One of the most important problems regarding machines for stamping sheet metal is the loud noise and the vibrations created when the punch penetrates the sheet metal. This is due to the fact that the force acting on the punching tool is built up continuously until the penetration suddenly occurs and this force is usually of such a magnitude that the frame of the machine is stretched and regarding certain types of frames, brought to "yawn". Then, when the tool is penetrating the sheet metal this occurs suddenly and the frame resumes momentarily its original shape which produces the loud noise. The "yawning" also results in a certain displacement of the tool out of alignement which in turn results in that the punching will not be absolutely correct. Further, there will result an extra hard wearing of the punch and die.
The object of this invention is to eliminate these drawbacks and, according to the invention, this has been achieved by a method for damping noise and vibrations in machines having transient force-relieving of the type appearing in punching or cutting presses of the kinds indicated above and characterized in that the force is relieved slowly by having it pre-compress a hydraulic fluid trapped in a closed chamber the pre-compressing being carried out via a first piston to a predetermined pressure corresponding said force prior to relieving same, and by having the force then act on a second piston immediately before the force relief, whereafter, to obtain the slow force relief the hydraulic fluid is exhausted from the chamber through a line having a valve which is opened at a said predetermined pressure.
The invention also has an object to provide an apparatus for carrying the method into effect. This apparatus is charcterized by a housing defining a closed chamber, a first piston exposed to the force for pre-compressing a hydrualic fluid in the chamber, and by a second piston to be exposed to the force immediately before relieving said force and by one or more valve in an exhaust line from the chamber said valve being designed to open at a predetermined pressure.
Further aspects of the invention are disclosed in the sub-claims.