Seeing that you are from my neighborhood and a Cornellian-

Has you professor specified a single flat screen deck or an inclined screen?-he did not tell you that part obviously.

Not to nit pick but vibratory feeders and screeners are two different animals

What is the desired tonnage per hour for this 8 by 12 screener?

magnetic and non magnetic vibratory feeders have a high speed that shakes a pan feeder at a very high frequency to feed material. the oil cooled magnetic ones were the simplest and best in my opinion

flat screeners- low incline screeners perform at a lower speed to increase screening ability-cut

From what I rememeber of the flat screeners at international salts old detroit mine prior to closure they had a very low speed and cable supported flat screens

using a small electric motor with a gearbox wheel eccentric and shaft connected to the screen body

simple perfect screens with excellent screening for 30 mesh size fines

Large elliptical screens can throw forward or backward under motion.

Smaller elliptical screens typically have one motor with a flywheel weight on the exposed shaker arm shaft with an second flyweight and shaft on the other shaft that is passive.

the idea of two counter rotating shafts is misleading as the rotation would cancel the entire business and eventually kick out a circuit breaker.

Is it simply a case of the motor drive and the shaft drive traveling in different rotation? screen deck counterclockwise and motor clockwise?

two motor screens are usually set on the same cross plate between the side plates to shake the side plates of screener in tandem either forward or backwards.

fine screeners can be either employed with a cross plate mounted oil cooled electric motor in elliptical motion for fine screening or gearbox driven like the rotax and others that shake from side to side in an elliptical orbit at a very low rpm

through the gearbox and eccentric.

==================================================================================================== ==================================

Bearings have specific B10 life and it is entirely dependent on the application-

the type of bearing needed will depend on the speed and shock loading of the application to determine the bearing needed.

roller bearings in an oil bath or grease run for hundreds of hours or thousands depending on the application as they are secured in a bearing housing or a machined chamber to accept the out race of one or more bearing sets. The inner race and balls do the work and the outer race is stationary - and hopefully the out race does not spin which is a big problem for the machined part.

the bearing lives or dies depending upon

its quality of materials

application

speed

greased bearings operate at a lower RPM

oil fed bearings run at a faster rpm

oil seals

grease seals

shock loading

duty cycle

lubrication

PROPER INSTALLATION

oil bath with splash lubrication or pressurised with oil seals.

the shaft seals hold the oil and gease in place

A gear box driven screener driven screener like a rotex will be:

cable supported

have two bearings, counterweight(s)

an electric motor with a V belt drive having a counter weight to create motion at the top of the screener.

The screen will be an incline screen where the material enters at the top encountering a gyratory motion, the next part of the screen has a circular motion, the third part has an elliptical motion and the last part of the screen has a straight motion.

The wire rope allows it to create the motion desired at a very low speed.

Attachments

rotex (PDF)

screens (PDF)

eriez-hd98b-brochure (PDF)

■

aagoyle1

As Izaharis rightly points out, such a design project in the big bad real world would require consideration of a lot of factors such as the required duty etc. etc.

However in this academic exercise let’s just work on the brief you have been given, which as I understand it is to “design a twin shaft geared exciter to vibrate a 7000 kg vibrating mass at 10 to 12mm linear amplitude".

Nevertheless I think one thing needs to be clarified. You have stated that amplitude is to be 10-12mm. The amplitude is defined as half the total motion, i.e. you are asking for a 20 to 24mm (peak to peak) stroke which would be quite extraordinary for a conventional linear motion screen'. In the examples below I have assumed a 10-12mm stroke (5-10mm amplitude) which would be reasonably common in the quarry/mining industry.

Then to answer your questions:

Question 1 - How do I decide the rotating frequency of the gearbox?

Answer - Refer answer to question 2.

Question 2 - Is the rotating frequency dependent upon the amplitude created by the geared exciter?

Answer - Yes. To protect the structural integrity of a screen of this size, it is usual to limit the acceleration to a max of 5g, with a common good practice limit of about 4g (they commonly operate down to about 3.5 g).

Therefore for 12mm stroke x 4g the max rpm would be:

= square root of ((g x 1,800,000) / stroke) = square root ((4 x 1,800,000) / 12) = 775 rpm = lets say 750 rpm

Question 3 - Is it correct to say that the static moment for the screen is equal to the static moment of the geared exciter?

Answer - Yes. In other words the total vibrating mass of the screen x the vibrating amplitude will be equal to the total mass of the eccentric weights x the distance from the exciter shaft to the centre of gravity of the eccentric weights.

In our case 7000 kg x 0.6 cm = 4200 kg.cm total exciter moment (2,100 kg.cm per shaft)



Question 4 - Is the formula kW = (mR x n x 1.25) / (97,400 x LRT) applicable for linear motion machines?

