RIGGING
Be sure to properly support a large press when lifting it from the truck. Do not lift and drag just one end, as it is possible for the frame to deflect, which in turn can shift the screw positioning within the press. Interference between the screw and the screens can result. Use a spreader bar when lifting long presses.

INSTALLATION
Do not bolt or weld the press down to a level foundation! Instead, first place the press where it is to be installed. Next, place shims between the press frame and the steelwork (or concrete pedestal or foundation) to fill any gap where the press is to be anchored. Only after shimming or grouting should the press be pulled down tight. Doing otherwise may rack the frame of the press, and this can cause misalignment.

The press must be mounted solidly to a foundation or structural steel. If a press draws more than two thirds of its rated horsepower without the press being anchored to the floor, the frame of the press can twist.

The screw is removed from the cone-end of the press. Allow the space required.

To suit individual situations, the press can be installed tilted upwards. Also, hollow bore gearboxes can be rotated 90deg; or 180 o. Consult the factory in these situations, as the oil level in the gearbox may have to be adjusted.

Material can be fed into the press many ways. Always allow for return feed of overflow material in the event that more is fed to the press than it can take. Spill containment is a consideration.

START-UP
Before putting power to the screw press, it is important to rotate the screw by hand. This will prevent damage to the press in case the screw has shifted so as to hit the screens. Also, the presence of tramp material in the press will be revealed. To turn the screw by hand, remove the motor's fan cover and turn the fan blade. This will cause the screw to turn.

The screw of the press turns in a counterclockwise direction, when viewed from the drive end of the press.

INSTRUMENTATION
The most useful instrument to have when testing a press is an ammeter. The load drawn by the drive motor of the press is indicative of how much work the press is doing. The higher the amps, the better the dewatering. Also, the higher the amps, the closer the press is to jamming. Very low amps indicate little dewatering is being done; the screen is blinded (covered over); or low compression is taking place.

A moisture balance is valuable for measuring the moisture content of the press cake as well as the inbound material. If an oven is used to dry samples, be sure it is set at 200deg; F or less.

Suspended solids (fibers) in the press liquor can be measured by filtering and washing a sample before drying the filter paper in an oven.

In the case of pressing liquids that contain dissolved sugars, a refractometer is valuable for assessing press performance. The Brix of the inbound flow, the press cake, and the press liquor will all be the same. The higher the Brix, the lower the moisture content of the press cake.

COMPRESSION
A screw press achieves compressing using several methods: (1) The discharge cone of the press causes back-pressure on the material being dewatered. The higher the cone pressure, the greater the liquid removal. (2) The pitch of the flights of the screw tightens as the material is conveyed through the press. This forces liquid to go through the screen. (3) The diameter of the shaft of the screw may be increased, forcing material outward, against the screen. (4) Force-feeding the press and applying a vacuum to the outside of the screen are two additional ways of achieving compressing. These are used infrequently because the performance results are uncertain.

CAPACITY MEASUREMENT
The best way to measure capacity of a press is to collect timed samples of press cake and press liquor. The samples should be collected during a period of steady operation, rather than by running a batch through the press. If only one flow (press liquor or press cake) can be measured, it is possible to calculate the total press throughput if the inbound solids content and press cake solids content are known. It is assumed that there are zero suspended solids in the press liquor, although this is never the case.

Press cake is generally captured in a tarpaulin, and press liquor in a 5-gallon pail or 55-gallon drum. When the press liquor drain is at floor level, a 3-mill plastic bag can be used.

FEEDING
Material can be fed into the press many ways. Commonly, screw conveyors, pumps, and transition chutes are used. Always make provision for return of overflow material, in the event that more is fed to the press than it can take.

Sometimes a rotary drum screen or static (sidehill) screen is mounted over the inlet hopper to prethicken the flow ahead of the press; the tailings (solids) from the screen can be funneled into the press. Also, material can be dropped from shredders or cyclone separators into the press.

