SCREENS & FEEDERS
MOTORBASE SOLUTIONS FOR VIBRATING SCREEN DRIVES
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lmost 30 years ago, I designed some of the first stored-energy motorbases for vibrating screens. These were simplistic in design, using rubber torsion springs that were well known for providing wheel suspension in caravans and trailers for many years. I have since used and built many motorbases, such as tilt bases, slide bases and other configurations of the same in quarries and extractive industry operations. In times past, inclined screens dominated the quarrying industry. Today, there is a range of new generation screening machines, including horizontal/variable slope and banana screens. Many new types of screen are driven via lay or jack shafts. For the purpose of this article, I will focus on those screens and/or feeders directly connected from the screen to the electric motor by vee-belts. These will be referred to as dynamic applications.
LINEAR-ACTION SCREENS More than 25 years ago when visiting a mining operation, the manager showed me a problem he was experiencing with breaking the motor shafts on his linear-action screens. These were 30kW, D200L four-pole motors, which were fitted with six-groove, B-section belts. They were driving 2400mm x 4800mm horizontal dewatering screens (Figure 1). The motor was adjusted with slide rails. Breaking of motor shafts occurred during resonance (stopping or starting) when a significant percentage of the mass of the screen was transferred to the electric motor shaft. To resolve the problem, I designed a simplistic stored-energy motorbase consisting of a rubber torsion spring, to which a flat plate was welded to carry the electric motor. The spring was pre-positioned to provide sufficient force to tension six vee-belts. There was enough resilience left to allow the electric motor to follow the wild movement of the screen during resonance. No more motor shafts were broken. The client went on to fit up to another 28 screens. LINEAR-MOTION FEEDER DRIVE A quarry maintenance supervisor contacted us about problems with a 55kW, D280S,
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Quarry June 2019
Figure 1. Linear-motion horizontal dewatering screen.
Figure 2. A linear-motion feeder drive.
Figure 3. A circular-motion screen. The correct method of tensioning the drive is to apply sufficient force (tension) to allow the screen to start without the vee-belts slipping.
six-pole motor fitted to a linear-motion feeder drive (Figure 2). A site visit and a check of the stroke angle revealed the vee-belt drive angle was incorrect and that it resulted in energy from the feeder being transmitted via the vee-belts into the supporting structure. This resulted in the cracking of steelwork and welds and an ongoing maintenance problem. To correct the problem, we relocated the drive position 90 degrees to stroke or line of action and fitted a stored energy motorbase. The mounting position was vertical down. In this position the mass of the electric motor assists in tensioning the vee-belts. The driven pulley now pivots around the
drive pulley centre line and minimises the transmission of energy to the supporting structure.
INCLINED SCREENS Inclined screens are most recognised in quarries. Larger screens may have twin shafts (drives). These are circular-motion screens in which gravity is used to assist the screening process (Figure 3). Horizontal screens are becoming more popular and depending on the type of action produced, whether linear or elliptical motion, the positioning of the motorbase can be critical to the vee-belt and pulley life. A passing conversation with a quarry manager resulted in a site visit to investigate
