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O objetivo deste trabalho foi determinar o rendimento dinâmico na barra de tracao de diversos tratores agricolas com tracao dianteira auxiliar (4x2 TDA), em diferentes condicoes superficiais (concreto, solo firme, solo preparado e solo com cobertura vegetal) e compara-los com os dados teoricos e praticos da bibliografia (fator 0.86 de Wendel Bower e norma D497.4 da ASAE, 1999). Foram avaliados onze tratores agricolas utilizando as diferentes pistas do NEMPA – Nucleo de Ensaio de Maquinas e Pneus Agroflorestais, da FCA/UNESP de Botucatu/SP, variando-se o tipo construtivo de pneus, as suas pressoes de inflacao, as relacoes entre o peso e potencia do motor e a velocidade teorica de deslocamento. Os dados de rendimento maximo na barra de tracao do trator, obtidos nos ensaios, foram dispostos em grupos, obtendo-se o valor medio do rendimento maximo na barra de tracao para cada condicao de superficie. Para a realizacao do experimento utilizou-se a Unidade Movel de Ensaio na Barra de Tracao – UMEB, pertencente ao NEMPA, como carro dinamometrico, estando equipado com sistemas de aquisicao e armazenamento de dados. Em pista de concreto, os resultados mostraram que o valor medio de rendimento maximo na barra de tracao obtido (63,2%) foi inferior aos valores teoricos da bibliografia. Em pista de solo firme utilizavel, o valor obtido (56,5%) confirmou os valores da bibliografia. Em pista de solo mobilizado, o valor obtido (44%) confirmou o valor proposto por Wendel Bowers (42,8 %) e foi inferior ao determinado pela ASAE (53,9%). Em pistas de solo com cobertura vegetal, o valor medio de rendimento maximo na barra de tracao obtido foi de 49,7%, nao havendo valores de comparacao na bibliografia. Palavras-chave : Ensaio de maquinas, superficie trativa, patinagem. DRAWBAR PERFORMANCE DETERMINATION OF AGRICULTURAL TRACTORS WITH FRONT WEEL DRIVE (4 WD) SUMMARY: This research was accomplished with the objective to determine the dynamic drawbar performance of several agricultural 4WD tractors in different surface condition (concrete, firm soil, tilled soil, and soil covered with vegetation) comparing them with the theoretical and practical bibliography values (Wendel Bowers "0.86 Factor" and D497.4 ASAE standard, 1999). Eleven agricultural tractors were tested at all different tracks of NEMPA – agricultural and forest tractors and tires test nucleus, College of Agronomic Science, UNESP, Botucatu/SP, Brazil, being varied the tire constructive type and models, tire inflation pressures, relationship between the tractor weight and engine power and theoretical.  The maximum tractor drawbar performance obtained in the tests was arranged in groups, being obtained the medium dynamic drawbar performance and standard deviation for each surface condition. For the accomplishment of the experiment the UMEB – Mobile Unit for Drawbar Test, belonging to the NEMPA, as a dynamometric car, equipped with an acquisition and storage data system. In concrete track the results showed that the medium value obtained for maximum drawbar performance (63.2%) was less than proposed by Wendel Bowers (79.2%) and for ASAE value (72.2%). At firm soil track, the obtained value (56.5%) confirmed the values of the bibliography. In mobilized soil tracks the obtained value (44%) confirmed the value proposed by Wendel Bowers' (42.8%) and it was less than the correspondent value for ASAE (53.9%). In vegetable covering soil surface the medium value obtained of maximum drawbar performance was 49.7%, and there are not bibliography data for this surface. Keywords : T ractor test, surface condition, tractor slippage.

DETERMINAÇÃO DO RENDIMENTO NA BARRA DE TRAÇÃO DE TRATORES AGRÍCOLAS COM TRAÇÃO DIANTEIRA AUXILIAR (4X2 TDA)