Abstract:
A tuned mass damper is adjusted by turning a screw inside the damper that engages coils on a spring, reducing or increasing spring stiffness without displacing the spring.

Description:
BACKGROUND 
     A tuned mass damper (TMD) provides improved damping to structures and devices at a single frequency by tuning the damper&#39;s natural frequency to be at or close to the single frequency. TMDs are attached to the structure at an effective position, usually the anti-node, to counteract the device&#39;s vibration. The vibration stimulates the TMD to oscillator independently, 180 degrees out of phase, reducing the device&#39;s vibration. 
     A TMD typically is adjusted at the factory by changing springs or removing material from the oscillating mass, estimating the frequency of the device to be damped. The typical TMD comprises a mass, a spring and a damping means which form a system with a specific natural resonant frequency and because of that structure it is difficult to tune that frequency. 
     SUMMARY 
     A TMD according to invention is adjustable by utilizing an adjustment screw that is retracted or advanced, changing the number of active coils in a spring that engages a damping mass in a sealed TMD. The screw adjustment changes the spring rate and the natural frequency of the spring-mass combination but does not compress the spring. 
     Objects, benefits and features of the invention will be apparent to one of ordinary skill in the art from the drawing and following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a cross section of a tubular adjustable tuned mass damper that embodies the present invention. 
    
    
     DESCRIPTION 
     The TMD  10  shown in the drawing comprises a tubular structure attached by a bracket  12  to a device or structure  14  that is subject to oscillations which are damped by the TMD  10 . The TMD  10  has two removable end-plates  16 ,  17  providing access to the interior of the TMD where a cylindrical mass  18  is located in a cylindrical chamber  19  and supported by a plurality of ball bearings  20 , constrained within grooves  22  running lengthwise along the mass (arrow A1) to enable sufficient lateral movement for the mass to oscillator back and forth (arrow A1). Gas flow, arrow, across the mass  18  damps those oscillations as it moves in the chamber  19 . 
     A primary coil spring  24  is placed between one end of the mass  18  and internal wall  10   a  of the chamber  19 . A screw  26  is threaded into a threaded passage  10   b , entering the center of the spring  24  where it captures one or more of the spring&#39;s coils, which should have the same screw pitch as the adjustment screw  26  if no movement of mass  18  is desired during adjustment. Rotating the screw  26  changes the spring stiffness and thereby the natural resonant frequency of the mass  18  and spring  24  combination. Rotating the screw  26  does not, however, displace the spring. It only grabs one or more coils, making them effectively rigid. One end of the TMD is sealed from the atmosphere when the end plate  16  is attached by screws or rivets, not shown. The other end plate  17  is similarly attached for sealing the side with the screw  26 , but also contains a small passage  17   a , closed by a removable plug (not shown), through which the adjustment screw  26  can be turned. A sealing and locking material can be included between the wall  10   a  and the screw  26 , for example at location  10   c , producing a screw friction lock and also sealing the interior of the chamber  19  from the atmosphere through the screw threads. 
     While the invention envisions in its most basic sense manually turning the adjustment screw  26 , it is also feasible to automate the process by attaching a screw actuator  30  such as a low speed motor  30  to the housing  10  with a shaft extending through passage  17   a  to the adjustment screw  26 . With this variation of the invention, the spring can be automatically adjusted on device  14  through a sensing apparatus  34  and  32 , such as an accelerometer  34  to fine tune the TMD during operation, further reducing the vibrations of the device  14 . 
     One skilled in the art may make modifications, in whole or in part, to a described embodiment of the invention and its various functions and components without departing from the true scope and spirit of the invention.