Help me understand the HST

[ACKBill]

Skidoo:

I agree with the guys that say that you can't easily hurt the HST by hitting the relief valve as long as you don't sit there with the pedal mashed to the mat for a long period of time (loud HST whine). What happens is that the fluid heats up more quickly as it is forced through the relief cicuit instead of through the pump/motor circuit. A certain amount of bypassing through the relief valve occurs all the time during normal operation anyway (occasional quiet HST whine) so it's really no big deal. The HST transmission will handle this without any problem at all as long as you don't sit there holding the pedal mashed. You would really have to be oblivious to things (coolant temp & decreased performance for instance) to actually push the HST so far as to damage it.

The bottom line is that as long as you keep up with the regular maintenace (filters etc.) your new HST will last for a long long time.

BILL

 

[n8wrl]

I've been following this thread with interest. I do a fair amount of ground-engagement on my L5030HST. Typically this is pulling an arena drag in low speed. It takes about an hour and I do hear the HST whine over the engine but temps never rise. I'm assuming all is well, but the whine worries me. Should it?

-Brian

 

[ACKBill]

Most HST's whine, some more than others. Is your coolant temp rising more than you think it should?

 

[skidoo]

Thanks again for the responses. I did some experiments with the results as follows:

With the rear blade heavily engaged facing uphill (loaded tires), at 1000 rpm, it will decrease the rpm's as I increase the pedal travel and stall the engine if depressed far enough. At 2500 rpm, it does decrease the rpms slightly, but whines louder as I depress the pedal. It does spin the tires under heavy load on flat ground, except when it has great traction such as going uphill with the liquid filled tires. So, I must conclude that it is working fine.

I also loaded it such that it will barely move with the pedal depressed at 1000 rpms. I then increased the rpms to 2500. It worked no different. In other words, it seemed like the forces (lifting or pulling) were not any greater at the higher rpms. But, it did seem to have a greater pedal range and more opportunity to whine loudly. I also did a transport comparison. The higher rpms simply provided for faster speed.

So, I have concluded that in general, the forces, or torque are about the same from 1000 to 2500 rpms and that the pedal depression just controls the speed (flow) at which the work is done. The increase in rpms, provides for more power for which higher speed (flow) can be done.

If this is true, then indeed the HST is not quite like a variable ratio transmission. With a gear reduction, one would get a corresponding increase in torque to the wheel. But, the experiments above tell me that the torque to the wheels are about the same regardless of rpms or depth of pedal depression.

The experiments above were empirical observation only. So, I may have missed something that actual data could have revealed.

It looks like when doing heavy ground engaging work, about 1500 rpms will work well. Less chance to overheat the HST and if it won't move at 1500 rpms, then it will not likely move at 2500 rpms. I'll reserve the higher rpms for transport or mowing or such...


Dieselmotorhead, It is interesting the mention of a flat plate. My dealer could not tell me what was wrong with the old unit, but did say something about the operation now being smooth as opposed to choppy with an uneven plate. I do recall that before there were times where the HST shuddered a bit and then finally seemed to catch and move. Maybe a warped plate had something to do with that.

 

[Chris2520]

Skidoo , I disagree completely. i seen a graph somewhere. at 1500 RPMS the 2520 has not even reached its maximum torq curve. also you say running at lower RPM's is helping cooling. this is incorrect as well. tractors have direct drive fans. no clutch like a car. slower RPM's less air it draws over the radiator and---> Hydrolic cooler keep this in mind. also i doubt you have reached maximus pump presure. i cant say for sure. as i am not an expert on hydrolics. but what i can tell you from my own experience. is at 2000 RPMS and up. if i have extreamly heavy load in the bucket. i have better steering and responce than i do at low RPM's . so i know them pumps got to get spooled up. JD tells you what operating range is right on the tach. digging in dirt or pulling a heavy blade , would be no different than than running a large PTO implament. and it says run at 2600. i personaly find 2400 is a sweet spot for digging and 2100 best for running the hoe. at 1500 doing heavy digging i feel you are straining it. I know i been digging at a lower RPM and it started to bog. i kicked it up to 2500. and it plowed right through it. .
if i was you i would run it like it says to. Specialy since you done had trouble.
Just my view.
its your machine. so you run it the way you feel best....

PS , loved the pics of the tractor shed work.

Chris. ..

 

[n8wrl]

Most HST's whine, some more than others. Is your coolant temp rising more than you think it should?

Coolant temp is steady - no change after warmup. Good, solid pull - everything seems fine except for the whine, and I must have to get used to that. If everything uses the same coolant I'm probably ok. Just recalling days of towing where transmission temp in the truck had to be monitored seperately.

Thanks for the reply

-Brian

 

[skidoo]

I should clarify that the 1500 rpms are working well because I am simply doing things slowly. If work needs to be done faster, then the higher rpms are needed...

The engine torque may not be at peak, but it sure seems like the force (torque to the wheels, or lifting forces) are about the same. It just does the work slower. In that small area, work space is tight, so working slow was the order of the day... I'll do some more comparisons on another day in another spot.

 

[dynasim]

I would suggest looking up swashplate motor and swashplate pump on wikipedia. They have a an illustration of this variable speed pump and motor(my NH TC45D has both, most have one or the other). It is quite amazing.

For a given range, the maximum torque is independent of engine motor speed, in that the maximum torque is developed at the relief valve pressure. Slow engine speed leads to stalling susceptibility, or slower actuation.

Chris

 

[hemiguy]

skidoo,
This is a pretty long thread and I may have missed something and also I'm also not going to get too technical (even though I'm an engineer by profession - electrical not mechanical). All I can offer is my experience with 3 different HST tractors I've owned including currently a 2520. I use these on landscaping and small grading jobs and would offer the following advice
1) A very noticeable whine is always normal with any HST as others have stated.
2) I may have missed this but I certainly hope you are using low range for this ground engagement work
3) For this type of work you really should be running near rated RPM to get all of the engine HP you paid for when you bought the tractor. You would definitely want do the same with PTO powered attachments. If you spin the tires (in 4wd with or w/o diff lock) or stall it is simply a signal that you are taking too big a bite.
Lift up on your 3pt a little. These things can do they same work as a 75HP,15000lb dozer but its just gonna take a LOT LONGER. You have to be patient.
4) I have used my loader, 5ft box blade and 6ft rake in all types of soil including heavy clay and have never had the tractor just stop and whine. I would have the dealer check their HST work again (including the relief valve). But again, I would forget anything you've tried at less than about 2200 RPM.

 

[n8wrl]

...
4) I have used my loader, 5ft box blade and 6ft rake in all types of soil including heavy clay and have never had the tractor just stop and whine. I would have the dealer check their HST work again (including the relief valve). But again, I would forget anything you've tried at less than about 2200 RPM.

Exactly my experience. It either spins the tires or stalls.