What is going on in an HST?

[SPYDERLK]

I have the opposite problem. I can spin the ag tires anytime. My tires are filled and I use a 1,300 pound block when doing heavy digging. The engine will bogg when I lift the bucket while the tires are spinning though. I think I am getting the max out of the tractor.

Im guessing you have a 2mph or less top speed in low range. Either that or your relief is set a bit high. Maybe both. On my Mahindra 7520 [gear] I can push without bogging on good soil when there is about 18,000 pounds on my AGs. The down force/weight is an estimate based on Tractor [7300], loader/bucket/frontguard/canopy [2400], BH/w subframe [2200], 4x 90% loaded AGs [2000], pushing against a 16" tree with the bucket 9' high [4000 down force on the loader mounts]. I touch the tree @800 idle, ease the clutch in and punch the accelerator. In the first few inches of motion it climbs to, and will hold the max torque point of 1500 rpm in 1st [1.7mph@2300rpm]. The tires either spin or the tree goes down. Sometimes both.
larry

 

[gladehound]

I agree that it would be hard for a hydro (the way the are typically designed for CUTs) to put down as much force to the ground as a gear tractor.

Using the LB1914 as an example it has a 0.75 mph gear @2800 rpm. The 36 inch rear tires have to go around 560 times to go a mile. 0.75 mph is 420 revolutions per hour. Divide by 60 gives 7 revolutions per minute (RPM). So at and engine speed of 2800 rpm I get 7 RPM at the rear tires. That is a 400 to 1 gear reduction.

19 hp at 2800 rpm = 35.6 ft-pounds of torque at that rpm but that is not peak. There is probably a 15-20% torque rise to peak so about 42 ft-pounds at peak. 42 x 400 = 16,800 foot pounds of torque at the rear wheel at 1600 rpm (peak torque) or about 0.4 mph. Divide by the tire radius of 1.5 feet and you get ~11,000 pounds of pulling / pushing force if you could ever get the traction. Which I can't. With just the right soil conditions I may be able to get a pound of horizontal force for every pound the tractor weighs, but that is only 4000 pounds with loader, filled tires, rear impliment and me. Even at idle it will either spin the tires or lift the front if it cant pull something in low gear.

As larry mentioned, the extra force at the wheels can be handy when doing things like pushin trees over.

 

[Albi]

My CK20HST seemed very under powered when new. It would only run on level ground in high range. After 50 hours it was much better and now I use high range for most work as long as I'm on hard ground.

Frank

My Ck20 HSt seems stronger after just 16 hours. It would bog down in high gear while pulling a 60" finish mower up a small incline, it's much better now and I am hoping it gets even better.

 

[RandyT]

The pressure relief valve is primarily used as a pressure spike valve, for example, when shoving the bucket into a pile of dirt, when the loader stops moving into the pile the resulting pressure spike from the sudden stop can lift the hydro pump off of its mating surface to relieve the pressure. The pump can then slam back down on the mating surface after the pressure drops off causing the pump to gall the mating surface. the damaged mating surface can then start contacting the pump and causing further damage eventually leading to pump failure.