The Pump, the Heart of Any Hydraulic System
Since at least five decades all snow removal trucks use a hydraulic system to operate their equipments. Hydraulic systems have seen major improvements, so this topic deserves a serious update.
Hydraulic system functions
Hydraulic pump location
Important: Eliminating the drive shaft, reducing hose length and positioning the pump lower than the tank oil level should be the main consideration when choosing where to install the pump.
Possible pump choices
Systems are offered that combine a single fixed displacement pump (B) with a sectional valve that incorporates all functions.
A third possibility is a variable displacement piston pump (C) (load sensing, LS) combined with a valve incorporating all truck hydraulic functions.
Selecting the pump
Every system built around a fixed displacement pump, simple or double (A and B), faces a dilemma: As the truck engine RPM vary greatly (600 to 2000 RPM), it is difficult to have a pump with sufficient flow at low RPM without having problems due to the surplus of oil at high RPM. Choosing valves capable of higher flows makes precision difficult, but smaller valves will create over heating problems.
If a single fixed displacement pump (B) is chosen, load sensing valves combined with an inlet section that evacuates all surplus oil are a possible solution. But each drop of oil moved by the pump is pressurised to the highest pressure required by the system, which makes these systems very inefficient and still very slow for the hoist.
A variable displacement pump (C) does not suffer from any of these limitations. It is possible to combine just about any size of pump with any size of valves, without any problem. Associated with load sensing valves the pump moves oil on demand only. This gives excellent flow at low engine RPM while having speed and precision according to function needs. These pumps also may benefit from ratios higher than 1 to 1 in PTOs and REPTOs, which increases potential pump flow from the same pump, and without any additional fuel consumption.
All load sensing pumps (C) occasionally work in the «pressure compensated» mode, which means that they try to simply maintain a pre set pressure in the system. This feature allows combining load sensing and pressure compensated valves. For all intermittent functions these pressure compensated valves offer a very interesting alternative to load sensing valves, while maintaining the benefits of the variable displacement pump.
Important: The pump is always part of a complete hydraulic system integrating various other components.
Therefore, choosing the pump must always be done with the carrier truck and its various equipments in mind. Defining the functions and performances that the truck must deliver remains the first task before even starting to consider eventual technical solutions.
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