Henry Guenter is a former service manager for Massey Ferguson. He has compiled advice on the workings – and dangers of hydrostatic systems.
Hydraulics involve stored energy. A break in a fitting or a line can send a blast of oil that can penetrate the skin and kill or maim someone.
I once watched a mechanic put his hand around a relief valve that was set at 2,800 psi. When it blew, it shot oil under his skin, and he lost his arm.
In hydrostatic drives, the pressure can be as high as 9,000 psi.
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Accumulators can have stored energy even when nothing is running.
As well, implement hydraulics will be dangerous when a cylinder is left up and loaded. This is especially true when left out in the sun for a while.
I’ve seen combine table lift cylinders expand just from the differential in day and night time temperatures. If a combine is parked in fall with the header all the way up, by next fall the producer might see a bulge in the cylinder. That’s an example of the power in hydraulics.
Combines and swathers built in the last 20 years are driven by a hydrostatic drive. One manufacturer uses a hydrostatic system to run the rotor.
A hydrostatic system has an infinite range of speeds within its limits. It is easy to change speeds and maintain constant engine rpm’s.
A hydrostatic drive is a pump that is turned by the engine connected hydraulically to a motor, which turns the wheels. The pumping action is from a round block of axial pistons with a movable slipper on one end, which rides on a movable swash plate.
As long as the swash plate stays level, as compared to the block, nothing happens. When the swash plate is tilted it starts to make the pistons go in and out as well as round and round, which causes the unit to start to pump.
The pistons that are going uphill on the swash plate are pushing against a set of pistons in the motor that are sitting on the downhill slope of the swash plate. The slippers want to slide down hill, which makes the block turn.
The block is attached to the wheels. When each one reaches the bottom of the hill they start going back up, pushing the oil back into the pump pistons, which are now going down hill.
There is no pressure on this side because there is no weight against it. If the machine was put into reverse, the swash plate would tilt the other way and now that side would have pressure.
Nine pistons in each block are doing exactly the same thing. As one piston leaves the pressure side, another one enters, so three or four pistons are doing all the pushing. This is a closed loop system and if there was no leakage, that’s all we would need.
Oil supply
There is always some leakage at the valve plates where the blocks meet the housing, where the slippers meet the swash plate and where they swivel.
A charge pump that puts oil into the low pressure side of the loop keeps the loop full. It does this using a shuttle valve that is pushed over by the high side of the loop. This opens the low pressure side to the pump.
The demands on the oil in a hydrostatic system are tremendous.
Imagine five sq. inches of metal carrying the entire weight of the combine, with a full hopper, across a soft field. The slippers have to be lubricated well enough so they will slide with all that weight on them, and yet not too much because then they will generate a lot of heat.
To handle this, the lube comes from the high pressure circuit through a hole running down the centre of the piston. As a result, when more pressure is applied to the piston by the weight of the load, there is more pressure in the oil circuit for squeezing into the slipper area.
The faces of this area meet so precisely that even a small scratch will cause a leak, and enough heat is immediately generated to discolour the valve plates. After that it won’t take long and the entire system will be destroyed.
Manufacturers say more than 80 percent of the failures happen because of the way the operator handled the oil.
Dirt could have gotten into the oil, or in a pinch to get running, the operator might have used old oil that contained tiny amounts of iron filings. The wrong oil could have been used or there might have been a dry start after a repair.
In the factory, the entire combine’s hydrostatic system is filled from the bottom up, and in the lowest part of the system are the hydrostatic motors. This is to make absolutely sure that the pump and motor are full of oil before starting the engine and that there is no air in the system.
Usually the first sign of trouble is heat. Hydrostatic drives should, unless the manufacturer says otherwise, run about 190 to 205 F.
I think the red light comes on at about 225 F. That’s brought down five to 10 degrees just by running in a lower gear, so the control stays in the top quarter of the range. The lower the gear, the lower the pressure.
Check relief valves
In many machines, bolted to the side of the pump is a big block with a bunch of plugs in it. Under the two largest plugs are a pair of identical forward and reverse relief valve cartridges. It could be that the relief valves are running too low because they are worn out or contain debris.
They can be switched around because the reverse cartridge is hardly ever used and is probably like new.
It’s a much bigger problem if iron filings are found.
Do not try to hook up gauges to this pressure port because it’s contents can be as high as 9,000 psi.
If switching valves did not solve the heating problem when running in a lower gear, producers should see how much wear they have in their pump or motor. This is done by checking the charge pressure.
This block has a charge pump and a shuttle valve. The pressure in the plug right beside it should be around 220 psi when in neutral. The pressure reading is not as important as what happens when a load is put on the hydros using the brakes. The pressure should be around 160 psi in either forward or reverse.
Leakage problem
If the pressure drops more than this, producers know that the leakage exceeded the capacity of the charge pump. If they find too much leakage, they will probably also find a lot of wear.
Unless they feel confident taking the unit apart, now is the time to call the professionals and tell them what was found. The mechanics will appreciate receiving this information.
There is one other potential problem – the operator could have difficulty finding neutral. That is when the gear selector is in neutral and the combine or machine wants to crawl away, which is dangerous.
The two canisters on each side of the pump hold a heavy spring. They should hold the swash plate in neutral when there is no oil pressure. The canisters can be set by a measurement marked on them.
There is a way to set them in the field, but I think that method is too dangerous. Instead, take off the unit and measure and set them equally so the swash plate is perfectly straight.
