hydraulic fluid pressure in the cold

[jimg]

The other day my daughter reported that the tractor at the farm where she works sprouted a fairly substantial hydraulic leak as it was warming up (at an idle I suppose). That got me to thinking about the system pressure when its cold. I know the fluid in my tractor is *very* thick when cold and to pump it Im sure the system is under a higher pressure than when its hot. Of course this must have all been thought thru when the tractor was designed. I figure there must be a relief mechanism in the system to limit the max pressure....which must also affect performance until the fluid warms a bit.

Ive observed a similar behavior in my truck. When cold the oil pressure is very high. As the engine warms the pressure slowly decreases to the 'normal' range. Again, there must be some limiter on how hard the pump can pressurize the system.

Id like to understand how it works. jimg

[Hambone]

There is a relief that send ecsess pressure back to tank. The orfice is still relatively small. If you try and work it too much when fluid is thick it could cause problems.
It is the same valve that reliefs when you hold your FEL lever over after it hits the stops.