Hydro gear vs Piston to piston

[pfrederi 17,193]

I have been thinking a bit (always dangerous) about hydro gear vs piston to piston Sundstrand units.  I have 4 hydrogears none of which have serious issues even though 2 have had their parking brakes ground down.  The piston to piston on my D200 had major issues.

 

I do not see the advantage to the piston to piston.  The weak spot is usually the brass slippers and valve plate.  They are highly susceptible to damage from parking brake shavings, dirt and other crud. In a piston to piston if the pump side is damaged you can be almost dead certain the motor side will have similar issues.  In a hydro gear the motor side had steel gears which are more resistive to damage by dirt crud etc..  This means in a piston to piston that has internal damage you have weak pump trying to drive an inefficient motor compounding your problems.  In the hydrogear the damage may not cause as big a performance issue as the motor side will still be reasonably efficient.

 

From a production standpoint making the slippers and piston block seem to me to be more complex (and expensive) than the gears.  (Gear pump and motors were widely used for years)

 

Wonder why WH changed.  Also if i wind up with another messed up piston to piston I may put in a hydro gear.

[oldredrider 2,548]

Time seems to be the problem in getting answers to questions like this. Age has taken away the engineers who designed these components. Unfortunately for us, because old blue prints and decision makers of the time have disappeared, we may never know these answers. Perhaps reverse engineering is the only hope available to decipher why certain decisions were made.

[Aldon 4,826]

Well I'd like to hope someone chimes in. I'll keep tabs on this thread.

[Sarge 3,462]

All the original Sundstrand engineers around here are long dead as far as I know , as well as most of the factory workers . It seems they had a very low turnover rate and it was pretty much all older folks that worked there . It's a shame that 20yrs ago someone didn't have enough forethought to go ask a lot of questions or possibly get some more detailed info from them - years ago in the gearhead community around here there were several Sundstrand employees and one was an engineer that designed most of their equipment . Odd part is - there is very little of Wheel Horse equipment around here in comparison to areas east of Illinois .

 

Here's a snippet of some general research -

" During the early 1960s, a team from the company's Hydraulics division experimented with applications of CSD technology on off-highway equipment. To commercialize these applications, the company merged this team with another from its Aviation division to form a fourth operating unit, Hydro-Transmission. In 1965 Sundstrand built a separate facility in LaSalle, Illinois, especially for this division, and began supplying hydrostatic transmission components to the automotive industry. This operation was subsequently expanded to include yet another facility, located in Ames, Iowa. "

Source: http://www.fundinguniverse.com/company-histories/sundstrand-corporation-history/

 

As with many of the older businesses that built the high quality machinery like Wheel Horse and Sundstrand - they are no more of what they once were , taken over by the corporate ideal of quick profits...sad .

 

Sarge

[R. L. Addison 299]

Agreed. I like the piston>piston because it is more flexable, more servacable, and I think more durable than a geared unit. Usually (at least in my experience) when a geared unit died, it was beyond resurection and replacement was the only resurection. Also the piston type is is much more variable in many ways. A piston pump can  go to no output with centralization of the swash plate, and require almost no power until the demand arises. (Big power saving for industry) 

 

Once a chip goes through gears in a geared unit usually it will wipe out the gears and pockets and unless a cartridge is available (I have never seen one)  the unit cannot be repaired practically. I have seen this in several hydrogear units. The castings in the piston to piston rarely are damaged is my experience. Therefore industry goes to the cheapest and most practical direction. Most of the pump's in my early past were Vickers vane type but as time went on more piston type pumps and motors were taking over. My $.02.

[pfrederi 17,193]

5 minutes ago, R. L. Addison said:

Agreed. I like the piston>piston because it is more flexable, more servacable, and I think more durable than a geared unit. Usually (at least in my experience) when a geared unit died, it was beyond resurection and replacement was the only resurection. Also the piston type is is much more variable in many ways. A piston pump can  go to no output with centralization of the swash plate, and require almost no power until the demand arises. (Big power saving for industry) 

 

Once a chip goes through gears in a geared unit usually it will wipe out the gears and pockets and unless a cartridge is available (I have never seen one)  the unit cannot be repaired practically. I have seen this in several hydrogear units. The castings in the piston to piston rarely are damaged is my experience. Therefore industry goes to the cheapest and most practical direction. Most of the pump's in my early past were Vickers vane type but as time went on more piston type pumps and motors were taking over. My $.02.

 

 

But in a hydro gear the pump side is a piston pump and functions just the same and in a piston to piston.  (the swash plate goes flat and there is no output...

[R. L. Addison 299]

Yup. I agree. That doesn't happen with a fixed displacement pump. Unless you had a bleed-off system to dump total pump out-put there would be no control, be just like a solenoid controlled system, with open center to reservoir , either go or stop. 

 

[953 nut 52,177]

On my one and only Sundstrand Hydro-gear unit in a '70 GT-14 the problem child was the acceleration valve springs were broken and without them it had very little power. It was so bad that the broken springs had destroyed the valve bodies. I changed it out with a good unit from a Charger and it came back to life, had to change the output gear because the GT- 14 has a different final drive ratio than the Charger/Electro have. I have nothing but praise for the Hydro-gear, never had a piston to piston unit, but the Hydro-gear would push a house down if it could get enough traction.

[R. L. Addison 299]

Not a thing wrong with a good hydrogear unit at all. I've had several that performed very well.

[DennisThornton 4,769]

Figured it would happen but I didn't know how long it would take.  Now there's a thread that I don't understand at all.  I own a few but man am I hydro ignorant!  Where do I go for hydro 101? 

[pfrederi 17,193]

11 hours ago, DennisThornton said:

Figured it would happen but I didn't know how long it would take.  Now there's a thread that I don't understand at all.  I own a few but man am I hydro ignorant!  Where do I go for hydro 101? 

 

 

Check out the Sundstrand  manual here for description of the two systems

 

http://www.wheelhorseforum.com/files/file/24-transmission-hydro-sundstrand-sm-492-4206pdf/

[Sarge 3,462]

Hydraulics is a different animal and it's a common misunderstanding that they are weaker than a manual transmission . Pressurized fluid can move mountains if everything is sealed correctly . I have a C-160 that the original owner actually took great care of , changing the hydro oil yearly and he said a few times even twice as it was worked hard over a hot summer . Wheel Horse filters , every time as well . The day we tried pulling a dead truck with it on concrete it actually started to smoke the tires , until the belt broke anyway...lol . My old 1277 with the old style pump and the 2-pinion automotive style differential has pulled well over 2 tons - to the point the drive belt gave up and had to be towed a block home on the front axle - rear axle in the bucket of a Deere CUT ....

 

Both pump designs are fine - it's the pickup circuit to the charge pump in my opinion - with only a fine screen to filter any debris from wear or broken parts such as those springs . If they had only designed those circuits slightly different they would have a much better chance to live .

 

Sarge