Degree Type


Date of Award


Degree Name

Master of Science


Agricultural and Biosystems Engineering


The tailings return system is an important functional part of modern combine harvesters, but published research on tailings return systems is very scarce. The tailings return system is intended to take crop material that is not completely threshed and/or not completely cleaned from the combine cleaning shoe back to the main threshing cylinder. Returning the tailings is regular practice on modern combine harvesters, but is not necessary and may be detrimental in some crops and conditions. The primary objective of this thesis is to research the function of the tailings return system with emphasis on developing a tailings rethresher. A tailings rethresher rethreshes the material from the cleaning shoe and returns it only to the cleaning section of the combine for re-processing. There are several potential advantages to rethreshing returning the tailings only to the cleaning section of the combine: 1.) Threshed grain in the tailings will not be exposed to the main threshing cylinder to be potentially damaged. 2.) The combine could be simplified and reduced in cost. 3) The rethresher will effectively reduce the loading of the main threshing cylinder, adding threshing capacity to the combine. 4.) Returning tailings to the cleaning system offers some advantages in distributing the load on the cleaning shoe for more efficient operation.;5.) With the addition of an innovative bypass system, tailings can be returned to the cleaning system without rethreshing. Rethreshing is not necessary in many crops. This thesis addresses the data collected in the field and its implications. The function of the tailings returns has been further quantified in this study. The information is directed toward development of an improved tailings rethresher/returns system. Fourteen field studies were performed to measure tailings flow rate and composition and grain purity and damage in four different crops and numerous different conditions. Two prototype rethreshers were constructed and laboratory tested. One prototype was installed on a combine for lab testing, but it was not field tested.



Digital Repository @ Iowa State University,

Copyright Owner

Brian J. Hollatz



OCLC Number


File Format


File Size

134 pages