Поставьте 3-4 вопроса к тексту, раскрывающих его содержание. combine harvesters bushels per acre, not acres per day, must be the aim of the combine operator who wants the maximum amount of grain from a given crop. to maintain an even flow of material through the machine is possible by varying land speed according to crop volume. no specific recommendations can be given in this respect – the driver must make his variations almost instinctively to maintain the even rhythm characteristic of well-fed threshing mechanism. cylinder speed controls threshing; therefore it should be matched with both the volume and condition of the crop. heavy and damp crops require higher speeds than do light and dry crops. a cylinder speed which is too low for damp material will leave unthreshed grain in the head. on the other hand, to thresh dry grain at too high a speed causes excessive chaff. the concave is as important for separation as for threshing. it is an adjustable stationary grid against which the drum rubs out the grain. in reasonable harvesting conditions, it should be about 5/16 in. from the front of the cylinder. if cracked grain appears, lower the front of the concave and reduce the cylinder speed; if grain tends to remain in the head, raise the front end and increase the cylinder speed. straw-walkers should be used to separate only the minimum of grain. loss over the walkers indicates that the straw has been overloaded with grain, which would have been removed at the cylinder and concave had they been correctly adjusted. as a general rule, sieves lose more grain through lack of wind than from excess. grain goes out with the chaff when the latter is allowed to sit on the sieves. this can be prevented by adjusting the fan air-inlet shutter so that the outlet blast is strong enough to keep the chaff riding above the grainˡ. before starting a combine, even for a preliminary run, lubricate it according to the manufacturer’s instructions. this routine must be observed throughout the season, for it usually takes longer 2 to cure a fault caused by lack of lubrication that it takes to lubricate the complete machine.