Questions You Should Know about Screw Conveyor Systems

As a Product Manager at JMS, one of my jobs is to work with Owners and Engineers on a day to day basis answering their questions.  I recently had a conversation with an Engineer regarding recommendations for conveyance of compacted screenings from one unit to two adjacent dumpsters.  This is a common application; however, some pertinent questions came up from both sides. Learn more about Screw Conveyors for Raw Screenings with this article. 

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My first question was to define the amount of raw screenings that we are talking about transporting.  It is important to know the maximum amount of solids in volumetric flow (ft3/hr.) that will be transported per unit of time and whether this is batch or continuous flow. The layout of the area is important as well so distances and lift can be accounted for in the design and Hp.

We normally oversize screenings conveyors to allow for large objects to pass, especially if the plant has a CSO, to handle peak event discharge from the screens, and to handle large plugs from compactors.  The screw needs to be as large as the potential plug from the compactor. Belt conveyors are a good choice for handling large objects often associated with a CSO during storm events. A belt conveyor downside, however, is that ragging may occur on the wiper and plows requiring extra housekeeping. The decision to pursue the design of a screw conveyor in this app was based on the ragging issue.

Several questions were returned from the engineer.  Some of these may be questions may be of interest to other Engineers as they are working through the system design process.  For simplicity, I will run through the list of questions and answers as they came up:

Screw Conveyors for Raw Screenings Q & A

Q: Is there a shafted screw option rather than shaftless?

A: Yes, the shafted screw is available as well, however the screenings tend to rag on the shaft of a shafted conveyor, so a pushing shaftless is preferred. The non-drive end of a shaftless is free and not connected, so screenings are discharged without impediment.

Learn more about shafted vs shaftless screw conveyors with this JMS Learning Center article. 

Q: What is the maintenance involved with a shaftless conveyor?

A: There is a liner that will need to be replaced.  The timing on this depends on how much grit is present in the screenings, the diameter, speed and run time.  At a minimum I have seen 25,000 hours liner life in a high grit app, the average is approximately 50,000 hours.

Learn more about typical screw conveyor wear parts with this JMS Learning Center article.

Q: Does the screw need to be removed from the top to access the liner?

A: No, it only needs to be lifted one inch off of the liner to take the liner out in four-foot sections. The liner is rotated over the top of the spiral and lifted out through an open top cover.  A spiral is very flexible so it can be lifted in just the 4’ area. 

Q: Does the shaft/screw need greasing/maintenance often?

A: Some of the maintenance items are the same for the shafted and shaftless screw conveyor, others will differ.

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Shaftless Screw Conveyor Maintenance tips:

Establish routine periodic inspections of the entire conveyor to insure continuous maximum operating performance.

Weekly:

• Ensure all trough covers, guards and drive guards are installed whenever conveyors are operating.
• Check drive for unusual noises or vibrations.
• Check screw conveyor for any unusual noises or vibrations.
• Check for good transfer of material, material spills at inlets and outlet.
• Check for reducer oil leaks.
• Check shaft seals for product leakage, adjust or grease as required.

Quarterly:

• Open covers and inspect trough liner for wear.
• Inspect screw flights for wear and wear pattern.
• Emergency stop switch, trip to test, reset.
• Check speed switch controller lights for active function.

Reducer oil:

• Initial oil change for drive at 500 hours.  Drain and refill oil in gear reducer.  Regular oil changes every 10,000 hours or 2 years whichever comes first.

Shafted Screw Conveyor Maintenance tips:

Weekly:

• Ensure all trough covers, guards and drive guards are installed whenever conveyors are operating.
• Check drive for unusual noises or vibrations.
• Check screw conveyor for any unusual noises or vibrations.
• Check for good transfer of material, material spills at inlets and outlet.
• Check for reducer oil leaks.
• Check shaft seals for product leakage, adjust or grease as required.
• Manually grease or check lubricant in cups for: hanger bearings.

Quarterly:

• Check shaft and bolts couplings for tightness
• Check for elongation of cross holes in screw or drive shaft. 
• Inspect trough & screw flights for wear and wear pattern.
• Inspect hanger bearings for wear and shaft looseness.  
• Manually grease or check lubricant in cups for: hanger bearings, tail bearings, & drive bearings. 
• Emergency stop switch, trip to test, reset.
• Check controller lights for active function.

Reducer oil:

• Initial oil change for drive at 500 hours.  Drain and refill oil in gear reducer.  Regular oil changes every 10,000 hours or 2 years whichever comes first. 

