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Troubleshooting vee-belt drives for screens and feeders

Almost 30 years ago, I designed the first stored energy motor base for vibrating screens.
Figure 1. A linear motion horizontal dewatering screen.
Figure 1. A linear motion horizontal dewatering screen.

These were simplistic in design, using rubber torsion springs that were well known as wheel suspension in caravans and trailers for many years previously.

We have built many motor bases over the years, such as tilt bases, slide bases and other configurations of the same in quarries and extractive industries operations.

In times past, inclined screens dominated the quarry industry. Today we see a range of new generation screening machines such as horizontal/variable slope, banana screens, etc.

Many of these new screens are driven via lay or jack shafts. In this article, I intend to focus only on those screens and/or feeders directly connected from the screen to the electric motor by the vee-belts. These will be referred to as dynamic applications.

Case study 1

More than 25 years ago, when visiting a mining operation, the manager showed me a problem he was experiencing with breaking of the motor shafts on his linear action screens.

Figure 2. A linear motion feeder.
Figure 2. A linear motion feeder.

These were 30kW, D200L 4-poles motors. The motors were fitted with 6-groove, B-section belts. They were driving 2400mm x 4800mm horizontal dewatering screens.

The motor was adjusted with slide rails, and the 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 motor base 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 (Figure 1). 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.

Case study 2

A quarry maintenance supervisor contacted us about ongoing problems with a 55kW, D280S 6-pole motor fitted to a linear motion feeder drive.

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.

Figure 3. A circular motion screen.
Figure 3. A circular motion screen.

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 motor base. The mounting position was vertical down. In this position the mass of the electric motor assists in tensioning the vee-belts (Figure 2). 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 those 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).

Case study 3

Correct Tension
Correct Tension

A passing conversation with a quarry manager resulted in a site visit to investigate the high wear of vee-belts and pullies on an inclined vibrating screen.

Upon inspection it was clear the screen was being over-tensioned, resulting in the screen being skewed on its suspension springs and the misalignment of the drive and drive pulleys.

Maintenance staff had been advised to tension the vee-belts by the deflection method, according to the vee-belt manufacturer’s specifications. This method was incorrect for this type of drive.

Over Tensioned
Over Tensioned

The correct method of tensioning the drive is to apply sufficient force (tension) to allow the screen to start without the vee-belts slipping (Figure 4). Check that the pulley grooves and vee-belts are in good condition.

The OEM would have designed the drive with pulley diameters and vee-belt numbers to start the screen DOL (direct online starting) without the belts slipping.

Over-tensioning may pull the drive and driven pulleys out of line, resulting in extreme belt and pulley wear.

Drive angle

With all screen drives the most common fault we find is incorrect drive angle/motor mounting position. This may have one or more of the following consequences:

  • High belt wear

  • Reduced drive and driven pulley life.

  • Belt 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.

 












ABOUT THE AUTHOR
Richard Sharp

Richard Sharp is the managing director of Leverlink Australia









Wednesday, 20 June, 2018 09:36pm
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