Two types of hydraulic steering?

[1stDeuce]

I participated in a thread this weekend about steering, and it appears that thread has now been deleted, but I wanted to ask those in the know about tractor hydraulic steering.

Several people had suggested that if the tractor wheels won't turn, or they are against the steering stop, their steering wheel can still be turned, which was the thread poster's problem. I fessed to never having tried this on my MF, but my recollection was that I could not turn the wheel if the front tires would not turn. (IE, really heavy on the front and trying to steer while stopped.)

Yesterday, while doing loader work with only the box blade for ballast, I had the opportunity. On my tractor, when the hydraulic pressure is not enough to dry steer, I can NOT turn the steering wheel either. I hogged on it pretty good, whereas in videos, it didn't appear it took much effort at all to continue turning the wheel even after the tires stopped turning.

My tractor has full hydraulic steering, not linkage based steering with a steering gear.

Are there two different kinds of hydraulic steering? I've seen systems advertised as "hydrostatic", and posters suggest those are the ones where the steering wheel can be turned even after the tractor wheels stop turning.

I also noticed that I can steer my tractor with the engine off, with some effort of course, like a car.
So, what kind of steering does my tractor have?

School me.
And thanks!

 

[fatboycowen]

I'm wondering too. Good question.

 

[bigtiller]

Interesting question, my jd 2720 is like that.

If I an trying to jocky a heavy loan on my pallet forks into just the right spot, the front wheels will quit turning soon after I stop moving forward/backward. Sometimes I need to backup 6" to move the load over 2" and I can't do it. I have to back way back and come at it again.

 

[David_Kb7uns]

Of the mechanical link types of steering I have seen power assit steering where there is a cylinder mounted somewhere on the linkage, power steering where the hydraulics are in the steering gearbox and hydrostatic steering where there is a valve the steering wheel controls with only hoses going to a cylinder on the axle. On the hydrostatic system the valve turns into a hydraulic pump when engine is off and steering still may be done but with a high effort. Some leakage of the hydrostatic system may occur and the relative position of the steering wheel drifts.

Electric power steering is being used more often on ATVs hybrids and other equipment where a motor and ECU box provides the power.

Have a G2160 Kubota lawnmower and it has electric power steering and all the other Kubota equipment is hydrostatic.

David

 

[1stDeuce]

So my steering system is hydrostatic then, and I just have a "tight" steering valve, so very little leakage. I get zero drift of the steering wheel, from what I can tell.
Are newer tractors getting steering valves with a LOT of leakage then?

I watched a few vids that show people fairly easily turning the steering wheel with the front wheels stationary... Doesn't seem right to me.

 

[David_Kb7uns]

Sometimes hard to tell if the valve is leaking or the cylinder is. On the Kubota BX series there was a known batch of bad cylinders and when they go your spinning the wheel quite a bit.

David

 

[CalG]

IF the steering wheel turns without moving the wheels, something is defective.

Think how "un safe" that would be.

 

[kthompson]

The steering where there is no linkage but fully hydraulic between the steering wheel and the cylinders has been around I know for over forty years at least on some heavy equipment. Any machine that has a steering wheel in a cab that can rotate of course can not have linkage the full system. If you have a spinner knob on those systems you may notice they do not always line up the same way (the wheel).

I have a Kubota and JD with the hyd steering system and agree with CalG comment above. You should be able to steering your tractor when the engine if off with the full hyd system albeit very hard to. I have a "B" with hyd assist and it does have linkage the full steering system.

 

[TractorGuy]

My backhoe is full hydraulic and I think the seals in the steering cylinder are bypassing. It has a hydraulic motor on the steering wheel shaft that directs pressure to the steering cylinder. I can spin the steering wheel indefinitely when the wheels are at lock. If the tractor is sitting still with a load in the loader bucket the wheels may or may not turn at all. The more load I have on the front the more input I have to put in to get it to turn and usually have to be moving. Sometimes when I am stationary and the wheels are pointing the wrong direction I will lift the front wheels off the ground with the loader and turn them in the direction I want to start in. I have performed the pressure test outlined in the service manual and it passes so that is why I think the problem is in the steering cylinder. I only use the thing in the field on my property so I have learned to live with it for now.

Have talked to another owner with a New Holland 655 and his turns to lock and stops. Our tractors are built on the same platform and have the exact same steering system so I know mine has issues.

 

[zzvyb6]

There are actually 4 types of hydraulic power steering:

Open center with valve no t-bar (torsion bar).

Open center with valve + t-bar

Closed center with valve and t-bar

Closed center with valve and no t-bar.

The differences:

Open Center means oil flows all the time, the valve is a diverter sending oil to either side of a piston which provides force to cause turning.

Closed Center means oil flows only when the valve is opened.

The torsion bar is a precision specified part in the valve body which regulated how much steering force is from the t-bar itself and how much is to be provided by the hydraulics. There are stops on each side of the valve body which prevents you from breaking the t-bar in case rim force exceeds the maximum allowed twist in the valve body.

With the t-bar case, when you apply a force/moment to the steering wheel, the road wheels have not moved yet, so the valve is opened to provide assistance. The assistance turns the road wheels and the valve closes back to almost zero opening with the remaining rim force being provided by the t-bar. This gives you some "road feel". This has been the most common type of steering in cars and trucks.

Next case is the same device without a t-bar. Road wheel response closes the valve when the steering wheel angle commanded road wheel position is achieved. A lot less road feel with this type. Early Dodge and Chrysler vehicles had this type of assist. So, you could spin the steering wheel forever because no mechanical connection or limiter stops were in place.

Open center means oil is always flowing and bypasses the valve if its closed. Crack the valve and oil starts pushing. Response is felt almost immediately.

Closed center means oil in not flowing from the pump (it recirculates within the pump) until the valve opens and pressure builds up in the boost chamber. Response is not as fast as open center because the fluid inertia slows the pressure build-up. If you are driving a car VERY fast, this type of steering assist can be nerve racking. However, work by the pump/engine is only done when oil moves, so it helps fuel economy.

There are big cost advantages to the no-t-bar systems. Valve body profiles and t-bar diameters are required with .0001" or better precision. That takes some serious machine complexity and also requires sorting at the steering gear assembly plant. "curb-pushaway" is a required test to see what might break if you yanked on a steering wheel while parked with wheels parallel to the curb and touching. You are not allowed to break wheels, knuckles, tie rods, gear parts, i-shafts joints or steering wheel spokes. Bend, but not break.

There have been some models which have A/C clutches on the steering pump, so at low speed you get power steering, at high speed you don't, but people complain about the sloppy precision in driving the vehicle.

There are some tractors which have linear valves on the pitman arm coming off a manual gear instead of having a rotary valve, but the principles are the same.