Measurement of Tractor-Wheel Slippage

Abstract

A criterion of farm tractor performances is the ratio of travel reduction or slippage of the driving wheels for a given drawbar load. The conventional method of measuring drive wheel slippage is cumbersome and time consuming. The reason for the difficulty of finding an appropriate method for measuring instantaneous slippage of the driving wheels of tractors may be that we can not measure travel velocity or speed as a function of time.
The purpose of the present experiment was to develop a system of direct measurement of tractor wheel slippage using a gage wheel which measures travel speed of a test tractor on a packed and relatively smooth ground surface.
The results of the tests were as follows:
1) The gage wheel apparatus was suspended by a spring damper in such a way that the preload could be changed. The travel speed was measured by a pulse generator (600P/R) which was coupled to and driven by the gage wheel shaft. As the base value for computing the amount of wheel slippage the engine speed was measured by another pulse generator (60P/R) which was attached to the PTO shaft of the tractor. After two signals were converted to DC voltage proportional to each speed, they were lead to an analogue circuit to compute the amount of wheel slippage.
2) The performance of the gage wheel apparatus as a travel speedmeter gave a measurement accuracy of±1.0%, which was applicable to velocities ranging from 0.4 to 3.0m/s on a packed and relatively smooth ground surface.
3) The cut-off frequency of the low-pass filter used to average the data was determined using the travel reduction v. s. drawbar pull curve. The appropriate cut-off frequency may be considered to be 0.1 or 0.2Hz.
4) To investigate the practical application of this system a drawbar pull test was conducted by using a test tractor with 52 PS engine power on a concrete and a clay soil test track. The drawbar load was adjusted by the brake of the tractor for loading in such a way to produce drive wheel slippage ranging from 0 to 100% continuously. The non-slippage travel speeds were set at 0.45, 0.89, 1.35 and 1.66m/s.
5) The maximum drawbar pull on the concrete test track was developed before the drive wheel slippage reached 100%. It was further found that the amount of slippage with respect to the nonslippage travel speed decreased from 75.5 to 55.0% as the ground speed increased. On the contrary, the maxiumum drawbar pull increased by about 30% as the slippage speed increased. But the relation between the slippage speed and the maximum value of the drawbar pull showed opposite tendency.
6) On the clay soil test track, the maximum drawbar pull and the drawbar pull at 20% wheel slippage showed the same tendency as described in 5). No relation between the drawbar pull and the 100% wheel slippage was found in these tests.