September 10, 2003 

'Inside Hydraulics' Newsletter

Issue 19

1. Zero-Leak Gold® Plugs - guaranteed not to leak
2. Hydraulic pump control problem highlights the value of component testing
3. Hydraulic troubleshooting tips
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


Zero-Leak Gold® Plugs - guaranteed not to leak

The old-style, o-ring only SAE J514 port entry plugs have fundamental shortcomings. They require a spot machined port surface (1). And their large diameter o-ring may extrude out of the tapered interface or may be damaged by the threads (2). You're probably well familiar with the result: leaks, seeps and weeps.

In contrast, every Zero-Leak Gold Plug has two seals. The tapered metal-to-metal seal provides exceptionally high unit loading at the interface (3). And an o-ring fits below the taper contact point for a backup seal (4).

As a result, Zero-Leak Gold Plugs are guaranteed to provide leak-free performance. They won't back out - even during severe vibration, pressure spikes or other circumstances where old-style, SAE J514 plugs may fail. Zero-Leak Gold Plugs can be resealed repeatedly without loss of effectiveness, which makes them ideal for drain/fill applications. Available in steel, aluminum and stainless steel, Zero-Leak Gold Plugs come in metric and standard sizes as well as a magnet style. They work without chemical sealants that may lead to system contamination. They're fully interchangeable with the plugs you're currently using. Just take the old plug out, insert the Zero-Leak Gold Plug, and the leak is stopped for good. And each Zero-Leak Gold Plug has the size engraved on the head for simple identification.

For more information on Zero-Leak Gold Plugs, including test data from the Milwaukee School of Engineering's Fluid Power Institute, visit

plug cutaway

2.   Hydraulic pump control problem highlights the value of component testing

A client recently engaged me to design and build a hydraulic power unit for a specific application. The unit comprised a diesel engine driving an axial piston pump fitted with load sensing, power limiting and pressure limiting control.

What is load sensing control?

Load sensing control is so called because the load-induced pressure downstream of the directional control valve is sensed and pump flow adjusted to maintain a constant pressure drop (and therefore flow) across the valve.

For example, let's say we have a hydraulic pump driving a winch thru a manual, directional 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 decreases and the winch slows down.

The load sensing control senses the load-induced pressure downstream of the orifice and adjusts pump flow 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.

Because the 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, provides more precise control.

What is power limiting control?

A constant power or power limiting control operates by reducing the displacement, and therefore flow, from the pump as pressure increases, so that the power rating of the prime mover is not exceeded. The advantage of this type of control is that more flow is available at lower pressures, so that the actuators can operate faster under light loads. This results in better utilization of the power available from the prime mover. The power limiting control overrides the load sensing control.

What is pressure limiting control?

Pressure limiting control limits the maximum operating pressure of the pump. Also referred to as a pressure compensator or pressure cut-off. The pressure limiting control overrides both the load sensing and power limiting controls.

Pump control problem

Three, brand new pumps were ordered for the project from a leading manufacturer. When the three hydraulic power units where being commissioned, I found that the power limiting control on all three pumps was not functioning.

When advised of the problem, the manufacturer maintained that the pumps had been tested prior to delivery and that the cause of problem therefore must be elsewhere in the circuit. Possible external causes were quickly checked and eliminated. While waiting for the manufacturer to respond to the problem, I checked the schematic diagram of the pump's control and noticed that a vital part was missing. Click here to view the schematic.

Plug-in controls

The power limiting control on this particular pump is a modular, screw-in cartridge fitted to the standard pump with load sensing and pressure limiting control. The power limiting cartridge is a relief valve with a link to the swash plate that increases spring bias as swash angle decreases. This relief valve limits load signal pressure depending on swash plate position. When the allowable power setting is reached, the relief valve intervenes to reduce the load pressure signal to the load sensing control. This results in a decrease in swash angle and therefore flow. The lower the swash angle and therefore flow, the higher the load signal pressure and therefore operating pressure permissible. Because power is a product of flow and pressure, this limits the power draw of the pump.

If you examine the two schematic diagrams closely, you will notice that other than the addition of a power limiting relief cartridge, there is a second difference. An orifice is shown just below the load sensing signal connection or X port. Without this orifice to limit the flow from the load sensing line, the power limiting relief valve cannot effectively limit the load pressure signal. This means that the power limiting control cannot function.

I checked the pumps fitted to the power units and none of the three had this orifice fitted. I advised the manufacturer and requested that they dispatch three of these orifices urgently. I was astonished by the manufacturer's reply - the required part was on back order. To minimize any further downtime, I manufactured three orifices, fitted them to the pumps and handed the hydraulic power units over to the customer.

Test for success

Thoroughly testing new or rebuilt hydraulic components prior to dispatch, ensures that the component will work the way it should and will perform within its design parameters. It is possible that the manufacturer tested the pumps discussed above - but only their load sensing and pressure limiting controls. Had the functionality of the power limiting control been tested, the pumps would not have been dispatched without the necessary orifice. This would have avoided an embarrassing mistake for the manufacturer and many hours of downtime for the customer.

"This book has the potential to save many organizations lots of m0ney. It should be on the bookshelf of every engineer, supervisor, planner and technician that deals with hydraulic equipment... it's worth its weight in gold." Find out more

Alexander (Sandy) Dunn
Plant Maintenance Resource Center

3.   Hydraulic troubleshooting tips

For some practical tips on hydraulic troubleshooting, read Brendan Casey's article in the July-August Issue of Machinery Lubrication magazine, available here. To receive a complimentary subscription to this informative magazine (US and Canada only) go to:

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 to let us know where you'll be publishing them.

3. You include the following credit 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:

5. Help us spread the word

If you've found our 'Inside Hydraulics' newsletter interesting and informative, then chances are you have a friend or colleague who would too. Help spread the word about 'Inside Hydraulics' by forwarding this issue to a friend or colleague. If they share your interest in hydraulics, then they will surely appreciate being told about this newsletter.

New subscribers can join the mailing list by completing the form at

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:

Copyright 2000-2003 by All rights reserved.

If you have received this mailing in error, or do not wish to receive any further newsletter mailings from us, send a message with "unsubscribe inside hydraulics" in the subject line or click here.

To subscribe to our Inside Hydraulics Newsletter, go to the following URL:

This message has been sent to the following e-mail address: