The spiral pump uses the mechanical action of an eccentric screw to suck in and discharge the slurry. The rotation of the screw inside the stator then creates a series of hermetic chambers that move along the suction/discharge axis. The advantage of spiral pumps is that they only suck up until the liquid reaches their height (not like vacuum models to the highest point in the tank). They then send it to the tank or spreading implement. Sucking up and discharging thick slurry through long suction pipes become then possible. Storage lagoons over 3.5 m deep are emptied within a reasonable time. A 3-way valve allows, besides the filling and emptying functions, the mixing in closed circuit and the transfer from one pit to another one. All spiral pumps on JOSKIN vehicles are ergonomically and compactly positioned at the lowest point of the machine for an easy access and maintenance. The screw inside the pump is continuously immersed in the liquid to be transferred to reduce the risk of cavitation.

Unlike vacuum pumps, the slurry goes through the volumetric pumps and the flow rate mentioned here is the amount of liquid per minute. When used on slurry tankers, there could theoretically be an overflow. In order to avoid such a situation, an automatic filling stop system can be fitted. In this case, a signal from the float gauge gives the filling status and changes the position of the valves from "full" to "mixing in closed circuit". The pump then no longer sucks in slurry and does a mixing job.