Sheev,

The purpose of the screen mounting springs is to isolate the vibratory forces from the support structure, and for effective isolation the springs need to be selected carefully, otherwise there is the potential to aggravate matters. In favourable conditions the dynamic forces can be reduced to 2 or 3%, and in such cases vibration is practically eliminated.

Given the wish to do so, it is possible to go as far as to produce a screen so finely balanced that there is no perceptible escaping vibration. However this ideal isolation system is delicate and easily upset by fluctuations in screen loading or changes in the operating conditions.

The efficiency of an isolation system depends upon the flexibility of the supporting elastic elements, and the higher the static deflection of the loaded spring, the better will be the isolation efficiency.



Explained in simple terms - the greater the spring compression under load, the more efficient will be the springs isolation efficiency, and the lower the operating frequency the greater will be the compression required to achieve the same isolation result.

For most screening applications an isolation efficiency => than 95% is aimed for. Efficiencies of down to 90% can sometimes be tolerated, but generally percentages below this should be avoided.

The list below sets out the required spring compression to achieve 95% of isolation efficiency over the most common screen frequency range.

750 r/min34.0 mm

800 r/min29.9 mm

850 r/min26.5 mm

900 r/min23.6 mm

950 r/min21.2 mm

1000 r/min19.1 mm

1200 r/min13.3 mm

1400 r/min 9.8 mm

1500 r/min 8.5 mm

1600 r/min 7.5 mm

1800 r/min 5.9 mm ■

Sheev,

I assume that you are going to fit helical coil springs? There are alternatives, but coil springs are normally the most economic and if designed carefully will give good results.

I think that you should work closely with your spring supplier on dimensional and material specifications, but I would expect perhaps the material to be a Chrome Vanadium Steel – ASTM A231?

Ensure that the spring designer appreciates the heavy cyclical shock and impact loads.

The ends should be closed and ground square, and dimensional tolerances placed on free length, straightness and end squareness, and also load length.

Shot peening of the wire will improve resistance to failure by fatigue, and best results will be obtained on springs that have a reasonably smooth surface before shot peening. ■

Greetings Sheev

I think you have probably already selected the best material for your springs, but the diameter 16 wire seems a little small for a 5.5 -6 ton screen. It may be OK and depends on the spring outside diameter and the number of free coils.

At 210 free length and 160 compressed length you have a soft spring (which is good) but because it is relatively short make sure that you still have enough distance between the compressed length and its solid height so that the coils do not strike during the increased stroke at start and shutdown. I would think that this distance should be an absolute minimum of three times the screen stroke.

Side snubbers are sometimes used to laterally stabilize the screen during operation, but with good good spring selection I have never found them necessary. The possible exception is perhaps for primary scalping screens where there is high impact loading from large boulders. If you would still like a picture or drawing of a side snubber I could email you direct.

Have fun ■

see rubber straps.......restrict upward movement especially on shutdown, when box is most erratic.

George B

Attachments

horizontal screen cw invicta drives (ZIP)

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Dear friends I am designing a dewatering Screen for sand, with which I count with two vibrators 12 KW each and 1800 RPM, the weight of the structure and vibrators is 5730 kg, would like to know how to design the springs, if I should helical or rubber spring. ■

which causes the structure of a dewatering screen is crack ■

Hello,

Refer engineering text-books or more intensive literature on the subject to know about spring design, commonly used materials, etc.

The right design of spring is an engineering art, involving design trials by choosing various features, to arrive at most appropriate design. It has to satisfy:

1) Stiffness (tuning ratio) to suit vibration isolation efficiency.

2) Stiffness changes by wire diameter as well as spring diameter. So adopt acceptable combination.

3) With the known vibration amplitude values (from starting to steady running), see that spring is suitable with respect to its closed as well as free length. The spring should remain longer than the closed length during operation. However, the length to be also restricted such that if it becomes closed during installation / accident, the resulting stress in spring should be less than the yield stress.

4) Spring axial load at its base should remain positive, and its resulting friction grip should be sufficient to prevent horizontal rubbing between spring and its base during rated operation (it could be okay if it occurs momentarily during starting). This is preferred even though spring is positively held by concentric lug.

5) Vibration analysis becomes extensive due to speed range and load range.

Regards,

Ishwar G. Mulani

Author of Book: Engineering Science And Application Design For Belt Conveyors (new print November, 2012)

Author of Book: Belt Feeder Design And Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Pune, India.

Tel.: 0091 (0)20 25871916

Email: conveyor.ishwar.mulani@gmail.com

Website: www.conveyor.ishwarmulani.com ■