How many tippers does a plant need? Working it out from cycle time

Every site has an opinion on how many tippers it needs, and it is usually one more than it has. The number can be worked out instead, from a stopwatch and a weighbridge slip.

Time one full cycle of a tipper. Divide the working minutes in a shift by that cycle to get trips, multiply by the legal payload to get tonnes per tipper, then divide the tonnes the hopper needs in the shift by that figure and round up. That is the fleet, before spares.

The five parts of a haul cycle

1. Spot and load. Reversing under the excavator and taking the passes that bring the body to its legal weight.
2. Haul loaded. Face to hopper, or plant to road head.
3. Queue and tip. Waiting for the hopper or the weighbridge, reversing in, raising and lowering the body.
4. Return empty.
5. Queue to load. Standing behind another tipper at the face.

The first four are work. The fifth is waste, and it is the one part that grows when you add tippers. Time several cycles at different hours of the shift and average them.

Two formulas for two different questions

The textbook formula is: number of trucks = truck cycle time divided by loading time, rounded up. It answers a narrow question: how many tippers keep this excavator loading with no gap. Below that number the excavator waits. Above it, trucks queue. Planners call the balance point a match factor of 1.

The second formula starts from tonnage. Trips per shift = working minutes divided by cycle minutes, rounded down to whole trips. Tonnes per tipper per shift = trips multiplied by payload. Tippers needed = tonnes required in the shift divided by tonnes per tipper. This is the question the project is asking. When the plant sets the pace and not the excavator, the second answer is the smaller one, and it is the one to buy diesel for.

A worked example

Every input below is an assumption chosen to show the arithmetic. The speeds in particular are illustrative: we have no published figure to offer for haul speeds on a site road, and yours should come from timing your own. The 15 tonne payload is likewise a placeholder: the legal payload of a tipper comes off its registration certificate.

Read the last two rows together. One excavator loading a tipper every five minutes puts out 90 loads in the shift, 1,350 tonnes, so 2,000 tonnes needs two loading points whatever the road is like. On the maintained road nine tippers carry the tonnage between them, and each excavator stands for part of every cycle. That is not a fault: it has a face to trim and oversize to set aside. On the rutted road the same tonnage takes fourteen. Five more tippers, five more drivers and their diesel, for no extra stone at the hopper.

Matching the fleet to the plant

The 2,000 tonnes a shift comes from the plant. One of our 250 TPH three-stage plants is rated for up to 1,00,000 MT a month on two shifts. Assume 25 working days and the hopper wants about 4,000 tonnes a day, or 2,000 a shift, when the plant runs at its rating. Size the face-to-hopper fleet against the tonnage the programme asks for in its peak month, then add the spares your breakdown record justifies.

The delivery fleet from plant to road head is a separate sum with the same method and a much longer lead. On a run of 30 or 40 km the two travel legs are nearly the whole cycle, and loading time hardly registers. Lead distance and the delivered rate follows that through to cost.

What stretches the cycle

Surface comes first. Caterpillar's published rolling resistance factors run from 1.5 percent on a hard, smooth surface and 3 percent on a firm, maintained, watered road to 8 percent on rutted soft dirt where tyres sink about 100 mm, 14 percent at about 200 mm and 20 percent in soft mud. A loaded tipper on the last of those spends the whole haul in a low gear. Gradient is added on top: DGMS Safety Alert 20 of 2025 gives 1 in 16 as the steepest a haul road should be, as best practice.

• Queueing at the hopper or the weighbridge, when tippers arrive in a bunch or the feeder is choked with oversize.
• Single-lane sections, where a loaded tipper and an empty one cannot pass and one of them waits.
• Rain, which can take a road from the 3 percent end of that range to the 20 percent end in an afternoon.

Cheaper capacity than another tipper

In the example, the difference between the two roads is one tipper in four: a vehicle, its drivers, its diesel and its tyres, for as long as the job runs. A grader, a roller and crushed stone on the haul road are spent once. In our experience a properly built haul road pays for itself in tyres and cycle time inside a month. Building a quarry haul road covers width, gradient and drainage.

The other cheap fix sits at the ends of the cycle. If tippers queue to load, a second loading point removes the queue without adding a vehicle: another excavator on the face, or a wheel loader on a stockpile. If they queue to tip, the hopper or the weighbridge is the bottleneck, and more tippers only lengthen the line.

We plan bulk haulage on our own tippers in that order: tonnage first, the cycle timed on the actual route, the fleet last.