TO: All fluid power engineers and designers. FROM: The Fluid Power Safety Institute™. SUBJECT: Recommended hydraulic system design improvement. Introduction - On behalf of the many fluid power industry maintenance and assembly line personnel, the FPSI™ respectfully urges engineers and designers to consider the following hydraulic system design improvement. What's the problem? - Maintenance and assembly-line personnel are experiencing great difficulty when: 1. Setting up and adjusting new pressure-compensated (PC) and load-sensing pumps (LS). 2. Setting flow adjusting screws on PC and LS pumps. 3. Testing PC and LS pumps. 4. Checking inlet restriction on PC and LS pumps. Why is this a problem? - The vast majority of PC and LS pumps fire to "dead-head." This simply means that fluid power systems that use PC or LS pumps are invariably equipped with closed-center or float-center directional control valves. Both configurations cause the PC or LS pump to de-stroke when the prime mover is running. Many systems utilizing PC and LS pumps are specifically designed to use less than full pump flow at any given time, to avoid one or more actuators from slowing down during a machine's cycle. In a nutshell, a PC or LS pump is generally capable of producing more flow than a system needs at any given time, and herein lie the problems: 1. How is it possible to safely set a PC or LS pump's volume adjusting screw if the pump cannot produce full-rated flow, under normal operating conditions? 2. The ideal method of testing a PC or LS pump, especially if the flow rate is unknown, is to observe the "no-load" (low-pressure) flow, against the "full-load" (maximum pressure) flow. How is it possible to achieve this if the pump cannot be unloaded to produce full flow? 3. This problem also makes inlet restriction difficult to test and evaluate. According to the majority of pump manufacturers, maximum inlet restriction should be observed while a pump is operating at full-rated flow, with the oil at normal operating temperature. Once again, we are confronted with the problem of "unloading" the pump to execute the inlet test. What is the current practice? - It is not good! The FPSI™ has learned that the majority of maintenance personnel, and others who need to "unload" PC and LS pumps, use these common methods: 1. They remove the transmission line from the outlet port of the pump and place the open end in a receptacle - usually a bucket! 2. They remove the transmission line from the outlet port of the pump, and place the open end in the reservoir fill port - in most cases it is unsecured! 3. They haphazardly adjust flow-adjusting screws. Why is the current practice unacceptable? - It is clearly unsafe! It is a well-known, and documented, fact that exhausting hydraulic oil to atmosphere can cause severe injury, death, or substantial property damage. The FPSI™ is deeply concerned that these "trial-and-error" procedures are going to hurt people. What can fluid power engineers and designers do to stop this problem in its tracks? - After careful consideration, the FPSI™ would like to make the following recommendation: We would like to see a quick-disconnect (suitable size) located at the inlet port ("T") of the return-line filter (diagram A) of every machine equipped with a PC or LS pump. When it is necessary for a person to set up pump, or check inlet restriction, an adapter could be used to connect the pump outlet port transmission line directly to the quick disconnect at the inlet side of the return-line filter (diagram B). This design improvement is safe, cost-effective, and practical! This recommended design-improvement has additional attractive features: 1. It allows a person to execute a "direct-access" pump evaluation outside of the circuit. If there were a possibility that a pump is rapidly degenerating, the particles, or the majority thereof, would be contained in the filter while the pump is being evaluated. The term "direct-access" simply suggests that a pump can be tested and/or evaluated independent of its circuit. This minimizes harmful contamination ingression, and lessens the "collateral damage" that a failing pump can cause. 2. Many problems associated with contamination are said to be "incidental to maintenance." When adding oil to a hydraulic system in the field, especially if the field service vehicle has to travel along dirt roads, it is very difficult to avoid the introduction of contamination through the various fill ports. Dirt ingression can be greatly reduced if the "make-up" oil is introduced through a quick-disconnect that is conveniently located at the inlet side of a return line filter. Conclusion - This is the first of many design improvements that the FPSI™ is going to recommend to fluid power engineers and designers. We believe that it is fair to say that the majority of frustrations that maintenance personnel experience is not because engineers are inept, it is because there is a flaw in our system based on the fact that "the people who design hydraulic systems don't usually work on them, and the people who work on them don't usually design them." This problem is exacerbated by the fact that we don't communicate very well to one another! The FPSI™ would like to, in a sense, "mediate" the frustrations that both fluid power engineers and maintenance personnel face. We cannot hide the fact that there is dissension in the ranks. However, we can encourage constructive and meaningful dialogue.