Basic Principles of Kidney Loop Filtering Hydraulic Fluid Systems {0680, 0768, 5050, 5056} Caterpillar

Basic Principles of Kidney Loop Filtering Hydraulic Fluid Systems {0680, 0768, 5050, 5056}


MT700 018

Caterpillar Products
Off-Highway Truck/Tractor
794 AC (S/N: MN51-UP; MT51-UP)


Table 1
Revision  Summary of Changes in SEBF8840 
04  Updated Introduction
Added new serial number prefixes for New Product Introduction (NPI) 
03  Updated Introduction.
Added new serial number prefixes for New Product Introduction (NPI). 
02  Updated Introduction.
Added Reference section. 
01  Converted to SIS Authoring Format 

© 2019 Caterpillar All Rights Reserved. This guideline is for the use of Cat dealers only. Unauthorized use of this document or the proprietary processes without permission may be violation of intellectual property law.

Information contained in this document is considered Caterpillar: Confidential Yellow.

This guideline enables dealers and customers to benefit from cost reductions which were made possible through an established Contamination Control Program. Fluids used in Caterpillar machines are highly effective unless the fluid contains damaging amounts of contaminates. Therefore, every effort must be made to reduce the contamination level within Caterpillar fluid systems. Caterpillar makes ongoing changes and improvements to the Caterpillar products. This guideline must be used with the latest technical information that is available from Caterpillar. Updated technical information will ensure that such changes and improvements are incorporated when applicable.

For technical questions when using this document, work with your Dealer Technical Communicator (TC).

To report suspected errors, inaccuracies, or suggestions regarding the document, submit a form for feedback in the Service Information System (SIS Web) Interface.

If a Contamination Control Guideline is required, but not available in SIS web, submit a form for feedback in the Service Information System (SIS Web) Interface.


Off-line filtering procedures, or kidney loop filtration, for machine fluid systems are often complicated. A prior knowledge of filtering basics will help ensure the desired outcome. Passing oil through a filter is only one step to managing the total system cleanliness. Successful results are obtained when a holistic approach to the system management is applied. Issues covered in this article relate directly to off-line filtering procedures. The following information should be applied when filtering machine fluid systems.

Important Safety Information

Illustration 1g02139237

Most accidents that involve product operation, maintenance, and repair are caused by failure to observe basic safety rules or precautions. An accident can often be avoided by recognizing potentially hazardous situations before an accident occurs. A person must be alert to potential hazards. This person should also have the necessary training, skills, and tools to perform these functions properly.

Improper operation, lubrication, maintenance, or repair of this product is dangerous. Improper methods could result in injury or death.

Do not operate or perform any lubrication, maintenance, or repair of this product, until you understand the operation, lubrication, maintenance, and repair information. Safety precautions and warnings are provided in this manual and on the product. If these hazard warnings are not heeded, bodily injury or death could occur to you or to other persons.

Hazards are identified by a safety alert symbol. Safety alert symbols are followed by a signal word such as "Warning" that is shown below.

Illustration 2g01032906

The following is the meaning of this safety alert symbol:

  • Pay attention!

  • Become alert!

  • Your safety is involved.

The message that appears under the warning explains the hazard. The message will be written or pictorially shown.

Operations that may cause product damage are identified by "NOTICE" labels on the product and in this publication.

Caterpillar cannot anticipate every possible circumstance that might involve a potential hazard. Therefore, the warnings in this publication and on the product are not all inclusive. When a tool, a procedure, a work method, or an operating technique that is not recommended by Caterpillar is used, ensure that the tool, procedure, or work method is safe for all personnel around the machine. Ensure that the product will not be damaged or that the product will not be made unsafe by the operation, lubrication, maintenance, or repair procedures.

All the information, specifications, and illustrations that are in this publication are based on information that was available at the time of publication. The following specifications could change at any time: specifications, torque, pressures, measurement, adjustments, illustrations, and other items. These changes can affect the service that is given to the product. Obtain the most current and complete information before starting any job. Caterpillar dealers have the most current information that is available.


------ WARNING! ------

Sudden movement of the machine or release of oil under pressure can cause injury to persons on or near the machine.

To prevent possible injury, perform the procedure that follows before testing and adjusting the steering system.

------ WARNING! ------

Personal injury can result from hydraulic oil pressure and hot oil.

Hydraulic oil pressure can remain in the hydraulic system after the engine has been stopped. Serious injury can be caused if this pressure is not released before any service is done on the hydraulic system.

