The short answer
Tippers needed = tonnes required per hour x cycle time in minutes, divided by 60 x payload in tonnes, rounded up, plus one spare for every eight to ten. A cycle is loading, the loaded haul, weighing and tipping, and the empty return. A plant dispatching 100 tonnes an hour over a 20 km lead on 18-tonne tippers needs nine trucks running.
Fleet size is arithmetic, and most sites still get it wrong in the same direction. They count the trucks they have, divide the day's target by them, and discover the answer in the afternoon when the loader is standing idle and the laying gang is on the phone.
01The formula
Start with what has to arrive each hour, not each day. Then time one complete cycle of one truck with a watch, from the moment it pulls under the loader to the moment it pulls under the loader again.
- Loading and trimming
- 6 min
- Weighbridge, out and back
- 5 min
- Loaded haul, 20 km at 30 km/h
- 40 min
- Queue and tip at the road head
- 9 min
- Empty return, 20 km at 40 km/h
- 30 min
- Cycle time
- 90 min
- 100 t/h x 90 min / (60 x 18 t)
- 8.3
- Tippers running
- 9
Nine running means ten on the site. A fleet with no spare is one puncture away from missing its tonnage, and tyres, springs and clutches fail on a schedule of their own.
02Trips per shift, by lead
The same sum turned round shows what one truck delivers in a shift. The table assumes 20 minutes of fixed time per trip for loading, weighing, queueing and tipping, 30 km/h loaded, 40 km/h empty, an 18-tonne payload, and 500 working minutes in a ten-hour shift.
| Lead | Cycle | Trips | Tonnes per tipper | Tippers for 1,000 t a shift |
|---|---|---|---|---|
| 5 km | 38 min | 13 | 234 | 5 |
| 10 km | 55 min | 9 | 162 | 7 |
| 20 km | 90 min | 5 | 90 | 12 |
| 40 km | 160 min | 3 | 54 | 19 |
| 60 km | 230 min | 2 | 36 | 28 |
Running trucks only, before spares. Trips are rounded down because a truck cannot deliver part of a load.
Two things stand out. Doubling the lead from 20 to 40 km does not double the fleet, it adds more than half again, because the fixed time matters less as the haul grows. And at 60 km a truck manages two loads a day. That is the distance at which it usually becomes cheaper to stand a plant nearer the work than to keep adding tippers.
03Where cycle time hides
- Under the loader. A bucket matched to the tipper fills it in four or five passes. A small loader takes nine, and every truck in the fleet waits for it.
- At the weighbridge. One bridge serving both directions makes loaded and empty trucks queue for each other.
- At the tipping point. A road head with room for one truck to reverse turns nine tippers into a convoy.
- On the haul road. Ruts and standing water take ten kilometres an hour off the average speed and put it onto the tyre bill.
- At shift change. If every truck returns to the yard at the same time, the first hour of the next shift has no trucks at the loader.
Five minutes saved in each cycle on a 20 km lead is worth half a truck across a fleet of nine. It is nearly always cheaper to find those minutes than to hire the truck. Grading the road the tippers run on is the usual place to start, and haul road design covers it.
04Matching the loader to the fleet
A fleet can be too large as well as too small. If trucks arrive faster than the loader can fill them, they queue and the extra trucks deliver nothing. If the loader finishes a truck and has nothing to load, the loader is too big or the fleet too small. The aim is a loader that is almost never waiting and a queue that is almost never more than one truck.
The check is simple. Divide the truck cycle by the time the loader needs for one truck. With a 90-minute cycle and a loader taking 9 minutes a truck including manoeuvring, the loader can serve ten trucks. More than that will stand in line.