The pressure increases directly according to load. As a for instance, you want to pull a stump. The chain is around the stump, the throttle is at 1/2 and you slowly apply drive. The pressure increases to a low level as the tractor takes up the slack. Once the slack is out, the tractor starts to take up the strain and the pressure rises quickly. At this point, either the wheels spin or, more likely, the relief pops due to lack of horsepower from the low drive speed. Advancing either the drive control or the throttle will make more horsepower available which will temporarily reduce the pressure, followed immediately by a rise in pressure again (it didn't really drop that much) to the relief setting, or the wheels spin. Repeated increases in flow will generate the same results until either the wheels do spin, or the stump moves. Throughout these attempts, the pressure remains very close to the relief setting, until the wheels break traction. At that point it drops dramatically and the horsepower from that additional flow takes over and the tractor digs holes.
If the stump starts to give before the tires do, the same thing applies. With forward motion available to make use of the flow, the pressure begins to drop and more horsepower goes to motion instead of heat over the relief valve. As the stump continues out of the ground with reduced resistance, the pressure drops in tandem until only enough is required to drag the stump along the ground. About the same as simply moving the tractor.
In the above scenario, the pressure spiked when it came under max load, even though there was only a little flow. Advancing either the throttle or the drive control has little effect at max pressure unless the wheels can turn. The additional horsepower produced by the hydro goes over the relief valve as heat.
Second scenario, towing a 10 cu-ft cart with a level load of dirt (1000 lb+).
Throttle at 1/2 and by advancing the drive slowly, the pressure will rise immediately to what is required to get the rig moving, then it will fall off some as inertia is broken and will settle to accomodate the slow advance of the drive control with the resultant accelleration. When the limit is hit with the drive control and there is no more accelleration, the pressure will drop a little more as horsepower takes over the duty of maintaining speed. A sudden increase in throttle at this point will drive the pressure back up to accommodate the accelleration to the new cruise level, where it will once again drop back to the required (slightly higher due to higher speed) cruising pressure and the horsepower again does its thing.
In this scenario, the pressure increased in tandem with the application of drive speed, with both the drive control and the throttle, and dropped back when speed became stable.
There are so many variables involved with pressure, load, horsepower, speed and terrain, that it is difficult to lay out precisely what will occur. As operators, we learn quickly what does and does not work and automatically adjust to all of these variables to some extent at the beginning, and we get better at it as we gain experience until it becomes second nature. Explaining it and keeping all of those variables accounted for is a lot tougher than actually doing it while operating the tractor. An increase in grade will cause an immediate increase in pressure, as will coming to the end of pavement and transitting to sand, both with no change in throttle setting or the drive control. The governor will, of course, open the throttle to get more horses hooked to the wagon for the increased load.in both situations.
For your last question, I'm not sure how much the pressure will change, if at all, since the horsepower is doing the work at steady speeds. Torque accellerates, horsepower maintains. A 500 lb load isn't all that much anyway. GTs weigh anywhere from 500 to over 1000 lb and then there are the operators, ballast, and any attachments added to that weight. Your 1855 weighs over 1000 lb, empty, and won't even notice carrying or pulling another 500. My 1655 weighs over 2400 with me on the seat and a 4000 lb trailer is barely noticeable, to me, at 2/3 throttle. The pressure may say something different with that load, but it won't be much. I've moved heavier loads with that tractor.
Edited by TUDOR, April 02, 2012 - 01:41 AM.