A silo unloader is usually suspended by a cable from the top of the silo so that the unloader mechanism is just above the topmost level of the silage in the silo. The silo unloader incorporates a mechanism that travels around the inside of the silo wall such that it simultaneously loosens and conveys the silage for delivery through conveying tubes downwardly to feed bunks in adjacent animal pens. As the silage is used, the silo unloader must be progressively lowered in the silo so that it will stay in contact with the top surface of the silage in the silo.
Accordingly, the silo unloader must be very slowly lowered in the silo at a rate which is equal to the rate at which silage is loosened and conveyed out of the silo. It will therefore be recognized that the cable suspending the silo unloader must be payed out from its control winch very, very slowly.
It is also very desirable that the silo unloader be rapidly raised and lowered so as to lift the unloader off the silage after a sufficient quantity of the silage has been discharged to accomplish feeding the animals; and of course it is desirable to lower the silo unloader quickly onto the silage whenever the silo unloader is again to be put into operation. This lifting of the unloader off the silage minimizes the likelihood of problems due to extremely cold conditions wherein the silo unloader may actually freeze to the surface of the silage in the silo.
Previously, in order to very slowly lower the silo unloader to keep the unloader working into the face of the silage, it has been common practice to briefly operate the winch motor, by simply instantaneously touching the start button, and immediately releasing it. This is sometimes called "bumping" the switch; but this practice does not accomplish the maintenance of a uniform pressure by the silo unloader against the top of the silage so that the silage will be uniformly loosened and conveyed out of the silo.