Impact damper

Disclosed is an impact damper which is attached to an object of damping forming a main vibration system having a main natural frequency and serves as an additional vibration system. The natural frequency of the additional vibration system ranges from 60% to 80% of the main natural frequency, and the space between a weight of the additional vibration system and a stop attached to the main vibration system as measured when the object of damping is not vibrating ranges from 0% to 80% of the resonance amplitude of the object of damping.

BACKGROUND OF THE INVENTION 
This invention relates to an impact damper comprising a bed plate attached 
to an object of damping forming a main vibration system with a main 
natural frequency, an additional weight, additional elastic means 
supporting the additional weight so as to be able to vibrate the same in 
the same direction as the vibration of the object of damping, mounting 
means for mounting the additional elastic means on the bed plate, and stop 
means mounted on the bed plate to strike against the additional weight in 
vibration. 
From a vibrational point of view, the object of damping may be regarded as 
a main vibration system including a main weight and main elastic means and 
having the main natural frequency. Likewise, the impact damper of the 
invention may be regarded as an additional vibration system including the 
additional weight and additional elastic means and having the additional 
natural frequency. 
The impact damper is attached to the object of damping, and vibrates in 
concert with the object so that the additional weight strikes against the 
object to damp the same with high efficiency. 
In the well-known impact damper of this type, the frequency range providing 
a satisfactory damping effect is restricted, limiting the fields of 
application, unless the characteristics of the main and additional 
vibration systems and the relative positions of the object of damping and 
the additional weight are determined properly. 
SUMMARY OF THE INVENTION 
The object of this invention is to provide an impact damper free from the 
aforementioned drawbacks of the prior art impact damper and capable of 
producing a satisfactory damping effect throughout a wide frequency range. 
To this end, an impact damper according to this invention is so constructed 
that the natural frequencies and relative positions of the main and 
additional vibration systems fulfill the following two requirements: 
(1) The distance between the additional weight and the stop means as 
measured when the object of damping and the additional weight are not 
vibrating is to range from 0% (in this case the additional weight is in 
contact with the object) to 80% of the resonance amplitude of the object 
without impact damper. 
(2) The additional natural frequency of the additional vibration system is 
to range from 60% to 80% of the main natural frequency of the main 
vibration system. 
When the impact damper of the above-mentioned construction is attached to 
the object of damping, the object and the additional weight strike against 
each other during vibration, so that the ampitude of the object is 
attenuated by a change of momentum thereof and an energy loss caused by 
the impact. With the aforesaid construction, moreover, the object of 
damping can effectively be damped and prohibited from vibrating with a 
great amplitude in a wide frequency range including frequencies higher or 
lower than the main natural frequency, not to mention the main natural 
frequency. 
The requirements of the impact damper are deduced from many tests conducted 
with regard to a lot of parameters other than those referred to above. 
Effects obtained with use of this impact damper will be mentioned later in 
conjunction with the preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows an impact damper 10 according to an embodiment of this 
invention. In FIG. 1, a bed plate 10a is mounted with a retaining stand 14 
retaining an additional elastic means or an additional spring 12, and a 
screw retaining stand 18 rotatably supporting a screw rod 16. The 
additional spring 12 is a leaf spring which extends in the horizontal 
direction of FIG. 1, and has an additional weight 20 fixed on the left end 
portion thereof. The right end portion of the additional spring 12 is 
fixed between the retaining stand 14 and a retaining member 22 by means of 
a clamp screw 24 so as to be adjustable for the location of the weight 20 
in a desired horizontal position. The retaining member 22, the retaining 
stand 14, and the clamp screw 24 form a mounting means for mounting the 
weight 20 and the spring 12. A nut 30 is attached tight by means of a 
screw 32 to the right end portion of the spring 12 which is projected to 
the right side of the mounting means. A threaded portion 34 of the screw 
rod 16 is screwed in the nut 30. The screw rod 16 is rotatably held by the 
screw retaining stand 18 so as not to move horizontally, and can easily be 
rotated by means of a square portion 36 protruding from the right end of 
the screw rod 16. The effective length of the spring 12 can be varied by 
loosening the clamp screw 24 and rotating the square portion 36 by means 
of a suitable tool. A tapped hole 40 is vertically bored through the left 
end portion of the bed plate 10a, and a stop 46 is fixed on a base 44 on 
the top of a screw 42 which is fitted in the tapped hole 40. The stop 46, 
the base 44, and the screw 42 form a stop member. The vertical position of 
the stop 46 is set by rotating the screw 42 and fixed by means of a 
setscrew 45 so that the weight 20 attached to the tip of the spring 12 can 
strike against the stop 46 with proper strength when it moves downward by 
vibration. The space between the under surface of the weight 20 and the 
top surface of the stop 46 as measured when the weight 20 is not vibrating 
is designated by d in FIG. 1. 
