To Filter, Or Not To Filter, That Is The Question.

Last week I received an email from a past student of mine asking me to explain in broader terms why I vehemently disagree with equipping a closed loop system with in-the-loop filtration.

Note, a closed loop system is also referred to as a hydrostatic transmission.

I decided to share my response with my friends; and enemies, on Linkedin. To give readers a little background, my reasons are based on two factors:

1.  I was originally invited to come to America as a "troubleshooter." Apparently, the underground mining machinery manufacturer for whom I worked was getting inundated with complaints from clients because the mechanical availability of their machines (underground loaders and drilling rigs) was hovering around 45%. According to reports, the lion’s share of the breakdowns stemmed from hydraulic systems failures. Moreover, the machines that were experiencing the highest number of breakdowns had a common dominator; they were equipped with closed-loop systems.

2.  The knowledge I gained from analyzing open loop and closed loop hydraulic pump failures over a period of two decades.

Here are the reasons why I am vehemently opposed to in-the-loop filtration:

1. Service related:

There is no point in filtering oil in one direction only. Consequently, two filters are needed to filter the oil; one for each direction of flow. If the closed loop circuit is reversible, which the majority are, the system would have to be equipped with a total of four filters. An alternate solution is to equip the systems with two-filters with rectifier valves. Both would be plumbing nightmares!

A closed-loop system is for all intents and purposes a “sealed system.” If the smallest amount of contamination is introduced into the loop, it will cause the pump and motor to fail.

Most mechanics don’t know much about oil filtration. Fifty out of fifty mechanics and hydraulic instructors I asked did not know anything about ISO 4406; how to target a hydraulic system’s oil cleanliness level; what a range-code number is, or what the term beta-ratio means.

Consequently, most mechanics have no idea about how dirt intolerant hydraulic systems are. Accordingly, when they service hydraulic filters they typically remove the canister (s) and screens (if equipped) and walk to the nearest solvent tank. The filter heads are left exposed to the elements, while the canisters and screens undergo a "contaminant infusion bath." Another name for this hydraulic component life ending process is a solvent tank.

Solvent tanks are where metal filings, mud, debris, etc., are washed off miscellaneous parts and pieces. It is arguably the most contaminated liquid in any repair shop. Washing filter canisters and screens in a solvent tanks epitomize the term “cross-contamination.” For the record, filter canisters and screens should be cleaned with Isopropyl alcohol or an equivalent cleansing agent.

If the filters are serviced every 500-hours, this equates to "infusing" large amounts of life-ending contaminants into a closed-loop every 500-hours.

2. Cost-related:

Most maintenance managers don’t know much about filtration. Accordingly, the price of hydraulic filters is put in the “cost” column of their maintenance budgets instead of the “investment” column, where it belongs. As we are all aware all items in the “cost” column of a ledger automatically become targets for cost reduction. Now the destiny of a plant's hydraulic systems is placed in the hands of the purchasing agent - the person that purchases everything from toilet rolls to dump trucks. It's hardly fair to expect them to be experts in oil filtration.

The order goes to the lowest bidder, and plant and machinery reliability take another turn for the worse. This is where the owners and operators of hydraulic systems take a very expensive course titled "all filters are not created equal." It’s remarkable that many maintenance managers work very hard at investing in the demise of their company's hydraulic systems.

3. Ignorance related:

The loop in a closed loop system is the oil reservoir for the pump and motor. The average closed loop holds 1-quart (0.95 Liters) of oil. Most heavy-duty closed-loop systems are equipped with a “loop conditioning" system, which is enabled through simple charge pressure adjustments.

Problem is, most pump and motor manufacturers, machinery and equipment manufacturers, and hydraulic component rebuild centers, are so naive about closed-loop systems that they believe charge pressures are pre-set by the respective pump/motor manufacturer, rebuilder. They obviously pedal the misinformation to their clients. Consequently, there is a huge population of closed-loop systems that are equipped with vital loop conditioning systems, which are disabled.

The problem could be detected with a temperature gauge. However, thanks to poorly trained design engineers, you will find that the few systems that are equipped with temperature sensors, the sensors are in the wrong place. Closed loop temperature sensors should be in the loop. Over 99% are in the reservoir, which, by the way, is also the wrong place. This design oversight was a cunning, but very profitable, marketing strategy.

Of all the reasons I have been given by end-users for installing in-loop filters, the one that topped the list for being the most absurd was that if the pump suffered a catastrophic failure, the hydraulic motor would be spared. This logic shows that many owners of hydrostatic transmissions are as ignorant about closed loop systems as the folks that sell filters.

4. Design related:

There is only one way (typically) for oil to enter a closed loop system, and that is through the charge pump. Clearly, the most logical place to put the best filter money can buy is on the inlet side of the charge pump. Since the charge pump flow is continuous, if there is a high-quality/efficiency filter at the inlet of the charge pump, the system has, for all intents and purposes, a “kidney-loop” filtration system, which circulates, on average, 10% to 15% of the over-center pumps’ flow continuously.

