needle cannot be immersed in the melt stream during the production process, except at startup or shutdown, when the line can be broken, but the extrudate is best measured by the needle pyrometer after exiting the die. There are some operating techniques involved with the needle pyrometer that can effect the final reading. Experience shows that the most accurate and repeatable technique is to allow a small quantity of the melt to be collected upon exiting the die and immediately immerse the needle into the center of the collected extrudate. An alternate, but less consistent method is to insert the needle into the extrudate at or near the die orifice while the machine is running. A non-contact reading of the product exiting the die can be made via an infra-red (IR) pyrometer. Keep in mind that the IR reading is a surface measurement and can be influenced by external light sources, operational techniques and by emmisivity calibration errors. Comparing melt IR readings from one extrusion line to another using the same IR device should yield reasonably accurate results, assuming environmental and emmisivity issues. Comparing IR readings from one IR unit to another may have inherent inaccuracies. The actual reading of the IR can be compared to a needle pyrometer at startup and the emmisivity can be adjusted to improve accuracy. Some applications provide sufficient exposure to the extrudate to enable occasional or constant monitoring of melt temperature with an IR sensor.

See the graph below for a better understanding of the relationship between these three measurement techniques and the variation in indicated melt temperature. While the flush probe basically measures the die/adapter metal temperature near it's tip, you can see that the indicated melt temperature does increase, albeit at a lower rate than the "actual" melt temperature. As the true melt temperature rises with screw speed, the internal die/adapter metal will also rise above the 350 degrees Farenheit set point due to the added heat from the hotter melt passing inside.

The immersed probe's readings increase to a point above the true melt temperature of the "actual" melt temperature (pyrometer reading) due to the shear heating effect discussed earlier. Each method may have their benefit, but true readings are better understood when the above graph's reality is taken into account.

Which method is best for you?

An immersed probe is generally the most cost effective and accurate single solution for in-process measurement (real time readings, no operator influence), however there are several criteria to consider:

	Immersed probes should be positioned approximately 1/3 to 1/2 of the distance across the adapter bore for most accurate temperature readings.
	Immersed probes should not be used in production for degradable compounds such as PVC, wood composites, cross-linked, and foamed materials.
	While the indicated temperature is repeatable with a given material at a given output, it can be 5 to 50 degrees F higher than the hand held pyrometer reading. This depends on screw speed, viscosity, screw tip pressure, position of the probe in the flow, etc.

In almost all cases, a flush-mount melt thermocouple will not provide a reading that is accurate enough to be useful. If your process and budget allows, a continuous infra-red system may also be a viable option. And finally, a hand held (needle) pyrometer is always a cost effective and useful tool for non-production process development.

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TECH TIP No. 3
From the AKcelerator - September Edition

FEED SECTION CHOKING YOUR PROFITS?

The feed section is one of the most commonly overlooked components on an extruder. Let’s first explore the multiple functions that a feed section performs in the operation of an extruder.

1. The size and shape of the vertical opening in the feed section can restrict the flow of raw materials limiting throughput of the machine.
2. The geometry and configuration of the feed section around the screw flights can significantly improve or disrupt the efficiency of conveying the material from the hopper to the melting (transition) section of the screw.
3. Because most feed sections have a water cooling jacket, they prevent the heat from the first barrel zone from migrating back to the gearbox. In many extruders the thrust bearing is immediately behind the feed section in the gearbox and it is very important to keep the bearing and its lubricating oil cool.
4. The water jacket also prevents the incoming pellets, regrind, or fluff from melting prematurely causing bridging on the in-feed. This bridging may occur in the vertical opening of the feed section, or may result from hot resin sticking to the root of the screw.
5. Furthermore, existing equipment in the field can suffer from additional problems resulting from years of use, such as:

   A. Increased clearance around the screw flights due to wear and/or damage (grooves resulting from metal contamination), further reduces the feeding and conveying of pellets, regrind, and/or fluff – resulting in reduced output or quality.
   
   B. Partially or completely clogged water-cooling jacket preventing proper water flow, and therefore accurate temperature control – causing intermittent or permanent feed interruptions and long term damage to the gearbox and/or thrust bearing.
   
   C. Wear in the rear bushing/seal area that allows material (regrind, fluff, and fines) to escape and build up in the air-spacer area of the feed section – creating a housekeeping mess.
   
   D. Older extruders often have much smaller feed openings (some machines have only a 1 diameter round feed opening) significantly restricting material flow and throughput compared to today’s large rectangular feed openings.