Make sure all of the attachments have been lowered, oil is cool before removing any components or lines. Remove the oil filler cap only when the engine is stopped, and the filler cap is cool enough to touch with your bare hand.

------ WARNING! ------

Care must be taken to ensure that fluids are contained during performance of inspection, maintenance, testing, adjusting, and repair of the product. Be prepared to collect the fluid with suitable containers before opening any compartment or disassembling any component containing fluids.

Refer to Special Publication, PERJ1017, "Dealer Service Tool Catalog" for tools and supplies suitable to collect and contain fluids on Cat® products.

Dispose of all fluids according to local regulations and mandates.


Table 2
Media Number  Title 
PEGJ0045  Reporting Particle Count by ISO Code"Reporting Particle Count by ISO Code" (1) 
SEBF8482  Obtaining an Oil Sample for S·O·S Analysis 
PEGJ0047  How To Obtain A Good SOS Sample 
PERJ1017  "Dealer Service Tools Catalog" (2) 
NEHS0848  "Tool Operating Manual for 204-7392 Filter Cart Group" (2) 
(1) Visit Caterpillar Media Information Center to download or purchase the document.
(2) Visit to download or purchase the document.

Kidney Loop Filtration Basics

Kidney loop filtration is a process where a certain amount of fluid is removed from a system, filtered, and then returned to the system. Kidney loop filtration is different than full flow filtration, which is common in machine return filter applications. In a kidney loop filtration process:

  • Oil is removed from the system

  • Filtered

  • Returned to the system for mixing

The contamination level of the whole system is reduced slightly during each pass of the kidney looped oil. The total contamination level will decrease steadily, over time. Kidney loop filtration is a slow process. However, kidney loop filtration is less costly than oil changes and the resulting labor or other types of system clean-up. The kidney loop filtration process can be implemented during a scheduled PM interval or after a catastrophic failure. Kidney loop filtration is an addition to your normal maintenance procedures.

As the oil viscosity increases, ease of filtration decreases. The filter elements are not designed to filter out around or above 600 particles.

Filtering Time

S·O·S sampling is the best method of determining the system contamination level. On-site S·O·S information will guide you with this decision. When the contamination level has been lowered to the recommended ISO target or cleaner, discontinue the filtering process. On-site S·O·S information can easily be obtained using a Caterpillar Portable Particle Counter. Information on the use of a Portable Particle Counter during kidney loop filtering is located in various machine filtration guidelines.

A time measurement is another method of determining the proper system filtering duration. Time measurement is a general method, but can provide some guidance when a Portable Particle Counter is not available.

Oil Filtration Guidelines:

  1. The filtering system must pass each fluid unit of compartment oil through the kidney loop filter. To accomplish kidney loop filtration, the filter unit must move seven times the compartment fluid capacity through the filter. The filtration of seven times the compartment fluid is due to the mixing mechanics and dead compartment areas.

    Example: If a tank has a capacity of 37.9 L (10 US gal), the filter unit must move 265 L (70 US gal) of oil through the filter one time for every unit of oil to pass through the filtering unit.

  2. To clean and lower the system contamination to a level determined by the filter media. All the filtered oil must pass through the filter media 5 times.

    Example: A system is being filtered through a 10um filter. Due to the filter media efficiency, all oil needs to pass through the filter media 5 times (multi-pass). Incorporating a multi pass filtration increases the number of times the filter sees each unit of oil. The filter cart removes greater amounts of contaminants with each additional cycle.

Given the two guidelines above, filtering time can be summarized in the following example.

  1. System capacity is 113.56 L (30 US gal).

  2. 7 x 113.56 L (30 US gal) = 794.94 L (210 US gal) (to pass each unit of oil through the filter, one time).

  3. 5 x 794.94 L (210 US gal) = 3974.68 L (1050 US gal) (to properly filter the particles out of the oil down to the rated size of the filter media).

  4. The example kidney loop filter cart capacity is rated at 15 gallons per minute. The filter cart must operate for 70 minutes (1050/15=70) to achieve Caterpillar filtration guidelines. Remember, using this time formula is not exact. To verify the final fluid cleanliness, approved methods must be used.

Filter unit capacity is to be properly selected to move the required amount of oil ( 3974.68 L (1050 US gal)) through the filter in the targeted time frame. Other parameters for the filter cart operation and efficiency will be discussed later in this article. Vehicle-specific filtration times are also located within the various Caterpillar Contamination Control Guidelines.