Answer - Almost! The above formula is applicable to circular motion screens, and will give a conservative power requirement for liner motion.

For linear motion, multiply the answer x sin y, where y = the angle of vibrating action as measured from the horizontal plane, and would normally be between say 40 to to 60 degrees depending upon application.

In our case let’s assume ‘y’ = 60 degrees, and LRT (the locked rotor torque of the drive motor) =200%, then:

Design power = (4,200 x 750 x 1.25) / (97,400 x 2.0) = 20.2 kW x sin 60 = 17.5 kW

Question 5 - What should be the minimum life span (hours) for bearings in such exciters?

Answer - For an 8 hour day operation the L10 bearing life should be say => 20 – 30,000 hours which is usually acceptable, while for a 24 hour day operation a L10 life of => 40 – 50,000 hours should be aimed for.

It is important to appreciate that the L10 life of a bearing, is that attained or exceeded by a large batch of bearings before fatigue is evident. However it should be noted that the L10 life is exceeded by at least 90% of all bearings, and that 50% of them will attain a fatigue life that is 5 times as long.

Fatigue life is used as a calculation basis because it is the only cause of failure that is subject to statistical laws. Other causes such as inadequate or unsuitable lubrication will render a bearing unserviceable after a working life that cannot be calculated. Indeed experience shows that it is from these other reasons that the majority of bearings fail before fatigue. ■

1.The screen size has to be 16 feet by 6 feet. :^)

2.The tare weight(dead weight tons) is the weight of the screen body -dependent upon whether the screen deck is installed.

3.

16.3 cubic meters

it has the potetial to carry that much but it will not screen very well

4. the screen is not vibrating it is moving you dont have a sifting seave.

5.

the electric motor is connected to the reduction gearbox and using a V belt drive the pulley and weight or weights creates the movement to screen the material.

please look at the Timken bearing catalog as the FAG explanation is a bit mis- leading. ■

aagoyle1

To answer your questions:

1. A 16’ x 6’ (96 sq feet) screen is very common size in the industry. I don’t know where the extra 6’ (in the 16’ x 6’ x 6’) comes from ..... except perhaps it refers to the height of the actual screen box which is feasible, but of little relevance to your design of the exciter.

I would guess that the 1.6 tones per square metre figure is the bulk density of the material being fed to the screen (this is a usual figure for quarry aggregate in its loose state).

It is common just to use the weight of the screen in calculations, because the effective weight of material acting on the deck is typically quite small in comparison. If you design the stroke to give 12mm unladen, then the actual stroke under material load would normally easily fall into the 10 to 12mm specification you have been given. If you would like to allow for material load on the screen we can talk about that, but you would need to know the feed rate of material and the % that would pass through the lowermost screen deck.

2 Regarding the bearings, your Prof is quite bright (they usually are).

The bearing load will vary according to a sinusoidal function when the shafts are in rotation.

Fr min = the momentary bearing load at the point furthest from the axis of oscillation

Fr max = the momentary bearing load when the counterweights are 90 degrees to the plane of linear motion.

The equations for Fr min and Fr max can be combined and rewritten as you suggest to give the bearing load .

ie: Fr = (0.68 x Fr max) + (0.32 x Fr min)

The equivalent dynamic load (P) used in the bearing life calculation = Fr x 1.2

(The 1.2 is an application factor used on vibrating equipment ).

The dynamic load rating C will be listed in the bearing manufacturers tables.

ps: thinking further about my previous answer to your question about the max speed allowable for the 12mm stroke --- I would be quite comfortable to increase the acceleration up to say 4.5 g which would = 820 rpm ■

It is probable that practical considerations are going to determine the radius of the swing weights.

I imagine a good design is going to be compact, and the radius of the weights will need to be somewhat in proportion to the diameter of the gears.

I would think that within reason a thinner and larger radius weight will better than and thicker and smaller radius with regard to the bending moment on the shaft.

If you are still talking about a box with a static moment of about 4200 kg cm, then a look at commercial exciters of about that size will reveal a general height from the base of the box to the shaft center line of the shaft of about 325 to 375mm, and so the max radius of the swing weight would be a little less than that – lets say about 300mm. I would start on this premise until convinced of good reasons to alter.

The application safety factor for the gears is a little outside my direct experience, but I would imagine it to be about 1.5. Perhaps someone else may car to comment on this ■

You are on the right track.

G1 is just the combined mass of the swirling out of balance weights.

The remaining exciter components should be included in the vibrating mass of the screen G ■

Originally Posted by aagoyle1

Hi John & lzaharis!

I am stuck yet again ...well this project has its twists and turns!

I am stuck on this formula found in the FAG catalog I attached sometime back...I dont know which weight to take:

Fr max (centrifugal force on bearing) = 1/z x G1/g x R x (pi X speed/30)^2

Now z is the number of bearings.

R is the radius of center of gravity.

