Almost 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 motor-bases, 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.
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, 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 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 the high wear of vee-belts and pulleys on an inclined vibrating screen. Upon inspection it was clear the screen was overtensioned (Figure 4), resulting in the screen being skewed on its suspension springs, leading to the drive and driven pulleys becoming misaligned. Maintenance staff had been advised to tension the vee-belts by the deflection method, according to the vee-belt manufacturers’ specifications. This method is incorrect for this type of drive.
The correct method of tensioning the drive is to apply sufficient force (tension) to allow the screen to start without the vee-belts slipping. To ensure they are in good condition, the pulley grooves and vee-belts should be checked beforehand (Figure 5). The OEM would have designed the drive with pulley diameters and vee-belt numbers to start the screen direct online without the belts slipping. Overtensioning may pull the drive and driven pulleys out of line, resulting in extreme belt and pulley wear.
Drive angle/motor mounting position
With all screen drives, the most common fault is incorrect drive angle/motor mounting position (Figure 5). Inevitably, this may have one or more of the following consequences:
- High belt wear.
- Reduced drive and driven pulley life.
- Belts slippage on start-up.
- Belts jumping off.
- Energy (vibration) being transmitted to the supporting structure.
- Mechanical failure.
- Integrity of the supporting structure at risk of failure.
- Higher maintenance costs than would be expected.
- Unplanned downtime.
- Motor failures.