Hydraulic Pumps and Motors have Different Types of Pressure Limits

Pressure Limits:

1) Closed Loop Circuit- The highest pressure in the closed loop circuit is bridled by a high pressure relief valve to a valve that is not greater than 5000 PSI. The gauge pressure will read 150 PSI higher that the relief valve setting, due to the increase of charge pressure in the low pressure loop. The least amount of pressure in the closed loop cannot be less than 130 PSI above the case pressure. The pump and motor will have a 30 PSI drop in hydraulic lines.

2) Charge Pressure Settings – The least amount of charge pressure setting, can’t be less than 160 PSI above case pressure. The reason for these settings is to ensure satisfactory control pressure and closed loop settings.
Factory issued hydraulic units which use charge relief valves are made to give these types of charge pressure levels.

3) Case Pressure – The transmission case pressure cannot go under 40 PSI in any operating conditions. The case drain system (hydraulic lines, heat exchanger, etc.) needs to be designed so as not to go over this limit. Also, the line losses between the pump and motor cases if it goes over 10 PSI at regular operating conditions.

4) Charge Pump Inlet Pressure – The highest level of vacuum at the charge pump inlet doesn’t need to go passed 10 inches Hg at regular operating conditions.
If it is a cold start up, it is alright for the inlet vacuum to expand past 10 inches Hg. When a new filter is present, it is made to be three to five inches Hg at regular operating conditions. The filter will need to be replaced when it becomes clogged and it exceeds 10 inches Hg at the charge pump inlet.
We welcome any comments on this topic. Discussion on how to apply these applications or ways you have found to valuable.
Our phone number is 800-361-0068 be glad to answer any questions you have.

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