The selection of a motor for a hydraulic application can be complicated. The following technical article breaks down the process discussing fundamental selection factors.
The ideal in hydraulic system design is to match overall efficiencies to the application performance expectation. This requires the designer to first match the motor, then the pump to a specific system performance expectation. Whether the requirement is to do something within a specific time frame, or in handling a given amount of load, the design of the entire system will change depending on the motor selected.
A hydraulic motor is a hydraulic actuator that, when properly connected into a hydraulic system, will produce a rotary actuation. This can be unidirectional or bidirectional depending on the system design. Motors are similar in design to pumps only where a pump takes a rotary actuation to move hydraulic fluid out of the unit, whereas a motor will take flow into itself and put out a rotary actuation.
The motor selection comes first in the process because application design best practices require that you start with the load requirement, then work back to the prime mover—the pump that will put the fluid power into the motor selected to deliver the performance goal.
Each motor type—gear, vane, in-line piston, bent-axis piston and radial piston—has a specific performance profile. So, knowing the application performance requirement and which motor type best meets the objective is the first step. Then it’s necessary to evaluate the design advantages of your motor options along with the degree of complexity you want for the overall system.
In the end, it all goes back to the application’s performance expectations. Some have severe duty cycles, while others do not. If, for example, you consider running a low-efficiency, lighter-duty motor into a higher-duty cycle application, the life of the motor will be less than the life of a higher-duty cycle motor that is designed to operate in those types of environments. It is important to understand what operating pressures and flows are required for the motor selected to achieve the application performance expectations.
Each motor type has its own set of applications where they are a better choice than others. For example, if a small gear motor designed to operate at a max of 3,000 psi and 1,000 rpm is put into an application that requires it to run consistently at 3,000 psi and 1,000 rpm, the motor will be running in a “corner” overstressed condition and have a reduced life—even though it is technically within its ratings. The better motor choice would be a motor with higher ratings that will live longer in the application. Granted, there is a greater cost in going with a higher rated motor. The final decision always will depend on what is required in terms of application performance and motor life versus where you want to be with cost.
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Article by Justin Wheeler, CFPHS, C-Series/Bent Axis Project, Manager, Hydraulic Pump Division of Parker Hannifin and originally appeared in the October 2015 issue of Fluid Power World. A 2000 graduate of The Ohio State University, Wheeler has been involved in hydraulic and electrical systems engineering for 15 years. Wheeler is listed as an inventor on two U.S. Patents — US7134513 (“Drive Mechanism for a Boring Machine”) and US7500530 (“Control System for Construction Equipment’)."
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