You are a brave man, Douglas. Congratulations for tackling a problem you know nothing about.

I use a technique called Failure Mode and Effects Analysis - FMEA for short. It involves getting together with your mates and brainstorming all the things which can go wrong with a screen, what would be the effect of this, and what would have caused it to happen.

1 - too much feed

2 - unbalanced load

3 - lack of maintenance

4 - wrong size of eccentric

5 - uneven feed

6 - wrong type of product for the screen cloth

7 - wrong type of screen cloth

8 - wrong springs

9 - inadequate supports

10 - wrong type of bearings

11 - fatique cracking

12 - spillage

13 - dust

and so on and so on until you feel you have learned more about screens than your professor. ■

Since my reply to you did not get posted last night I will try again.

Now that you have opened a can of worms to go fishing on a monday EST.

You are investigating failure of welds/rewelds, metal fatigue from elliptical motion, vibration, overheating, overloading, poor design, not buying adequate screening capacity to handle overflow if needed.

For one thing you have to narrow your search:

1. what is the flow rate of the material in question? That is a huge part.

1.What is the product being screened?

Is there a wobbler feeder to do primary scalping after the primary crusher?

What is the motive power for the screen?

(a)electrical-if so what voltage and amperage?

480 or 600 volt AC?

50 cycle or 60 cycle?

Is the electrical feed to the screening plant independent/self generated or relying on line power stepped down from 4000 volts or more? Is the power regulated/conditioned due to line drop and its inherent woes?

(b) engine driven with hydraulic driven pumps and motors or clutch drive and v-belts?

What size are the drive an driven pullies?

FIRST AND FOREMOST WHAT TYPE OF LUBRICATION METHOD IS DESIRED?

(A) What type of screen?

Primary or tertiary?

(A1)Elliptical orbit?

(A2) Forward or reverse throw?

(B)Flat shaker/thrower type?

(C)True vibrating type/pan?

(D) Is the screen an orbiting incline screen-such as "Rotax" etc? for fine screening-30 mesh or less

(e) Is the screen an elliptical fine screen such as a Derrick stationary/ eccentric inner frame with screen cloth?

Now that I have that out of my system this early in the morning we will start with primary screens:

In lieu of a wobbler feeder to do primary scalping the primary screen takes all the crusher product and does the primary screening for that product.

The primary screen can have as many or as few screen decks as one wishes for the job.

How many screen decks? this will depend on the material size desired and the downstream processing if any and or recirculation of material to be rescreened.

Primary screening units can have as few as one or a many as six decks for the job at hand.

How much screening do you wish accomplish at the first screening?

How big a screen do you want to use? the larger the better to screen material thouroughly in one pass.

(C) What is/are the desired screen mesh sizes of product desired?

The top deck gets all the material at once and is the first defense or aid for the other screen decks below it. If the top deck is flooded the lower screens cannot work properly -all the material gets sent down stream to overload the secondary screens if there are any or if the material is fine it will pour right through it-the top deck-bad for rock salt.

2. Wet or dry screening?

Are you making/screening for mason sand or concrete sand? You need a lot of water to push for that sand and two augers to move the sand grades down the line to a stacking conveyors.

Asphault make up sand-rock dust? Are you making binder dust from crushed rock for asphault product/binder?, zero water is desired here as it is going into a bituminous hot or cold mixer plant.

3. Primary screening or tertiary?

4 How much screening in one scalping?

Screen failures are a fact of life do to the nature of the work done by the machine and or the lack of maintenence done or required by same.

If the screen is supported by springs? Are they to stiff?, are they broken?

If it-the screen is supported by airbags(the best way in my opinion) are the air bags holding air or developing pinhole leaks?, an easy and quick repair.

The failures are many and vary quite a bit from lubrication failures which lead to bearing overheating and eventual failure to stress cracking of primary screen frames due to the elliptical orbit of a primary screen and its resonanace.

The shock loading of primary screens with product is also a big factor if the screen has a high capacity delivery system to the top deck of same.