What’s the ideal temperature of a feed section?
Ideally the feed section casting should be warm to the touch, between 100°F and 140°F. For most extruder sizes and applications, this translates into approximately 1/2 to 3 GPM flow rate of 50°F (or higher) plant water. Colder may cause condensation that can result in introducing moisture into the process. Too hot may cause partial or complete bridging of the feed from the hopper in the vertical opening of the feed section casting. Elevated feed section temperatures may also raise the screw temperature to the point where resin sticks to the screw causing bridging on the screw root. And finally, excessive feed section temperatures will increase the risk of elevated gearbox oil and thrust bearing temperature, ultimately reducing the longevity of those components.

Which feed section geometry is best for my process?
North American extruder suppliers and processors have historically used “smooth feed” designs for the vast majority of extrusion applications. However, Europeans have predominantly used “grooved feed” feed section designs to process most resins.

This grooved feed approach uses a much longer design that also requires a much higher level of water cooling than smooth feed. The grooved feed design also requires a unique screw design that is much shallower in the feed (compared to smooth feed screws) to insure that pellets effectively “lock-in” to the grooves. The higher torque (larger drive, motor, and gearbox) required by this more aggressive grooved feeding method also translates into higher extruder prices for given size of extruder, however, the higher output often justifies the higher price. Nevertheless, the more common smooth feed section can be adapted to various internal geometry designs that can positively effect processing throughput and consistency. AK has developed field replaceable liners with grooves, tangential undercuts, and other design changes to specifically solve feed related problems related to a resin’s frictional characteristics (fractional melt HDPE for example), regrind feed problems (size, shape, and low bulk density), and early melting and feeding problems (urethane, PBAX, etc.).

If you are interested in a detailed review of your specific application(s) please do not hesitate to contact your local American Kuhne sales representative or feel free to e-mail Ed Steward or call him at (860) 886-7745, ext. 114.

Tech Tip No. 4

From the AKcelerator - December Edition

Pressure Stability 101..

Zen and Art of Pressure Stability 

The fundamental processing difference between extrusion and injection molding is the unavoidable fact that in extrusion, dimensional tolerances are controlled by the pressure and temperature stability produced by the screw design.  While the importance of accurate melt temperature measurement and stability has been covered in the May issue of the AKcelerator, this issue will focus on improving pressure stability - or eliminating surging.

Screw designers and suppliers often promote and focus their marketing efforts on extruder output.  While output is of primary concern, pressure stability is a close second in the race for profitability.  Optimizing pressure stability provides two critically important benefits:

1.    A stable extrusion output enables processors to meet their customers' requirements for dimensional tolerances.  This, of course, leads to happy repeat customers enhancing cash flow and profitability.

2.   Maximum pressure stability enables processors to tighten product tolerances beyond the customers' requirements minimizing raw material usage (and/or yield) - further leading to increased profits as well.

What are the potential sources of pressure instability?

There are four basic sources of pressure stability:

1.   Mechanical feeding issues relating to insufficient feed stock bulk density and/or bulk density variation.  Typically, a bulk density of 20 pounds per cubic foot is sufficient to sustain consistent feed delivery and pressures.   A feed bulk density below 20 pounds per cubic feet will likely require feeding assistance in the form of crammer feeders, pneumatic ram feeders, etc.

2.   Inconsistent feed stock particulate size and shape combined with an incompatible feed section size and/or geometry can also lead to pressure instability (see the September Technically Speaking for more details).

3.   The screw design is incompatible with the polymer(s) to be processed such that it does not process well at any speed or output.  Or, the screw is mismatched with the performance goals.  That is, the screw may work well at lower speeds and lower rates, but falls short in performance at the speeds and outputs required to make the extrusion job profitable. 

4.    Poorly tuned barrel temperature controllers that drift significantly from set-point (say more than +/- 5⁰ F) can also induce melting or feeding variations that result in pressure instability.

How much pressure stability is enough?

For most extrusion processes, pressure stability of +/- 1% at maximum output will typically enable you to meet or exceed most dimensional tolerances.  Many barrier screw designs can achieve +/- 0.5% pressure stability further enhancing processors' material savings and ultimate profitability.  As a reminder from our May AKcelerator, melt temperature stability should ideally be no more than +/-1⁰F.

How does screw design effect pressure stability?

