Nine things you need to know regarding vibrating screens

Radian Engineering has extensive experience in designing, manufacturing, and installing vibrating screens, pan feeders, and vibrating grizzly feeders (VGF) for the mining industry. Over the next few weeks, we will share some critical aspects of vibrating screen equipment. In our pursuit of Engineering Success, we will provide relevant, interesting and valuable information to our customers and the industry at large and continue delivering our services with Integrity, Reliability and Precision.

1. Lifespan predictions of Vibrating Screens.

It is difficult to predict actual vibrating screen lifespans because of the varying operating conditions, erratic external factors and diverse applications and maintenance regimes.

In general, screens are designed to be functional and efficient within the design and operating parameters. The design lifespans are for “infinite” fatigue life for the major structural components. Should the operating stresses and strains be low enough, the vibrating components will surpass the 100 million cycles (in three months) and therefore have a theoretical “infinite” fatigue life. This means many years of operation without failure.

The 100million cycles are reached within 3 to 4 months if the equipment is operated in the normal production cycle of 500 hours per month in most mines. A vibrating screen with typical six poles (1000 rpm) motors vibrates at 16.7 Hz (16.7 times per second), equating to 6 million times a week. Therefore it can be seen that 100 million cycles are easily reached within 12 to 16 weeks (3-4 months).

2. VIBRATING EQUIPMENT vs STATIC STRUCTURES

Dynamic alternating loads of vibrating equipment are far removed from the static loads of fixed components such as chutes, underpants, conveyors and other structures. Therefore, it is imperative to understand the influence of component stiffness interactions, bolted connections, and welded connections’ thermal stress relieving (heat treatment). We will elaborate on bolted and welded connections in subsequent posts and maintenance and repair philosophies and methodologies.

3. BOLTED CONNECTIONS

Suppose a static structure has a bolted connection with, for example, six M20 grade 8.8 bolts. In that case, it can still operate safely with 4 of those six bolts missing or loose because of the inherent safety factors of the design and the static nature of the operating conditions. But try that with a vibrating screen, and you will have oval holes and loose components within days, material cracking and catastrophic part failure within weeks.

Fatigue cracking and oval holes due to loose fasteners within weeks.

Bolted connections, albeit hex bolts and clevelock nuts or huck bolt fasteners, need the correct torque to create the preload required to clamp the components together with a friction force large enough to withstand the dynamic alternating forces. If this is not achieved with the correct tools and skills, the bolted connection cannot do its job and will not last long. Loose fasteners are one of the most significant causes of concern and imminent vibrating screen failure.

4. WELDED CONNECTIONS

Welded connections can only last if thermal stress relieving is done on the components after welding to normalize the heat-affected welding zones. Therefore, to attempt in situ welding or site welding without stress relieving is futile, and the weld will only last a couple of days before cracking at the brittle heat affected zone, causing failure.

In situ welded repairs and fatigue failures within weeks.

5. DESIGN LIFE COMPARED TO ACTUAL OPERATING LIFESPAN

Vibrating screens are designed to last between 5-10 years with excellent and regular skilled maintenance. The drive beam and side plates will last up to 7-10 years, depending on corrosive conditions. The deck beams, stringers and cleats up to 3 years and infeed and discharge components up to 5 years depending on screen loading and wear abrasion. Lifting beams and spray bars are dependent on corrosive wear and could last 3-5 years. Wear items are difficult to judge since it depends on the type of product, the top size of material, wet or dry screening, PH of water/coal, the abrasiveness of material and screen loading.

Abrasive wear and corrosion

If a screen experiences abuse or prolonged periods (1 – 3 weeks) of unmaintained operation, it is doomed and will not even reach a fraction of the design life. Just as important is correct and skilful maintenance timeously. Breakdown philosophy type maintenance will only buy a bit of time before other catastrophic failures occur at the same component and neighbouring components, which have also been affected and damaged. Therefore, it is crucial to install factory-made and quality-controlled parts soon after temporary breakdown repairs.

