Mobile Hydraulic Actuators
The actuators are the hydraulic components that actually perform the physical work in the system. They are the components that convert fluid power into mechanical power. Hydraulic cylinders and motors are the system actuators.
Hydraulic cylinders convert fluid power into linear motion to raise a dump body or angle a plow, for example. Hydraulic cylinders may be single acting or double acting; single stage or multiple (telescopic) stages.
Single acting cylinders, also referred to as “power up/gravity down” type, and are extended by supplying flow to the base of the cylinder, pushing the cylinder’s piston, and extending the rod. The single acting cylinder relies on an opposing force, weight and gravity, to retract. This type of cylinder is commonly found on dump bodies and snow plow lifts. Most telescopic cylinders are single acting.
Double acting, or “power up/power down,” cylinders have a port at each end and are hydraulically powered in both directions. Double acting cylinders are used where the cylinder will be mounted in a horizontal position or where greater control of the return stroke is desired. Snow plow angling and crane arms are common truck mounted applications. While most double acting cylinders are single stage, there are applications which require the longer stroke of a telescopic cylinder.
Pressure = Force ÷ Area
The theoretical hydraulic system pressure required to move a hydraulic cylinder can be calculated by dividing the load (weight to be lifted) by the area of the hydraulic cylinders piston. Notice that this is the theoretical pressure requirement. In actuality, to begin movement from a standstill approximately 30% more pressure is required. To accelerate movement approximately 10% more pressure is required. This added pressure is necessary to overcome friction within the cylinder. Obviously, other factors come into play in determining pressure requirements. Is the load being lifted straight up or pivoted, as in a dump body application? If the load is being moved horizontally, what frictional forces must be overcome? Does the load shift during cylinder travel?
How fast does a hydraulic cylinder extend or retract? If you desire to extend a cylinder in a given amount of time, how much flow will be required of the pump? How does pump efficiency affect cylinder speed? All of these questions can be answered by a simple calculation. Remember your basic high school geometry class and the formula πr2. You used it to calculate the area of a circle. The “π” , or pi symbol stands for 3.14, “r” is the radius (1/2 the diameter) of a circle, and to square a number (x 2) means to multiply it times itself. The formula then is r X r X 3.14 to get the area that is referred to in the above formula. This would be the surface area of the hydraulic cylinders piston in square inches. With the piston area known, you can determine the rate of extension of the cylinder by using the formula:
GPM X 231 ÷ Area = Cylinder velocity in inches per minute
To take this one step further, once you determine the area of the cylinder’s piston, the volume of the extended cylinder can be calculated by multiplying the piston area by the length of the stroke and dividing by 231. Using a 6" diameter cylinder as our example we would calculate a volume of 4.4 gallons if the length of the cylinder’s stroke were 36 inches.
28.27 X 36 ÷ 231 = 4.4 gallons
If we wish to fully extend this cylinder in one minute we would require a pump flow rate of 4.4 gpm. To extend it in 30 seconds a flow rate of 8.8 gpm would be necessary (twice the flow = twice the speed). If we are using a fixed displacement pump to provide the flow, we must turn it at twice the input speed to achieve twice the output flow. If we are using a power take-off to provide power to the pump we must then run the engine at twice the speed. An alternative would be to change the power take-off to a faster (2X) model. Yet another alternative would be to change the hydraulic pump to a model of twice the cubic inch displacement.
Things get a bit trickier when working with multiple stage, telescopic cylinders. How do you determine extended cylinder volume when you have three, four, or five successively smaller cylinder sections to extend to reach an overall extended length? See the chart on next page to get an estimated volume for various common sizes of telescopic cylinders. Two capacities are shown, “Fill” and “Extend”. The “Fill” capacity represents approximately how many gallons are required to fill the empty cylinder before it begins to move. (This will only apply the first time the cylinder is extended, thereafter these areas will contain oil.) The “Extend” capacity represents the volume of oil required to fully extend the cylinder. This information is also available from the hydraulic cylinder manufacturer or distributor.
Cylinder maintenance and troubleshooting. When first installed, hydraulic cylinders must be “bled” to remove air. Air trapped in a cylinder will cause the cylinder to move in a jerky manner as air, under pressure, will compress. The compression of air in a hydraulic system also creates heat. Cylinders are equipped with a bleed screw to facilitate the removal of air. Air can get into a cylinder, and oil can leak out, if the packing seals become damaged or if the rod becomes rust pitted or scored by contaminates.
Watch a telescopic cylinder extend and retract. Extension should always begin with the largest diameter section and progress to the smallest. If it does not, there is a problem with the cylinder, not the pump. Retraction should be just the opposite; smallest section first, largest last.
Cylinders that have to control heavy loads are sometimes equipped with a counterbalance valve to prevent the cylinder from bleeding down or retracting too rapidly, creating a dangerous operating condition. The counterbalance valve is typically non-adjustable to prevent tampering and allows for greater operator control of the loaded cylinder.
Cylinders should be stored in a vertical position to prevent seal distortion and with their ports plugged to keep contaminates out.