Even the most basic metering or mixing screw designs can provide impressive pressure

stability at low screw speeds and moderate outputs with some polymers.  Don't let your screw supplier sell you a more expensive barrier screw when all that is needed is a simple metering or mixing screw.  However, as screw speeds are increased above, approximately 30 or 40 rpm, the inherent deficiencies of the basic metering screw begins to become evident.  When metering screws are pushed to higher speeds, pressure (and temperature) instability rapidly increase leading to out of tolerance product or at it's worst, an extrusion process that cannot sustain itself - a line that keeps breaking, hanging up in vacuum sizers, a failed blown film bubble, or a sheet with "lakes" caused by  in-line or transverse dimensional instability.  A phenomenon called "solid bed break-up" usually occurs when simple metering screws are pushed beyond their inherent design capabilities.  

How do barrier screws eliminate solid bed break-up?

When pushed for moderate to high performance levels, the simple design of metering screws creates an environment for solid bed break-up.  With some polymers, even low screw speeds will exhibit this unstable melting behavior.  As the material progresses through the early part of the screw and starts up the transition section, there are forces that can pull the solids bed apart.  The barrier (or secondary) flight is designed to separate or segregate the melted polymer from the un-melted polymer, mechanically maintaining a continuous solids bed.  This eliminates the melting instability that causes surging and usually enables the use of a deeper screw.  This deeper screw produces more pounds per screw RPM, which in turn provides more pounds per hour at lower melt temperature - and with the added benefit of improved pressure stability.

How can pressure stability be measure accurately?

During the development and design of high performance barrier screws, a high speed data acquisition program is used to monitor and record extrusion pressure (and temperature) stability up to 50 times per second at various points along the barrel length.  This data is then analyzed to determine if the barrier flight position and clearances are positioned properly for maximum performance.  This data also indicates if the volume of the separate solid and melt channels (produced by the dividing barrier flight) match the melting rate of the polymer.  If existing performance is not optimal, this detailed data can pinpoint the necessary changes in the screw design.  Screw designs can only be properly developed and optimized on a well instrumented extruder with pressure ports and transducers properly placed to collect the correct data.

Can this analysis be performed on my extruders in my plant?

American Kuhne has a portable extrusion analyzer that can be connected to processors' existing pressure transducer(s) to measure the machines existing performance and develop a "base line" of performance.  Furthermore, it may be possible to diagnose the exact incompatibilities in barrel set-point temperatures and/or deficiencies in screw design, especially if you have an American Kuhne extruder equipped with a standard "diagnostic pressure port and plug" between barrel zones 1 and 2. 

Should I purchase a melt pump to solve my existing pressure instability?

Melt pumps can certainly provide improved pressure stability, but at what cost?  The most cost effective solution to a pressure instability problem is a new screw design.  Melt pumps have the following costs that must be considered prior to the investment:

1.   The initial capital investment cost of the melt pump, pump stand, drive, motor, pressure control system can be substantial.

2.   The utility costs of operating the melt pump system on a continuous basis will add to the overall cost of producing your product.

3.   The additional costs of maintenance and cleaning of the pump especially during color changes, material changes, etc.

Conclusion:

If you are experiencing pressure surging or pressure instability, with the inherent dimensional instability that results, you should contact a proven, reputable screw designer/supplier to discuss potential solutions.  Make sure that this supplier uses a scientific approach with quality instrumentation when developing their screw design.

If it is determined that screw design alone cannot solve the problems then a melt pump may be the best solution.  However, extrusion processors should take advantage of the modern screw development technology available to provide the most cost effective solutions with focus on improving product quality and maximizing profitability.

 If you are interested in a detailed review of your specific application(s) please do not hesitate to contact your local American Kuhne sales representative or feel free to e-mail Ed Steward or call him at (860) 886-7745, ext. 114.

Ed Steward

Director of Process Technology
American Kuhne

TECH TIP No. 5
From the AKcelerator - February 2004Edition

LIVE LONG AND PROSPER... LET'S TALK GEARBOXES

Why do single screw extruders use gearboxes or gear reducers?  Think of the extruder gearbox as a torque multiplier.   Extruder manufacturers typically use 1800 rpm or 1200 rpm motors because they are readily available, affordable, and small enough to comfortably fit on the extruder base.  However, extruder screws typically run in the 30 rpm to 150 rpm speed range.  The role of the gearbox is to reduce the motor's speed and, in turn, multiply the available torque from the motor in order to produce sufficient power to melt, mix, and pump the resin.

