The invention relates to a cleaning sieve for the harvested crop of agriculatural machine and in particular harvester threshers.
In present-day modern harvester threshers, two sieves one disposed above the other are generally employed wherein the upper one is shaped as a lamellar sieve which separates the coarse chaff and straw particles and the lower one sieves the finer remaining particles. The two sieves are passed by the air current from a blower from below.
The air current lifts the lighter chaff and straw particles so that they are transported by the air current above the sieves and the vibratory movement of the sieves to the rear of the machine and out of it whereas the specifically heavier grains fall through the sieves and are collected in a manner known per se in a grain tank or are filled into sacks.
In modern harvester threshers, the sieves are longer than one meter. Chaff and short straw have to be transported over the whole length of the sieves. The air current is not in this case able to keep the chaff particles coming up at the beginning of the sieve floating over the whole length of the sieve and to transport them toward the rear. Otherwise the air current should be so strong that it could also keep grains afloat and blow them out of the machine together with the chaff.
It has therefore been tried again and again by using mechanical means to keep the chaff and straw particles as far as possible above the upper sieve and contrary thereto conduct the heavier grains from the very beginning to the upper sieve in order to separate grains and chaff at least partly from each other before they fall onto the upper lamellar sieve passed by the air current.
In German Patent Specification No. 908,080, a structure of this sort has been described wherein on the gliding bottom from which the grain-chaff mixture is conducted onto the upper sieve, a rake-like extension is provided which keeps the chaff and straw particles over a certain height above the upper sieve while the grains drop through the rake onto the upper sieve.
A nearly identical apparatus has been described in GDR Patent Specification No. 227,604. In order to loosen the chaff-grain mixture on the upper lamellar sieve, most various sieve structures have been suggested in the past. German Utility Model No. 7,632,151, for instance describes a ramp sieve which is similar to a lamellar sieve in function but has the disadvantage as compared thereto that it comprises square hole openings instead of slit openings as in case of the lamellar sieve.
Such sieves were not successful since an adjustment of the sieve openings, which was obviously necessary with a view to the various kinds of fruit and humidity grades, is not possible in most cases and, secondly, longer straw particles passing through the holes and slipping into the lower sieve will stick fast and after a while will block the whole sieve.
In order to loosen the grain-straw mixture on a lamellar sieve, it has been suggested in German Patent Specification No. 1,238,707 to provide individual lamellae with finger-shaped extensions similar to a rake whereby these lamellae are vibratory relative to the other lamellae, whereby these finger-shaped extensions perform an up and down movement relative to the remaining lamellae and so loosen up the chaff-grain mixture.
This structure was not successful since the vibrating lamellae, on one hand, perform an on-off movement relative to the remaining sieve lamellae, i.e. do not permit a fixed preadjusted value corresponding to the kind of fruit to be harvested, and the wear of the pivotably moving lamella axes, considering a frequency of a couple of hundred movements per minute, leads to a breakdown of the sieve after a short time, on the other.
In German Offenlegungsschrift No. 2,751,842, a rake-like apparatus disposed above the sieve and provided particularly for the corn harvest has been described which however is completely unsuited for loosening a grain-straw mixture considering that the straw and chaff particles dropped under the rake prongs are kept down by these very prongs and hence cannot be pressed upward by the air current. A sieve provided with such an apparatus will be clogged within a short time.
U.S. Pat. No. 4,548,213 describes a sieve also particularly developed for the corn harvest which is inserted without a lower sieve and on which, on one hand, ribs are provided which laterally support the material gliding over the sieve when in inclination, and ondulated transversal ribs, on the other hand, prevent that the corn cobs glide too fast over the sieve and are lost. Such a sieve is not suited for cereals since the effect of a lamellar sieve for controlling the air current is lacking.
In German Patent Specification No. 3,720,696, a rake-like cover device provided above the oversweep area of a sieve box has been described which is to prevent that the grain-chaff mixture collecting on one side, when in an inclined position, is guided in total to the oversweep device of the harvester thresher. This cover is said to effect a last separation of the grains from the mixture. The performance of the sieve above the working zone as such, i.e. before the oversweep area, is not in this case influenced nor is it increased. A similar apparatus has been described in German Offenlegungsschrift No. 4,106,814 wherein, above the oversweep area, enlarged lamellar intermediate spaces are provided which when in an inclined position will guide the collecting grain-chaff mixture to the oversweep. This apparatus, is not in any way suited either to increase the performance of the sieve above the working zone as such.
In both the apparatus according to German Patent Specification No. 3,720,696 and German Offenlegungsschrift No. 4,106,814 referred to above the very opposite effect can be experienced. The oversweep amount obtained in excess because of the larger passages in the oversweep area starts a second trip over the sieve device and increases the crop amount on the sieves to be processed. This can lead to a situation where after a second trip, a third and even multiple trip develops which will overload the sieve to an extent that the machine has to run idly before mowing can be resumed.
The sieve apparatus most effective for the time being has been described in German Offenlegungsschrift 3,704,348. This apparatus comprises a plurality of separated upper sieves provided on different levels. By this arrangement, drop steps are provided between which the air current blows the chaff and straw particles towards the rear portion, in the travelling direction, of the following sieve provided on a lower level. With a view to this arrangement, the dreaded "carpet" which blocks the air current through the sieve cannot develop. The disadvantage of this sieve arrangement however is to be seen in that a vertically much higher space requirement of the sieve box is necessary. The harvester thresher has to be constructed from the very beginning to hold such a sieve arrangement. For space reasons, it will almost never be possible to add this arrangement to machines already available.