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A Worm Harvester is basically just a Trammel Screen, which is a rotating cylinder with a screen mesh for the sides. There are some Harvesters that work with a vibrating or shaking flat screen, but the most common types are all trammels. As the cylinder rotates, the material inside is rolled across the screen as it moves from the input end to the output end. This forward motion is accomplished by the angle that the harvester is tilted.  As the material is rolling, anything smaller than the holes in the screen will fall through the screen, and the rest will continue till it comes out the output end. At the output end, a cone is installed that can separate most of the worms from the other material that did not go through the screen.

There are several factors that can affect the efficiency of a Worm Harvester. We will look at each one of these below.
Moisture Level

If too much material is put in at one time, the screening process will not be efficient, as some of the material will never reach the screen where it could fall through. The moisture level is important in this, as wet materials tend to stick together and "ball" up from the rolling action. If you take castings that have been screened through an 1/8-inch screen, and soak them in water, very little of it will go back through the same size screen. They will instead form large balls, and roll out of the harvester. This demonstrates the need to have your worm bed as dry as possible before harvesting.

Worm beds are all wet to some degree. When this moisture level becomes excessive, less slope on the Harvester, and less material put in at one time, can increase the efficiency of the screening process. When the material is dryer, more slope can be used, and more material can be added at one time. The ability of a Worm Harvester to be easily adjusted in slope can be an advantage.

Most worm farmers will hit their Harvester's screen with a stick from time to time, to knock off particles that are stuck on the screen. Finish nails can be taped to the Jets' rotating hoop where the castors will run over them. This causes the screen to jump, which can dislodge stuck material, just as in hitting it with a stick. With the Jet Harvesters though, this vibration can be made automatic, and either used or not.

Speed of Rotation

There is no perfect speed, in RPM's (Revolutions per Minute) for a rotating Worm Harvester. If the Harvester rotates too fast, it will sling worms and material to the side from centrifugal force. If the speed is too slow, it takes much longer for the material to move through the Harvester. As the diameter of the rotating screen increases, the RPM needs to be reduced, to maintain the same speed at the circumference of the barrel. This is the important speed, the inches per minute that the screen moves, not necessarily the RPM that the Harvester moves. Jet Harvesters have the proper speed for both diameters.


The slope of a Worm Harvester will determine how fast the material will move from the input to the output.  A Jet Harvester is designed with the most often used slope built in, when the legs are adjusted so that the frame is in a level position. The slope can be easily changed with the adjustable legs, to suit any type of material.

In general you will increase the slope when the material being screened comes through the screen easily. This will be drier material, material with few worms, and when using a larger size screen mesh.

The slope is decreased when most of the material is not going through the screen, but rather going out the end. This can happen with wet material, material with many worms, and when using a smaller mesh screen.

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