Filter Bypass Valve
The filter bypass valve is a spring loaded check valve that lets oil pass around the filter when the element is clogged or when the oil is cold and very viscous. Cracking pressure is typically selected at 2-3.5 bar. It protects the system from running dry of oil, but while it is open filtration has effectively stopped.
The bypass is a safety measure, not an operating mode. When the pressure difference between the element and the housing exceeds the set value, the valve opens and dirty oil goes straight into the system. There are two typical causes for this: a clogged element, or excessively high viscosity on a cold start.
- Cold start: at around -10 C the viscosity of an ISO VG 46 oil is tens of times what it is at operating temperature; the bypass can stay open in the first few minutes.
- Reading the indicator: the clogging indicator reports that the bypass is about to open; if the warning is lit, the element must be replaced.
- Selection without bypass: a bypass is not wanted on a pressure filter protecting a proportional valve.
A common field mistake is postponing the element because "the system is running" even though the indicator is warning. At that stage the oil that does not pass through the filter goes straight to the valves; this is often the silent reason why proportional behavior degrades within weeks.
Where this term is used: How Are Manual Hydraulic Levers Controlled Remotely? A Comparison of 6 Methods