Usually presses work the best with only atmospheric pressure in the inlet hopper. Nevertheless, there are ways to force-feed a press, either with a headbox over the inlet or by pumping the inbound flow directly into the press. We are generally seeking a pressure of zero to no more than five psi in the inlet hopper, so headboxes are made from two to ten feet tall. Force-feeding may increase both the dewatering and throughput capacity of the press; however, it is more likely to cause the screen to blind, reducing capacity.

Material can be pumped to a press with either an open or a closed system. With the open system, no pressure exists in the inlet hopper so the press will never be force-fed. If the piping is piped through a cover bolted to the inlet hopper, force-feeding is possible. Pressure in the inlet hopper is controlled either by pumping with a variable speed pump or by providing a recirculation line which allows material to by-pass the press. At higher inlet hopper pressures (over five psi), fiber can be forced against the screen so as to blind (cover over) the screen, resulting in poor performance. At pressures of 40 psi and above the shaft seals can be blown out of their housing. At pressures above 60 psi the screen frames will start to separate from their mountings, resulting in bypassing of unpressed material directly into the press liquor flow.

At pressures above 10 to 15 psi in the inlet hopper, it is possible to blow the "plug" of press cake that forms at the solids discharge end of the press. Liquid will erupt from the cake discharge. Exercise caution if hazardous or hot material is being pumped into a press.

A vent line open to the atmosphere should be provided in the recirculation line to prevent siphoning material in the inlet hopper out through the recirculation line.

INTERMITTENT OPERATION
In the case of intermittent operation, the control panel for the feed pump or conveyor, and the press should have a timer. This timer should be set to have the press run for three minutes after the feed pump (or conveyor) shuts off. This will partially clear the press so that it will not trip out on overload when it is re-started.

Minimize the time that the screw press is run with no material being fed into it. Since the screw is supported to some extent by the material inside the press, running dry might allow the screw to rub the screen. The last material admitted to the press will dry to powder, and. as such, it will not provide the lubrication required to prevent abrasive wear. (Initially the press will likely be run dry in order to check rotation; negligible wear will occur so long that this period is kept to a minimum.)

BRIDGING
Sometimes bridging will occur in the inlet hopper, stopping the flow of material into the press. One way to overcome this is to direct a stream of water into the inlet hopper so as to break the bridge. Sometimes it is convenient to pump a jet of the press liquor into the inlet hopper for this purpose. The nature of the screw press is that essentially all of this added liquid will be removed in the subsequent pressing operation.

If a surge hopper is mounted over the inlet to the press, it should have at least one, preferably two or three, vertical walls. This will minimize bridging.

Coating the inlet hopper of the press with Teflon sheet is a remedy that has been used to reduce bridging of sticky materials.

DISCHARGE CONE
The principal adjustment of the press is made with the cone. The cone is the component at the cake discharge end of the press that acts as a stopper plug, or choke, to prevent material from leaving the press. The more pressure exerted by the cone, the drier the cake material will be. The motor amps can also be expected to increase.

The cone is moved in and out (actuated) by a pair of air cylinders. There is an FRL (filter, regulator, lubricator set) provided with the press, along with a four-way reversing valve. Filling the lubricator jar with light oil will assure long air cylinder life.

The presses come with a 4-way Parker valve, to be installed at the cone end of the press, which allows manual selection of the shut, open, or "neutral" position. This valve connects one side of the air cylinder to the regulator while simultaneously opening the other side to atmosphere. The vent line on the 4-way valve allows air to escape when pressure is switched from one end to the other of the air cylinder; continuous air flow from this line indicates a leak inside the air cylinder.

Presses are generally started up with the cone in the withdrawn (open) position. Once material is going through the press, the 4-way valve is set so that the cone goes to the closed position. With many materials it is satisfactory to start the press with the cone in the closed position. However, with materials that are dry to begin with, such as sawdust or wash tank sludge at a plastic recycler, it becomes more important to start with the cone in the open position as the press may tend to jam and overload with these materials.

