Introduction to Vibrosifters
Vibrosifters, also known as vibrating sieves or vibrating screens, are industrial screening machines used to separate materials by size. They operate by vibrating a screen or sieve to segregate materials into separate fractions.
The vibrosifter consists of a rigid frame, which holds one or multiple layered screens or sieves of varying mesh sizes. The frame is mounted on springs and connected to a vibratory motor that oscillates the frame and sieves at high speeds. As the material flows onto the screen, the vibration separates particles by size. Smaller particles pass through the mesh openings while larger particles are retained on the screen surface.
The vibrating action is essential for several reasons. First, it creates momentum for material movement down the screen. Second, it dislodges particles caught in the mesh openings. Finally, the vibration helps convey the particles down the inclined screen surface towards the discharge.
Vibrosifters are used across a wide range of industries including food processing, pharmaceuticals, chemicals, agriculture, mining, and more. Common applications include:
– Sifting and scalping: Removing oversize contaminants and particles from materials. The coarse particles discharge off the screen surface.
– Grading and classifying: Separating particles into size fractions using multi-layer screens. Each screen grade catches particles of diminishing size.
– De-dusting: Allowing dust and fines to pass through to remove them from coarser bulk materials.
– Dewatering: Draining excess moisture from materials through the vibrating screens.
– Media recovery: Recovering useful material from waste streams.
The vibrosifter is highly customizable with options for screen sizes and angles, number of decks, discharge configurations, and feeding mechanisms. This allows the vibrating sieves to handle high capacities for both wet and dry applications. Proper sieve selection and maintenance are critical for optimal performance.
Importance of Sieve Inspection
Regular inspection of sieves used in vibrosifters is critical to ensure proper operation and prevent contamination of the end product. Overlooking faults in the sieves can lead to a number of consequences:
– **Product contamination** – Holes, tears or mesh detachment in the sieves can allow oversized particles to pass into the product. This contaminates the final material with particles that should have been screened out. For products like pharmaceuticals and food, contamination can pose significant health hazards.
– **Loss of screening efficiency** – Worn, damaged or clogged sieves allow undersized particles to recirculate back into the feed. This decreases the effectiveness of the screening process and results in a product that does not meet specifications.
– **Equipment damage** – Severely damaged sieves with large holes or tears can allow oversized particles to pass through and potentially jam or damage equipment located beneath the vibrosifter. Replacing downstream equipment is costly.
– **Product loss** – Undetected holes or tears in a sieve cause good product to be lost into the undersized screened particles. This results in reduced yields and profitability.
– **Regulatory noncompliance** – In regulated industries like pharmaceutical manufacturing, improper sieve screening from lack of inspection can lead to regulatory action and production shutdown.
By following regular inspection procedures and quickly identifying and replacing faulty sieves, processors can avoid these negative consequences and ensure efficient, safe vibrosifter operation. The small investment of time in inspection pays off in product quality, safety and profits.
Sieve Materials
Sieves are constructed from materials that can withstand the high frequency vibrations and abrasive nature of the products they are screening. The most common materials used are stainless steel, polyurethane, nylon, and woven wire.
– Stainless steel sieves provide excellent durability and resistance to abrasion. They are suitable for screening a wide variety of dry materials from powders to large aggregate. The most common stainless steel alloys used are 304 and 316L.
– Polyurethane sieves are ideal for high impact applications. They have good elasticity to resist blinding. Polyurethane bonded mesh and molded polyurethane sieves are common types used.
– Nylon sieves are lightweight and economical. They are often used for general purpose screening of dry materials. Nylon holds up better than standard wire mesh. Monofilament woven nylon is commonly used.
– Woven wire mesh sieves employ various metals like brass, bronze, and stainless steel. They are often used for wet screening applications. Mesh opening size can be customized but options are more limited than perforated plate sieves.
The material selected depends on the application requirements including the product being screened, mesh size, required open area, particle shape, temperature resistance, and cost. Consulting with sieve manufacturers is recommended to select the optimal sieve material and construction.
Sieve Mesh Standards
Sieves used in vibrosifters are identified by mesh size, which indicates the number of openings per linear inch. For example, a 100 mesh sieve means there are 100 openings within one linear inch of the mesh. As the mesh number increases, the size of the openings decreases.
