Pump selection 101

by Tom Nash

The basis of much of today’s hydraulic systems is explained in Pascal’s Law, which in part, explains how pressure applied on a confined fluid acts with equal force on equal areas. 

In the centuries since Pascal’s time, there have been great advancements in the invention and development of the hydraulic pump. Unlike the force and areas stated in Pascal’s Law, however, not all hydraulic pumps are equal.

There are three basic types of hydraulic pumps: gear, vane and piston. Each offers unique characteristics and performance advantages. Different pumps are better suited to certain types of applications. The differences between these pumps also include price, available pressure, energy efficiency and contamination resistance.

When specifying a new hydraulic pump for a specific application, these differences must be taken into consideration.

Gear pumps
Gear pumps have a number of advantages over other types of pumps: they are generally the least expensive and are built to operate in the most punishing applications, particularly those with large amounts of debris and extremely high temperatures, such as steel mills, foundries and mining operations. An operational fact to consider, however, is that gear pumps are typically limited to 3,000 psi.

Operators prefer gear pumps for a variety of applications, including:
• conveyor drives;
• compactors;
• vibration tables in quarries and mines;
• cutters, augers and ditchers;
• mobile equipment;
• lift gates;
• plows;
• various dumping applications; and
• lubrication systems.

From a cost perspective, gear pumps not only have a lower purchase price attached, they also offer the longest life expectancy of all hydraulic pumps due to their tolerance of high levels of contamination. Due to their low cost, gear pumps are not normally repaired when broken or damaged. Instead, they are traditionally scrapped, and a new replacement is purchased.

Vane pumps
Despite the cost advantages of gear pumps, vane pumps are the most commonly found variety of pump in industrial applications. Within this category, there are two types: fixed displacement and variable volume pressure compensated (or just variable, for short). Both can be used in practically any application and are limited in their usage only by psi.

Fixed displacement pumps are highly suited to a number of operating environments, particularly those with repetitive applications. These include:
• saw mills;
• packaging applications;
• gang nailers;
• blow molding;
• light presses;
• stacking; and
• assembly machines.

Fixed displacement pumps have a maximum pressure limit to 4,000 psi, but most operate in the 2,000-2,500 psi range efficiently.  They are always “on” and generate constant flow.  Pressure will vary based on the load and system requirements.

Because of the heat generated during their operation, fixed displacement pumps require special considerations in the design of the valves in the circuit to keep pumps running cool. This results in significant amounts of wasted energy.

However, fixed displacement pumps generally have low purchase costs, are easily repaired and stand up well to contamination.

Variable pumps come in pressure ratings up to 3,500 psi, but are most efficient within the 1,200 psi to 2,250 psi range. Unlike their fixed displacement counterparts, variable pumps only provide flow and pressure when called upon, and that flow and pressure can be changed. In fact, variable pumps can be preset to self-adjust to a specific load as measured by sensors.

Operating applications well-suited to variable displacement pumps include:
• drilling, punching and polishing equipment;
• balers (including paper, banding and composite);
• bottling lines;
• injection molding;
• walking beam steel applications;
• pick-and-place equipment; and
• railroad tie presses and stackers.

More energy efficient than fixed displacement pumps, variable units control their heat output better and feature vibration dampening, resulting in quieter operation. Purchase price is generally a bit higher, and while they tolerate a fair amount of contamination, they can’t handle as much as fixed displacement models.

Piston pumps
Piston pumps pick up with pressure levels where most vane pumps leave off, providing pressures of 3,000-6,000 psi. Smaller in size than other pumps, piston pumps are highly suited to such applications requiring higher pressures and speeds, including:
• flying cut-off saws;
• shears and benders;
• presses (including steel, fiberglass and oriented strand board);
• boring and drilling machines;
• injection molding;
• extrusion machines; and
• adhesive machines.

Piston pumps offer greater accuracy and control than other pumps, and also offer vibration dampening and energy efficiency. However, these pumps are quite sensitive to contamination. Build-up of dirt and other particulates can cause these pumps to loose efficiency due to the extremely high tolerances in the internal components.

Pumps come in models and sizes to the specific needs of any application. For a better understanding of how to match the right model to your particular production application, contact your distributor or manufacturer’s representative.

Wouldn’t it be interesting to see which model Pascal would try first?

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