July 20, 2005 

'Inside Hydraulics' Newsletter

1. Case drain filtration - without the pressure
2. Reducing hydraulic cylinder repair expense
3. Hydrostatic transmission fundamentals
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


Case drain filtration - without the pressure

The logic for installing filters in piston pump and motor case drain lines is similar to the logic for locating filtering media in the return line, that is, if the reservoir and the fluid it contains start out clean and all returning fluid is adequately filtered, fluid cleanliness will be maintained.

The main disadvantage of installing conventional barrier filters in piston pump or motor case drain lines is that the restriction created by the element can cause high case pressure.

High case pressure results in excessive load on the lip of the shaft seal. This causes the seal lip to wear a groove in the shaft, eventually resulting in leakage past the seal. If case pressure exceeds the shaft seal's design limits, instantaneous failure can occur. The subsequent loss of oil from the case may result in damage through inadequate lubrication.

The effect of high case pressure on axial piston pumps is similar to excessive vacuum at the pump inlet. Both conditions put the piston-ball and slipper-pad socket in tension during inlet. This can cause buckling of the piston retaining plate and/or separation of the slipper from the piston, resulting in catastrophic failure.

High case pressure can cause the pistons of radial piston motors to be lifted off the cam. This occurs in operation during the outlet cycle. The pistons are then hammered back onto the cam during inlet, destroying the motor. If residual case pressure remains high when the motor is stopped, loss of contact between the pistons and cam can allow the motor to freewheel, resulting in uncontrolled machine movement.

For the reasons described above, most manufacturers do not recommend the installation of conventional depth filters in pump or motor case drain lines. And the capture of silt-size particles between the case of a piston pump or motor and the reservoir has not been possible - until now!

The Magnom Midi Module incorporates new, patented filter technology that allows filtration of ferrous particles down to less than one micron - with no restriction and therefore no possibility of damage caused by excessive case pressure.

Magnom Midi Module

The Magnom Midi Module - designed to
replace conventional filters in case drain applications.

Designed to replace conventional filters in case-drain applications, the Magnom Midi Module will capture and hold 45 grams of ferrous contamination - with no pressure drop across the filter. This makes the unit virtually service free.

When installed in a case drain line, the Magnom Midi Module is also an excellent condition-monitoring device. The unit can be inspected at regular service intervals to assess the amount of wear metal being generated by a particular component.

For more information, visit www.magnom.com or in the U.S. contact Keith Day on 312-738-1147 or email keith.day@magnom.com

Get the hydraulics knowledge you need...

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. Find out more

2.   Reducing hydraulic cylinder repair expense

As a product group, cylinders are almost as common as pumps and motors combined. So if you operate a lot of hydraulic equipment, it's likely that cylinder repair expenses are a significant portion of your total maintenance costs.

It is often stated that up to 25% of mechanical equipment failures are failures of design. If we extrapolate this to hydraulic cylinders, as many as one in four hydraulic cylinders are not adequately designed for the application they are operating in. This doesn't mean that the cylinder won't do the job asked of it, it will - but not with an acceptable service life. If you have a particular cylinder that requires frequent repair, you may need to address one or more of these design-related problems:

Bent Rods

Bent rods are a common cause of rod seal failure. Bending of cylinder rods can be caused by insufficient rod diameter or material strength, improper cylinder mounting arrangement or a combination of all three. Once the rod bends, excessive load is placed on the rod seal resulting in premature failure of the seal. The permissible rod loading for a cylinder in an existing application can be checked using the Euler formula. A detailed explanation of how to do this is contained in Industrial Hydraulic Control.

Ballooned Tubes

Ballooning of the cylinder tube is usually caused by insufficient wall thickness and/or material strength for the cylinder's operating pressure. Once the tube balloons, the correct tolerance between the piston seal and tube wall is lost and high-pressure fluid bypasses the seal. This high velocity fluid can erode the seal and localized heating caused by the pressure drop across the piston reduces seal life.

Insufficient Bearing Area

If the internal bearing areas in the gland and at the piston are insufficient to carry the torsional load transferred to the cylinder, excessive load is placed on the rod and piston seals. This results in deformation and ultimately premature failure of the seals.

Rod Finish

The surface finish of the cylinder rod can have a dramatic effect on the life of the rod seal. If the surface roughness is too low seal life can be reduced through inadequate lubrication. If the surface roughness is too high, contaminant ingression is increased and an unacceptable level of leakage can result.

In the context of extending cylinder service life, consider the surface of the cylinder rod as a lubricated wear surface and treat it accordingly. In some applications, the use of an alternative rod surface treatment with superior mechanical properties to conventional hard chrome plating, such as black nitride or High Velocity Oxygen Fuel (HVOF) metal spraying, can increase the service life of the rod and its seals. The installation of a shroud to protect the rod surface and seals from impact damage and contaminants can afford similar life extension benefits.

Repair or Redesign?

Not all hydraulic cylinders are made equal. So if you have hydraulic cylinders that suffer recurring failure, it's likely that modifications to the cylinder are required to break the vicious circle of failure and repair.

"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.   Hydrostatic transmission fundamentals

Produced by JI Case for training technicians on heavy-duty hydrostatic transmissions, 'Hydrostatic Transmission Fundamentals' explains axial pump and motor operating principles, pump and motor rotating group composition and terminology, displacement control principles and typical transmission charge and over-pressure protection circuits. 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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6. Tell us what you think

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