Mesh size is inversely related to the size of the openings. Higher mesh numbers correspond to smaller openings, while lower mesh numbers have larger openings. The actual dimensions of the openings depend on the type of weave used in the mesh – plain square weave, plain dutch weave, or twilled square weave. However, generally a 100 mesh sieve will have openings of 150 microns.
While mesh sizes are commonly used, especially in North America, sieve openings may also be specified in microns. Micron ratings provide a more precise measure of the actual opening size. Converting between mesh and micron sizes is possible using standard tables and formulas, but micron sizes give a clearer picture of the particle sizes that can pass through the sieve.
When selecting and inspecting sieves for a vibrosifter, the mesh or micron rating must align with the required particle size separation for the application. Using the correct mesh and weave allows proper screening and separation efficiency. Regular inspection of the sieves then ensures the mesh remains intact so the desired particle sizes continue passing through the sieve.
Inspection Frequency
The frequency of sieve inspection depends primarily on the application and material being sifted. Sieves handling highly abrasive materials like sand, minerals, and some chemicals will require more frequent inspections than those sifting food materials.
As a general guideline, sieves should be visually inspected daily for any obvious holes, tears or clogging. Dimensional inspections should be conducted every 1-2 weeks for heavy industrial applications with abrasive materials. For food and pharmaceutical applications with non-abrasive powders, dimensional inspections may only be needed every 1-2 months.
New sieves should be dimensionally checked after the first week of operation to establish a baseline. After repairs or replacements, dimensional checks should be made after 1-2 days of running.
The operating temperature can also impact the frequency. Sieves that operate at high temperatures or experience frequent temperature cycling will need more frequent inspections than those operating at ambient temperatures.
Proper documentation should be maintained for each sieve detailing the inspection date, findings, measurements, and any repairs or replacements made. This provides traceability and can help optimize future inspection intervals.
Following regular inspection frequencies tailored to the specific application and conditions will help maximize sieve life, efficiency and product quality. When in doubt, err on the side of more frequent inspections. The cost of replacing a worn or damaged sieve is far greater than the minimal time investment of regular preventative maintenance inspections.
Visual Inspection
A key part of the sieve inspection process for vibrosifters is visual inspection. This involves carefully looking over the entire sieve to identify any holes, rips, tears, or other damage in the mesh. The inspector should check for:
– Holes or tears in the mesh – Look along each mesh opening to make sure there are no gaps or holes present. Even small holes can allow particles through and impact classification.
– Mesh damage – Inspect for any mesh that is deformed, bent, creased, or shows signs of wear. This can indicate the sieve needs replacement.
– Corrosion – Check for any corrosion spots or rust, which weaken the sieve. Stainless steel sieves should not corrode under normal operation. Corrosion indicates a bigger issue.
– Screen blinding – Determine if product has become stuck in the mesh, blocking openings. This reduces screening efficiency over time as more material builds up.
– Mesh security – Confirm the mesh is securely attached to the frame with no loose areas. Mesh should lay flat across the frame.
The visual inspection provides valuable information on the current state of the sieve. Inspectors should look carefully at the entire screen area, not just sections, as damage can occur anywhere on the sieve. Things like small holes are easy to miss on casual inspection but can have a big impact on particle classification. Take time and care when visually examining vibrosifter sieves.
Dimensional Inspection
A key part of vibrosifter sieve inspection is checking the sieves for dimensional accuracy and mesh opening size. Over time and repeated use, sieves can experience damage and wear that affects the precise sizing function they are meant to provide.
One of the main dimensional checks is measuring for excess sag or warp. Sieve frames can bend and deform from the vibrations and impact forces in the vibrosifter. Operators should lay the sieves on a flat surface and look for gaps between the mesh and surface, which indicate sagging. Warping can also be detected by deviations from a flat profile.
Comparing the mesh opening dimensions against manufacturing specs is also important. This can be done through visual inspection with calipers or taper gauges. The mesh openings must meet size tolerances, usually around +/- 5-10% of the designed dimensions. Overly stretched or compressed openings will lead to improper sieving.
New sieves should always be checked for correct opening size before use. And worn sieves that are outside of tolerances must be replaced to maintain proper sizing. Periodic dimensional inspections allow detecting increased sag, warp, and mesh deviations over time before they severely impact performance. Keeping sieves in good dimensional condition ensures accurate and reliable sieving.
Replacement Criteria
Knowing when to replace sieves is an important part of maintaining a vibrosifter. Sieves undergo a lot of wear and tear during operation, so determining the right time to replace is critical.
