Tutorial on hydraulic cylinders

Written by pauline gill
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Tutorial on hydraulic cylinders
Hydraulic cylinders can generate tons of force and are common in construction. (Hydraulic excavator at work. Shovel bucket against blue sky image by Andrei Merkulov from Fotolia.com)

Hydraulic cylinders are capable of generating many tons of linear force at low to moderate speeds. They are used to apply brute forces to large loads in machinery, construction equipment, and transportation. Hydraulic jacks consist of integrated hydraulic cylinders that can apply the forces to lift heavy objects, while hydraulic presses can either squeeze or pull objects according to specific movement requirements. Specifying hydraulic cylinders consists mainly of defining their required forces and range of motion, also known as stroke.

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Things you need

  • Calculator
  • Design pad and pencil

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  1. 1

    Define the application requirements for a hydraulic cylinder. Suppose you need to construct a boom arm for a 600-pound (lb) hoist on the back of a truck that lifts objects straight up and swings them around and over the 4-foot-high truck bed. The boom arm is 10 feet long (120-inches) and is capable of a total of 8 feet (96-inches) of vertical lift. The cylinder is connected to both the hoist's centre mast and the boom arm only 1-1/2-feet (18-inches) from the mast. Assuming that your engine-driven hydraulic pump generates 726 Kilogram-per-square-inch (psi) pressure at a flow rate of 1 gallon per minute, you can design a cylinder to do the hoisting job.

    Tutorial on hydraulic cylinders
    These hoists are made possible with modern hydraulic cylinders. (Hebebuehnen image by HARD from Fotolia.com)
  2. 2

    Calculate the maximum force the cylinder will need to develop, and the total stroke it will need to have. Since the cylinder is pivoted to the mast only 18 inches from the mast, the forces against the cylinder will be multiplied 120 inches/18-inches = 6.67 times. A 272kg. load will put a force of 600 x 6.67 = 1814kg. on the cylinder. The stroke of the cylinder will be the 96-inch vertical lift/6.67 = 14.39 inches.

    Tutorial on hydraulic cylinders
    Flexible hose guards allow cylinders to extend many feet. (hydraulic machine 3 image by Heng kong Chen from Fotolia.com)
  3. 3

    Compute the minimum square-inches of area for the piston in the cylinder based on the 1814kg. required force, and the maximum available pressure from the pump of 1600-psi. 1814kg/1600-psi = 2.5-square-inches (sq-in). Since conservative cylinder design might add a 50-percent safety factor to the cylinder's capacity, you can multiply the piston area by 1.5 and solve for piston diameter. 2.5-sq-in x 1.5 = square root (3.75/0.785) = 1.68 inches.

  4. 4

    Select a standard cylinder with a 2-inch bore, and a 16-inch stroke. This will allow you some mounting flexibility, and account for the slight mechanical loss due to the angular (hypotenuse) orientation of the cylinder, as the boom arm moves around its pivot.

  5. 5

    Calculate a total lift time speed for your application. A cylinder with a 2-inch bore and a 16-inch stroke is only 2-squared x 0.785 x 16 = 50.24-cubic inches. The 1-gpm pump (231-cubic-inches-per-minute) will only take 50.24/231 x 60-seconds/minute =13-seconds to lift the load eight feet, or about 8 inches per second. Prudent hydraulic designers will put a one-way flow restrictor on the hydraulic cylinder line that will significantly slow the rate of lowering the load to prevent damage or injury.

  6. 6

    Specify the particulars for the cylinder application. Each end of the cylinder will need a mounting clevis that will receive a pivot pin or bolt, and fitting connections that are compatible with your hydraulic system.

Tips and warnings

  • Check the type of hydraulic oil the pump uses to assure compatibility with the cylinder's seals.
  • Always consult a professional when constructing hydraulic lifting equipment that will hoist loads to a point where they can cause serious injury.
  • Always include safety provisions such as warning bells or buzzers when designing lifting systems for overhead or vehicular use.

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