Device for generating an axial load in a drill string assembly

An axial load device configured to be placed in a drill string assembly with a downhole motor, the axial load device comprising a hollow cylinder body with a sealing bush, and a spring-loaded flow-type piston rod with a key, wherein:

• the hollow cylinder body comprises two-way longitudinal key slots along an inner generator line of the hollow cylinder body,
• the key is inserted in the two-way longitudinal key slots, the key comprising longitudinal and transverse gaps and a profiled surface adapted to move along the two-way longitudinal key slots together with the spring-loaded flow-type piston rod,
• the transverse gaps are configured so that the key has at least two degrees of freedom in the two-way longitudinal key slots and in slots of the flow-type piston rod for axial movements relative to the cylinder body, thereby allowing mud on the front of the key to flow around both sides of the key,
• a natural vibration frequency of the key is directly proportional to a length of the key and a velocity of propagation of generated flexural wave over the key and inversely proportional to the square of a thickness of the key, and
• a length of the two-way longitudinal key slots is determined based on a maximum stroke of the flow-type piston rod operating in the Eulerian area of stable equilibrium, considering its moment of inertia of a cross section and a permissible critical stress generated by an optimum axial load on a drill bit of the drill string.