In the drilling world, and more particularly in deep horizontal intervals there is often a need to provide the drill string (whether jointed drill rods, or continuous coil tube) with a level of axial excitation to minimise the frictional forces which can dramatically slow or stop a drilling or re entry operation. In addition this type of tool would be very beneficial to have within a drill string (or multiple such devices within a drill string) to help free drill strings once they have become stuck.
Ideally such a device would have the ability to:                Be engaged as and when necessary.        Generate sufficient force to minimise friction—and/or free stuck drill strings        Allow a substantially unrestricted fluid path through the length of the tool for drilling fluids, lost circulation medium etc.        Have a controllable level of force, from gentle to strong—adjustable as required from surface.        
In addition to the above the device could also be used as a seismic signal generator, or used for settling cement, or any other application where an axial excitation is useful.
It is a further or alternative object to provide an “on demand” capability downhole whereby, as and when wanted, a mechanical force generator can be activated. A further alternative is such a capability that is always or usually always operative.
The device options mentioned below aims to achieve or lend themselves to at least one or more of these objectives.