My Senior design project - a "Swamp Thing" tracked vehicle clone

[Fallon]

I've always heard HST transmissions are 10-15% less efficient than gear machines. At least with regards to straight power delivery. That's why they tend not to be used used on machines bigger than CUTs. Beyond that tends to be ag machines where the utility of a HST isnt nearly as handy & fuel/power efficiency is. Really HST is just an optimized & nicely matched pump & motor in the same package. But the optimizations & very short internal only plumbing is going to mean it's a bit more efficient than a generic pump & motor.

A torque converter is kind of a pump hooked to a motor. But modern ones lock them together except when starting moving & shifting for minimal efficiency losses.

I assume you could diminish or eliminate the dynamic braking aspect on a road going vehicle. But why go through the effort when it's still less efficient. The dynamic braking, complete lack of clutches & some of the other behavior is a bigger bonus in the off-road world than on-road world though.

 

[aczlan]

Generally speaking, a hydraulic system (IE: pump and motor) is about 80% efficient.
That's fine for smaller stuff, but it's unacceptable for large ground engaging equipment which is why they don't use it for larger equipment.

Aaron Z

 

[ArlyA]

Hydrostatic transmissions vary quite a bit in efficiency depending on where the are in its power curve. It varies from about 15 to 30% loss. Here is great video another fellow posted that shows how they operate, but not how efficient they are.. So yes, if you want X torque to move your machine forward, you must add HP to compensate for the Hydrostatic transmission....

Tigercat Hydrostatic Demonstrator Part One - YouTube

 

[balsam]

Thanks ArlyA: I have seen that video a number of times. It’s very well done.
Hydraulics often introduces inefficiencies but that has to be weighed against mechanical ineffiencies of mechanical systems where a lot of gearing is used to reduce the speed and increase the torque between an engine and wheels or track.
I’ve seen huge motors, delivering thousands of ft.lbs. of torque to winch drums or bull wheels on ski lifts simply because they are more efficient and provide easy changes in rotation and speed. It’s the faster lighter machinery where the mechanical gear boxes shine. Hydraulics is best for great lumbering machines or where compactness and remote force is needed, such as the wing flaps on a 747.
Love discussing this stuff. Learning all the time.

 

[ArlyA]

Mechanical transmissions ak gear boxes, operate with looses in the 3 or 4%+- range. That's pretty hard to beat. CVT's run a tad higher losses at 4 to 6%.

Thanks ArlyA: I have seen that video a number of times. It’s very well done.
Hydraulics often introduces inefficiencies but that has to be weighed against mechanical ineffiencies of mechanical systems where a lot of gearing is used to reduce the speed and increase the torque between an engine and wheels or track.
I’ve seen huge motors, delivering thousands of ft.lbs. of torque to winch drums or bull wheels on ski lifts simply because they are more efficient and provide easy changes in rotation and speed. It’s the faster lighter machinery where the mechanical gear boxes shine. Hydraulics is best for great lumbering machines or where compactness and remote force is needed, such as the wing flaps on a 747.
Love discussing this stuff. Learning all the time.

 

[John_Mc]

->At this time, I do not have a scheme in place for a parking brake. I could easily rig one up by using a spring-loaded arm that could be dropped into the unused CV axle tripod opening. I welded up the diff gears so that arm would securely lock the drivetrain. I thought with a diff feeding another diff, the reduction along with putting the auto trans in park would be sufficient to hold it on most any hill. Do you always use a parking brake?

You guys are WAY beyond my knowledge of hydraulics and HST transmissions. I know the HST on my CUT does a great job of slowing and stopping the tractor. It's 17 years old, and th emechanical brakes are as good as new. However, I don't trust the HST at all to hold the tractor in place when the tractor is on a hill. The tractor will drift down the hill. It's not like leaving it in neutral, but you simply can't count on the tractor to stand still using just the HST (unless I'm sitting there with the engine running and my foot on the HST pedal).

 

[balsam]

Yea, I find the same thing with my HST. I have to use the parking brake. I do love the constant variability of the system once it’s in one of the three ranges. Mine is only a 35hp machine, though.

 

[MossRoad]

If the OP is looking at parking brakes, there's a couple things he can do. My little machine has a "star" type wheel with 6 slots that is mounted behind the two front wheel motors. I pull a lever and that lets a spring loaded pin slip into the nearest slot. Sometimes I have to inch forward or backwards an inch or two to get it engaged, but it is a very simple and effective design. Push the lever to disengage.

Another thing is that there are hydraulic motors with built in brakes.

 

[Garauld]

If I wanted to add a parking brake, I could readily add a spring-loaded plug that would drop into the unused CV housing as shown in attachment photo. There's other options elsewhere too. I don't have plans to add one at this time.



In other developments, I could not get the outboard CV joint apart. I feel I need to so I can check trueness in my lathe when I weld the shortened halves together. A vendor who specializes in rebuilt and custom axles says they will come apart grudgingly. I built the following mini press to pull it apart. I'm waiting for the HP hoses and fittings so I can connect it to my Porta-Power.

 

[bindian]

Thanks ArlyA: I have seen that video a number of times. It’s very well done.
Hydraulics often introduces inefficiencies but that has to be weighed against mechanical ineffiencies of mechanical systems where a lot of gearing is used to reduce the speed and increase the torque between an engine and wheels or track.
I’ve seen huge motors, delivering thousands of ft.lbs. of torque to winch drums or bull wheels on ski lifts simply because they are more efficient and provide easy changes in rotation and speed. It’s the faster lighter machinery where the mechanical gear boxes shine. Hydraulics is best for great lumbering machines or where compactness and remote force is needed, such as the wing flaps on a 747.
Love discussing this stuff. Learning all the time.

Uh.......Primary Boeing flap drive systems use hydraulic motors turning jackshafts (driveshafts) and gearboxes to jackscrews. Alternate drive is electric, driving the same jackshafts and gearboxes.
hugs, Brandi