Lubrication failures:

Grease:

If the screens are not greased every day to drive in new grease and purge out the old it will fail eventually due to degradation of the lubricant and its inherent properties. blown grease seals are a problem as they fail to keep grease in to tdo its job.

Oil lubrication:

bad oil lubrication pump-systems, blown oil seals,unfiltered lubricating oils. lubricating oil degradation from reuse and circulation, poor filtration of same.

Stress failures are a huge factor as well:

Primary screens have a tendency to shake them selves apart due to the orbit/motion/loads they carry in question even with proper maintenence.

The stress cracks can and do start at the screen cloth angle iron holding the screen decks in place.

The screen side plates that hold all the screen decks/shaker arms and motor drive are under tremendous strain at all times due to the simple fact that they are throwing the combined weight of the screen and the product in question in an elliptical orbit to be screened during its work.

The shaker arms and bearings are doing a tremendous job of absorbing energy/making linear energy to keep every thing in line and if the screen arm mounting rubbers are not kept clean the orbital energy is transmitted further down the frame to the side plates and bearings putting further stress on an already stressed screen.

Are the shaker arms painted with circles/are black dot used to observe orbital rotation of same? if it is really screwed up/out of orbit it will show quickly.

Are the shaker arms weightedproperly-this also includes removal of dust build up a nescessity when screening very dry dust pruducing materila with open screen decks.

The supporting cables if any hold the assembly in its entirety within a given area-suspending the shaker in mid air to allow free movement and screening.

The shafts extending out from the shaker arms are transmitting all the energy of inertia to the screen decks to screen the product. typically one shaft is driven by the drive unit and the other is a carrier/balance/along for the ride etc.

The screen side plates are also a huge factor as they are being moved in concert with the screen decks at the same time.

The primary screen moves at hundreds of revolutions per minute and all this energy is transmitted through the shaker arms and shafts and

the stress is eventually transmitted the side plates and stress cracking will result and entire side plates must be chaged due to the energy transmitted. a lot of this happens because the mounting rubbers are not kept free of dirt/dust to allow absorption of the screens tranmitted energy.

And if they are out of square/ouch!!!!!!!!!!!!!!!!!!!!! no allowance for errow !!!!!!!!!!!!!!!!!!!!!!!

A better way:

This in contrast with a "Pennsylvania Crusher"

"Bradford Breaker" where it is a rotating drum with screen panels in the drum utilising gravity to break material and screen it at the same time, much simpler, much less work with adequate screens for primary and secondary scalping in the same drum if so desired.

they must be doing something right as they have been building these machines since 1905.

Just as important:

Forward ever forward:

Seconndary screens mostly contend with ripped screen cloth and worn out drive motors/gearboxes/bearings for eccentrics.

Flight conveyors for feeding secondary screen systems:

You have to worry about chutes plugging and overloading a screen near the plugged chute or chutes increasingf the load on the ext screen in line with the flight conveyor.

Secondary screen size plays a huge part here as the larger the screen the larger the screen areas and reduced load on screens. The more secondary screens the lighter the load the better product.

Screening is as simple or complicated as the end product desired so unfortunately there is no simple answer to your question.

George , john my humble apologies if I missed something-its late in the AM here and I have to do errands.

leon z ■

You must recognise & distinguish fault rather than abuse & ignore the latter because there is nothing that can be done about abuse or neglect. For instance, if a screen can be overloaded then it must have been wrongly selected in the first place. There is no other explanation. Such things are not faults of the screen!

Fault diagnosis for screens is quite straightforward. Fatigue failures regulary occur because the frame materials are inadequate in terms of quantity & quality. Commercial pressures prevent progress to the quality levels operated by eg pressure vessel suppliers. Downtime losses far exceed the expenditure on material certification, approved weld procedures & NDT.

There's not much to say about bearing failures. A selected bearing has been manufactured to stringent quality controls and the major causes of failure rest in the shafting & mountings.

Other industries use crack propagation theory to predict the remaining life of equipment. Once the crack starts in a screen frame it will have extended to disintegration size before the calculation sheet is finished.