Extruder buyers need to pay extra attention to the design and manufacture of the gearbox when evaluating and selecting a single screw extruder because it's the single most expensive component on the machine.  The HP rating with its service factor is the single most important specification that defines the capacity and ultimate longevity of the gearbox.  Within the gearbox, the single most important and most expensive component is the thrust bearing.  The type and life (B-10 or L-10 rating) are the two most important specifications when evaluating the thrust bearing.

Single screw extruder gearboxes are normally rated for torque or horsepower at a specific rpm based on common calculations and standards. These uniform standards allow engineers and users to compare gearboxes from different manufacturers on an equal basis.  Extruder manufacturers have historically followed these guidelines, allowing prospective buyers to accurately compare machines as long as they allow for different service factors and applications. 

The overall rating of a gearbox is based on the individual ratings of all its components, including the gear teeth design, gear hardness, shaft dimensions, bearing selection, housing design, and thermal considerations.  The overall rating of a gearbox should obviously be limited by the proverbial "weakest link". 

The calculated HP is most commonly adjusted with a service factor.  The formula for determining the calculated HP of a gearbox is simply: Calculated HP = quoted rating X Service Factor.  Typically, service factors of 1.25 or 1.5 are applied when quoting the operating capacity of a single screw extruder gearbox.  For example, a 2.5 inch extruder gearbox with a calculated rating of 120 HP would have a quoted rating of 96 HP with a 1.25 service factor.  Similarly, using a service factor of 1.5 on this same 120 HP gearbox would yield a quoted rating of 80 HP.  When comparing gearboxes, always multiply the quoted HP rating times the service factor to get an accurate comparison of HP/torque capacity.

We will review three (3) key criteria of a gearbox, including

1.  Basic design and construction, including:

      A. Gear design, construction, and hardness,

      B. Shafts

      C. Radial bearings and seals

      D. Housing design and construction

2.  Thrust bearing 

3.  Serviceability

1. A. Gear design, construction, and hardness

The gears themselves are rated for horsepower or torque based on their strength and durability ratings, which are calculated according to an industry standard (American Gear Manufacturers Association, AGMA) rating systems.  Major factors include the gear tooth pitch, center distance, (which determines diameters), material, and hardness. 

1. B. Shafts

The gear shafts must be designed to transmit the full HP and torque capacity of the gears.  The length and diameter of these shafts is critical to their ability transmit this torque without excessive deflection, fatigue and ultimately failure.  The diameter of the input shaft must be adequate to properly support sheaves (on belt driven machines) or a coupling (on direct coupled machines).  The output shafts must be properly designed to handle the correct range of screw shanks that will be inserted.  Adequate access to the drive keys is an added benefit in the event that they become worn or damaged and need to be replaced.

1. C. Radial bearings and seals

Radial bearings must be adequately designed and sized to handle the load forces and speeds.  The dynamic load capacity of these radial bearings must also be considered when evaluating the design and longevity of the gearbox.  Radial bearings must also be properly lubricated and sealed.  Historically, lip seals or labyrinth seals have both been used to keep the oil in the box and off the floor. 

1. D. Housing design and construction

Typically cast iron is the most cost effective material of choice, however steel fabrications have also been produced, usually for low volume and/or custom designed gearboxes.  Historically, cast iron gearboxes have been made in two pieces, split either horizontally or vertically.  More contemporary designs have the gearbox housing cast as one piece with an opening in one side for the insertion of the gears.  This approach provides a more rigid and compact design, able to handle higher HP applications.  The lack of a vertical or horizontal seem where the two halves come together reduces the potential for leaks.

                  Split Casting                                              One-piece Casting

2. Thrust bearing

The basic function of the thrust bearing is to isolate the rearward forces from the screw, that result as an equal and opposite reaction from the screw pumping material forward into the die.  The larger the screw and/or the higher the back pressure, the greater the rearward thrust forces.  The radial bearings are designed to support the rotational forces of the gear shafts, but have very little capacity to handle significant thrust forces, therefore the separate thrust bearing is necessary to prevent these forces from reaching the gear shafts and radial bearing. There are several types and styles of thrust bearings available. 

The B-10 Life (sometimes called L-10 Life) of the thrust bearing is based on an engineering calculation that estimates the number of hours of operation at which 10% of the bearings are likely to fail.  In single screw extruders, this calculation is most commonly based on a head pressure of 5,000 psi and 100 rpm.  Additionally, rating adjustment factors may be applied to the basic B-10 life based on several application factors including how the bearing is mounted.  For example, a thrust bearing that is mounted between two radial bearing is more likely to insure very accurate thrust bearing alignment, and will therefore have a higher rating adjustment factor.  Conversely, a thrust bearing that is mounted "cantilevered", that is outside or beyond the radial bearings is more likely to experience misalignment, and should therefore have a lower rating adjustment factor.  However, many manufacturers of these "cantilevered" gearboxes do not properly adjust their thrust bearing ratings in their extruder quotes.

