February 8, 2005 

'Inside Hydraulics' Newsletter

1. See you in Las Vegas?
2. Understanding load sensing control
3. Hydraulic troubleshooting - from your lap top
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


See you in Las Vegas?

The tri-annual International Fluid Power Expo (IFPE) kicks off on the 15th of next month at the Las Vegas Convention Center. I'm looking forward to the event and will be presenting a paper at the National Conference on Fluid Power. If your're attending, exhibiting or presenting at IFPE or the co-located CONEXPO-CON/AGG I would welcome the opportunity to meet with you there.

So if your are going to be in Las Vegas March 15-19, send me an email and, our schedules permitting, the coffee is on me.

2.   Understanding load sensing control

Load sensing is a term used to describe a type of pump control employed in open circuits. It is so called because the load-induced pressure downstream of an orifice is sensed and pump flow 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 used, depending on the application.

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 1). 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.

Load sense circuit.

Exhibit 1. 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% 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 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 position the load-signal port is vented to tank and the pump maintains 'standby' pressure equal to or slightly higher than the load sensing controller's delta P setting.

Because the pump always receives the load signal from the function operating at the highest pressure, high-end load sensing directional control valves feature a pressure compensator (not shown) at the pressure inlet to each section. The section pressure compensator works with the spool-selected orifice opening to maintain a constant flow, independent of the pressure variations caused by the operation of multiple functions at the same time. This is sometimes referred to as 'sensitive load sensing'.

A load sensing pump only produces the flow demanded by the actuators - this makes it energy efficient (fewer losses to heat) and as demonstrated in the above example, provides more precise 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 maximum displacement is reached.

Load sensing pump controls usually incorporate a pressure limiting control, also referred to as a pressure cut-off or pressure compensator. The pressure compensator limits maximum operating pressure by reducing pump displacement to zero when the set pressure is reached.

"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.   Hydraulic troubleshooting - from your lap top

'Fundamentals of Hydraulics and Troubleshooting' explains when and how to use diagnostic tests, including the direct pump test, system T test, spool valve leakage test and cylinder piston seal leakage test - all from your lap top or desk top computer! 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

5. Help us spread the word

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New subscribers can get the newsletter by completing the form at http://www.insidersecretstohydraulics.com

6. Tell us what you think

We would love to hear what you think of this issue of our 'Inside Hydraulics' newsletter. And of course, if you have any suggestions for future issues, please send us those too.

Just e-mail the editor at: newslettersuggestions@hydraulicsupermarket.com

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