Alternatively, very thin or soupy materials, like fruit pomace or clarifier underflow, might tend to purge right through the press if the press is started with the cone in the withdrawn position. With these materials it is frequently better to start up with the cone in the shut position.

As the air pressure on the cone is increased, not only will the cake become drier, but the flow through the press might be reduced because of the backpressure. With very slippery inbound material it is sometimes possible to apply enough cone pressure to stop the flow altogether. High cone pressures will result in increased quantities of suspended solids in the press liquor.

Typical cone pressures are in the range of 30 to 60 psi. Some materials will press only in a low range, say 5 to 20 psi. Other materials may press best with a pressure of 60 to 100 psi. Air consumption is minimal in all models, 1 to 2 cfm.

A press can be operated with the cone in the withdrawn position. The screw by itself may do enough compressing and dewatering to produce a cake at the discharge. If the material being pressed dewaters very readily, the press can jam even with the cone in the withdrawn position. If this happens, a lower compression screw is called for.

It is generally quite acceptable to open the cone while the press is in operation. This will give the operator a chance to observe operation with minimum dewatering and maximum throughput. It allows inspection, while in operation, of the discharge end of the screw and screens. It is also a good technique for purging bad material (generally, fermented or spoiled) from the press. (Do not try this trick if you are pressing hot or chemically aggressive materials.)

Where very low air pressures are required for proper operation, it may be practical to put the 4-way valve in a neutral position, half way between open and closed. (Keep in mind that a slug of cake will push the cone open, and it will not re-close on its own afterwards.)

The cone rides on the shaft of the screw. There will be either one or two bushings in the cone to support and guide it, and to protect the journal surface of the screw shaft. Sometimes these bushings are lubricated with liquid from the material being pressed, such as the juice from apples.

Grease fittings are provided for lubricating the bushings. This is extremely important when materials that are dry (like paper mill screen rejects) are being pressed. By the time such materials reach the discharge of the press, they do not have enough free moisture left in them to adequately lubricate the cone bushings. Sometimes presses are supplied with additional lubrication fittings so that water, in addition to grease, can be metered to the bushings as a lubricant.

Automatic grease systems are available. These can be spring, battery, or 110-volt powered. Vincent provides these for critical applications, such as pulp & paper and alcohol extraction.

Liquid leaking past the cone bushings drains out the back of the cone (at the air cylinder end of the press). A pan can be provided to collect this liquid and drain it into the main flow of press liquor.

One unusual technique is to set the air pressure so that the cone normally stays completely shut. A timer is used to periodically open the cone. The closed period is determined by the amount to time required for press cake to accumulate in the press. This type of operation is used with slippery or slimy press cake that cannot be dewatered to sufficient firmness to force the cone open. The duration of the "cone open" period is long enough to dump the press cake that has been formed.

CHANNELING
Very slimy materials, like concord grapes or canned dog food, will tend to channel or squirt past the cone at the discharge of the press. Channeling can be reduced by lowering the air pressure on the cone

Another way to reduce channeling is to slow the speed of the press. This allows greater dewatering. It also reduces the throughput capacity of the press.

PRESS SPEED (RPM)
A small change in screw speed, 20% or less, will generally not result in a measurable change in performance of the press. Generally, screw press speed (rpm) is changed by switching to a different rpm drive motor (900, 1200, or 3600 versus the standard 1800 rpm). Consult the factory to be sure the torque rating of the gearbox is not surpassed. Operation with a 3600 rpm motor requires the use of synthetic oil in the gearbox.

It is rare for variable speed to be used with a Vincent single-screw press, except when testing to determine optimum speed.

SCREEN
The screens of the press are made profile bars (wedgewire). These screens can be reversed when wear occurs at the discharge end; this allows double screen life. Profile bar screens are one piece weldments all of which must be replaced once excessive wear has occurred.

Screens made of profile bar will generally have slots that are 0.015" to 0.020" wide. With slot widths less than 0.012" there is a tendency for the screens to blind with the material being pressed. Changing the slot width has little impact on the clarity of the press liquor or the dewatering capacity of the press.

