Understanding the Traction Circuit

To fully understand how the traction system works on turf equipment you first have to understand some basic principals-

When talking about hydraulics there are two key terms- Flow and Pressure.  Flow is the movement of fluid (how we get speed) and Pressure is the resistance of flow (how we get torque).

 

Hydraulic flow moves to the path of least resistance, just like water, and electricity.  Think of this as an open differential on a vehicle.  If you jack one side of a vehicle up all the power goes to the wheel with no resistance.  This same reason it allows you to take a tight turn without the tires binding.  If your vehicle is in 4WD (or you lock and axle) and you make a tight turn it is much tougher and if you are on grass you are going to tear turf.

It is the same with turf equipment.  The reason being that with independent wheel motors when they are both on the ground and both have equal resistance you have “all wheel drive”  When you make a turn there is more resistance on the inside wheel motor so more fluid goes to the outer motor allowing them to spin at different speeds and makes it so you will not scuff turf. 

Now imagine you are going in a straight line through a very wet spot.  One wheel starts to spin.  What happens to the fluid?  It goes to the path of least resistance and you stop.  If you were to apply brake pressure to the wheel that is spinning, (with split brakes) you would increase the hydraulic pressure forcing the fluid to balance out.  Training your operators to do this will allow them to drive through more difficult situations. 

There is also an option called a Flow Divider found on many Toro pieces.  This is a momentary switch that you hold down while driving through a bad spot.  What this does is split fluid to both the front and rear wheel motors allowing balanced flow even if a motor starts to spin.  It does this in FORWARD only and the way it works is that it forces fluid over a set of orifices or small openings in the block of the same size. 

Now that we have some basics down we can look at the schematics.  Here is the traction circuit of a 4000- D  You will see this same circuit duplicated in many other machines.

Toro 4000-D Traction Circuit

The pump is on the left. (pumps have arrows pointing out)  There is an arrow through it because we can vary the amount it puts out depending on how the traction pedal is pushed.  There are arrows in two directions because we can go in both forward and reverse.  The motors have two arrows going in because they accept fluid in both directions. 

In the drawing you can see the closed loop circuit.  The solid darker lines are pressured fluid the dashed darker lines are return fluid.  We need to run a closed loop to for several reasons, but one of them is that it is very hard to filter fluid at 4000-5000PSI in two directions.  So how do we filter and the fluid?  -Charge Fluid.  Charge is make up fluid that fills the back side of the circuit refilling the circuit with cooled, filtered fluid.  As you can see in the schematic both the front and rear wheel motors leak off some fluid by design - dashed lines coming out of motors.

The flow in the diagram shows the circuit in Mow or when we are in 4WD.  If we flip the switch to go to High/Transport, PD1 &PD2 valves shift to block off flow to the rear motor and all the flow goes to the front wheel motors.  This is why we go faster in high.  Remember flow is speed and we are sending more flow to the front motors. 

One last thing I want to add is a pump and a motor in hydraulics are basically the same thing.  The only difference is that a pump is spun by an outside force and the motor accepts fluid.  This is why if you push a machine with a closed loop circuit the motors turn into pumps and produce flow until it hits what was the pump.  It now turns into a motor and has the resistance of whatever was spinning it.  That is why the wheels lock up, and why there is a bypass / tow valve on these machines.  When you open that up the closed loop goes to the path of least resistance and bypasses the pump allowing the fluid to spin freely through the circuit.  Only problem being that we are still bleeding off some fluid and now charge flow is coming in to make up for it.  This is why you should not tow the machine very far or fast or you can cause damage to the wheel motors. (you run them dry) 

In a future post I will cover the 4WD manifold in more detail.