Adding power steering - why a separate pump?

[rectifier]

I'm planning to replace my old B-275 steering gear (worn out and was always hard to steer anyways) with a hydraulic setup pulled from a scrap combine.
Looking at assorted PS drawings, I think I have all I need:

- flow from my main hydraulic pump
- control unit ("orbital valve") and wheel
- double acting ram
- hydraulic lines

and that's it. Seems too simple!

So why do so many systems have a separate pump or splitters/load sensing valves to run the power steering?
As far as I see it, these things are open center just like a spool valve. Why can't they just be stacked with my spool valves in the main hydraulic circuit? There must be something I am missing?

 

[J_J]

In any steering circuit, you want steering flow available at all times.

Mainly for safety reasons. :You do not want the main flow to take flow from the steering or you be in trouble.

So in a single pump tractor, the flow is usually split so the steering has priority over all other hyd.

In a two pump circuit, the steering is independent or the attachment hyd .

If yo are going to use your main hd pump flow, yo need to spslit the flow using a priority valve .

You can find these at Surplus Hydraulics.

Select the valve for the GPM you want the steering to have.

 

[rectifier]

Makes a lot of sense. The only thing I wonder is if you put the steering valve first in the circuit, wouldn't it be able to access the main flow and starve the remotes instead of the other way around? I'm no hydraulic expert so maybe it wouldn't work this way.

On this little chore tractor, I never operate any hydraulic motors - only the cylinders on the loader, or the 3 point hitch via hand operated levers which return to center when you let them go.

In this case, would it be safe to share the flow directly? I can't think of very many cases in which I would be operating the cylinders and steering at the same time. In fact right now I require both hands on the wheel to steer at all, so I never can touch the levers!

 

[J_J]

Steering usually operates on different pressures, in the 1000 to 1500 psi range.

This is a splitter /priority valve .

2/30 GPM PRINCE PRIORITY FLOW DIVIDER W/RELIEF

 

[rectifier]

So, does using a valve like this result in the permanent loss of 2 GPM into the steering circuit, even when the wheel is not being turned?
Unfortunately the tractor has a weak pump as it is, it only is rated for about 6 GPM.

I can pull the PS pump from the combine as well, but there is no space to mount it on the B-275. I was also considering an electric pump from a car, I believe some Toyotas had them. That way I would be free to mount it anywhere I choose.

 

[J_J]

Yes, that is dedicated fluid just for the steering.

My Power-Trac uses a fan belt driven pump for the steering and loader and works quite well.

You can do the same.

The pump cost about $130.

Pump is very small.

 

[boggen]

looking at... TractorData.com International Harvester B-275 tractor information

i would say separate steering pump. mount it to front if you have some sort of shaft you can connect to out in front, or make a bracket and get a larger belt for the fan/coolant pump, alternator, steering pump...

i think the steering would get way to "jerky" trying to use FEL / 3pt hitch at same time, as trying to steer, when using a single pump.

one moment ya thinking ok just a little bit of turning of steering wheel. and then ya do something with FEL or 3pt hitch. and suddenly wheels turn a good amount more than expected. or they do not turn enough.

safety reason... when ya loose power for what ever reason.... what'cha going to do? a separate isolated steering circuit. should keep you in control of the steering.
having things together on same pump = you just lost steering completely and no control over which way the wheels suddenly turn on you, if they turn at all.

separate pumps = more GPM, and getting PSI were needed for both systems. or rather a more responsive system. wanting to say raise 3pt hitch up and turn at same time with a single pump. may actually mean stopping as you wait for 3pt hitch to rise enough, and then once done, put the rest of GPM to steering quicker.

 

[rectifier]

Yeah, that's about where my tractor's existing pump is mounted right now. There really isn't anywhere else to mount a belt-driven pump available - the tractor has side fairings over the engine that don't give me space to hang extra stuff off. The commercially available PS kit replaces the factory pump with a fancy 2-circuit pump which costs $1000+.

I suppose I could swap my old pump for one that puts out more GPM, and use the priority valve. How many GPM does the pump you are using put out?

 

[rectifier]

loose power for what ever reason... ...you just lost steering completely and no control over which way the wheels suddenly turn on you, if they turn at all.

Won't the steering valve operate as a manual pump in this instance? Unless of course you have lost all the fluid, in which case... things are going to be bad.

 

[J_J]

rectifier

I suppose I could swap my old pump for one that puts out more GPM, and use the priority valve. How many GPM does the pump you are using put out?

3 GPM at 3600 rpm.