An unusual option in the Series CP presses is screens made of perforated stainless sheet metal. Another unusual option is screening on the lower 180deg; of the inlet hopper.

SCREEN BLINDING
It is not uncommon, especially when dewatering slimy materials, for the screen of the press to become blinded (covered over). When this occurs, the flow of press liquor coming through the screen diminishes. In some cases, the screen can be cleared by continuously reversing the direction of rotation of the screw. This can be programmed with many VFD's, so that the press runs forward for a given period and then reverses direction briefly when the screen starts to blind. This is one of the easiest possible solutions to test.

Many other methods are used to prevent blinding: (1) Adding press aid to the flow. (2) Adding notches to the screw. (3) Reducing or eliminating the pressure in the inlet hopper. (4) Changing to a screen with a different opening. (5) Reducing the screw-to-screen clearance. (6) Employing a back-flush with caustic solution, acid, or high pressure spray.

SCREW-TO-SCREEN CLEARANCE
Generally the clearance between the screw and the screen is 1/16", plus or minus 1/16". The screw should not rub the screen hard, as it can cause wear and premature failure of the screen. Reduced clearance is used with materials that blind the screen, such as tomato skins. With a clearance greater than 3/16", the dewatering performance of the press can start to deteriorate; this depends a lot on the type of material being dewatered.

Checking the screw-to-screen clearance after a press has seen service is done by withdrawing the cone. With the cone in the withdrawn position it is possible to see the tips of the last two flights on the shaft. The most severe wear occurs on these tips. Judging by the amount of wear visible, it can be decided if more detailed inspection is required.

The screw-to-screen clearance can be checked by removing one half of the screen and bolting the other half fast to the resistor bars. Inspection is made from the side from which the screen half has been removed.

With profile bar screens, a feeler gauge can be slipped through a slot until it hits the edge of the screw. Subtract the thickness of the profile bar from the measured depth to determine the screw-to-screen clearance. (Profile bars usually are either 3/8" or 1/2" thick.)

Worn screws are returned to Vincent for rebuilding. The cost of a screw rebuild is around one third the cost of a new screw.

SCREW CONFIGURATION
Almost all Vincent Screw Presses use the interrupted screw design. The interruptions leave room for stationary resistor teeth, which reach almost to the shaft of the screw. This design of screw press stands in contrast to a continuous screw design. The main advantage of the interrupted design is that solid material must accumulate in the interruptions until sufficient consistency is reached for the solids to be pushed toward the cake discharge. There is a reduced tendency of the material being pressed to co-rotate with the screw. Also, there is more agitation within the press and, consequently, quicker dewatering.

The screw starts with a feeder section of continuous flight. This picks up material in the inlet hopper and pushes it into the screened part of the press. The feeder section ends at the first pair of resistor teeth. The feeder section of the screw is followed by compression stages where the flights have reduced pitch. The reduction in pitch of the flights causes compression of the material going through the press. Series CP presses are made with four and five stages of compression, depending on the model.

A screw configuration referred to as Sterile Butterfly is fairly common. There are a reduced number of flights on this screw, and the flights do not wrap as far around the shaft as is normal. This design screw is good for high throughput of materials that are easily dewatered. Medical waste and knots are typical examples.

Sometimes the compression of a screw is reduced, in the field, in an operation called "pie cutting". This involves cutting pie-shaped segments from certain flights of the screw. It is useful in modifying a press so that excessive compression and jamming are avoided. Check with the factory for assistance before making this modification.

RESISTOR TEETH
The interrupted screw design press has teeth that protrude into the flow of material as it passes through the press. The resistor teeth are an integral part of the Resistor Bar. These resistor bars are positioned axially, parallel to the screw, with one bar above and one bar below the screw. The resistor bars are bolted between the B and C plates; they form a part of the structural frame of the screw press. The screen frames bolt onto the resistor bars.

Sometimes the resistor teeth can be shortened, usually by half, to increase the capacity of the press. Removing the teeth altogether can result in co-rotation and jamming.

