The P.D.M is one of the two kinds of the downhole motors. The Positive Displacement Motor is a hydraulic motor. Its pumping system depends on the Moineau principle to rotate the bit. The main four components are:
A) By-pass valve (dump sub) :- This valve allows the pass of the drilling fluids to the hole while tripping in, besides the drainage of the fluids while tripping out. The common type is the spring piston valve which allows the mud to flow through the tool under pressure. When the downward pressure is removed, the spring opens a channel of flow to make the fluid extract through the annulus.
B) Motor section:- This part depends on the reverse application of the Moineau principle. It consists of a rubber stator and a steel rotor. The rotor is forced to rotate transferring the motion to the bit because of the pumped mud filling the gaps between rotor and stator.
C) Connecting assemblies :- The rotor traces a back and forth motion and does not rotate concentrically as it has spiral shape. This motion is converted to a concentric motion to be transmitted to the bit using rod assembly. The main two types are Universal-joint and Flex rods.
D) Bearing section:- There are three sets of bearing: one set of axial thrust bearing and two sets of radial bearings. Thrust bearing is used to support the on and off bottom loading. It consists of series of balls stacked on each other. The number of these balls is determined based on the size of the tool. Due to the presence of the tungsten carbide inserts, these bearing can support the concentrically rotating drive shaft against lateral loads.