There are many factors that are often considered in selection of drive size, placements, and their economic and plant impact.

1. What is the cost effectiveness

2. What is the reliability effectiveness

3. How will this affect plant spares inventory

4. How does this affect maintainability

Can you use multiple drive sizes on the same conveyor? Yes. Again it is a muliple choice problem and answer.

One example is the 20 km Curragh NOrth conveyor. We used 2x1000 kW on the mid-station drive, 2 x 1000 kW at the dual head station drives, and 1 x 250 kW on the return at the mid-station. The carry strands are selected to minimize the belt strength, while standardizing the motor and reducers. The return 250 kW is selected to minimize belt vibration effects. Fortunately, its also was located at the mid-station carry side drives, so that the impact of an additonal electrical sub-station did not penalize the design.

Curragh was commissioned in 2007. The drive selection and placement did a proper job and worked as predicted. ■

Dear Shaklee,

A proper discussion is not possible on this forum. A brief note is offered:

1. Cost Effectiveness - Why use more than one drive motor? Again I use Curragh North as an example. Each driven pulley is fitted with 1000 kW motor. It could be fitted with 2 x 500 kW or 1x 2000 kW that repaces the two driven pulleys at the mid and head stations. At 2000 kW, the counterweight or take-up would need to be doubled in size. At 500 kW, we have twice as many drives. Larger, gives economy of scale, but, at a significant cost to belt strength. More means lower cost of spares (500, 1000 vs. 2000 kW) drives.

2. Reliability - more motors mean higher likelihood of a failure, but, the size of the drive is also an issue. Do you want to operate with one drive failure? Larger drives mean larger cranes to handle them, higher voltage to keep motor size in perspective, etc.

These issues have more discussion than i wish to expand upon here. If you design these types of machines, the method of selection should become apparent. Every design decision is a trade-off amony many alternatives or options. ■

The belt has a natural resonance, as it is spported between idlers, depending on:

1. Belt tension

2. Idler spacing

3. Belt elastic modulus - longitudinal and transverse

4. Belt mass

This resonance must not couple or be near a critical the spin frequency of the idler depending on:

1. Roll diameter

2. Belt speed

When the belt and idler natural frequencies are equal in the first through fourth mode, damaging forces may exists that reduce idler life, emit unacceptable noise, or damage the idler supports or other structural members. We attempt to keep the critical frequencies and near neighbor vibration spectrum from inducing unacceptable belt flap. CDI criteria is less than +/- 15% of the resonant frequency must be avoided, especially for overland conveyors.

Placing a return drive, drops the tension after the drive, which may control the tension to a narrow range that avoids the coupled resonance. This is only true with gravity take-up systems. Winch controlled take-up systems are not as easily adjusted to mimic a gravity take-up. ■

Hi Shakee

Just a point, but if you are using fluid couplings and different drive sizes, you normally find that the exact gear ratios of the boxes are not the same.

You often find that the motor rpm will not be identical either.

Often the detrimental effect the above has on proportional load sharing is way beyond the calming effect of the fluid couplings, especially if the lagging wear condition is different, and not in your favour.

Not worth it. Use identical drives if fluid cplgs.

Cheers

Taggart LSL Tekpro ■

In support of Graham's point, the exact gear ratios are sufficiently different that I have occasionally chosen to use identical reducers in order to get perfectly matching ratios at drives that were not of the same power. This of course means wasteful over-sizing at the lesser drive. The motors, of the same design though of different power ratings, do tend to behave proportionally if their power ratings are not dramatically different.

Multiple drives (boosters) along the conveyor length allow for high throughput high powered conveyors with a belt of modest strength. The decision on what to do ultimately comes as a result of technical economic study that considers all of the alternate solutions.

Joe Dos Santos ■