The safe and effective operation of hydraulic equipment requires an awareness and understanding of a few critical pieces of information. Among the most important of these is the speed at which the actuators move, a variable that is directly tied to the gallons per minute (GPM) of the hydraulic fluid. Anyone trained to work with fluid-powered machinery must be able to calculate GPM to ensure their equipment is functioning optimally.
Fortunately, learning how to calculate GPM is a fairly simple process once you understand the other variables involved in the operation of your hydraulic machinery. In the following blog post, we’ll break down the standard formula, including a detailed explanation of each component, so you can apply this crucial calculation in real-world scenarios.
Gallons Per Minute: The Basic Formula
In any fluid transfer system, the basic formula for the speed of a hydraulic pump is used to determine the volume of liquid the system can transfer within a specific interval. In the United States, this volume is measured in gallons, and knowing the number of gallons that the system can transfer within one minute allows operators to calculate other important variables, including the pressure levels of the machinery and its current efficiency.
Operators must recognize that there are different types of hydraulic actuators before they attempt to calculate their system’s GPM. In most cases, when operators are looking for GPM, they are attempting to calculate the output flow rate of a hydraulic pump motor. The basic formula for this is the RPM of the motor multiplied by the displacement volume of the pump and then divided by the number of cubic inches in a gallon (approximately 231).
GPM = (RPM x Displacement Volume) / 231
Once you have the correct inputs, the output can be determined using the calculator on our website, accessible via the calculator icon at the top of this page.
The Aberdeen Dynamics calculator can be used to determine a great deal of other important information about hydraulic pumps and motors as well. For example, knowing the GPM can allow you to determine other variables, such as torque and motor speed, as well as the required pump size if a specific GPM is needed.
Determining the RPM of a Motor
The first of the basic inputs to put into the GPM calculator is the revolutions per minute (RPM) of your hydraulic pump. This is, of course, a factor that can change depending on the settings you use, but the safe range for a specific piece of equipment can be found on the manufacturer’s specifications. Always keep your pump well within the safe operating range, or you will risk overheating and eventually burning out.
While the RPM of your pump is directly connected to its GPM, it’s worth noting that in some cases the flow rate can be weak even if the machinery is operating at a high RPM, a fact we’ll discuss further later in this blog post. If this is the case, it indicates a problem with the system: There may be a leak, or the hydraulic fluid may be contaminated, among other possibilities. If there’s an issue, the machinery will need to be professionally repaired before you can successfully calculate the correct variables.
Displacement in Hydraulics
Each time the cylinder in your hydraulic pump rotates, it displaces a specific amount of fluid, and this amount is the other variable you need to determine GPM. Once again, this information can be found on the manufacturer’s specifications included with the equipment.
If you don’t have access to the manufacturer’s displacement specs, then you can calculate this information in other ways. Having the GPM and RPM as inputs can allow you to use the calculator to determine it. Of course, if you do not already have the GPM, you won’t be able to do this. If that’s the case, you’ll need to measure the internal components of the machinery to ascertain the displacement of the specific type of pump you have. Half the diameter of the piston squared, multiplied by the stroke length, and then multiplied by pi (3.14) can give you an accurate measurement of the displacement of your pump.
Displacement Volume = (Piston Diameter / 2)2 x Stroke Length x π (3.14)
Efficiency
The above inputs will allow you to determine the ideal theoretical flow rate of your hydraulic pump. However, since pumps do not operate at 100 percent efficiency in reality, this is unlikely to be the actual output achieved by your machinery. Many factors can reduce the efficiency of the machine, including wear and tear on the parts, the viscosity of the fluid used, and the presence of any contaminants that reduce its flow rate.
To determine the efficiency of your hydraulic pump, you’ll need to attach a flow meter to the equipment and compare the readouts to the theoretical rate. Divide the true GPM by the theoretical GPM to get the efficiency percentage of your equipment.
The laws of physics make it impossible for any pump to operate at 100 percent efficiency, and most stay within the range of 75 to 90 percent. If the efficiency dips below 75 percent, however, the pump is not functioning optimally and will need professional repair.
Using this Information on the Job
Learning how to calculate GPM and the efficiency of a hydraulic pump is important for several reasons. As mentioned above, it can be used to determine when repairs are needed. It can also be used to determine if your specific pump is the correct one for your current job. For example, if you have a construction project that requires a certain amount of digging or lifting to be accomplished in a certain amount of time, then you’ll need to find a pump with a higher GPM. To ensure you have the right equipment for any task, contact Aberdeen Dynamics to request a fully customized setup tailored to your specific requirements. We’ll work with you to find the right sized pump to achieve your goals at the optimum level of efficiency.