Many factors can affect the wear life and performance of your cross-tensioned screen media. Screen cloth tension is critical for maximum wear life and can be achieved with right support bar capping rubber and good even crown across the deck. There is, however, another equally important and often neglected component critical to the successful performance of your screen media – the side clamping bar.
The purpose of the side clamping bar is to stretch and tension the screen cloth over the crown of the screen.
Screen box manufacturers design their side clamping bars so the side clamp sits in the base of the side hook, applying force on the side hook at the correct angle, pulling the side cloth hook down towards the side wall and side support rail of the screen box (Figure 1).
Quite often, replacement side clamps are sourced from a local fabricator and sometimes the profile angles can be different to the OEM bar or the bolt holes can be punched at the wrong height.
{{image2-a:c-w:600}}Incorrect profile side clamping bars or bolt hole position can cause the hook to apply pressure to the screen cloth side hooks at the wrong angle, sometimes lifting the hooks up and away from the side support rails (Figure 2) or causing the middle of the screen cloth to bow upwards, creating a gap between the screen cloth and the support bar capping rubber.
These conditions will lead to inefficient screening and eventual premature screen cloth failure.
Side clamping bars overlapping from one screen cloth to another is a common and serious problem that will affect screen cloth tension and wear life.
No two screen cloths are identical in width, so one cloth or part of one cloth will be tensioned while the other is under-tensioned. A loose cloth will move with the vibrations of the screen box and reduce cloth wear life.
{{image3-a:c-w:600}}The number of side clamp bolts is important. Some screen box manufacturers have only two bolts for a 1220mm long clamp bar, while others may have three or four bolts. Ideally, the clamp bolts should be spaced at 300mm centres. Where there are too few bolts or the clamp bars are made from thin steel, the clamp bar will flex and the cloth will only be under full tension closer to the bolts, while the remainder has little tension at all.
The self-cleaning wire screens are very sensitive to this uneven tension and localised wear areas can result.
Increasing the clamp bar?s steel thickness will help spread its load and reduce flex.
Worn bars will flex and bend, applying uneven tension across the screen cloth and reducing screen cloth wear life.
It costs much more to replace a failed screen cloth than it does a side clamp so replace your side clamp bars regularly, before they are ?worn out?.
Some screen box manufacturers supply new screens with springs for side clamp bar tensioning. From a screen media manufacturer?s perspective, it would be better to see the springs removed and replaced with domed washers. Unless the springs are fully compressed, the screen cloth will pull on the clamps as the screen box vibrates and the cloth can move.
TIPS
? Only use the OEM profile side clamping bar.
? Replace bolt springs with domed washers.
? Use bolts with full length thread.
? If your clamp bars only have two or three bolts, increase steel thickness. It will even out tension over the cloth, increasing cloth wear life.
? Never overlap side clamps and screen cloths. One hook = one clamp bar.
? Rubber-line the side clamps for bolt head protection and longer wear life.
Quarrying attracts a diverse range of people from career professionals with extensive resources and technical expertise to farmers? wives looking to supplement their family income. Not everyone has access to their own in-house technical gurus to solve problems and advise on best maintenance practices. Too often, we gain our knowledge the hard way, through mistakes.
Source: Nepean Rubber