DOUBLE PRESSING
Many processes benefit from what is called double pressing. This means that the cake coming from the press is run again through a second press. If little moisture is removed in the second (double) pressing, then it is known that the liquid removed in the first pressing is all of the free liquid that there is to be pressed out.

Sometimes water is added to the cake in between the first pressing and second pressing. This is done to enhance the recovery of dissolved sugars in the original press cake.

Molasses can be added to press cake between the first and second pressing. This is used to diffuse dissolved sugar into the cake, increasing the final press cake solids content.

Capital-effective double pressing can be achieved by using an inexpensive Soft Squeeze Series KP screw press for the first pressing, following with a tighter-pressing Series CP in the second position.

PRESS AID
If apples are run in a press, apple sauce will come through the screen. However, if a press aid is added to the apples, then apple juice will come through the screen. Press aids include cottonseed hulls, rice hulls, and paper fiber. These are most commonly used in producing juice from deciduous fruit. The press aid gives the press something to get a bite on. Press aids also tend to hold back suspended solids (short fibers) and prevent them from going through the screen with the press liquor.

Typically, the amount of press aid used is only 1% to 3% by weight of the flow going through the press. This will look like more than such a small percentage because press aids have a much lower bulk density than the wet materials that are pressed.

HYDRATRED LIME
Lime (calcium hydroxide) must be added to citrus peel before it can be pressed effectively. The lime breaks down the pectins or cell walls so that the press can remove moisture. Less than 1% by weight is used. A reaction time of several minutes must be allowed prior to pressing. Lime has been used successfully in the same manner with potato, carrot and pineapple waste. It works well on acid materials such as onions, strawberries, and coffee bean pulp. Vincent offers liming systems.

POLYMER
The addition of polymer can be indispensable in achieving adequate screw press performance. Polymers are added to dilute waste streams, especially to those containing very small size suspended solids. The polymer will flocculate the solids, agglomerating them to the point where they can be pressed. Under the right conditions, drastic improvement can be observed in press throughput, press cake moisture, and pressate clarity. Low speed operation of the press is usually required in order to achieve good performance.

NOTCHES
Sometimes it is necessary during press operation to have the screw wipe the inside of the screen clear of blinding material. Cutting shallow notches (1/8" by 1/8") in the outer edge of the screw flights best achieves this. Notching is done from just inside the B plate to the second resistor tooth. Fibrous material accumulates in the notches and wipes away slimy material that may be blinding the screen.

FLUID INJECTION
It is possible to inject either steam or water into the press. This is done by drilling holes through the resistor teeth and piping these holes to a manifold outside of the press screen. This arrangement is used in dewatering raw fish, citrus and pineapple waste (applications where the steam partially cooks the waste), as well as in pressing materials that need to be washed as they go through the press.

The moisture reduction that results from steam injection is related to a chemical change that comes with blanching, or parboiling, a material. Tests run with steam injection in a Vincent Twin Screw press at Anheuser-Busch showed little benefit. The material being pressed, spent grain, had already been "cooked" before steam was added in an effort to squeeze out more moisture.

PRESS LIQUOR
A screw press produces relatively "dirty" press liquor as compared to a Filter Press or Belt Press. Suspended solids will pass through the screen of the screw press along with the liquid being expressed from the inbound material.

If suspended solids are to be removed from the press liquor, the most common method is to pump the press liquor over a static (sidehill) screen or through a rotary drum screen. Generally the screen tailings (sludge or solids) are blended into the feed of inbound material going into the screw press. Most of these solids will be captured with the solids of the inbound material and end up in the press cake. Although some of these tailings will once again go through the press screen with the press liquor, equilibrium is reached and satisfactory pressing operations are realized.

If the press liquor is to be concentrated in an evaporator, better screening than can be achieved with a static screen may be required. Decanters and centrifuges can be effective. Another Vincent machine, the Fiber Filter, provides premium performance.