There are several visual signs that indicate a sieve may need replacement:
– Excessive holes, tears or openings in the mesh. Too many enlarged openings means the sieve can no longer effectively separate particles by size. As a rule of thumb, if openings increase by 50% or more, replacement is recommended.
– Mesh detaching from the frame. If the woven mesh is coming loose or detaching, it’s time for a new sieve. Proper tensioning of the mesh in the frame is required.
– Warping of the frame. Over time, the metal frame may warp or bend out of shape from fatigue. A non-flat sieve will not screen properly. Replace once warping exceeds 3mm from flat.
– Damage that cannot be repaired. Things like metal fatigue, broken welds, and mesh unraveling usually can’t be fixed. Attempting repairs at this stage is not worth the time and cost.
Even if a sieve is visually intact, replacement is recommended once it’s been in use for 1-2 years. The sieving accuracy declines over time as mesh openings enlarge microscopically with repeated use and cleaning. Periodically replacing sieves helps ensure separation efficiency.
Proper storage and handling extends useful sieve life, but replacement will eventually be required. Routine inspections make it easy to determine when the time is right. With a quality replacement sieve, vibrosifter performance can be restored.
Repair Methods
Sieves used in vibrosifters can become damaged through regular use. Fixing damaged sieves extends their usable lifetime and saves on replacement costs. Common damage includes corrosion, holes forming in the mesh, and wires becoming loose or breaking. There are a few repair methods that can address these issues:
Fixing Corrosion
Corrosion is common with stainless steel sieves exposed to certain materials over time. Light corrosion may be removed through gentle cleaning with a wire brush or abrasive pad. More severe corrosion requires sanding the affected area with progressively finer grit sandpaper. Start with a coarse 80-120 grit paper to remove corrosion and finish with a fine 220+ grit for a smooth surface. Be sure to sand evenly across any repaired area.
Repairing Holes
Small holes can be patched using a thin gauge stainless steel sheet and welding, spot welding or brazing, depending on the sieve material. The patch should be cut to overlap the hole by at least 1 inch on all sides. Larger holes may require replacing an entire mesh section. Take care not to warp the sieve when welding – use shallow welds and allow cooling between passes.
Fixing Mesh Damage
Loose or broken mesh wires can be re-secured by welding. Use a skilled technician to spot weld loose wire junctions. For broken wires, trim the broken section and butt weld the cut ends together or use a small wire insert of similar gauge to bridge the gap. Take care not to alter the sieve aperture size or damage other nearby wires during welding repairs.
Properly repaired sieves should function like new. Careful inspection after repairs ensures the sieve integrity has been restored before putting back into service in the vibrosifter.
Sieve Handling
Proper handling of sieves is crucial to ensure they remain undamaged and maximize their operating life. Here are some best practices when installing, removing, and cleaning vibrosifter sieves:
Installation
– Always handle sieves carefully by the frame or edges to avoid damaging the mesh. Do not pick up sieves by grabbing the mesh.
– Before installing, inspect sieves visually for any holes, tears, or mesh detachment from the frame. Damaged sieves should be repaired or replaced.
– Check that the sieve frame fits snugly into the vibrosifter mounts with no gaps. Sieves must be securely mounted or vibration can cause them to shift and tear.
– Make sure sieve mesh and frames are clean and free of residual materials before installation. Previous materials left on the sieve can contaminate products or lead to clogging.
Removal
– When removing sieves for cleaning or replacement, avoid banging or forcing them out of position. This can bend or distort the frame.
– Use the appropriate tools specified by the equipment manufacturer to dislodge stuck sieves if needed. Never use makeshift tools that could damage the mesh or frame.
– Wear proper PPE like gloves when handling used sieves to avoid contact with hazardous residual materials.
Cleaning
– Rinse sieves with water first to remove larger loose particles. Do not use high pressure sprays directly on the mesh.
– Use non-abrasive brushes and soft sponges to gently clean the mesh. Scrubbing too aggressively can lead to mesh tearing.
– When needed, use recommended cleaners and degreasers compatible with the sieve material. Avoid harsh chemicals that could degrade the mesh.
– Allow sieves to completely dry before reinstalling to prevent material buildup and clogging.
Following these handling best practices will maximize sieve life, efficiency, and effectiveness in the vibrosifter.