Servopumpe mit höherem Druck

TR-Spider, Samstag, 06.01.2024, 10:39 (vor 117 Tagen) @ Tobi_M50

Ich hatte mich vor kaum 20 Jahren mal damit beschäftingt, hier Auszüge was ich damals rausgefunden hatte:

However, I will now start digging a bit deeper into the hydraulic steering system, as I had some unpleasant surprises during my last drifttraing.
When moving the steering wheel real fast, it occasionally got extremely heavy...not what you want when you try to countersteer quicky.

So, how does the pump regulation work?
The pump itself has quite a high flowrate, something like 7 cm^3/R, so if it rotated with 4000RPM, it will flow 28Liters/min (with 1liter=1000cm^3)

ZF quotes something like 6liters/min...so where does the flow go?

Lets look at the pump crosscut:

where yellow is the inflow side and red the high pressure side.
(In our pump we do not have the pin on the right side of the piston. My guess is that this pistons shape dictates the flow over RPM characteristics.)
and now let's take a closer look on the regulation at different states:

the first pic shows zero flow into the steering rack, all flow goes through the pressure relieve valve.
The second pic shows a low flow state: After the pump we have a pressure P1 and a Volume flow V1.
When the Volume flows through the regulating orifice, pressure loss occurs and the pressure P2 after the orifice is lower than P1. This pressure difference acts on the piston and compresses the spring, making the piston move to the left.
The third pic shows the high flow state: increasing flow lead to a larger compression of the spring, the piston mooves further to the left and opens the inlet to the pump, effectively splitting the pump delivery V1 into V2 towards the rack and V3 back into the pump. V3 depends on the area opened up by the piston movement
So the combination of orifice diameter and spring rate defineds the flow rate and the total pressure is controlled by the separate overpressure valve.
The dp across the piston (P1-P2) is rather small, I estimate 2-5 bar (with dp=fluid_density / 2 * orifice_resistance_coefficient * Volume_flow^2 / orifice_area^2). All units SI.

That now answers the question what could happen due to aging of the piston spring: If the spring shortens, the Volume flow will go down, as the V3 backflow will start earlier.

I opened and measured the stock steering pump and the before mentioned other BMW ones.
My stock pump is a 32 41 2 226 222.
That's how they basically look inside:

Inside the center bore sits the spring and the piston (with the pressure relief valve inside).
The assembly is clamped in and easy to disassemble.
The brass part is the metering orifice and is pressed into the housing (very difficult to remove).
On the right side there lays a removed one.

The housings were all the same (except the "054" type which hade a additional flow vs pumpspeed valve with 3 holes consecutive closing with pump speed).
The pump is exactly the same in all units.
Also the spring wire diameter, outer diameter and number of coils are the same (spring rate is 0.81N/mm)
Thats what came out measuring the different types:
So basically #386 and #594 are perfectly the same...
My guess is that the difference in springlength comes from aging only.
However, I programmed me a small model of the flow regulating device and it showed that the spring length was not overly sensitive.
As example, a spring saged by 5mm results in a decreased flow of 7%.

I assembled then a #386 housing with the largest orifice and the longest spring and put it back in the car.
That means I increase the flow from 4.6l/min to 8.2 l/min (based on my calculated values, which do not take the into account the squeezed location of the orifice, which may alter the flow coefficient with increasing flow somewhat).
The nominal pressure has increased from 110 bar to 130 bar (which is, as I understand, the pressure where the overpressure-valve iside the piston opens - usually only when steering against the locks)

The good news is that the before existing loss of assistance during fast steering movements is completely gone.
That is excellent news as this loss of assistance was rather annoying and is usually happened at a bad timing...
On the other side, the resistance of the steering is decreased, so it moves somewhat lighter now.
Not excessively light, but definetly lighter than before.
That's certainly something I need too get used to.
The reasoning is clear - pushing more flow through the system increases the working pressure, thus reduces steering resistance.
Obvoiusly there is a temptation to play with that a bit, to find a sweet spot.
There may be th possibility to move the orifice to the outside, so would make adjustments more conveniant...we'll see.

Thomas