PRE-THICKENING
If the flow into a screw press is too dilute, the high volume of liquid going through the press screen can cause either of two problems. The flow may flush a lot of the solids right through the screen. Or it may plaster solids against the screen, thus blinding the screen.

To prevent these things from occurring it may be necessary to pre-thicken the flow ahead of the screw press. This is commonly done with a static screen (sidehill) or a rotary drum screen. The Vincent Fiber Filter offers premium performance for pre-thickening dilute flows.

WING FEEDERS
Frequently there are blades welded to the outside tips of the last two flights of the screw. Called "Wing Feeders", these are mounted parallel to the discharge screen surface. Care must be taken that wing feeders are not made so long that they hit the face of the cone when the cone is in the closed position.

Wing feeders can serve three purposes. (1) In the case of materials that want to channel out the discharge of the press, like pineapple and spent brewer's grains, the wing feeders break up the channeling flow. (2) For abrasive applications, short knobby wing feeders are provided as sacrificial wear elements. (3) In the case of press cake that is very hard and which tends to jam at the discharge, the wing feeders can break up the cake to encourage a discharge flow.

Clumps of dry material can accumulate between a wing feeder and the shaft of the screw. This in turn reduces screw press capacity. The problem is most apt to occur with small size presses (10" and under). Should the problem occur, either grind off or at least shorten the wing feeders by half.

Dry material can also pack between a wing feeder and the screen. This will wear out the screen in the area where the wing feeders are located. Removal of the wing feeders is recommended.

In abrasive applications, the thickness of the flights at the discharge should be monitored. When the flights wear very thin, it is easy for the wing feeders to break loose.

A-B-C PLATES
There are several vertical plates welded to the frame of the press. Starting from the drive end of the press, the first one is the A Plate. This plate forms one wall of the inlet hopper; the shaft seal housing is bolted to this plate.

The second plate is the B Plate. It forms the other wall of the inlet hopper. The screen frames start at the B Plate.

The final plate, the C Plate, supports the discharge end of the screen frames. The discharge cone lightly touches the C Plate when in the closed position.

The bolted plate on which dual air cylinders are mounted is referred to as the D Plate.

CORD CUTTERS AND STRIPPERS
Sometimes long stringy material will be pinched where the feeder portion of the screw goes through the hole in the B Plate. This material will co-rotate with the screw and build into a bundle that reduces the flow through the press.

A groove, like a keyway that is 1" wide and 3/8" deep, can be cut through the hole in the B plate. We call this a Cord Cutter. Material trapped between the flight and the hole in the B plate will pop up slightly as it passes the Cord Cutter. The result is that the material is sheared loose.

Alternatively, a part called a "Stripper" can be welded to the B Plate, inside the inlet hopper. It is welded in a position so that the flight lightly kisses the stripper as it goes past. This strips the material away. Strippers are made of key stock.

JAMMING
Should a press become jammed a series of steps can be taken to un-jam the press. Generally, the easiest thing to do is to reverse the leads on the electric motor drive. This will cause the screw to rotate in the clockwise direction. The press should be run backwards in this fashion for about ten seconds to half a minute.

Generally jamming is caused by over-pressing excessively dry material. Running the press backwards will break up this material. If the jamming was caused by tramp material, hopefully this can be found and retrieved from the inlet hopper following operation in the reverse direction.

If a press has had extensive use in an abrasive application, the flights can be worn away at the discharge of the press. This will lead to serious jamming and can ultimately result in a burst screen.

If a press is operated in the reverse direction it is possible that solid material in the press will be forced against the A Plate. This can deform the inlet hopper or cause a failure of the shaft seal. For these reasons, care must be taken when the press is run backwards.

If running the press backwards does not clear the jam, the press screens should be removed so that the cause of the jam can be determined.

CLEANING
Material will leave a press only if there is inbound material forcing it out. This makes it difficult to clear all material from a press without removing the screens. One technique used successfully is to feed crushed ice into the press. Water must be fed along with the ice to prevent jamming. When the ice melts, the press will be relatively clean inside.

