I like to tell people that I prefer to learn from someone else’s experience rather than my own because it tends to hurt a lot less. In keeping with that, here are some things I’ve experienced with electro-hydraulics and some simple, easy solutions that will help you take some of the legwork out of developing an EH system.

1. Flashing an ECU on a Machine That’s Running . . .

1. Consider your operators and technicians.

Switching from manually operated valves to electro-hydraulics is a big step. You are, in a sense, pulling the operator one step further away from the machine. For some people, this is great and they love a color display that allows them to endlessly tweak a hydraulic system without getting soaked with oil. Others enjoy the feel and perceived reliability of a manually configured electronic device system, and find a more traditional technology, such as an analog valve driver that is easily tuned with a screwdriver and trim pots more appealing.

When designing a hydraulic system, you want to optimize your design for minimal pressure drop. Savvy system designers have found that using a custom manifold with the right combination of cartridge valves is one way to optimize a system. When using this approach, it is important to correctly size your valves, drillings, and flow paths within the manifold. It is just as important to “right-size” the hydraulic hose and pipe connecting the manifold to the rest of the installation. Hose and tubing need to be the right diameter, length, smoothness and shape to handle the demands of the pressurized hydraulic flow. Undersized hose or tube can cause turbulent flow and excessive heat buildup. Over-sized hose or tube can add cost, size and weight to a system and decrease the rate of flow.

To understand what “right-sizing” means in terms of hydraulic hose and tube, you must first understand the nature of fluid and friction. Whenever fluid flows, there is a loss of mechanical energy to overcome viscous forces within the fluid. In a hydraulic system, this loss is seen as a pressure drop in the direction of flow.

Each component within the hydraulic system will contribute toward the pressure drop, i.e. cartridge valves, tubing, fittings, hoses, filters etc. This lost energy is dissipated as heat energy in the oil.

Frictional losses in pipework are mainly dependent upon:

• Length of pipe

• Cross-sectional area of pipe

• Roughness of pipe surface

• Number of pipe bends

• Velocity of flow

• Viscosity of fluid

The total allowable pressure drop of the hydraulic system must be chosen with care, as the power loss is a product of the system flow rate and pressure drop. This is an efficiency loss that has to be balanced against the cost of larger pipework/hoses and fittings. The wasted energy is dissipated as heat energy in oil, which may lead to cooling problems and shortening of the oil life.

Pressure losses in pipework will depend upon the flow condition. There are two distinct flow conditions: 

1. Laminar Flow and

2. Turbulent Flow.

Laminar flow is the condition when the fluid particles travel smoothly in straight lines, the inner-most fluid layer travels at the highest speed and the outer-most layer at the pipe surface doesn’t move, as shown in Figure 1.

Figure 1 – Laminar Flow

Turbulent flow has irregular and chaotic fluid particle motions, such that a thorough mixing of the fluid take place, as shown in Figure 2. Turbulent flow is usually not desirable, as the flow resistance increases and thus the hydraulic losses increase.

Figure 2 – Turbulent Flow

The Right Calculations

To determine the right size of hydraulic piping, you must first do the right calculations for the nature of the hydraulic flow in your system.

Osborne Reynolds discovered that the flow condition depended upon the mean flow velocity, the diameter of the pipe and the kinematic viscosity of the fluid, formula 1.

Q = Flow (m³/s)

d = Pipe internal diameter (m)

ν = Kinematic Viscosity (m²/s)

A Reynolds number of 2000 or under is deemed to be laminar flow. A Reynolds number above 3000 indicates turbulent flow.

Pressure loss in straight pipe can be calculated using Poiseuille’s equation (for laminar flow only). See Formula 2.

A more general equation used for turbulent flow and laminar flow is D’Arcy equation. See Formula 3.

ΔP = Pressure Loss (Nm^-2)

F = Pipe Friction Factor

L = Pipe Length (m)

D = Pipe internal diameter (m)

ρ = fluid density (kg m ^-3)

U = Fluid mean velocity (ms^-1)

The friction factor (f) depends upon the nature of the flow in the pipe. The most convenient form of depicting friction factors are from a Moody Diagram. However for hydraulic systems it is often assumed the pipe conditions are smooth.

A quick and easy and more common method of determining pipe/hose sizing, is to calculate the diameter size based upon recommend fluid velocities. See Table 1.

 Table 1 – Recommend Mean Fluid Velocities

 Based upon using the mean fluid velocities, the appropriate hose/pipe diameter is determined using Formula 4.

Rearrange the formula to get:

d = Diameter (mm)

Q = Flow (lpm)

V = Fluid Velocity (m/s)

This quick check calculation is useful to ensure that potential pressure drop and energy loss due to hose/pipework is not excessive when designing your manifold installation. For critical long runs of pipe or hose, the pressure losses should be checked using Poiseuille’s or D’Arcy equations as briefly discussed earlier.

Don’t let the wrong size of hydraulic hose or pipe keep your hydraulic system from reaching its maximum efficiency. Do the right calculations, and specify the right size of hydraulic hose and tubing to keep your installation at optimum working pressure.

Here at HydraForce our Technical Services department typically receives multiple question daily regarding our Proportional Valves.  Common questions include, "What is the proper dither frequency for a particular proportional valve?' and "What coil should I use with this type of system?".  Following these five simple rules when applying  proportional valves  will prevent field failures, reduce valve instability, and ensure that your system performs as desired without having to reach out to a tech service representative.

1. Applying dither to a valve reduces actuator friction and reduces hysteresis.  A good rule of thumb is to use 70 -250 Hz dither frequency on all SP, ZL, and PV valves (flow and directional control valves), and 200-300 Hz dither frequency on TS Valves (proportional pressure control valves). Otherwise, at lower frequencies the actuator will follow the dither signal and the valve output will appear unstable.

The idea to develop Multifunction Valves came from our engineers noticing that particular combinations of valves were used over and over again in standard hydraulic circuits.  For example, a 2-way, 2-position SP proportional valve is typically paired with a pressure compensator to provide pressure compensated flow control.  This particular example drove the development of the PV Proportional Flow Control family of valves, which include the flow control and pressure compensator inside the same cartridge.  There are many other multi-function options, including the SPCL, SVCL, SVRV, EPFR, FRRV, and others).

As Lisa pointed out in a previous post, we skipped the gimmicks this year and went back to the basics, our broad product offering.  That appears to have been the right decision because the show was a great success for HydraForce.  The number of visitors to our boothand their optimism about the future were refreshing.  We were mentioned as a ‘company to watch’ in several pre-show news posts, our new products generated a lot of interest and we received many compliments on the design of our booth, which seemed to work very well for our purposes.