Hydrostats: How they work

[J-Mech]

There seems to be a lot of questions here about hydrostats. How they work, how they should work, what they are capable of, and how to repair them. Thought I’d try and help everyone understand them a little better. I will cover some basics of hydrostatic transmissions. Then, describe specifics about the ones used on the cub cadets.

Now, before we get too deep into specifics, let’s break it down. Hydraulic pumps come in many different styles, sizes, and capabilities.

Positive and non-positive displacement:

Non-positive displacement pumps output flow will slow down or stop altogether depending on restriction to flow. An example of this kind of pump is a water pump (impeller pump) on your car engine.

Positive displacement pumps output flow will deliver an exact amount of fluid and continue to do so regardless of restriction to flow. Examples of these types of pumps are a piston pump, gear pump and vane pump.

Fixed displacement and variable displacement:

A fixed displacement pump moves the same volume of oil with every cycle. The volume is only changed when the speed of the pump is changed. Volume can be affected by the pressure in the system, but this is due to an increase in internal leakage in the pump. This occurs when pressure rises.

A variable displacement pump can vary the volume of oil they move with each cycle, even at the same input speed. These pumps have an internal mechanism, which varies the displacement and thus the output.

Note:

Displacement is the volume of oil moved or displaced during each revolution or stroke of the pump. It is usually expressed in cubic inches. The flow is expressed in gallons per minute.

Remember: A hydraulic pump does not create pressure; it creates FLOW. Pressure is caused by RESISTANCE TO FLOW.

Gear Pumps:

Gear pumps are a positive, fixed displacement pump. Gear pumps can suck. What I’m referring to is, they can draw the fluid up out of the reservoir. The piston pump used to make the power cannot “suck”, or draw its own oil. (We will cover that next.) The gear pump on these systems has several functions. 1.) It provides the hydrostat the fluid it needs to do its job. 2.) Provides extra fluid for cooling and lubrication. 3.) If your machine has hydraulic power lift or remotes, it provides the oil for those functions. This is what we call the “charge” pump because it charges the piston pump with oil. It is located at the very front of the hydrostat unit where the driveshaft hooks up.

gear pump.jpg

Parts:
Gear pumps don’t have many parts. They are made up of a gear and the rotor. (See pic above)

GP1.jpgGP2.jpg
In this first pic, you can see the internal and external gear. The second pic is of the housing. Both gears turn inside the housing. The pump pictured is from a cub hydrostat.

The center gear is attached to the drive shaft of the motor and turns anytime the engine is on. As the pump rotates; one side of the gears separate, drawing oil into them. Then the oil is carried in the cavity until the gears come back together where it is forced out the outlet of the pump. The oil then travels to the piston pump.

Piston pumps:

Piston pumps are a positive, variable or fixed displacement pump. Piston pumps cannot “suck”, or draw their own oil into them. They have to be “charged”, or supplied with pressurized oil from another pump. There are two kinds of piston pumps: the radial piston, and the axial piston. The hydrostat in the cubs use a axial piston pump

piston pump.jpg

The cylinder block is attached to the drive shaft. On the cub hydrostats, this is the same shaft that goes through the center of the charge pump. As the cylinder block rotates the pistons are riding on a “ramp” known as the swashplate. As a piston travels down the ramp, oil is forced into the pistons bore from the charge pump. Then, as the piston starts up the ramp oil is forced out of it and directed to the hydrostats motor. The swashplate in the pump is what controls how much oil is moved, or which direction it goes.

The swashplate is what is directly connected to the lever on your dash that you control the speed and direction of the tractor. When you move the lever, you are changing the angle of the swashplate. The angle of the swashplate adjusts flow. The steeper the angle, the more it will flow and the faster it will move. The swashplate also controls direction (see pics below)

2012-08-05 16.12.33.jpg
In this pic: Top is with the swashplate in the forward position. Middle pic is neutral. Lower pic is reverse.

Hydraulic Motors:

I will not go into these. Just know that the motor that your cub hydrostat uses is a positive, fixed displacement motor and looks EXACTLY like the pump. The only difference is the shaft that runs through the cylinder block is the shaft the goes into the small gear at the back of the unit, which drives the reduction gears.

Motor.jpg

Although it is hard to tell from the pictures, the motors pistons ride on a ramp that is made into the housing of the unit itself, and is not adjustable.
The motor itself works in just the reverse as the pump. As oil is forced into a pistons bore, it pushes the piston down the ramp causing the cylinder block to turn and consequently, the reduction gears, differential and then the tires. When the piston reaches its max travel, the rotor continues to turn forcing the used oil out of the pistons bore where it enters a passage way and returns to the pump. The process is then repeated over and over …….

So, now we know that the hydrostat uses a positive, fixed displacement gear pump for charging, a positive, variable displacement piston pump for power and a positive, fixed displacement motor for drive. Here is how the hydraulic schematic would look.

2012-08-05 16.12.08.jpgschematic 2.jpg

Some other stuff:

There are three other valves in the system. One is on the charge pump and is a system relief valve. It’s a relatively low setting. Service manual says 500-625 PSI. That is the maximum pressure that the hydraulic accessories (aka, hydraulic lift and remote outlets) have to work with. Understand also, that this gear pump is relatively small and does not produce much flow, or GPM. The other two valves are for the hydro itself. One is on the foreword circuit. The other is on the reverse circuit. They have a much higher relief setting. (The manual doesn’t state it) The service manual calls them check valves.

Outside parts:

Now that you know how the inside of your hydro works, or, at least I hope you do . Let’s take a look at the outside.
On the outside you will see cooling fins made onto the unit itself, a fan on the driveshaft, and a bunch of linkage.

The hydro unit is air cooled via the cooling fins and the fan. It is important that you keep the hydrostat clean, as a build-up of dirt, oil and grass will prevent it from cooling properly. The fan on the driveshaft is of the most importance, as without it there is no air moving across the unit to keep it cool. The fan, when installed properly blows air TO THE REAR of the machine, across the hydro and out the back.

The internal swashplate is connected DIRECTLY, via all that linkage, to the handle on the dash. When you move it, you move the swashplate. Now, understand that this linkage takes a severe gear reduction as the amount you move the lever on the dash really doesn't move the swashplate very much. It is of the most importance that this linkage be: free moving, have no play or “slop” in it, and be adjusted properly. If the linkage is not tight (no slop) it will take more hand lever travel to get your machine to move, and then, once moving, the swashplate may push against the linkage until the “slop” is again tight and the machine will speed up on its own.

Repair:

When having trouble with your hydro-static unit, do these things first:
1.) Change the oil and filter on the unit. (see technical section on changing hydrostat oil)
2.) Adjust the linkage, replace as necessary (see technical section on adjustment)
3.) If the unit still does not work properly, and you are sure that the adjustments are correct, then it may have an internal problem. At this point, consult the service manual for your machine and test the hydro as stated in that manual. Repair as necessary OR find another unit.

As long as the oil and filter is changed regularly, the unit shouldn’t have an internal failure. Most of the problems with these units is external, or at least able to be fixed externally.

If you are not very “mechanically inclined” DO NOT, I repeat DO NOT attempt to tear this unit apart. The inside of the hydrostat units are very sensitive to dirt and the tolerances are very tight. MUCH tighter than in an engine. If you do decide to work on it, make sure that you have a VERY clean work area and possess the proper tools for the job. And PLEASE, use the service manual.

Thanks for reading!
Jonathan AKA J-Mech

Sources: Power Trains, Hydraulics. Both are Fundamentals of Service books produced by John Deere. And of course, pics from stuff at the shop and knowledge I've learned over the years