Now there will be described the operation of the impact damper. FIG. 2 
shows how the impact damper 10 is attached to a pipe 52 as an object of 
damping. For the simplicity of illustration, the pipe 52 is drawn in an 
unduly reduced scale as compared with the impact damper 10. FIG. 3 is a 
side sectional view corresponding to FIG. 2. Since the impact damper 10 of 
FIG. 2 is substantially the same as the one shown in FIG. 1, reference 
numerals are used to designate the principal members or portions only. The 
bed plate 10a of the impact damper 10 is mounted on an upwardly extending 
mounting member 50 with a T-shaped section by means of fitting screws 10b. 
The mounting member 50 is attached to the pipe 52 by means of fitting 
members 58 which each consists of a pair of substantially semicircular 
curved members 54 embracing the pipe 52 and bolts and nuts. A multitude of 
the impact dampers 10 of the invention are attached to the pipe 52 at 
suitable intervals so as to damp and prevent the pipe from vibrating 
substantially when an external vibration or shock such as an earthquake 
shock is applied to the plant, thereby protecting the pipe from damage. 
For example, the pipe may be one of those pipes which are used in piping 
systems in plants. In this embodiment, the additional natural frequency of 
an additional vibration system representing the vibrational characteristic 
of the impact damper is limited from 60% to 80% of the main natural 
frequency of a main vibration system representing the vibrational 
characteristic of the pipe 52 as the object of damping, and the space d 
between the additional weight 20 and the stop 46 as measured when the 
weight 20 is not vibrating is limited to 80% or less of the resonance 
amplitude D of the pipe 52 in vibration without impact damping. The ratio 
.epsilon. of the space d to the resonance amplitude D, i.e., 
d/D=.epsilon., is referred to as a specific space. Thus, the space d may 
be zero. 
Now there will be described results of a test on damping effect conducted 
with use of the impact damper. FIG. 4 shows a testing device 100, in which 
a pipe 106 is rotatably mounted at point B on a support 104 standing on a 
floor 102, the pipe 106 is vertically vibrated at the left end or point A 
with an amplitude of .+-.0.25 mm, the impact damper of the invention is 
attached to point C where the amplitude of the pipe 106 is measured on the 
pipe 106, and the right end of the pipe 106 is supported at point D (where 
the amplitude of the pipe 102 is measured) on a stand 108 by means of a 
U-bolt. The pipe 106 has a diameter of 48.6.phi. and a natural frequency 
of 8.1 Hz, and is subjected to a weight of 32 kg. In this case, the 
resonance amplitude obtained is 4.8 mm. 
FIG. 5 is a graph prepared by plotting the change of the resonance 
magnification .xi. obtained as the ratio .eta. between the additional 
natural frequency of the impact damper and the main natural frequency of 
the object of damping is changed and where the space d between the 
additional load 20 and the stop 46 is 0.8 mm. The resonance magnification 
is defined as the ratio of the amplitude of the actual vibration of the 
object of damping to the amplitude of the vibration applied to the object 
of damping. As may be seen from FIG. 5, the resonance magnification .xi. 
is minimized at a point where .eta. is approximately 0.7, and takes 
relatively low values elsewhere. It was confirmed that substantially the 
same result may be obtained where the space d is approximately 0.8 mm or 
less. 
Thereupon, the damping ratio .gamma. was measured for three values of 
.eta., 0.6, 0.7 and 0.8, as compared with the specific space .epsilon. 
varying from 0 to 0.8. Here the damping ratio .gamma. is defined as the 
ratio of the amplitude of the pipe obtained with use of the impact damper 
to the ampitude obtained without the use of the impact damper. FIG. 6 
shows curves corresponding to the three values of .eta.. The axes of 
abscissa and ordinate represent .epsilon. and .gamma., respectively. 
Although the measurement was made for more varied values of .epsilon., the 
plotted spots are thinned out for the simplicity of illustration. If we 
have .eta.=0.6 to 0.8 and .epsilon.=0 to 0.8, .gamma. is reduced to 
approximately 0.5 or less. 
As may be seen from the aforesaid test results, it is resonable to set 
.eta. within a range 0.6 to 0.8 and .epsilon. within a range 0 to 0.8. 
Namely, according to this invention, a substantial damping effect may be 
obtained for a wide range of the frequency .eta., 0.6 to 0.8, with use of 
.epsilon. ranging from 0 to 0.8. 
In the damping effect test, the mass ratio .mu. of the additional weight to 
the pipe is 0.027. In testing, various values are used for .mu., and 
substantially the same damping effect may be obtained with use of the 
values of .eta. and .epsilon. within the aforementioned ranges.