Recommended System Operating Levels

Caterpillar Machine operating levels do not dictate an allowed level of system contamination. Contamination control guidelines are available to enhance the fluid system health for the life of the product. Guidelines are not specifications, but are the maximum contamination operating levels recommended by Caterpillar. Operating the machine systems at or below these contamination levels will help achieve the engineered life of the system components. A system management strategy would be difficult to implement without these guidelines. Based on historic S·O·S data, component life is directly related to proper fluid health and cleanliness management. Contamination Control procedures utilizing high efficiency filters or off board (kidney loop) filtration procedures will reduce or eliminate the contamination-related failures so often seen in poorly maintained vehicles.

Older machine filtration may not be adequate to achieve the new recommended cleanliness levels. Older machines will benefit from a higher level of system contamination control management. The engineered life of any component is attainable by eliminating wear and contamination-related failures.

ISO 4406 Method for coding the level of contamination by solid particles.

ISO (International Organization of Standardization) is a worldwide federation of national standards bodies (ISO member bodies). ISO 4406 specifies the code to be used in defining the quantity of solid particles in the fluid used in a given compartment fluid power system. An automatic particle counter derives the equivalent size of a particle from the particle cross-sectional area. Particle counts are affected by various factors. These factors include procurement of the sample, particle counting accuracy, the sample container, where used, and fluid cleanliness. Proper care should be taken during sample procurement to ensure that the sample obtained is representative of the fluid circulation in the system.

The code for contamination levels using automatic particle counters comprises three scale numbers, for example X/Y/Z.

X = The number of particles equal to or larger than 4 microns.

Y = The number of particles equal to or larger than 6 microns.

Z = The number of particles equal to or larger than 14 microns.

Caterpillar does not require the reporting of the X scale number since compartment tolerances exceed the 4 micron value, therefore, a "-" is used. For example, a particle count is read as -/Y/Z, meaning that there was no requirement to count particles equal to or larger than 4 micron.

System Intervention

When should off board fluid filtration be implemented? When should action be taken on a contaminated system? The following are several symptoms or signs indicating possible system intervention is required:

  • Opening any hydraulic system for repair or service

  • Identification of poor maintenance practices

  • Leaks

  • System overheating

  • Poor system efficiency

  • Normal/possibly elevated wear

  • S·O·S analysis indicates high contamination levels for any reason

What Intervention is Appropriate?

There are several levels of intervention to consider when addressing system contamination.

  • Customer education

  • High efficiency filters

  • Kidney loop filtration

  • Complete oil changes

  • Complete system flushing

Any of the above measures may be appropriate for a given system contamination level and condition. Incorporate the proper intervention level as required.


After a catastrophic component failure of any kind, the complete fluid system MUST be thoroughly cleaned. Kidney loop filtration as the first or only system cleaning procedure is not an option. Manual cleaning may include some or all the following steps:

Flushing all system lines

Major component removal and cleaning or exchange

Steam cleaning large housings or tanks

Complete oil replacement

Caterpillar filter carts are NOT designed, nor intended, to be the sole filtration after a catastrophic system failure. The hydraulic system must have the proper degree of manual cleaning intervention prior to filtration cart use.

The service provider must determine the proper intervention measures. The provider is the one closest to the machine operation and history.

Kidney Loop Filter Cart Applications

Caterpillar offers a complete line of kidney loop filtration carts to support kidney loop procedures. Caterpillar filter carts are designed to reduce elevated levels of contamination in the oil. The filtering efficiency of any filter cart is determined by many factors:

  • Oil viscosity (temperature and weight) plays a large part in determining how fast the oil will flow through the filter and filtering unit. Oil viscosity also determines the contaminant mixing or lack thereof in the compartment oil. Oil viscosity can also dictate GPM.

Oil movement (GPM/LPM) through the filter may be impacted by:

  • Viscosity

  • Filtering unit air pressure

  • Filter unit plumbing

  • Filter media size

  • Filter media dirt saturation

  • Filter bypass condition

All Caterpillar filter unit capacities are rated with 10W oil at 70° F. If moving oils with different viscosity ratings than the parameters above, the rated oil movement or filtering capacity of the filter unit will change proportionately. If any part of the filter cart is changed or modified from the as-shipped condition from the manufacturer, the filter cart may not operate as designed. Filter Cart changes include:

  • Hose length

  • Hose diameter

  • Fitting size

  • Filter media

Common Filtering Issues

Following are common issues, which may affect your filtering results:

Amount of oil to be filtered

100% of the system oil is not located in the reservoir. 30-40% of the system fluid capacity is located in the system piping and components. The system must be exercised to exchange the dirty oil with the clean oil in the reservoir or compartment where the oil can then be filtered.

