Normal and, therefore, hydraulically challenged people ("normies", as many IH
readers would call them) when confronted with the abbreviation "PC"
think of a) Personal Computer, b) Portable Computer, c) Pocket
Computer, and d) Pepsi Cola. Hydraulically advanced individuals -
skilled technicians, mechanics, engineers, and IH visitors - when confronted with the same two letters think of Pressure Compensation
in the first place, and then recall that "normies" used the same
letters some years ago to name their newly appeared gadgets...
In present-day industrial hydraulics we have pressure
compensated controls (controls, that automatically adjust something in
response to changes in pressure) for everything. In this post I'll talk
about PC controls of variable displacement hydraulic motors, which are
widely used in hydrostatic transmissions, and from what I often see,
cause all sorts of confusions when in need of adjustment. The article
will discuss the control's function as well as different adjustment
techniques, hopefully straightening out the crooked PC matter once and
for all.
A simple PC motor control, or as it can also be
called, an automatic pressure compensated control, functions as
follows - the motor starts at minimum displacement, and when the load
induced pressure reaches the compensator setting, the control increases
the displacement, reducing the motor's speed and increasing the
torque/pressure ratio. In other words, this type of control allows the
motor to adjust automatically its displacement to current system torque requirement.
This is a very useful quality for hydrostatic drives, which allows the
vehicle to be fast in plain terrain (low displacement - fast speed, low
torque), and to have enough torque to go uphill (higher displacement -
lower speed, more torque), functioning as an infinitely variable
automatic gearbox.
Common options include:
- Override option (hydraulic or electric) - for situations when we
don't want out vehicle to go fast, and put our drive motor into maximum
displacement by supplying pilot pressure or electric signal. A reverse
type of control is also possible - when the motor starts at maximum
displacement, and goes into "auto-fast" mode (pressure compensator
function) when pilot pressure or electric signal is supplied.
- Brake pressure defeat option - for making sure that the PC
doesn't swivel the motor to maximum displacement during braking or
moving downhill (situation when the motor becomes pump) and cause
uncontrolled abrupt deceleration or the prime mover over-speed. When a
PC motor is equipped with this option, it uses only one side of the
loop as the high pressure reference for the given direction - and so
the PC is not affected by the pressure rise in the opposite line during
braking. The brake pressure defeat spool can be piloted hydraulically
by the pump's servo-pressures, or it can be operated by a solenoid.
- Feedback spring option - the use of a feedback spring in the
servo-mechanism allows the control to "stretch" the high pressure zone
where the displacement transition takes place. For example, the Rexroth
A6VM HA2 control requires a 100 bar pressure increase over the PC
threshold setting to go from minimum to maximum displacement. The
existence of the feedback spring also permits proportional displacement
control, which can be a useful feature for certain applications.
All variable displacement motor manufacturers
produce motors with this control. For example - Rexroth A6VM HA1
and HA2 controls, Sauer Danfoss series 51 PC control, Linde BMR,
as well as newer HMV-02 (optional), Parker V12 (V14) AC, AH and AE
controls, Eaton BAV motor PA and PB controls (not sure if there's one
of these here in Portugal...), and so on. Despite different
engineering solutions, all of these controls pursue the same goal -
automatic displacement selection to guarantee optimum speed and torque
delivery at all times.
When PC threshold is set incorrectly, you wind up
with a vehicle too slow, when the threshold is set too low, or
experience "hill climbing" difficulties, when the threshold is set too
high. From what I've seen around, the average PC threshold setting is
around 70-90 percent of the maximum system pressure setting, but the
best reference for a particular machine should be factory information
(at least when you can get it...).
The adjustment of PC control threshold setting,
while easily performed on a test bench, can become a challenging
task in the field.The
first and most obvious reason for that would be reduced motor access
due to compact nature of modern machine designs, but the main
challenge is to define exactly the pressure, at which the
displacement transition starts to take place. I will describe several
techniques, that can be used for this purpose, although the choice of
the best technique will depend on the actual machine "in question".
1. A good reference point to define when the displacement transition occurs is the moment when the system pressure stops climbing (or even experiences a slight pressure drop in some cases) during constant load increase.
The main difficulty is to create the controlled load increase, which
can be performed only on vehicles equipped with a well adjusted braking
system. The process is very simple - you install an analogical pressure
gauge in the pressure line, set the machine in motion at constant
speed, and then increase the system pressure by applying the brakes progressively. Upon reaching the PC setting, the pressure climbing rate will decrease or even stop, indicating the moment when the PC kicks in. Advantages
- you use only one pressure gauge, most of the times simple to install
using the machine's test ports. The moment, when the pressure stops
rising, indicates exactly the displacement transition pressure, independently from the servo mechanism of the motor. Disadvantages
- you will need space and plain terrain to move the vehicle around, the
braking system must allow for progressive braking (which, I must say,
is rarely the case), you might need to operate the vehicle yourself
(which is cool, but can become dangerous).
2. Another technique you can use to define
when the motor starts to swivel towards a higher displacement is direct
servo-pressure reading. Normally you will have to read servo-pressure
in the side of the servo-piston, that pushes the axis towards maximum
displacement. For most types of controls the pressure rise in this side
of the servo-piston will indicate the start of the PC function. In this
case you can turn on the parking brake, and slowly increase the system
pressure while monitoring both the line pressure and the servo pressure, and noting the system pressure when the servo starts to rise. Advantages - the machine remains stationary during the adjustment procedure. Disadvantages
- you will have to give attention to two readings at the same time
(requires some practice, analogical gauges are best for the job). The
moment the servo-pressure starts to rise does NOT necessarily indicate
the displacement change, which will depend on a number of
factors, like the relation of servo-piston areas, presence of bias
springs, whether the opposite area is connected to high/low loop
side... That is why you will have to do some research on the motor
before applying the technique. For most motors I've encountered so far
the pressure rise of 30-50 bars safely indicates the start of
displacement transition, but in some cases it can be as high as 150-200
bars (around half of the PC setting). On most vehicles, operating
transmission with the parking brake on requires disabling some safety
features or operating the brake manually (which can be a tricky task if
you are not familiar with the machine). Often it is difficult to rise
the system pressure slowly enough to take accurate pressure readings (in this case you can use the next technique).
3. When it is impossible to accurately
control system pressure, you may resort to the following - instead of
controlling the system pressure by controlling the pump or increasing
motor load, you adjust the pump's pressure cut-off (pressure
limiter) to the desired PC setting level. In this case all you have to
do is screw the motor's PC adjusting screw in till it stops, apply the parking
brake and give the pump a "full throttle" (the system pressure will be
limited by the pump's pressure cut-off), and then turn the PC
adjustment screw out till the rise of the servo-pressure indicates the
displacement change. Advantages - the
machine remains stationary and you don't have to worry about precisely
controlling system pressure - the pump's pressure limiting function
does it for you. You can give all your attention to one pressure gauge
- the servo pressure one, which is easy. You work with lower than the
maximum system pressure, which is safer. Disadvantages - basically the same as in the second technique, plus the necessity to re-adjust the pump's pressure limiting system.
You must understand that these are very basic
descriptions, which are a good point to start, but don't cover all
possible scenarios. The best action before going into the actual
nut-turning is to get as much technical information on the component to
be adjusted as possible.
