Why Aren't PTO's Hydraulic?

[glennmac]

Why Aren\'t PTO\'s Hydraulic?

Why have we been shafted instead of oiled?

It seems that hydraulics power all the heavy lifting attachments: buckets, hoes, blades, cranes, derricks, etc. So, why aren't tractors designed to transfer power from rear and mid PTO's via hydraulics instead of rotating shafts.

Hydraulics must be more powerful, given their use on all the heavy lifting stuff. The attachment would also be easier to perform. A few light quick-coupling hoses instead of heavy, clumsy shafts. You have to be a real contortionist to get the shaft on a belly deck, a process that gets real old as you get real old.

Actually, Ingersoll makes a line of garden tractors that are completely hydraulically driven. I have always been impressed by them. They even give a lifetime warranty on the system. I don't know whether they put hydraulic PTO's on the larger Case tractors when Ingersoll was part of Case.

In a sense, this question is somewhat academic. Tractor manufacturers would now be deterred from offering hydraulic PTO's because they would be incompatible with the universe of shaft implements that now exist. But why didn't they offer hydraulics originally, or did they?

Glenn

 

[Harv]

Re: Why Aren\'t PTO\'s Hydraulic?

Glenn -

Just a wild guess here, but since you ultimately need to translate your hydraulics into rotary motion for most implements, you would be shifting that task to each implement. I assume that would make them more expensive.

Like I said, just a guess...

 

[Rowski]

Re: Why Aren\'t PTO\'s Hydraulic?

Glen,

I'll take a guess too... Cheaper (less complex) to design, maintain, repair/replace?

Derek

 

[Bird]

Re: Why Aren\'t PTO\'s Hydraulic?

I'm not real sure, but I think John Deere has a hay cutter that has a sickle bar operated hydraulically instead of via PTO. I suppose it would be a relatively simple matter to put a hydraulic motor on each implement instead of the current gearbox, but I'll bet it would cost a lot more, and then of course, your tractor would have to have adequate flow rate to operate it. In other words, I think it boils down to cost (and if someone knows better, I'll admit to not really knowing)./w3tcompact/icons/wink.gif

Bird

 

[Mike H]

Re: Why Aren\'t PTO\'s Hydraulic?

The comments on cost are accurate but a hydraulic PTO would probably be less powerful. All the energy in the tractor is coming from the combustion chamber. Every time the energy is converted, there are losses. Why add losses in the hydraulic system when you can connect right to the transmission? The losses are things like friction in the pump, line losses in the hydraulics, etc. Hydraulics are wonderful when your energy source (engine) and desired work (loader) are distant enough that connecting them mechanically is expensive.

 

[Anonymous Poster]

Re: Why Aren\'t PTO\'s Hydraulic?

FYI - Visit http://w3.one.net/~wduck/ for the history of the Colt/Case/Ingersoll garden tractors.

 

[Anonymous Poster]

Re: Why Aren\'t PTO\'s Hydraulic?

Don't know why the above link doesn't work. You'll have to copy & paste, sorry.

 

[scruffy]

Re: Why Aren\'t PTO\'s Hydraulic?

Dutchman, your above link worked just fine for me.

 

[TomG]

Re: Why Aren\'t PTO\'s Hydraulic?

The big augers on digging trucks that install electrical utility poles are hydraulic. I think I've heard of some smaller (maybe 3ph-post hole augurs) that are hydraulic. There are advantages to having something 'soft coupled' to the engine. However, augers on the digging trucks turn very slowly. I'm not sure how well a hydraulic engine would manage at 540 or 1000 rpm PTO speeds, and the engines certainly would be more expensive than gears.

Maybe the reason has to do with open centred hydraulic systems. Open centred systems flow oil through the system continuously. Oil is diverted to hydraulic cylinders on demand. My 8 gpm tractor pump is a not very big thing on the side of the engine. My 24 pto hp will run a pto pump somewhere in the 20's I think. So, to get full pto hp into the hydraulics, I'd need a very large pump on the engine that would just push a lot of oil around most of the time.

 

[art]

Re: Why Aren\'t PTO\'s Hydraulic?

There are losses that come with hydralic power as stated in the other posts. There is less horsepower to the wheels of a hydrostatic drive tractor than a gear drive. This would be the same as on a hydralic pto. Hydralics are expensive to build as well as repair costs and diagnostic time to fix. Post hole diggers are at there best when hydralic drives are used as when you catch a stone you can back it out instead of fighting to get the auger free. Larger tractors are using hydralics to engage the pto but it is still a positive drive with a shaft. If the numbers would be the same for a hydro trans as the pto for power loss you would be looking at a 7 to 10% power loss.