Q: Other than power, what other services need to be provided?

A: An emergency stop switch and loss of speed switch is standard. Optional services are: flush water connections for cleaning the conveyor, limit switches for pressure relief coves, actuators for slide gates (electric or pneumatic).

Q: What is the approximate budget for a 9” shaftless conveyor?

A: Typically, this will run something like $1,500 per foot, not including controls.  This can also be dependent upon support systems and other variables (e.g. slide gates, material thickness and grade stainless, drive size, chutes, etc.).

Q: What is the maximum angle from horizontal the screw conveyor can be placed?

A: We like to stay at 25 degrees or lower with a full pitch screw.  Steeper than that we will have to look at a reduced pitch screw and that will impact diameter and speed.

So, this is a summary of a recent exchange. I hope this will help answer some of the questions you might have regarding screw conveyors for screenings.

If you want to learn more, please visit our website Screw Conveyor Systems.

Shaft screw conveyors are strong and will last a long time. Very expensive though.

Shaftless screws can go around corners, but tend to wear the casing because the casing acts as the guide. OK for occasional use on coarse nonabrasive materials such as grain. Also sizes are normally small.

For heavy continuous duty use shaft.
For occasional light duty use shaftless.

I got landed with a shaftless screw for conveying sand. Total misapplication and I had to pull it out. We call them "chook-feeders"

Cheers

Steve McKenzie This may or may not apply to your application, a couple projects I was on used shaftless conveyors for dewatered sludge that were smaller then the discharge chute from the machine, which was sized for a shafted conveyor, requiring the chute to be necked down in size between the machine and conveyor. This caused serious bridging problems and required the replacement of the conveyor with a larger size in one instance and speeding up the conveyor in others. Smaller is not always better even when ratings are the same.

Shafted conveyors also seemed to convey better on inclines then the shaftless and the shafted lets water drain better when flushed. The shaftless conveyor runs on a teflon wear pad that seals the flights and prevents flushing water from draining. Mike Bensema
Did you notice that shaftless conveyers have profiled stators, that is static part of conveyor has a groove calculated to fit the shape of a "screw" itself. Such design gives more turbulation to moving mass and completely avoids dead corners.
I presume that is the reason for use it in sludge conveying applications.

On the other hand, standard screws with shafts have ability to be used as rather precise dosing devices in a wide range of applications. (I saw screws with 10 mm as well as 150 mm in diametar serving the same purpose).

From my background in waste water treatment plants, we always used shaftless spiral conveyors for inlet works screenings, grit transfer and dewatered sludges. In specifications I only ever see shaftless conveyors.

I'm doing a subject at Uni - Materials Handling and the lecturer is from the grain industry and unfortunately is only discussing shafted conveyors.
I was wondering the selection criteria for the conveyor type. Thanks for everyones input.

Andrew
There are two types of centreless screws used in screw conveyors :-
1) flexible bits of wire that go in small diameter (3"-4" bits of flexible tube for very light duty applications
2) 1/4" thick flighting designed to go in more conventional screw conveyors.

I was involved with a 12" screw conveyor handling fine wet coal which built up very quickly between the flight and centre tube. We changed to a centreless screw and the problem went away. I have used shaftless screw conveyors for sewerage screenings with relatively good results, bu now I have a client that wants to use one for vertical conveying. has anyone done this? He also wants calculations on the amount of compression (deflection) in the screw. Any idea on where I can find a reference? Thanks,

Bob 20 years in plant maintenance. Unless you have a very, very good reason to use the shaftless, Save yourself some money and your mechanics alot of grief. Send the shaftless to the scrapyard where it belongs. The shaft helps convey, as product tends to slip thru the hole where the shaft should be. If you bend a piece of metal back and forth enough times IT WILL BREAK, This applys to metals that are desighned to bend as well, IE: shaftless screws> they bend alot on every revoultion, a shafted bends a litle and will eventully break as well, unless you give them some flex at the joints. Genearly shafted performs better and lasts longer. gongmuro, I commissioned a couple centrifuge projects that used vertical shaftless conveyors on municipal wastewater. The vertical runs tended to plug easily when the solids were not dry enough or a water slug caused the sludge to slip on the flights. Spirac may have some info for you, they are located in Georgia.

Mike Bensema
Some of the comments are confusing the two different types of shaftless design. One is the smaller flexibe type, much like a thin 4" OD spring (see The other is heaver (think one inch thick spiral flighting up to 36"OD) used in WWTP & other industries(see or contact me for discussion at [ protected]