6. SCREEN MAINTENANCE AND COMPONENT REPLACEMENT.

New vibrating screens are fabricated and assembled to exacting specifications. Any misalignment or loose-fitting components will fail and affect the neighbouring parts. This is not always apparent, and the extent of the secondary damage which has occurred to an adjoining part is difficult to ascertain or see. This leads to unpredictable and continual failures on screens that have been repaired with the wrong methodology.

A disastrous knock-on effect on individual component integrity is due to neighbouring parts failing, affecting the complete screen. Screens are designed to be as light but stiff as possible, and this stiffness is achieved from the integrity of all the components working as a unit.

A screen needs to vibrate in a certain way in the direction of material flow. Even a tiny amount of sideways movement (0.8 to 2 mm) dramatically reduces the screen’s lifespan since a screen is not inherently as stiff in the transverse direction. Sideways (transverse) vibrations are generally caused by non-synchronizing motors, the wrong combination of motors, broken or incorrect coil springs/ suspension parts, non-level and or overloaded screens, broken deck beams, torsion beams, etc.

7. REPAIR METHODOLOGY

It is always best to repair a screen in a controlled environment. This is not always possible, and therefore it needs to be done correctly, with trained and skilful personnel, correct parts, fasteners, and the correct tools. We often see that technicians and engineers do not realize fasteners’ proper usage and purpose. People think a fastener is there to line up two components through a hole.

A fastener is a clamp and is not needed even to touch the sides of the holes. It is there to create a clamping force on the plates because of the correct torque. This torque ensures the proper preload (stretching of the fastener), making a clamping force holding the plates together because of the friction between the plates. Typically, an M12 bolt of the correct class can produce 8-10 tons of clamping force.

For example, a deck beam with six fasteners per side through the side plate is loose; it moves and bends and cracks. This has also strained the cleats, stringers, and fasteners.

8. REPAIR/REPLACE PHILOSOPHY

When to replace a screen and when to fix it? How much downtime will I save, and what does it cost? What is my plant availability?

For example, when deck beams, cleats and stringers are worn but not yet catastrophically failed, the components, complete deck, need to be replaced with the correct method, tools and skill. This can be done in situ, although it is time-consuming and more complex than on the ground or in a workshop and is nearly never as good. The result is a reduction in the overall screen lifespan.

When a side plate is cracked (less than 80 mm long), it can be temporarily repaired by drilling a hole and installing patch plates (doublers or fish plates). This will mainly prolong the life of a side plate by many months, but not indefinitely.

When the side plate is damaged beyond feasible repair, it needs to be replaced. This needs to happen in a workshop and is a specialized and time-consuming operation. Is it viable/economical to stop the plant production/operation for a week? Then it is time to replace the screen as a whole or at least the complete screen body and try in a twelve-hour shift to take the drive beam and motors of the old screen and install them onto the new screen body. This can only be done with an experienced and skilled team specializing in vibrating screens. Anything else is temporary and doomed for subsequent failures.

Alternatively, replace the screen as a whole and refurbish the old screen at a reduced cost.

When a drive beam is worn or cracked, it must be replaced, and it cannot be fixed in situ, and any reparations will only last a couple of hours or one shift cycle.

Repairs or replacement is done with a ‘Breakdown’ mindset only postpones the inevitable failure and is a waste of time and money if it is not followed soon by replacing the correct parts and sub-assembly or complete screen.

There are many variations and permutations, but it is at least not infinitely so.

9. SECTIONS and MAINTENANCE PLANNING

It is prudent to use a specialist vibrating screen company’s resources to assist with the regular inspection (monthly) of the vibrating equipment at your site. A reputable company will analyze the data, predict the screen lifespans, and suggest or assist with repairs and components. They will also be able to train the maintenance teams with the correct know-how regarding vibrating equipment and provide the right tools needed to do the repairs and maintenance.