January 17, 2006 

'Inside Hydraulics' Newsletter

1. Water in oil detection - instantly
2. Power saving with load sensing
3. Nail breakdowns - nail fluid cleanliness
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


Water in oil detection - instantly

Water contamination of oil can cause serious damage to bearings and other lubricated components - often without the equipment user being aware that damage is occurring. Water contamination:

  • Reduces lubricating film-strength, which leaves critical surfaces vulnerable to wear and corrosion.
  • Depletes some additives and reacts with others to form corrosive by-products that attack some metals.
  • Reduces filterability and clogs filters.
  • Increases air entrainment ability.
  • Increases the likelihood of cavitation.

Left unchecked, these problems can rapidly lead to costly breakdowns. The speed with which water-induced degradation can progress, means that in many applications, water contamination is a more serious threat to equipment reliability than particulate contamination.

Online monitoring for water contamination is becoming more widespread, particularly in applications where reliability and accountability are paramount. The ability of an instrument to accurately measure dissolved, emulsified and free water at the same time, with immediate response, is long overdue. The challenges that have faced previous attempts at dielectric based devices stemmed from electronic drift and poor design.

More than 8 years of research and development by EESIFLO has produced a loop-powered. mini PCB card that works with cylindrical tubes that act as capacitor plates to measure immediate changes in water content instantly or over time. Since oil has a low dielectric and water the opposite, it is possible to detect any changes, in ppm or percentage, using this well known principle with the aid of digital circuits, increased electronic sampling and faster processing.

EESIFLO's EASZ-1 water in oil analyzer

The EASZ-1 Water in oil monitor.

EESIFLO's EASZ-1 water in oil analyzer provides early warning of any increase in water content so that corrective action can be taken. It has a response time of one second, a resolution of 100ppm and a range from 0 -10,000 ppm, 0-1%, 0-3%, 0-10% or 0-25%.

The technology can measure water contamination in most types of oil, whether mineral or synthetic based - low or high viscosity. It is also suitable for fuels, vegetable oils, hydraulic oils and other hydrocarbon based liquids. An intrinsic safe version will be released in 2006 for measurement of water in gasoline, aviation fuels and crude oil.

For more information, or to locate your nearest EESIFLO representative, go to www.eesiflo.com

Get a lifetime of pneumatics knowledge...

Get 'Pneumatic Control for Industrial Automation'
. Find out more

2.   Power saving with load sensing

Load sensing is a term used to describe a type of variable pump control used in open circuits. It is so called because the load-induced pressure downstream of an orifice is sensed and pump flow is adjusted to maintain a constant pressure drop (and therefore flow) across the orifice. The 'orifice' is usually a directional control valve with proportional flow characteristics, but a needle valve or even a fixed orifice can be employed, depending on the application.

In hydraulic systems that are subject to wide fluctuations in flow and pressure, load-sensing circuits can save substantial amounts of input power. This is illustrated in Exhibit 1. In systems where all available flow (Q) is continuously converted to useful work, the amount of input power lost to heat is limited to inherent inefficiencies. In systems fitted with fixed displacement pumps where 100 percent of available flow is only required intermittently, the flow not required passes over the system relief valve and is converted to heat. This situation is compounded if the load-induced pressure (p) is less than the set relief pressure - resulting in additional power loss due to pressure drop across the metering orifice (control valve).

Flow-pressure-power diagrams.

Exhibit 1. Flow-pressure-power diagrams for fixed, variable
and load sensing controlled pumps (Peter Rohner).

A similar situation occurs in systems fitted with pressure controlled (pressure compensated) variable pumps, when only a portion of available flow is required at less than maximum system pressure. Because this type of control regulates pump flow at the maximum pressure setting, power is lost to heat due to the potentially large pressure drop across the metering orifice.

A load sensing controlled variable pump largely eliminates these inefficiencies. The power lost to heat is limited to the relatively small pressure drop across the metering orifice, which is held constant across the system's operating pressure range (see bottom of Exhibit 1).

