PT hydraulic drive vs. CUT HST?

   / PT hydraulic drive vs. CUT HST? #1  

davesisk

Platinum Member
Joined
Apr 18, 2002
Messages
864
Location
Raleigh, NC USA
Tractor
Massey-Ferguson MF 1220
Tech question, here. I understand how the hydraulic drive on our PT's work: hydrostatic pump and 4 wheel motors, pretty simple in concept. What's different about the HST drive that you find on many (traditional) compact utility tractors? I'm going to make a wild guess, and someone correct where I'm wrong. I'm guessing that there's still a gear-shift, differentials, etc., but that the main drive shaft is turned by a single hydrualic motor rather than having a direct mechanical connection to the engine. Is this correct? That way, you have the ability to select gear ranges while still having the ability to press the right pedal to go forward/left pedal to go backward. If this is correct, I'd also assume that you have some potential loss of traction due to the differentials (unless they can be locked up), where with the approach of using 4 wheel motors you do not (at least in theory).

When I was looking, I drove a few manual tractors (although I didn't drive any HST models because they were usually out of my price range) and I have to say, I really like the hydraulic drive, it's fantastic for my small property with a maze of trees...

Can anyone educate me on the mechanical differences?

Dave
 
   / PT hydraulic drive vs. CUT HST? #2  
Here's a link to a great discussion on how HST works <A target="_blank" HREF=http://www.tractorbynet.com/cgi-bin/compact/showflat.pl?Cat=&Board=genoperating&Number=99598&page=&view=&sb=&o=&vc=1>click here</A>
 
   / PT hydraulic drive vs. CUT HST?
  • Thread Starter
#3  
Hey Moss:

Yup, I've read all those before, but I guess I was thinking that there was some difference between the pump + 4 wheel motors on our PT's vs the HST tranny's you'd find on (say) a Kubota. I was thinking that the HST's on some CUT's (enough acronyms there?) were some combination of a mechanical transmission that's turned by a hydraulic motor rather than being turned directly by the engine (in other words, some combination of hydraulic and gear drive). Perhaps I was just day-dreaming about tractors again...<g>

Dave
 
   / PT hydraulic drive vs. CUT HST?
  • Thread Starter
#4  
Also, btw, I'm just curious what "shuttle-shift" trannys are. I'd assume this is a manual gear-drive + synchronizers (so that you can shift gears while the tractor is actually moving). Am I guessing right? Or am I way off?

Dave
 
   / PT hydraulic drive vs. CUT HST? #5  
I'm not sure. I know my big IH2500b has a 4 cylinder gas engine that turns the guts of the HST transmission, which is a two speed. The guts is just a huge pump, but it is contained within the transmission. The rear end is turned by hydraulic force that is generated by the pump. I never got into it more than the discussion thread that I linked to in the earlier post. All I know is that thing is powerful enough to knock a house down. In low range I was pushing over 12 inch trees that were easily 50+ feet high with the FEL just by driving up against them, locking the differential and stepping on the forward pedal. Over they went /w3tcompact/icons/smile.gif. Of course, 8000# of traction helps compared to the 1100 pounds of our PTs /w3tcompact/icons/tongue.gif
 
   / PT hydraulic drive vs. CUT HST?
  • Thread Starter
#6  
8000 lbs, eh? That is a big 'un!

Hmmm, thanks for the description. Sounds like there's definitely some combination of hydraulic and gears in the IH. Maybe someone with some knowledge on this type tranny will chime in...

Dave
 
   / PT hydraulic drive vs. CUT HST? #7  
<font color=blue>Also, btw, I'm just curious what "shuttle-shift" trannys are. I'd assume this is a manual gear-drive + synchronizers (so that you can shift gears while the tractor is actually moving).</font color=blue>

Not all shuttle shifts are synchronized. Shuttle shifts allow you to shift from forward to reverse or just the oppisite while the tranny is in any gear. If not synchronized you do have to depress the clutch and come to a complete stop before shifting. Well if you get real good at it you could get the rpms and your timing just right and manage to 'shuttle shift' without grinding the gears. I haven't gotten that good yet. /w3tcompact/icons/smile.gif
 
   / PT hydraulic drive vs. CUT HST? #8  
Your description of the HSTs sounds about right. My 4300 has a 3 speed (plus neutral) manual shift gear box on the output of the hydrostatic drive to change ranges. The rear axle differential can be locked by stepping on a pedal. My question is about how the system on the PTs works. Are the hydraulics for the wheel motors piped in series or parallel? It seems to me that if the hydraulic wheel motors on each wheel are piped in parallel that you effectively only have one wheel drive. Whichever wheel turns the easiest would get all the oil flow. If the wheel motors were piped in series then all the wheels would have to travel at the same speed. This would make the tires scuff badly while turning but would give much better traction. Now that I think more about this, I wonder if they are setup is a series/parallel combination. Maybe the wheels on each axle are in parallel with each other but the front axle is in series with the rear axle. This would allow the wheels on each axle to travel at different speeds while turning but the combined speed of the front wheels would have to be the same as the combined speed of the rear wheels. This would effectively give you two-wheel drive, one front and one rear. So now you've got me curious./w3tcompact/icons/smile.gif How does this really work?
 
   / PT hydraulic drive vs. CUT HST?
  • Thread Starter
#9  
I *think* you're assumption is correct, it's series/parallel with the front/back deal you described.

Dave
 
 
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