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Grades K–2 · Machines

How does a dump truck tip out a mountain of gravel?

A dump truck can carry a pile of gravel without anyone shoveling it out. Its big open box is called the bed. At the work site, the front of the bed rises. The gravel slides toward the back and spills out.

The back of the bed has a strong hinge. It stays joined to the truck while the front swings upward, much like one end of a seesaw rising. The whole truck does not have to tip.

Under the bed, a lifting part grows longer. A pump pushes oil into it. The oil pushes a rod outward, and the rod lifts the bed. This oil-powered lifting part is called a hydraulic cylinder.

Find the fixed rear hinge, the lengthened lifting cylinder and the downward gravel path.

A whole dump truck and its tipping mechanismThe exterior shows the cab, wheels and loaded bed. A numbered callout identifies the bed. Below, an extended cylinder raises the bed about its rear hinge so gravel falls out.A dump truckOutside view → detail below11 The bed becomes a ramp.A cylinder tips a dump-truck bedA truck bed pivots at a rear hinge while an extended hydraulic cylinder pushes upward. Gravel follows the descending bed and falls out.Lift the bed, tip the loadA simplified side viewBed risesOil pushes thelifting part longer.RearhingeThe bed tilts.Gravity moves the gravel.
A hydraulic cylinder raises the front of the bed around a rear hinge; gravel falls down the tilted bed.

Now the bed is a steep ramp. Gravity pulls the gravel down the slope through the open back. The lifting part made the ramp; gravity moves the stones. When oil can flow back out of the cylinder, the bed can come down again.

About this explanation

This shows one under-bed lift; other dump trucks use different lifting arrangements.

Starting knowledge: Recognize a hinge, a push and things moving downhill.

Sources

  • Hyva — Tipping solutions
  • Hutchin Hydraulic — The guide to dump truck hydraulic hoist
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