A load sensing circuit typically comprises a variable displacement pump, usually axial-piston design, fitted with a load sensing controller, and a directional control valve with an integral load-signal gallery (Exhibit 2). The load-signal gallery (LS, shown in red) is connected to the load-signal port (X) on the pump controller. The load-signal gallery in the directional control valve connects the A and B ports of each of the control valve sections through a series of shuttle valves. This ensures that the actuator with the highest load pressure is sensed and fed back to the pump control.

Load sense circuit.

Exhibit 2. Typical load sensing circuit. Enlarge

To understand how the load-sensing pump and directional control valve function together in operation, consider a winch being driven through a manually actuated valve. The operator summons the winch by moving the spool in the directional valve 20 percent of its stroke. The winch drum turns at five rpm. For clarity, imagine that the directional valve is now a fixed orifice. Flow across an orifice decreases as the pressure drop across it decreases. As load on the winch increases, the load-induced pressure downstream of the orifice (directional valve) increases. This decreases the pressure drop across the orifice, which means flow across the orifice decreases and the winch slows down.

In a load sensing circuit the load-induced pressure downstream of the orifice (directional valve) is fed back to the pump control via the load-signal gallery in the directional control valve. The load-sensing controller responds to the increase in load pressure by increasing pump displacement (flow) slightly so that pressure upstream of the orifice increases by a corresponding amount. This keeps the pressure drop across the orifice (directional valve) constant, which keeps flow constant and in this case, winch speed constant. The value of the pressure drop or delta p maintained across the orifice (directional valve) is typically 10 to 30 Bar (145 to 435 PSI). When all spools are in the center or neutral position the load-signal port is vented to tank and the pump maintains 'standby' pressure equal to or slightly higher than the load sensing control's delta p setting.

Because the variable pump only produces the flow demanded by the actuators, load-sensing control is energy efficient (fewer losses to heat) and as demonstrated in the above example, improves actuator control. Load-sensing control also provides constant flow independent of pump shaft speed variations. If pump drive speed decreases, the load-sensing controller will increase displacement (flow) to maintain the set delta p across the directional control valve (orifice), until displacement is at maximum. To further your knowledge on load sensing and other variable pump controls, visit http://www.IndustrialHydraulicControl.com

"Thanks for the great work on the two publications, Insider Secrets to Hydraulics and Preventing Hydraulic Failures. I have been in the hydraulics business for the past 20 years and it is very difficult to find any decent material on hydraulic maintenance, troubleshooting and failure analysis. These two books cover it all in easy to understand language... I conduct hydraulic training courses and plan to purchase copies to distribute to my students to share your practical approach to understanding a not so understandable subject."

Paul W. Craven, Certified Fluid Power Specialist
Motion Industries, Inc.

3.   Nail breakdowns - nail fluid cleanliness

The findings of a three-year study of 117 mobile and industrial hydraulic machines to determine the correlation between fluid cleanliness and breakdown frequency, has shown that maintaining fluid cleanliness at ISO 4406 14/11 will result in a tenfold gain in the average time between breakdowns when compared with a fluid cleanliness level of 22/19. Hydraulic Oil Cleanliness explains how hydraulic fluid contamination damages hydraulic components and outlines methods for its effective control. Find out more

4. Content for your web site or e-zine

Need some fresh content for your web site or e-zine? You now have permission to reprint these 'Inside Hydraulics' articles on your web site or in your e-zine, provided:

1. Each article is printed in its full form with no changes.

2. You send an e-mail to editor@hydraulicsupermarket.com to advise us where you'll be publishing them.

3. You include the following acknowledgement at the end of each article:
About the Author: Brendan Casey has more than 16 years experience in the maintenance, repair and overhaul of mobile and industrial hydraulic equipment. For more information on reducing the operating cost and increasing the uptime of your hydraulic equipment, visit his web site: http://www.InsiderSecretsToHydraulics.com

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