ToThink Engineering

DARREN TOTH, Director of ToThink Engineering reports on the importance of having efficient screens. 

Most vibrating screens and feeders in service are likely to be costing you in lost income or through excessive down-time and repairs. For many screens, both apply. This is a big statement, isn’t it?Let me explain.

Over the past decade (with a previous employer and, more recently, through my own business) I have tested hundreds of vibrating machines (of various makes and models). Of these machines, upward of 75% were either not screening at their full capacity or they displayed very inconsistent and potentially damaging motion. For some, it was both. I have no reason to think that the thousands of machines I haven’t yet tested are any different, hence my BIG statement.

In a perfect world, a vibrating machine would have the exact same shape and amplitude-of-motion on its’ left side as its’ right and zero displacement transversely (left-to-right). However, a well built, well set-up and reliable screen would only come close to this ideal but, in practical terms, close is enough. Regardless of age, these machines can be configured and adjusted to run at the appropriate speed and throw for their application and within limits that would ensure a respectable service life. The cost of doing so is considerably less than the cost of decreased productivity and excessive maintenance. From a reliability and service life perspective, fine tuning a screen to produce relatively consistent motion between its’ left and right sides in addition to minimal transverse vibration is crucial. Here’s why.

Significant variations in the throw of a screen, from one side to the other, and excessive transverse vibration indicate flexing (stressing) of its’ major structural components, resulting in premature failure. In terms of fatigue life, a 50% increase in stress on a mild steel component, subject to cyclic loading, will reduce its’ life five fold. This often is the reason why some screens fail within months of operation (very rare but it can happen) and others run issue-free for many years.

To perform efficiently and reliably, vibrating screens and feeders must be expertly designed and carefully built (including a balanced and aligned mechanism). The machine’s mass, including feed material, needs to be similar on one side of the machine as the other. The machine’s box, or frame, should have no natural (critical) frequencies close to its’ operating speed and it is vital that support springs have similar static compression between the left and right sides of the unit. The springs should have equal stiffness and none of them are to be broken or sagged. The equipment also needs to run with a motion and speed and that is appropriate for the feed material, deck angle and type of media. It may sound a little daunting but all this is certainly achievable.

All new screens should be thoroughly tested, when they are about to go into service, by someone with expertise in vibrating equipment using an appropriate vibration meter. Unfortunately, not all new screens are checked (to this extent) when put into service. This commissioning check is in addition to any factory testing carried out by the manufacturer. I also recommend that screens be checked once or twice per year and further checks should be performed when major repairs are carried out or changes are made. These changes can include speed or counterweight adjustments, media conversions (steel mesh to rubber/poly for example), support spring replacement etc. Slight changes can cause significant shifts in machine dynamics causing potentially damaging vibration which, if undetected, can result in significant damage.

My business offers a range of products and services to the aggregate, sand and recycling industries and is not focused solely on vibrating equipment. However, it is with vibrating equipment that I see a great need and where, I believe, opportunities exist for increased productivity and reliability. That is, money to be made and money to be saved.

If your screens are a problem or you think they could be running better, get them checked out.

For further information contact Darren Toth on 0412 881 824 or darren.toth@tothink.com.au

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