Commonly material is cleared from a press by stopping the inbound flow, setting the cone in the withdrawn position, and running the press for several minutes until no further material is discharged. This will leave some material inside the press, but this is generally insufficient to cause jamming on subsequent start-up of the press.

There are applications in which the press must be cleaned very frequently, such as once a shift, in order to meet sanitary regulations. At these installations, the screens are removed from the press in order to remove residual material.

The screen frames are bolted in place, making removal and replacement a simple matter. Frequently hinged screens are provided for operator convenience. Once the screen frames are removed (or swung open), the screw and screens are scrubbed with caustic solution.

In laboratory applications, the press may be disassembled for cleaning. The various components can be dipped in an appropriate solution or placed in an autoclave.

SCREW REMOVAL
The screw is removed through the hole in the C Plate, at the cake discharge end of the press. The operation can be difficult the first time, so we recommend consulting with the factory before getting started.

Start by removing the four bolts holding the shaft seal housing to the A Plate. This will prevent damage once the screw is loose.

The bars with the resistor teeth must be removed before the screw can be removed.

Series CP screw presses use a key and keyway to couple the screw inside the hollow bore of the gearbox. It may be that a great deal of force may be required in removing a screw.

The basic procedure is to push or pull the screw out of the gearbox. This can be done in three ways: (1) pry the screw using the B plate as a lever point, (2) construct a gear puller, using a hydraulic cylinder, combined with a cross bar and two long shafts threaded into the gearbox housing (metric), or (3) use a heavy fine-threaded rod to push the screw out of the gearbox. This rod in threaded through a heavy nut provided inside the hollow bore of the gearbox. This nut has a lug that fits in the keyway, to prevent the nut from turning. The nut backs up against an internal snap ring in the hollow bore of the gearbox. The result is that the rod pushes against the end of the screw. Loaner tool kits are available in Tampa; see Screw Change Kit Drawing B-92839-B.

SCREW REPLACEMENT
In the case of hollow bore gearboxes, when pulling a screw into the press it may be necessary to use a long allthread rod (English threads). This rod is screwed into the threaded hole on the end of the screw. The screw is slid into the press far enough that the allthread rod goes through the hollow bore of the gearbox. A large washer is slid along the allthread rod to form a brace against the gearbox. Running a nut on the rod, against this large washer, will pull the screw into the gearbox.

When reinstalling a screw, the screw must be pulled in until the step in the shaft seats against the thrust bearing of the gearbox. This will position the flights of the screw so as not to be hit by the resistor teeth. Be careful when guiding this step in the shaft through the shaft seals.

During re-assembly of a keyed shaft, be sure to apply Never Seize or Anti Seize to the portion of the screw shaft that goes into the gearbox.

There are two ways to avoid excessive rubbing between a new screw and the screen: Either shims can be placed between the screen frames and the resistor bars, or the interference can be ground off the edge of the screw. To eliminate the high spots, coat the edge of the screw with Prussian blue, bolt the screens in place, turn the screw, remove the screens, and grind the screw where interference has occurred.

SCREEN REPLACEMENT
For ease of maintenance, the screen halves are held in frames that are split vertically, being bolted to the resistor bars. This allows the screen halves to be removed from the sides of the press.

In the case of screen failure, frequently a solid patch can be welded onto the screen, from the outside. This is simple as the screen need not be removed from the press.

Profile bar screens may become smeared from being wiped by the screw or by hard press cake. Sometimes this can be corrected with a flapper wheel. Profile bar screens generally work satisfactorily with 20% or more of their surface smeared over.

To replace a perforated screen, the screen frame must be removed from the press. Damaged screens should be unbolted and discarded. The replacement perforated screen first must be clamped tightly against the supporting frame. This can be done by forcing the new screen against the frame, using C-clamps and wood or steel pieces. Be sure that extra screen material sticks out beyond the edges of the frame, to allow trimming with a hand grinder.

Once the new screen is tight against the frame, look through the screen and locate the holes in the frame where the attachment bolts go. Use a center punch to open holes in the screen. These holes must be large enough to allow the attachment bolts to go through the screen and thread into nuts on the far side of the frame.

