An experimental vehicle to harvest whole-stalk sweet sorghum was designed by a university research team. At present, the harvester is a pull-type machine, meaning it is towed behind a tractor and powered via a universal joint driveline.
The decision has been made to convert the harvester to a self-propelled machine. The configuration chosen is a single caster wheel at the rear and two drive wheels at the front. Your assignment is to design a hydrostatic transmission to drive the two front wheels. You may use high-torque, low-speed motors to drive the wheels directly, or high-speed motors mounted in planetary gear final drives.
Total weight on the two front wheels is 3310 lbf, and weight on the rear caster wheel is 3750 lbf. It is estimated that the addition of an engine, pump mount, pump, and associated accessories will add 1110 lbf to the front wheels. Additional weight due to wheel motors can be neglected.
Constraints:
1. You will specify the engine power required. The hydraulic circuits on the harvester (other than the HST) require a total power of 34 hp.
2. Initially, assume that the pump mount is a direct drive, meaning that the pump turns at the same rpm as the engine. You will want to operate the engine in the range 1800 to 2000 rpm when it is developing maximum torque for field operations. Do not plan to continuously operate the engine at a speed higher than 2000 rpm when it is under full load. It can be operated at 2200 rpm for road travel, as it will be developing only the power required for the HST. If needed, you may choose a two-speed pump mount with a ratio in the range 1:1.2 to 1:1.5.
3. As a starting point for your design, assume that the harvester has 42-in. diameter front wheels. You may choose larger or smaller wheels if needed to satisfy a design criterion, but do not go outside the range 28โ48 in.
4. Assume a pump volumetric efficiency of evp = 0.85 and a motor volumetric efficiency of evm = 0.85 at design pressure.
5. Do not design for a relief valve setting greater than 3500 psi.
6. If you use planetary final drives, assume they are 98% efficient. It is also appropriate to assume that the pump mount is 98% efficient.
Design for the following field conditions:
1. Rolling resistance R = 50.
2. Maximum grade in the field 8%.
3. The machine must be able to develop a maximum drawbar pull of 250 lbf when traveling at the desired harvesting speed of 2.2 mph. Assume a coefficient of friction, ? = 0.4 for field conditions.
Design for the following transport conditions:
1. When fields are less than two miles apart, the machine will be driven between fields. Design for a maximum road speed of 15 mph.
2. When fields are more than two miles apart, the machine must be loaded onto a tilt-bed trailer for transport. Design the HST so that the harvester can load itself onto a tilt-bed trailer at a 27% incline. Assume a coefficient of friction ? = 0.6.
Requirements:
1. Select a hydrostatic drive [variable displacement pump, fixed displacement motors (planetary final drive)] for the harvester. Specify:
a. Maximum pump flow
b. System operating pressure
c. Show that the HST can supply enough torque to spin the wheels.
2. Specify required engine horsepower for the harvester. Show that all power requirements are met.
3. What is the maximum drawbar pull with ? = 0.4? ? = 0.6? What drawbar pull can be maintained at a 2.2 mph field speed?
4. Show that the harvester can load itself onto the tilt-bed trailer.