Design engineers are na?ve about monitoring oil temperature in both closed loop and open loop systems. Most put temperature sensors in the tank, which is pointless.  Take a closed loop system for example. If the differential charge pressure is not set, which most aren’t, the loop conditioning (cooling and filtering) system is negated. This means that instead of replacing an average of 10% to 15% of the oil in the loop continuously, the only oil that is replaced is the oil that is lost in the pump and motor due to internal leakage in the pump and motor.

The charge pump flow simply circulates through the pump housing and back into the reservoir, where the temperature sensor sends out a signal, or, if you will a false positive, that the temperature is “normal.” In the meantime, the system’s other reservoir; the closed-loop, is overheating.

It begs the question. Do machinery and equipment manufacturers purposely "hide" a closed loop system's vital operating parameters i.e., oil temperature and charge pressure, in order to exploit untrained and poorly trained mechanics? Is the early demise of closed-loop systems a design objective?

5. Knee-jerk reaction related:

There are so few mechanics that know how to perform a post-catastrophic start-up procedure on a closed loop system that people actually believe in-the-loop filtration will solve the problem. Not! Once a closed-loop is contaminated with pump and/or motor fragments unless a mechanic follows a disciplined and well-orchestrated start-up procedure, order lots of pump and motors, a few pizzas, and a tent because it’s going to take a very long time before the transmission runs again; if it ever does.

I have seen many companies drop an average of US$100,00.00 on pumps and motors because their mechanics didn’t know how a closed loop system worked, let alone how to perform a post-catastrophic failure start-up procedure.

Maintenance managers are led to believe that in-the-loop filtration will stop the bleeding. Some go so far as to shoot projectiles through “wet” oil transmission lines.

Conclusions:

Closed loop system reliability is defined by a few simple things. Here a just a few:

1.  Install the best filter (no by-pass valve) money can buy in the oil transmission line that delivers oil to the charge pump. Avoid in-tank filters and screens/strainers. Spec only spin on/off filters.

2.  Install a vacuum monitoring device at the inlet port of the charge (available from www.hydracheck.com)

3.  Install oil sampling valves in the closed loop (available from www.hydracheck.com).

4.  Ensure the oil reservoir is pressurized.

5.  Charge pressure cannot be pre-set. Train assembly-line technicians how to set charge pressures.

6.  Sense oil temperature in the closed loop - a must if it is equipped with an "oil-conditioning system."

7.  Set a target cleanliness level for the system at 15/13/12 or better.

8.  Take oil samples (in the loop) every 30-days and make sure the targets are being achieved.

9. Attend my workshop where you will learn from someone that cares sincerely about your safety, and knows what it takes to be a competent fluid power technician. At least if you attend my workshop, you will be in a position to recognize that most of the component test procedures you see on Linkedin and YouTube are hazardous and ineffective.

Note: I strongly urge design engineers to boycott the companies (allegedly Dennison and Sauer Danfoss) that install horizontally mounted filters on their pumps. Have you ever tried to change filters on a machine that has six-pumps with horizontal filters mounted to a single gearbox? This while the machine is standing next to an oil-well. It's time to let a few hydraulic system design engineers face the wrath of an environmentalist!

Closed loop systems are a gift to machinery and equipment manufacturers from the profit Gods. America’s technical colleges are profit angels. As long as they continue to do a poor job of teaching closed-loop systems to America’s maintenance warriors, the fluid power industry will continue to profit off the backs of poorly trained technicians.

I am going to start posting the names of technical colleges that have invested in my MF500 Hydrostatic Transmission Training System. The MF500 is the most advanced hydrostatic transmission training system in the world. (https://www.fpti.org/simulators_hydraulic_MF500.php.)

Here’s an interesting point. Caterpillar allegedly “owns” sponsorship of SkillsUSA’s national contest. The hydraulic competition; at least when I attended, consisted of a machine with an open engine compartment. Contestants had to name the hydraulic components that had numbers next to them.

I offered to take four of the most advanced hydraulic training systems in the world (MF102-TSE (https://www.fpti.org/simulators_hydraulic_MF102-H-TSE.php) to the competition and let the contestants construct and troubleshoot actual systems in real time. My request fell on deaf ears because the person in charge of the contest at the time was allegedly a Caterpillar employee. It seems marketing is more important than nurturing competent technicians!

As long as machinery and equipment manufacturers' reps sit on the advisory boards of America's technical colleges, America's colleges will continue to nurture "new parts fitters." In my humble opinion, most advisory board members that work for machinery and equipment manufacturers are there to ensure the colleges "dumb down" mechanics so they can increase profits through robust parts sales.

Here's a novel idea: have your college's advisory committee members take my complimentary hydraulic competency test at (www.fluidpoweracademy.com). If they don't score at least 90% ignore their recommendations about hydraulic training. If a technical college is not teaching MicroLeak testing, that college is shortchanging America's students (hydraulic) and in turn, doing an injustice to America - plain and simple!

WARNING: DO NOT modify your hydraulic systems based on my comments and opinions. Before making any modifications to a hydraulic system consult the respective company’s design engineers. Caution: NEVER take a hydraulic system’s design engineer “out of the loop,” even if he/she is wrong!