Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Agricultural and Biosystems Engineering

First Advisor

Stephen J. Marley


The purpose of this study was to collect and analyse current data related to repair and maintenance costs of tractors and combines. Specific objectives of this study were: (a) to collect current repair cost data for tractors and combines; (b) to compare these data with yearly repair cost estimates derived from ASAE repair cost formulas; (c) to estimate expected tractor life; (d) to estimate expected combine life; (e) to study the factors affecting repair costs of tractors and combines; (f) to develop appropriate new repair cost equations for tractors and combines;Two questionnaire surveys were conducted by the author to collect current repair cost data for tractors and combines from a sample of Iowa farmers who owned these machines in 1984 and 1985. Factors affecting repair and maintenance costs of tractors and combines included make and model, age, annual use, and farm size. The results showed that there were no effects of tractor and combine make on repair and maintenance costs. The relationships between repair costs and age were highly significant. Also, the greatest single factor affecting hourly tractor and combine repair costs was the annual use;Testing the validity of the ASAE formulas for predicting repair costs indicated that these formulas, except for one equation, do not accurately predict yearly repair and maintenance costs for Central Iowa tractors and combines. The data show that the average tractor life estimated by the farmers was 8355 hours, and it was 3575 hours for combines. Using the STEPWISE regression procedure to fit the best model for predicting yearly repair costs for tractors and combines leads to the following two equations: For wheel tractor: Y =.072 TAH +.0096 P +.66 H + 78. For self-propelled combine: Y = 241.70 N +.016 P + 2.27 H + 1.07 A - 1894.9. Where: Y = Repair and maintenance costs (/year); TAH = Total accumulated hours of use, at end of year (hrs); P = Purchase price ( ); H = Annual hours of use (hrs); N = Age of the combine (yrs); A = Annual harvested acres (acres).



Digital Repository @ Iowa State University,

Copyright Owner

Ismail Ahmed Abdelmotaleb



Proquest ID


File Format


File Size

160 pages