HydraForce Insider Blog

Weighing the Benefits of Pre- and Post- Style Pressure Compensators

Posted by David Price on Thu, Mar 10, 2011 @ 08:03 AM

A pressure compensator maintains a constant pressure drop across a metering device regardless of the load induced pressure on the function. There are only two types of compensation methods used in hydraulic flow control functions. These are pre- and post-style compensation. Pre and Post refer to the position of the pressure compensating element relative to the metering element. A pre-style pressure compensator is positioned upstream of the metering element (a proportional valve) and a post-compensator is positioned downstream of the metering element. There is also a sub-category of these, which adds load sharing (this is sometimes called flow sharing). Using current cartridge valve technology, load sharing is limited to post-compensation circuits.

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So, when do you apply each of these, and what are the advantages and disadvantages?

In a single direction flow control circuit, such as proportionally controlling a single direction motor, there is no real performance advantage between pre- and post-compensating, unless you require load sharing (which I will cover later). There is usually a slight cost advantage to post-compensators (like our ECxx-30s pressure compensator) because they are cheaper to manufacture. Cartridge post-compensators have been around since the 1960s, but with the advent of proportional directional valves, the need for a pre-compensator cartridge valve emerged. However, it wasn’t until the development of the inverse-spring arrangement that they became practical.

Pre-compensators can be more economical in a proportional directional circuit. Trying to use post-compensation in this type of circuit becomes a complex and costly endeavor. You would need an individual compensator and a reverse flow check on each leg of the directional valve. In contrast, when pre-compensating, you only need a single compensator on the supply port of the directional valve and a load-sensing shuttle or checks between the work ports (or a directional control valve with a load sensing port like our SPxx-5xx series as shown below).

Pre- and Post-Compnsated Circuits

Post-compensation with load sharing (or flow sharing) is useful in circuits where multiple functions operate simultaneously, and the supply flow is less than required to satisfy the combined flow consumption of those functions. With traditional compensation, when the flow requirement exceeds the supply flow, the function with the lowest load-induced pressure receives flow first. Once a particular function's flow requirements are satisfied, the remainder of the flow supply always goes to the next highest load-induced pressure function until the supply flow is exhausted. This is not acceptable if multiple functions need to operate simultaneously in the application.

Load sharing provides a means of balancing the highest loaded function across all the functions. So instead of having the path of least resistance taking most of the flow, it is evenly distributed between all the operating functions according to the percentage of flow each metering element demands. For example, say you have two functions that would normally take 11 lpm (4 gpm) and 30 lpm (8 gpm) individually and you have only 30 lpm (8 gpm) of supply flow. When operated simultaneously, the two functions would flow 4/12ths (or 1/3) of and 8/12ths (or 2/3) of the 30 lpm (8 gpm) supply in a load sharing application.

Post-Compnsated Circuit

A very effective load-sharing circuit can by created by using pilot-to-close logic valves that are biased closed with a very low-pressure spring (like our EPxx-S35-x-x-10 Piloted, Spool Type Logic Element) down stream of each metering element. All of the pilots are connected to the work port via load-isolating check valve (see Fig. 3 below). When a work port is pressurized, the load pressure is communicated through the check valve to the pilot ports of the EP valves. This causes all of the EPs to remain closed until the working pressure exceeds the work pressure of the highest load-induced pressure, plus the bias spring (this is why we use a very low bias spring). All of the functions in operation see nearly the same resultant load-induced pressure and thus share the available supply flow.

Post-Compensated Circuit with Load Sharing

In summary, follow these simple rules of thumb when deciding between pre or post compensation and you should be fine:

• Single uni-directional or multiple uni-directional functions operated singularly;
advantage: Pre-compensation.

• Single bi-directional functions that operate singularly; advantage: Pre-compensation.

• Multiple uni-directional functions that need to operate simultaneously;
advantage: Post-compensation with load sharing.

• Multiple bi-directional functions that need to operate simultaneously;
advantage: Post-compensation with load sharing (though expensive
and complex with current technology).


About the Author:

Mark Decklar is an Application Engineer at HydraForce with over 29 years of hydraulic experience. Contact Mark

 

Tags: cartridge valves, proportional valves, pressure control valve, directional control valve, hydraulic cartridge valves, pressure compensators, pressure compensation, Piloted, Spool-Type Logic Element, proportional directional valves, pre-compensated hydraulic systems, post compensated hydraulic systems