Poor motion in vibrating screens and feeders can occur for many reasons, but one is arguably the easiest to prevent and yet seems the least understood – the adverse effect of support springs (the springs on which the machine sits) behaving differently from one side of the machine to the other. The motion of the equipment will vary between the left and right sides, increasing the internal stresses within the machine, reducing its service life. It will become a greater challenge to maintain.
What do support springs do? They allow free movement of the vibrating machine and isolate most of the unit’s vibration from its supporting structure. Something that is not broadly understood, however, is that they also magnify the vibration of the machine. While typically only by a few per cent, this magnification (caused by a dynamic response of the screen’s mass combined with the springs) occurs at each corner and will only be equal around the unit if the compression on each of the springs is equal.
A difference in spring compressions (between the left and right side on the equipment) means a difference in the amplitude of vibration (from one side to the other), which leads to excessive internal stresses within the machine body that are different to those typically considered in the machine design. To worsen matters, a variation in spring compressions (ie spring heights) forces a slight twist into the machine, creating yet another dynamic response that contributes to these adverse internal stresses!
Tips for support springs
What can be done to ensure a machine’s support springs aren’t causing it to fail prematurely? Consider the following tips:
- Check your spring heights. Make sure equivalent springs (ie springs in the same location along the screen but on opposite sides) have a similar free height and then end up at similar heights (within 5mm – note some equipment manufacturers’ requirements may differ) under the static load of the machine. If the height difference is greater than 5mm (or the variation recommended by the supplier) from one side to the other, then use packers to load up the springs that are sitting higher. In configurations where the springs in a set are parallel (typical of fixed plant but also in some mobile), ensure they are at similar heights.
- Keep your springs clean. Debris and fines build-up should be cleared from springs.
- Never run a screen or feeder with broken or collapsed springs. Have at least two new spares on hand to remove the temptation to continue operating with broken springs while waiting for replacements. If you replace a set of springs and don’t have spares, consider buying extras at the same time.
- Be consistent. Always replace equivalent springs in pairs (ie if a spring is replaced on one side of the screen, replace the spring in the same location on the opposite side).
- Make sure springs can be easily inspected.
- Monitor. As part of your regular checks, measure the heights of the springs while the equipment isn’t running (as they are under the weight of the machine) and record them. After each check, compare the spring heights with those previously recorded so that changes are identified. It will be obvious when a spring has catastrophically failed and, in most cases, will be evident when they are on the “way out”.
If there is suspicion about a spring, either replace it or lift the machine up and check the free heights of equivalent springs. Also compare these to their original (new) height.
|A screen mount bracket, with another example installed on a screen (right).|
Following these guidelines will not only keep your machines running evenly by minimising the equipment’s internal stresses, it will also help maximise the service life of springs without compromising the health of the machine. The life of a set of springs is that of the one most stressed. Keeping the spring heights consistent means stresses in the springs will also be consistent. That means a longer service life.
As well as selling various components for screens and feeders, including different types of springs, businesses also offer the service of testing vibrating equipment to identify adverse motion, from an equipment longevity perspective, and ways to improve performance. When a machine is running unevenly, we need to look for the various causes.
If the springs are in good condition and their static heights are reasonably similar from one side to the other, it becomes easier to identify the cause of the adverse motion. If the springs are in poor condition and/or vary significantly in height (left to right), it can often mean an additional site visit is required before there can be recommendations to correct the unit’s motion. This adds to the service and means the machine will run poorly for a longer period.
The three most common (but not the only) types of springs seen on sites, on vibrating screens and feeders, are steel coil springs, rubber springs and screen (or oscillating) mounts. Although they differ in their construction, they all perform the same function on vibrating equipment, and their behaviour is governed by their deflection from their free height when under the weight of the machines they support. In all cases, the laden height of the spring is simply the vertical distance between the surface of the mounting bracket sitting on top of the spring and the surface on which the spring is sitting.
The design intent for vibrating machines is for them to wear out after many years of reliable service, rather than fall apart due to fatigue. In many situations, regular consideration of screen and feeder support springs in a site’s maintenance checks will make the difference between years of reliable service and premature failure.
Once a vibrating screen or feeder is running well, which is how every machine should start life, the most likely cause for it to run poorly is the adverse effects of changes in the support springs.