Many engineers mistakenly think the use of a proportional valve for speed control gives precise and consistent speeds. They feel the spool positioning is very accurate and repeatable when LVDT feedback devices on the main spool are used. However, an expensive proportional valve is no better than a plain, old-fashioned needle valve.  The flow will change if the pressure drop across the valve changes.

When castings of different weights in this application were used, the load pressure on the outlet of the proportional valve changed, causing the flow across the valve spool to change. The addition of a pressure compensating “Hydrostat” module placed under the proportional valve solved this problem. The “Hydrostat” makes the proportional valve pressure compensated, so it ignores load changes — just the same as a pressure-compensated flow control valve does.


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