There are three basic types of thrust bearings, each with their inherent advantages:  cylindrical, spherical, and tapered.  Cylindrical thrust bearings are used primarily because of their low cost.    However, the nature of the cylindrical shaped bearing requires a "cage" that "steers" the bearing in a circle as it rotates.  Significant forces can develop at critical contact points between the bearings and the cage where the roller skids along the race that lead to localized high temperatures.  These intense temperatures may break down some of the oil additives in EP (Extreme Pressure) oils and actually convert them to corrosive by-products, causing permanent damage to the gearbox.  Spherical thrust bearings offer greater contact area, are shaped to rotate in the race without skidding, and have a built-in self-aligning feature.  They are most often used in extruders up to and including 3.5 inches because their cost becomes prohibitive in large sizes.  Tapered roller thrust bearings are expensive but represent the best solution for larger extruders.  The individual bearings are tapered, or conically shaped to provide the greatest contact area and thrust capacity.  Furthermore, the conical shape enable the bearings to roll naturally in a circle, eliminating the localized high temperatures caused when steering a cylindrical bearing in a circle.   The tapered and spherical thrust bearings are also less subject to wear in the event of marginal lubrication conditions such as viscosity loss due to excessive time between oil changes and/or lower than ideal oil levels.

                                        Three Basic Thrust Bearing Types

3. Serviceability

Access to affordable parts and service are important aspects of gearbox selection.  While there a few extruder manufacturers left that still manufacture their own gearboxes, the majority of extruder suppliers purchase their gearboxes from proven and reputable manufacturers that specialize only in gearboxes.  Extruder manufacturers still making their own gearbox are, in some case, having them made offshore, in China for example.  Access to parts and or service of these Extruder OEM gearboxes may be more limited and more expensive than those made by dedicated gearbox manufacturers.  Purchasing an extruder that uses a gearbox manufactured by a dedicated gearbox specialist enables dual sourcing of parts and service - from either the extruder supplier and/or the gearbox manufacturer.  Before selecting your next extruder supplier, be sure to determine where you gearbox is made as well as the cost and availability of potential parts and service.  The cost of these parts and service should always be factored in to the initial purchase price in order to determine the real long term operating costs of the machine.

The Rating Game

As you can see, it is the total gearbox design and construction, including the shaft and bearing sizes and the housing thickness and rigidity that must be designed around the gearing to provide sufficient support and capacity so that the whole assembly can comfortably handle the gear capacity, effectively transmitting the motor torque to the screw without significant distortion or failure. 

There appears to be an increasing trend toward ignoring these undeniable facts when quoting gearbox HP ratings.  For example, a gearbox designed years ago around through-hardened and shaved gears (then current technology), would have shafts, bearings, and housings designed accordingly.  Simply installing new hardened gears with a higher HP capacity, does not automatically allow the gearbox rating to increase without stronger shafts, bearings, and housing.

Today, gear manufacturing technology almost exclusively consists of carburized and ground gears, capable of delivering much more power in a smaller size.  When old gear designs are constructed using the new materials and process, the power calculations yield much higher gear tooth ratings.  But if the rest of the design is unchanged, and the same bearings, shafts, and housings are used, the total gearbox rating cannot simply be based on the new higher gear rating alone. The higher belt load or torque could never be applied to the original sized input shaft without causing bending or twisting.  The bearings  and/or shafts would be overloaded with the higher forces, and the housing would probably not have sufficient strength to resist significant distortion.  A legitimate gearbox rating must consider the whole design, not just the gear teeth.

Conclusion

When selecting your next extruder, you should pay particular attention to the gearbox and thrust bearing.  Compare the answers to the following questions:

§         Where is this gearbox made?

§         How long has this design been in service?

§         Are replacement parts readily available and how much to they cost?

§         Is the housing cast iron or fabricated steel?

§         Is the housing a one piece construction or split?

§         What is the design of the gear teeth and what are their hardness?

§         What type of thrust bearing is installed and what rating adjustment factor has been applied to the B-10 life rating?

§         Where is the thrust bearing mounted and what is the rating adjustment factor?

§         What type of oil should be used in the gearbox? 

For detailed information on this subject, please contact Bill Kramer at American Kuhne at (860) 886-7745, ext. 112.

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