April 1, 2012
These eight tips will help to keep all types of aggregate screens in optimal working condition.
Aside from all the proper adjustments and operating parameters required to gain the most in screening efficiency, the need for good preventive maintenance practices is a must for longer lasting screens and reliable performance. “Above all, be proactive,” says Deister Machine Co. Service Manager Scott Murphy, an industry veteran with more than 25 years of experience. “Develop a keen focus in forecasting, and then fixing, each factor that could cause potential problems. And take the time to do maintenance the right way — before, rather than after, component failures occur.” Murphy shares the following tips for superior screen maintenance.
1. Establish an oil sampling program.
Although a commonly overlooked practice, a regularly scheduled oil sampling is an operator’s best insurance against catastrophic component failure and costly downtime. The valuable insights provided by samplings — such as detecting a worn bearing — allow operations to cost-efficiently schedule maintenance downtime around periods of prime production.
Scheduled sampling and analysis establish a baseline of normal wear and can help indicate when abnormal wear or contamination is occurring. Oil that has been inside any moving mechanical apparatus for a period of time reflects the exact condition of that assembly. Oil is in contact with mechanical components as wear metallic trace particles enter the oil. These particles are so small they remain in suspension. Particles caused by normal wear and operation will mix with the oil. Any externally caused contamination also enters the oil. Identifying and measuring these impurities indicates the rate of wear, as well as any excessive contamination. Importantly, an oil analysis will also suggest methods to reduce accelerated wear and contamination.
2. Employ recommended lubrication practices.
Always consult your owner’s manual for the manufacturer’s recommended lubrication practices.
Install the correct amount of oil, and use the recommended type of oil. Change the oil at the proper intervals, making sure that the oil in storage is clean and that clean containers are used to transport the oil. Make sure that your machine is completely level so that oil does not flow to the low side of the machine.
3. Maintain proper belt tension.
Belt tensioning must be right on target for optimum screen performance — not too loose and not too tight. Ideally, the belts should only be tight enough that they do not slip during startup. If necessary, use a belt gauge to tension correctly. If belts squeal during startup or in operation, or whip excessively, this may indicate insufficient belt tension.
Importantly, over-tightened belts can cause serious damage such as the vibrating frame being pulled out of square with the support frame. Operating in this twisted position introduces stresses that may lead to spring failure, metal fatigue, or cracking and broken welds. Additionally, this twisting affects stroke amplitude and character, which then affects material flow and screening efficiency. Over-tightened belts also put an extra load on the mechanism bearings and may tear up motors and motor bases.
Additionally, to prevent drive belts from slipping, flopping, or coming off, belts and sheaves must be kept clean and properly aligned. Inspect sheaves for wear, and, if the grooves are worn, replace the sheave.
4. Prevent material buildup.
Accumulation of dust and stone around moving parts is one of the largest single causes of part failures, particularly for pivot motor bases, support springs, roller bearings, and the vibrating frame. Impact between the vibrating frame and accumulated material may lead to tower vibrations as well as potential side-sheet and support-deck cracking.
Note that sheaves and belts are susceptible to material jumping over the side sheets and causing damage. Where possible, use stationary skirt plates or rubber flaps to deflect airborne material.
It’s also important to avoid material buildup in bins, hoppers, and transfer points.
5. Maintain proper screen media support and tensioning.
Proper tension is a must. Uniform tension must be maintained on the screen surface to prevent whipping and to maintain contact between the screen surface and the bucker-up rubber on the longitudinal support bars. Improper tensioning may cause severe damage to costly screen media.
Also, do not operate a vibrating screen with screen cloth or other screen media sections removed, as this will result in accelerated wear on the support frames and the longitudinal support bars.
When doing screen media changeouts, it’s a good time to do a complete deck inspection. Inspect cross members for signs of premature wear — especially in wet screen applications where wear is accelerated. Cover and protect the cross members, decking, and housing tubes with rubber or urethane liners to extend wear life. Prior to installing screen media sections, make sure they are appropriately square and flat so that they will seat properly on the longitudinal support bars.
6. Monitor spray systems.
Use the required number of spray nozzles, and make sure they are open and fully operational. Maintain the proper water volume and pressure; avoid spraying perpendicular (at 90 degrees) to the screen surface, as this may result in a rapid deterioration of the screening surface. The spray should strike the screening surface at approximately 45 degrees. Nozzles can be positioned to spray against or with the flow of material. This depends upon the desired washing/rinsing efficiency and material properties. For most applications, a pressure of approximately 40 pounds per square inch is desired at the nozzles.
7. Operate with proper clearances.
Maintain adequate clearances around stationary structures, and never allow vibrating frames to hit on stationary structures. Wherever possible, a minimum of 24-inch side clearance should be provided on each side of the machine. This enables the operator to adjust screen cloth tension and check the unit’s condition and operation.
Allow sufficient clearance in front of the screen at the discharge end, or in the rear at the feed end, for replacing screen sections. A suggested clearance would be at least 1 foot longer than the longest screen panel. A minimum vertical clearance of at least 5 inches should be maintained between the vibrating frame and any stationary structures, such as the feed hopper or discharge chutes and bins. Avoid providing places for dust and stones to accumulate and interfere with the movement of the vibrating frame.
8. Be thorough
Murphy also points to the importance of such items as replacing worn plates to maintain the integrity of screen frames. And he recommends that operators stock key replacement parts. For example, it’s advised to have a mechanism rebuild kit in inventory during the prime part of the season.
Lastly, while most everyone wants more tons per hour, avoid overloading the screen. If you need to increase tonnage, contact your manufacturer regarding proper speed and stroke and the required types of support springs. AM
This article is provided by Deister Machine Co., a family-owned company currently celebrating its 100th-year anniversary as a manufacturer of vibrating screens and feeders.
PREVENTIVE MAINTENANCE CHECKLIST
Be proactive. Complete scheduled maintenance checks. Daily, weekly, monthly, and semi-annual inspections should cover the following points:
o Oil level (for one week after oil change)
o Clear away stones and dust build-up from any moving parts
o Screen cloth tension
o Wear on screen media and tension plates
o Oil level
o Even material feed and distribution
o Loose bolts
o Wear on bucker-up rubber wear strips at screen cloth changes
o Wear on snubber assemblies
o Drive belt tension
o Support springs
o Wear on V-belts and sheaves
o Type of oil being used
o Wear on cloth support decks
o Wear on feed and discharge wear plates (if applicable)
o Test oil samples at oil change intervals