Wobble Stick Switch

October 24, 2002                                                                                                                                                                                                    ISSUE #M25


The screws of screw presses used in manure are subject to abrasive wear. This wear does not occur in normal operation nearly as much as it does in a condition of "running dry". If a manure press is allowed to continue running when there is little or no cake being produced, the cake remaining in the machine becomes bone dry. This dry material, even without sand being present, is extremely abrasive. Because of this it is recommended that a wobble-stick switch be incorporated in the electrical control system.

The switch is mounted someplace below the cake discharge so that the wobble stick is in the path of the falling press cake. This is illustrated in the sketch below. The switch should be connected to a timer such that, if the switch does not detect activity for a while, like three minutes, the manure pump and press are shut down.

Shutting down the system when cake is not being produced is advantageous for several reasons:

1. Even though the press is not producing cake, as long as the screw is turning the flights are being worn down. In this stalled condition, the cake at the discharge remains stationary, tends to get very dry and even hot, and the amount of abrasion occurring to the circumference of the flights increases substantially.

2. If cake is not being produced, it is likely that the pit has been drawn down to where the capacity of the manure pump has been seriously reduced. This comes about because of the increased height that the manure must be pumped. (The pumped height is from the level of the pit to the inlet of the separator, not from the pump suction to the inlet of the separator.) It is time to shut down the system.

3. If very little cake is being produced, water on the inlet end of the screen will soak into the dry cake at the discharge. In this manner the lower portion of the plug at the discharge becomes sopping wet to the point where it breaks loose and flows out. The discharge door is held open by the dry material on the upper portion of the plug, and purging will begin. Shutting down the system before this happens can prevent the mess that comes with blowing the plug.

Square D offers wobble-stick switches and timers, as do many other companies. They can be bought through McMaster-Carr, Grainger, and Gulf Controls among others.