The Pump, the Heart of Any Hydraulic System

Richard Hacker, ACE, Accent Electronic Controls Inc.
Special Collaboration


This article is the first one in a series of five dealing with the hydraulic accessories of snow clearing trucks and their tools. These articles aim at supplying some essential information and, above all, make buyers and users ask questions before purchasing any snow clearing equipment.

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
A snow removal truck with plow, wing and sander has very varied demands for a hydraulic system:

  • the sander works continuously, 24 hours a day if necessary, with flows usually between 2 and 5 gallons per minute (GPM), with pressures between 400 and 1800 pounds per square inch (PSI);
  • the plough functions take a few seconds followed by long inactive periods and require between 10 and 15 GPM per function, with pressures rarely above 1000 PSI;
  • operating the ploughs must not interfere with the sander operation;
  • since four season bodies appeared on the market, raising the hoist has been added to the tasks of our hydraulic system. Rarely the hoist is used more than ten times a day, but when used it needs very high flow with pressures pretty high on occasion;
  • if we replace the gravel box with a standard V-sander in the fall, the high flow requirement for the hoist will be back in spring, and in the mean time we need a different hydraulic set-up for the sander;
  • for the sander and plough the pump must continue running even when the clutch disengages the manual transmission. Stopping the hoist during clutch operation is acceptable.

Hydraulic pump location
Traditionally the hydraulic pump is installed at the front end of the truck and driven by the engine crank shaft through a drive shaft.
On Allison automatic transmissions (3 and 4000 series) the pump may be driven by a power take off (PTO). Some configurations require a drive shaft, others don’t (direct mount).
Some engine manufacturers offer a Rear Engine Power Take Off (REPTO) as an option. Again some configurations require a drive shaft, others don’t (direct mount).

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
The most common system has a tandem fixed displacement pump (A), with separate valves for the sander and plough, and often a third valve that combines both pumps for increasing the hoist speed.

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
No matter where the pump will be installed, the pump performance will always be determined by the basic pump design, fixed or variable.

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.

Conclusion
The pump is more than a simple truck accessory: it is the heart of the hydraulic system. Its choice will determine to a large extent the manoeuvrability, performance and operating costs for the truck. Each vendor will propose its solution to the buyer, according to criteria that may not necessarily be those of the buyer or user. The absence of independent sources of information makes the buyer’s job much harder, but there seems to be no change in sight.

Watch for the upcoming article on: Oil distribution and hydraulic valves

 

Cet article est également publié en français


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