Re: Making my own tractor, still running the calculations for HP/torq

Posted by tubba_JD420C_ny on December 15, 2002 at 18:44:37 from (24.59.61.54):

In Reply to: Making my own tractor, still running the calculations for HP/torq posted by KURT on December 15, 2002 at 14:27:25:

Tractor designers think in terms of rimpull, which is the torque at the rear axle divided by the radius of the wheel. The torque at the rear axle is the engine torque times the overall gear reduction ratio thru the entire drivetrain, tranny, rear axle, final drive, etc. The indicidual ratio's multiplied together. The rimpull would be the drawbar pull if there was 100% traction without slippage. SO you would think of what you want to do with the tractor, how much pull or push force you need, then figure out the size wheels you will use and the engine you plan to use and its RPM and torque/HP performance as shown on a power curve. then back-calculate from the rimpull the torque at the rear axle, then consider the overall gear reduction in the drive train and consider the RPM/torque curve of the engine. If you have selected the engine and the wheels, you can select the gear ratios in the drive train so that you get the rear axle torque/rimpull you want at the time the engine is at somewhere between the RPM for its peak torque and the RPM for peak HP (which is always more than the RPM for peak torque), using the above methods. Of course it can get more complicated in terms of the torque rise of the engine, i.e. how much the torque goes up as the engine bogs down from the RPM of peak HP. That is where you need to look at the torque curve of the engine, so you can select gears so that the rimpull is what you want when the engine RPM is where you want it on the power curve. If the engine has a governor, you can select an RPM just above the RPM of peak torque. The governor will keep it there as the engine bogs down. With chains and proper ballast, you should get about 80-90% of the rimpull for traction depending on snow/ice/and surface. SOrry if this sounds confusing, and please excuse the long post, but what you are acturally doing is mechanical engineering, like the big guys at Deere, Case-IH, etc do when they design a new tractor. Just wanted go toss out some things to think about. Hope this helps some and good luck on your project.

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