G1 is the imbalance weight in kN. Now my question is does the "imbalance weight" also include the weight of the gearbox body (as in the shafts, gears, casing, bearings etc)? or is it only the swirling masses?

Now if it is only the weight of swirling masses then where should I consider the weight of the shafts, casing, gears, bearings? Do I include it the mass of the screen?

Thanks!

==========================================================

One would think that as the motor and gear box are stationary the issue is moot.

Leave payment for services rendered in molasses cookies on stoop entering Snee Hall at your convenience.■

Originally Posted by aagoyle1

Haha! Sure thing Izaharis! I am not in town at the moment as soon as I get back after the break I will bake ur cookies :P

Right now sitting in the wild west..I have a wild thought...why do people use this gearbox at all? Is it of any use? I have seen a simpler design where a big weight is supported on 2 bearings on the sides, the principle seems to be the same... whats the difference? which one is better?

1. Its numb, dumb and stupid

2. read number one

3. read number 2

4. read number 3

a. the initial cost of the gear box added to the cost of the machinery it is delivering energy to is the major factor.

b. the new replacement cost of said unit versus, when sent out to be rebuilt by the manufacturer with:

1. a rewound electric motor

2 new bearings

3. replacement shaft and keys

4. oil and grease seals

5. preshipment testing and warranty

Rather than doing it at the end users facility

c. the end users application.

d.duty cycle

5. a lot of energy is deliver to a small area with a small motor and gear box.



6. neither one is better than the other it all depends on the end used desired or

unfortunately built around it.

from what I remember following from top to bottom.



end use screener fine screen of 30 mesh cut for final product



2 groove keyed v belt pulley with attached counterweight with electric

motor and high speed pulley twin v pulley

Keyed pulley shaft from gear box

greased bearing and oil seal

vented gearbox cover plate

cover plate gasket

spur sun gear

one or more spur planetary gears

ball roller bearing or bearing pairs for low speed application

gear box casting allowing splash lubrication.



7. the end user is simply interested in a means to the end at the lowest cost per

ton to deliver a low value screened commodity that is extremely friable, has a

Rockwell of 4 and a Mohs of 3 and must be screened to minus thirty mesh

instead of creating a water based mix of the minus thirty mesh material that is

scalped and simply spraying the lquified dust on its end product to increase its

tonnage.

8. the end user of said machinery and thier pocket book especially when they

use the "triple declining balance method" of depreciation for all assets

and the same end users 20 percent ROI per year.

9.If the end user has a tonnage quota all bets are off!!!

maintenance becomes a secondary priority as it is a

parasite that lives off the production department.

just so you understand the real world

10.

a. everything is fine until it breaks simply because

"I dont understand it, they told us it would work".

b. reread a

c. reread b

11. aggregates do not flow well by them selves unless they free fall

which is why the Pennsylvania Crusher companies Bradford Breaker has been

a proven product for one hundred years now



If you PM me with your e-mail I will expand on number 11 to

demostrate to you why you must listen first and then ask questions ■

Originally Posted by aagoyle1

Hi John & Izaharis,

I feel I am comfortable with the design I have developed for my gearbox. I have started modelling it too... Now if I have to couple 2 gearboxes across the screen then what kind of coupling arrangement is the best? Is a universal joint recommended?

If the timing is the least bit off the gear boxes will be attempting to counter each other while under way and the gear backlash will be a huge issue with tolerances.

A single worm and spur gearbox with a flywheel pulley would be simpler with less work, but mounting and it is the issue but thats what i think.

lzaharis ■

Aagoyle 1

I think if you look at the literature of the major linear motion screen manufacturers, you will see that it is very common practice on large screens to mount twin (or more) exciters.

Provided that both the exciters are mounted on a single rigid support beam they will self synchronise.

Indeed there is some advantage in the design of the support beam for wide screens to have small exciter loads acting near each end of the beam rather than a single large exciter acting in the centre.

The two exciters are coupled together with a universal joint, and the screen would be driven by a single electric motor (statically mounted to one side) and connected to one of the exciters by a further universal joint.

Having said all that …… twin exciters are generally only used on large heavy screens where the required moment is beyond that of single commercially available exciter. The size and weight of your screen would not normally fall into that category. ■

1. bearing preload if any.

2. bearing quality

3. bearing type and actual speed

4. quality of casting and machining as it will tolerate no shortcuts

from poor machine tool work or inspection

a. total labor cost per unit of finished product

5. quality of steel casting ingredients and carbon content

6. end use life cycle



2, 3, 4, 5, and 6 become an opportunity cost question

7. profit margins

8. return on investment

9. why am I doing your homework with out cookies or a lunch offer at the dairy bar? ■

Almost right. A single unbalanced motor would produce a circular motion, but two contra rotating unbalanced motors will produce a linear motion the same as your exciter. ■