Kidney loop vs. full flow filtration

Kidney loop filtration has continuous mixing of the clean and dirty oil. Oil in sheltered parts of a compartment may not properly mix with the returning clean oil. Oil in sheltered parts may require longer filtering times for this sheltered oil to mix and then pass through the filter unit. Use the proper equation in the “Filtering Time” section above. Remember, if the contaminated oil never reaches the cart filter media, the oil will NEVER be cleaned.

Full flow filtration would seem to be fast and easy. If the system is an open loop system, the resulting filtration is still a kidney loop operation. Oil flows from the tank, through the pump and then to the filter. The oil is eventually returned to the tank resulting in a kidney loop type operation.

Single pass filtering is NEVER an absolute or efficient means of filtering. Multi-pass filtration is a superior type of filter process to reach your filtration goal.

Filtering media size

Filter media has rated efficiencies for different sizes of contaminants. A 10-micron (µm) absolute filter will remove most 10µm and larger particles and a decreasing percentage as the particle sizes decrease from 10µm. A 5µm filter will take out most of the 5µm and larger particles and a decreasing percentage as the particle sizes decrease from 5µm. In other words, a given absolute filter media size will only remove a certain percentage of a given particle size. The oil contamination level will eventually stabilize for a given filter media. Additional filtering from that point would NOT be valuable, or economical, unless a smaller micron filter media is used. See the chart below:

Illustration 3g06339830

Filtering Time

Filtering time is determined by all the above conditions, but can generally be based on the oil flow through the filter media. However, the filtering time will always be dependent on the specific situation. A portable particle counter provides a good indication when measuring contaminant levels.

Portable particle counters are not a substitute for lab particle counting. Portable particle count is intended to give an ongoing record of the reduction of contamination during the filtering process. Portable particle count is used as an on-sight trending analysis tool for machine systems in remote locations.

System contamination (mixing the oil) can only be thoroughly mixed by exercising the machine and system components. By exercising the machine system, the oil is warmed, viscosity is lowered, and contaminant mixing takes place at a higher rate. More importantly, when contamination is mixed and placed into suspension, contamination will be carried to the filter unit easier and quicker. Remember, if the contamination does not get to the filter unit, the filter CANNOT remove contamination!

The above information provides the basic filtration principals required to perform a system off-board or kidney loop filtration procedure and help troubleshoot poor procedural results.

Caterpillar Cleanliness Targets

Cat dealers need to help ensure that Cat machines meet Caterpillar recommended cleanliness targets. Caterpillar recommends that cleanliness targets apply to new equipment to which dealers add attachments, add work tools, or perform final assembly. Cleanliness targets are to encompass used, rental, or repaired equipment.

In general, the recommended cleanliness targets for Caterpillar machines operating in the field are the same as Caterpillar minimum roll-off cleanliness levels for new equipment. These targets are expressed in two-factor (X/Y) ISO Codes determined from Particle Count data obtained from the Scheduled Oil Sampling (S·O·S) program. The first factor (X) represents the number of particles larger than 6 microns in size. The second factor (Y) represents the number of particles larger than 14 microns in size. Refer to Contamination Control Guideline, PEGJ0045, "Reporting Particle Count by ISO Code" examines ISO Codes and Particle Counting in more detail.

Recommended Cleanliness Targets


Table 3
Hydraulic/Transmission Systems  ISO Code 
Hydraulic Systems (Implement & Steering)  ISO 18/15 or cleaner 
Electronic Transmissions  ISO 18/15 or cleaner 
Mechanical Transmissions  ISO 21/17 or cleaner 
Power Train Systems  ISO 18/15 or cleaner 

Note: Older technology machines in the field may not be able to maintain the cleanliness levels of newer models. However, the same Contamination Control service and maintenance procedures should be used on all Caterpillar equipment.


Machine operating cleanliness targets should not be confused with component cleanliness targets. Caterpillar recommends that cleanliness target for reconditioned/repaired Caterpillar components should be two ISO Code levels below (cleaner than) the recommended machine system operating cleanliness target.


Caterpillar recommended target for fill oils – hydraulic, transmission, and final drive – is ISO 16/13 or cleaner.

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