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Ch47 transmission oil system

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I have asked this same question and never really gotten a for sure answer. So heres my best guess; the aft shaft is so much higher then the aft transmission that even though its a 20psi pump, by the time it gets all the way up there all you can expect is 10. As for the c-box maybe we can get someone with more 47 experience to weigh in.

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On 7/18/2019 at 4:18 AM, sbox23 said:

why only the aft vertical shaft and combining transmission auxillary low pressure lights up at 10 PSI whereas the rest of the transmission low pressure lights up at 20psi?


On 7/23/2019 at 6:15 AM, sbox23 said:

was really thinking whether both system got to do with the low amount of oil flowing through that why it only require 10 psi.Still hoping that someone has the answer.


We need to understand the system we’re dealing with. The pump doesn't pump pressure. The pump delivers a rate of flow and that rate of flow meets with and hopefully overcomes resistance in the system. What you’re reading on the gage is not the amount of pressure the pump is putting out. What you’re reading is the amount of resistance being overcome downstream of the gage. Contrary to your quote, 10 PSI represents low resistance and adequate flow rates. The difference between pressure and flow is often misunderstood.


The CH47 aft transmission lubrication system, see photo below, is a parallel-series system where the pump is servicing multiple branches. The branch could have multiple series loads or additional parallel flow paths. These parallel and series combinations behave differently and have branch pressure that differs from the overall system pressure.


The transmission pressure is taken downstream of the filter. The filter and transmission resistance to flow causes a pressure drop. The physics of this series branch states the sum of the pressure drop must equal the system pressure. Under normal operations, 6-10 PSI is needed to overcome filter resistance resulting in a 6-10 PSI drop across the filter. The remaining 14-10 PSI is dropped across the transmission.


Problems with the system similar to filter blockage or blockage downstream of the filter will increase the pressure drop across the filter, and reduce flow into the transmission. The physics are satisfied by an increased pressure drop across the filter and decreased pressure drop across the transmission. The total of all pressure drops in the branch remains equal to system pressure.


Without the transmission pressure information, we would never have known there was a problem, since the system pressure is still at 20 PSI, see photos below.


click photo to enlarge










Edited by iChris
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On 3/28/2021 at 4:49 PM, sbox23 said:

@iChris will we be able to know the aux pressure if the pump is operating at a rated speed of 3326rpm and produces a flow rate of 1.5gpm?

Pressure is equal to force over the area in which the force is applied. The calculation that you’re referring to would only give the theoretical capacity or capability of the pump. You’ll need more configuration or design specifications for an exact pressure value. Begin by taking a look at the pump’s data plate. You need the pump’s horsepower and or torque specification along with what you have.

Hydraulic Pump Calculations_1

Hydraulic Pump Calculations_2

Example taking the pump below:

Horsepower = ( Q_Flow rate_GPM  x P_Pressure_PSI)  ) / (1714 x Eff )

 hp = (Q x P)/(1714 x Eff)    or   P = (hp x 1714 x Eff) / Q   or 

 P = (torque_inch_pounds  x  rpm) / ( Q x 36.77)  (less 10 -20% for efficiency)

 The Horsepower required to produce 2 GPM @ 1000PSI at 80% efficiency estimates as:

 hp = (2 x 1000) / (1714 x .80)  = 1.46hp


Edited by iChris
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