Pre-compensated vs. post compensated valves

[DriveHard]

This question is open to all - no rewards, just fun conversation.

In a hydraulic system, a hydraulic compensator is a part of a valve that helps to regulate the flow through your main metering orifice, and allow consistent performance. There are two main configurations, pre-compensated, and post compensated. There is a primary performance difference between the two (and a secondary difference) - as the valve asks for too much flow from the pump, and you become flow limited, how do the two valves differ in performance?

Please assume we are using a multi-section valve where there is more than one flow demand.

 

[JohnnyG]

In my opinion, the primary difference is in how quickly it should open. Both systems will regulate flow AFTER opening.
Secondary difference is probably how quickly and controllably the valve closure will respond to downstream flow.

 

[DriveHard]

In my opinion, the primary difference is in how quickly it should open. Both systems will regulate flow AFTER opening.
Secondary difference is probably how quickly and controllably the valve closure will respond to downstream flow.

While a in a pre-comp valve the compensator is normally open, and with a post-comp valve it is normally closed, in my testing, I have never seen much of a response difference, as normally the compensator frequency response is much faster than the main control spool.

I'm thinking more about machine performance characteristics that result from using one type of valve vs. the other, especially in flow limited conditions.

 

[DriveHard]

HINT:
Think about using either type of valve on a machine that has multiple demands...much like a Backhoe loader, and how the two different valve types would effect performance when the operator was asking for more flow than the pump could provide. How does the system respond? When the operator is "asking" for more flow than the pump can provide, the two valves behave in very different ways. Which would you rather have and why?

 

[DriveHard]

Ok, ok, ok...maybe a bit too niche for the crowd...

Precomp valves are extremely accurate in flow demands until you ask for more flow than the pump can supply. In that case, the function with the highest pressure will stop working.

Postcomp valves are not quite as accurate, but when you ask for more flow than what the pump can supply, they will "flow share" and all functions will slow down proportionally.

 

[wwillson]

Can you give us examples of where we would find pre and postcomp valves?