Once the new screen is bolted tightly in place, beat over and grind off the excess screen material.

There are two ways to avoid excessive rubbing between a new screen and the screw: Either shims can be placed between the screen frames and the resistor bars, or the interference can be ground off the edge of the screw. To eliminate the high spots, coat the edge of the screw with Prussian Blue, bolt the screens in place, turn the screw, remove the screens, and grind the screw where interference has occurred.

SHAFT SEAL
The Seal Housing is bolted to the A Plate. This housing may be solid UHMW (ultra high molecular weight plastic) or it may contain one or two Johns Manville (JM Clipper) lip shaft seals. There may be a grease fitting on this housing; the grease is used to help prevent trash material from entering the seal and damaging the screw shaft.

Generally, seals are allowed to drip once they start leaking. They are replaced only in conjunction with major maintenance, as when the screw is removed from the press.

In some cases we have found that leakage from a shaft seal can be stopped by simply loosening the four bolts holding the seal housing to the A plate.

GEARBOX BASICS
16" and under Series CP presses use hollow bore gearboxes, while the 24" and 30" presses use foot-mounted gearboxes. The foot-mounted gearboxes are concentric (the motor shaft is in line with the output shaft). The hollow bore gearboxes all use C-face mounted motors. The larger presses may use this arrangement, or a foot-mounted motor with a belt drive.

Gearboxes are rated, and sold, by torque. The manufacturers generally offer their designs in progressively larger sized castings, or boxes. The larger the box, the larger the torque rating. Each box size will be available with different gear ratios. In order to keep the torque fairly constant, larger horsepower motors are used with the high speed boxes. Similarly, smaller horsepower motors must be used when a low output speed is selected.

Many times screw presses are designed around the size of the gearbox that is selected.

MOISTURE CONTENT
A screw press separates free water. This will leave organic water in the press cake. The organic water is either bound to, or part of, the animal or vegetable molecules. Mechanical pressure alone will not remove organic water; it takes heat or chemistry. Frictional heat from the press can remove organic water, but this obviously should be avoided. For chemistry, see Hydrated Lime above.

To determine the moisture content of a material (feed to the press, press cake, or press liquor), a sample should be weighed and dried overnight at a temperature slightly less than 100deg; C. The sample should weigh ten or more times the tare weight of the sample tray or cup.

The moisture content of press cake varies considerably. Tomato press cake will be 95% moisture. Orange peel will be 72%, unless it is reacted with hydrated lime, in which case it will go down to 65% moisture. Dairy and hog manure will come out at 70% moisture, unless there is sand or sawdust in the sample, which will reduce the moisture content. Cellulose fiber from a paper mill (knots, screen rejects, primary clarifier underflow) will come out about 50%. However, if secondary (biological) sludge is added, then the moisture content of the cake will go up considerably. Moisture contents of only 25% can be achieved pressing things like glass and plastic chips.

The heat from steam injection can change the chemistry of the material being pressed so that cake with lower moisture content is produced. This works with fish and orange peel, for example.

A quick approximation of what to expect from a screw press is to squeeze as much water out with your fist, and figure that the press will do a little bit better.

REPLACEMENT PARTS
Most replacement parts are standard OEM components that can be purchased from their original manufacturer. The specification of these items is included in the O&M Manual. Only shaft seals and bearings are apt to require replacement.

The most common wear parts in the Vincent Press are the screens, the screw, and cone bushings. Vincent stocks these for the more popular models. Screws and cones are generally rebuilt at the Vincent factory. Be sure to specify the Serial Number of your press when ordering replacement parts or repairs.

SAFETY
These Operating Hints have left unstated the obvious safety hazard: A screw press, like any screw conveyor, is totally unforgiving. If clothing or a limb gets caught in a rotating screw, the screw will not stop. Vincent Corporation has heard of only one injury of this nature with a screw press; do no let yourself become the second. The use of common sense is all that is required.