How Hydraulic Machines Work

Hydraulic Pumps

One thing you can see is that the advertised "20-ton splitting force" is generous. A 4-inch piston has an area of 12.56 square inches. If the pump generates a maximum pressure of 3,000 pounds per square inch (psi), the total pressure available is 37,680 pounds, or about 2,320 pounds shy of 20 tons. Oh well...

Another thing you can determine is the cycle time of the piston. To move a 4-inch-diameter piston 24 inches, you need 3.14 * 22 * 24 = 301 cubic inches of oil. A gallon of oil is about 231 cubic inches, so you have to pump almost 1.5 gallons of oil to move the piston 24 inches in one direction. That's a fair amount of oil to pump -- think about that the next time you watch how quickly a hydraulic backhoe or skid/loader is able to move! In our log splitter, the maximum flow rate is 11 gallons per minute. That means that it will take 10 or so seconds to draw the piston back after the log is split, and it may take almost 30 seconds to push the piston through a tough log (because the flow rate is lower at high pressures).


From this discussion you can see that just to fill the cylinder with oil, you need at least 1.5 gallons of hydraulic oil in the system. You can also see that one side of the cylinder has a larger capacity than the other side, because one side has the piston shaft taking up space and the other doesn't. Therefore, big hydraulic machines usually have:

  • Large appetites for hydraulic oil (100 gallons is not uncommon if there are six or eight large hydraulic cylinders used to operate the machine.)
  • Large external reservoirs to hold the difference in the volume of oil displaced by the two sides of any cylinder

Now that you understand the basics of a simple hydraulic system, we can look at some really interesting equipment!