From the old school thinking I used to resent the growth of paperwork, especially as manifested in new age quality control & health & safety activites. Now I know better & even suggest that a screen properly designed & manufactured to the procedures in place elsewhere in heavy engineering, would not fail with the sad familiarity we expect at the moment. Fault diagnosis could be covered by reference to eg ISO 9000. After all that is one of the reasons for getting the certification..so they say. Job done.

I could have just written "refer to ISO 9000" but I'd written this speel already so I'm going to post it.

I also have to be honest (we really appreciate that on these forums)....Now go & enjoy the rest of the summer. ■

In the dim distant past I was involved with accoustic analysis of both process noise and potential machine failure. The primary use for this type of technique was to "listen" to a ball mill grinding slag and adding various chemicals to reduce the static charge between particles to increase grinding efficiency. The resultant signal was passed through a spectrum analyser and broken down in 50Hz segment and then displayed. The results were rather surprising. The amount of information contained in a frequency band of 10Hz to 10kHz was astonishing. The main problem was analysing the results. One test we did was to capture the spectrum when the machine was operating correctly, and then do a continuous comparison to show up any deviation. One result was that we detected a bearing failure about 6 weeks before the bearing actually failed. Unfortunately, we did not know what this sound signature meant until the bearing actually disintegrated. The original research on this technique was carried out by BP, and I am fairly sure that there is a company out there providing this type of equipment.

As a research student it may be an interesting way to look.

Jon Scarrott ■

Dig those worms. In some parts of Australia they are a delicacy.

Douglas,

I didn't answer your questions directly for a number of reasons.

1 - Professors love bearings. Given a bearing load, a few Factors, and a bearing catalogue you can calculate the bearing life to the nearest half-cycle.

On site life is a little different. If some grit gets into the tachonite seal the bearing will blow. The Vendor will say - impossible, it's not our fault. The maintenance crew will replace the bearing - no questions asked (you have to beg them not to throw it in the trash if you want to take a closer look).

2 - Once you know the bearing load, the next logical item is the girder - ultimate strength load factor, and all that.

But what do you mean "girders or beams"? If the floor steel is so close to the limit then something is wrong. Floor beams have to be designed to a deflection limit - not a stress limit. It's all about natural vibration of the deck.

But if you mean the side frames which form part of the screen structure - that's another kettle of fish.

I was a structural engineer called in by a major screen manufacturer in South Africa. They never called me in when they were designing their 6 m X 3 m double and triple deck screens - only when cracks appeared in the side frames. They didn't have any expertise in mechanical structures in South Africa, and very little in the US.

In those days I was a little naive, and thought I could solve the problem by looking up stress formulae in Handbooks. That was before Finite Element Analysis became available. We did all out analysis on a slide-rule.

I have seen cracks in vibrating machines welded up and, before you know it, there they are again. Yes we did do some failure analysis, but there are many manufactuers even today who have very little appreciation of fatique cracks and how to design to prevent them.

3 - ISO 9000 says you should have a procedure for everything. But it doesn't get down to details of what and how when it comes to Reliability Engineering and Failure Analysis techniques in mineral processing equipment. ■

Grit in bearing is not the Vendor's fault. Frequency beams rarely fail. If they do the building comes down. Frames do fail on a regular basis. ISO 9000 requires vendors to maintain design records, material test certificates & weld examination records as well as the good old inspection reports etc. In the cut throat business of mineral processing this is not generally acheived. If it was correctly implemented the garbage on the market would soon disappear. Strange how everybody thinks that their business operations & equipment are unique. What excuse do mineral processors have for buying & using cheap crap other than they've always done it when they could. ■

Bravo John,

My sentiments exactly.

leon ■

Are we trying to help a group of student at Beijing University learn about fault diagnosis of vibrating screens - or am I on the wrong thread?

Within that context I think

1 - the supporting structure is not a problem - it will outlast the life of the screen

2 - if the screen manufacturer has done his job properly then the bearing problem is a non-issue (apart from being an occassional interruption to production)

What would be helpful to the students?

