Positive Displacement Pumps: Important Things that You Should Know

Blog | December 13th, 2018

Positive displacement pumps rule the hydraulics sector. Granted, an attempt by the sector to switch to non-positive displacement could be made, but the equipment based on this pump type would be unable to produce a positive internal seal. Yes, the gear would create a continuous flow, but that flow would vary as pressure changes took place throughout the system. Back with positive displacement technology, proportional output occurs every time the pump cycles.

High Resistivity Pump Mechanics

Compared to its opposite number, non-positive models fall woefully short of the performance requirements operators have come to expect from their mobile hydraulics equipment. First of all, this pump principle generates slippage when there’s a system obstacle. If a capable non-positive displacement unit generates pressure when the system is obstructed, its flow rate falls to zero. Replaced with a positive displacement model, there’s next to no fluid slippage, so the pressure and flow both rise proportionally as the pump cycles, no matter the obstruction.

Fixed or Variable Displacement

Here comes that word again: displacement. Pumps, those that employ positive displacement technology, trap a set quantity of fluid, then they cycle that fluid package through the pump chamber. Keeping the flow constant, fixed fluid models use standardized fluid transferring manifolds and volutes. Opposing variable displacement technology requires a special type of pump chamber geometry. Incidentally, even though this pump type comes with many system-advantageous features, it cannot be operated without a discharge-side relief valve. A competent design engineer, tasked with creating a new mobile hydraulic system, must place an internal/external relief valve in the discharge line.

Deconstructing Hydrostatic Pumps

That’s just another name for progressive pumps, one that should take its turn because this article has overused the former label. Anyway, hydrostatic pumps are designed so that they have very little clearance between their moving parts, hence the low slippage feature. If there is a larger than expected quantity of fluid slippage taking place, the pump is malfunctioning. As for the form a deconstructed pump takes, it breaks down into vanes or gears, pistons or diaphragm parts. There are even long cylindrical housings, which use extended helical rotors. Of course, this configuration wouldn’t exactly be practical, not for a mobile piece of equipment. Other deciding factors, those that impact the chosen pump architecture, include volumetric capacity, angular constraints, pressure ratings, and mounting configurations.

Basically, positive displacement pumps are highly functional, highly efficient, and they’re extremely popular in the hydraulics sector. However, there are several factors that must be incorporated into their design when they’re used with high pressures. A discharge-side relief valve is essential, as is an architecture that removes the pulsing effect that can propagate through some of the above pump types.

Mobile Hydraulic Specialties Pty Ltd

Factory 89, 38-40 Popes Road
Keysborough, Victoria, 3173

Phone: (03) 9798-6511

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