A PRIMER ON FLUID POWER nce during a discussion about a mobile fluid power application, a colleague kept referring to what he called “voodoo hydraulics.” This curious phrase illustrated the many and often misunderstood differences between mobile and industrial fluid power. As with industrial fluid power, mobile fluid power automates a machine and makes it move. Most of the time you’ll find mobile fluid power in machines with tracks or tires that are outdoors in the elements. But mobile fluid power refers not only to the type of machine, it also refers to types of hydraulic components and circuits used to control the machine. This article discusses how mobile fluid power works and how it is different from industrial fluid power. In most cases the human-machine interface between mobile and industrial equipment is very different. Many industrial applications are automated. Programmable logic controllers and programming allow the machines to run a large number of functions based on sensor or user inputs. Most of the time, with mobile equipment the human-machine interface is much more intimate. The movement of a lever or joystick or the push of a proportional thumbwheel directly affects the machine’s movement. It gives the operator the ability to get the “feel” of the machine, much like driving a car. Another big difference between mobile and industrial equipment is the prime mover. Mobile machinery regularly uses the vehicle’s engine-driven system (its internal combustion engine) for the prime mover; in industrial applications, it’s usually powered with an electric 8
AUGUST 2021
By Joe Raccosta, Fluid Power Sales Manager, and Micah Taylor, Mobile Fluid Power Manager, Milwaukee Division, Motion
motor. However, there is an overlap. For example, open loop pumps (vane and piston) are used in both segments but are more prevalent in industrial systems, whereas closed loop pumps are more frequently used in mobile systems. Screw-in cartridge valves are used in many manifold assemblies across both platforms as well. Both segments use cylinders, but mobile tends to use more welded and telescopic types than industrial. As far as fluid conveyance, mobile applications often operate at higher pressures and need hose with higher pressure ratings. They also require corrosion-resistant fittings for environmental protection, as well as abrasive-resistant hose because of machinery movement. Later in the article, we will describe some of the more common components found in a mobile system.
As with other segments of industry, mobile fluid power has unique characteristics and challenges. Some of those characteristics are duty cycle, environment, modular size, and noise and shock. Duty cycle. Duty cycle is the ratio of time a load or circuit is on compared to the time the load or circuit is off. The duty cycle on hydraulic pumps is the number of minutes in each hour the pump is under load. If you place a lower-efficiency, lighter-duty pump into a higher-duty cycle application, the life of the pump is compromised. Each subsegment of mobile equipment has varying
duty cycles, and some applications may require higher-duty cycles than others. Corrosion resistance. Mobile equipment operates within a wide scope of environmental conditions. Mobile equipment must be compact yet powerful, with performance unhindered by environmental factors like rain, snow, ice, dirt, extreme heat, or moisture. These environmental factors typically don’t come into play in the design or performance of industrial equipment. With these environmental challenges, mobile hydraulic components require anticorrosion agents for preventive measures. Size, weight, and power output. Size, weight, and power output are crucial in mobile applications. Mobile components are typically smaller, modular, and lighter compared to industrial components. Take a reservoir, for example. On an industrial system, it is usually sized around three times the flow of the pump’s output, whereas in a mobile system the reservoir size could be as small as the output of the pump or less. Producing the power output required with less space and weight is crucial. That is why power density is a major factor in mobile hydraulic circuits. Power density is the power output per unit volume, or the power output as a ratio of the actuator size. Power density is one reason why hydraulics is so important to mobile machinery. Mobile systems require high-output power combined with minimum weight. Noise, shock, and vibration. Mobile systems tend to have higher noise, shock, and vibration challenges because of improper human operation or extreme and rugged operating conditions. Because of the environment in WWW.FLUIDPOWERJOURNAL.COM • WWW.IFPS.ORG