1 - perhaps some reliability statistics showing the real issues in screen maintenanance

2 - some briefing notes and graphics indicating how the problems presents themselves - without giving away the solution (that would be counterproductive to the university learning task).

3 - some notes on how to do the detective work involved in solving the crime ■

Unless he is willing to climb inside a screen and see whats what and get dirty from examining side frames, screen fabric or heavy screens there no point in him asking anything and expecting him to learn anything!!!

Learning why it failed is more important than examining failure!!!!!!! ■

A very impressive wide range of responses on this thread, so let's add some more fuel to the fire, shall we?

1. Here is one not previously mentioned.....An oldie but a goodie.

Comes from the Ripley's Believe It or Not Segment of vibrating equipment failure analysis.

My 73 year old travelling companion used to do this....AND I SAW IT....take a rounded edge broom handle (less the broom), stick it in your ear and put it against the sideplate of a vibrating screen when running near the bearing housing. He said you could hear the bearings "RATTLING" and change them before they SEIZED. I tried it once, fun but, that was also thee last time. Something bout jamming big wood in your ear that I found rather unappealing, It worked though....and I guess you classify that as "Old School"

2. I like to just listen to the ear piercing high pitched "HOWLING SOUND" to diagnose upcoming bearing failure.

3. Diagnosing cracks in sideplates. I saw a SONIC VIBRATION Analysis GUN.....we used in QUEBEC Canada to identify very tiny minute fractures in the sideplate steel indicating that a CRACK OR FAILURE of the sideframes was imminent. It actually worked nicely.

4. FINITE element analysis is used by a number of VIBRATING SCREEN MFRS to read frequency variations of different components on a vibrating screen that is CRACKING sideplates for whatever reason. Basically identifies an element that is running on a FREQUENCY...of it owns vs running on a FREQUENCY that the rest of the components are running at. By locating the area that is running on a ERRACTIC or different vibrating frequency, they can identify what to tighten up or what to replace to MAKE all the parts run as one again and avoid cracking.

5. Bearings: Failure analysis: SKF AND TORRINGTON for a couple of well know VIBRATING SCREEN BEARING mfrs have beautiful troubleshooting catalogues that show a pic of a BURNT inner race or MARRED INNER race, GOUGED outer race, FRETTING and corrosion damage and give the reason why this probably happened and the most probable fix.

6. If you want real answer, get the bearing with old, dirty, burnt, nasty grease still in it. SEND it to the BEARING mfr and they will inspect, diagnose failure and give full technical report on why it failed. This gives you real WRITTEN technical expertise to your company on how to stop this from happening.

7. FOR another WACK of canned possible failure recommendations....check CHAPTER 8 AND 9 of VSMA (Vibrating Screen Handbook)

"Shake til you Break" George Baker ■

Douglas,

We hope you appreciate the passion involved in the design, manufacture and operation of vibrating screens.

One last question - if a screen cannot pass the specified throughput, would that be considered a "fault"?

The Vendor claimed it was the feed chutes. We nearly came to blows on the Retention monies (also known as Bank Guarantees, Liquidated Damages, or Latent Defects. You'll soon enough find out about such terms).

We haven't heard from you for a while. We trust you are still alive and well, and able to make some use of the potpourri of wisdom freely offered to further your education. ■

If you have a bad weld or welds your are going to have

problems period.

You have not told us what the cross member is, is it:

A tube weldment. meaning a rectangular box piece of steel welded to the front.

A piece of channel iron? I it is it will not hold due to the reduced surface area for welding.

If it is channel iron does it have any gussets welded to the inside of the channel? since the frame does not appear to have any gussets welded to the front out side of the ends of the piece to add strength to the front I would say not!

IF the screen is not balanced period it will go sour.

Do not forget that those springs are transmitting vertical energy back and forth in concert with the exciter.

If they had air bags for a support system they might , might have

fewer problems. It all depends on the quality control or lack there of if any. ■