VERIFICATION OF TAPPING MACHINES
Håkan Andersson
SP Swedish National Testing and Research Institute
Acoustics Laboratory
Box 857, SE-501 15 Borås, Sweden
[email protected]
ABSTRACT
Six tapping machines of three different models have been verified. Only one of them was fulfilling the
specification. All the others fail in at least one of the tests, such as velocity at impact, interval of impact or
hammer head dimension.
The high ratio of tapping machines not fulfilling the specification any more points out the need for regular
verification.
1. INTRODUCTION
The requirements for tapping machines are specified in annexes to ISO 140-6 to 8 [1]. They are mechanically
defined such as number and mass of hammers, angle, timing and velocity of hammer impact, etc. According to
ISO 140, some of the parameters only need to be determined once and some should be verified regularly.
A procedure for regular verification of tapping machines has been established at SP. Verification
measurements have been made on a batch of two machines of each model B&K 3204, B&K 3207 and
Norsonic 211. Two of them were very old, three of them have been used for a while and one was almost brand
new.
2. METHODS AND RESULTS
2.1. General
ISO 140 requires that the tapping machine is verified on a test surface that is flat to within ±0,1 mm and
horizontal to within 0,1°. To comply with that requirement the tests at SP are carried out on a polished stone
table with a mass of 500 kg, designed for precision dimensional measurements.
Besides the tests described below, checks for exceptional sounds are made by listening.
2.2. Impact momentum
The momentum of each hammer that strikes the floor shall be of an effective mass of 500 g, which falls freely
from a height of 40 mm. The momentum tolerance is 5 %. As friction of the hammer guidance is not
negligible, the hammer velocity at impact has to be measured.
The method used at SP for measuring falling velocity is laser interferometry. A retroreflecting tape is
fastened to the impact surface of the hammer, which is positioned over a hole in the stone table. Below the
table the laser interferometer beam is pointing at the retroreflector through the hole in the table, measuring the
falling velocity.
Joint Baltic-Nordic Acoustics Meeting 2004, 8-10 June 2004, Mariehamn, Åland
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Picture 1. Test table with laser interferometer below.
As mentioned above, the impact momentum is a function of mass of hammers and velocity at impact. For all
tested tapping machines, the mass of hammers were within tolerances. However, three of the tapping machines
had at least one hammer with a velocity outside the tolerance. The result of the velocity measurements of the
six tapping machines is given in figure 1. The fact that the brand new tapping machine has all its hammers in
the two right bars in the figure indicates that the friction of the hammer guidance increases by the use of the
machine.
Cleaning and greasing the hammer guidance according to the manufacturer’s manuals did not in all cases
bring the falling velocity within the tolerance. In some cases the falling height had to be adjusted to a level
above the length of the height adjusting rod, supplied with the tapping machine.
Number of hammers
6
Tolerance
5
4
3
2
1
92
0
90
5
89
0
87
5
86
0
84
5
83
0
81
5
80
0
78
5
77
0
75
5
74
0
72
5
0
Impact velocity (mm/s)
Figure 1. Results of impact velocity measurements.
Joint Baltic-Nordic Acoustics Meeting 2004, 8-10 June 2004, Mariehamn, Åland
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2.3. Impact interval
12
Tolerance
10
8
6
4
2
90
10
0
11
0
12
0
13
0
14
0
15
0
16
0
17
0
18
0
80
70
60
50
0
40
Number of hammer pairs
According to ISO 140, the time between successive impacts shall be 100 ±20 ms and the mean time between
impacts shall be 100 ±5 ms.
The method used to measure the impact timing requires that an accelerometer and a storage oscilloscope
are used. The accelerometer is placed on the table with the same distance to the two successive hammers to be
measured. All the other hammers are damped.
The results of the impact interval measurements of the six tapping machines are given in figure 2a and 2b.
The five hammer pairs outside the tolerance belong to the same tapping machine.
Time between successive impacts (ms)
Number of tapping machines
Figure 2a. Results of impact interval measurements.
3
Tolerance
2
1
0
91
92
93
94
95
96
97
98
99 100 101 102 103 104 105
Mean time between impacts (ms)
Figure 2b. Results of mean impact interval measurements.
Joint Baltic-Nordic Acoustics Meeting 2004, 8-10 June 2004, Mariehamn, Åland
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2.4. Hammer head diameter and curvature
The tip of the hammer shall be spherical with a radius of curvature of 500 ±100 mm. The diameter of the
hammer heads shall be 30 ±0,2 mm.
The curvature measurements are carried out using a surface roughness instrument at the department for
dimensional metrology at SP. The curvature is measured along two perpendicular lines through the centre
point of the hammer tip. The curvature radius is determined using the least square method.
The results of the curvature radius measurements of the six tapping machines are given in figure 3.
Number of hammers
12
10
Tolerance
8
6
4
2
62
0
60
0
58
0
56
0
54
0
52
0
50
0
48
0
46
0
44
0
42
0
40
0
0
Curvature radius (mm)
Figure 3. Results of hammer tip curvature measurements.
The diameter of the hammer heads are measured using a micrometer.
The results of the hammer head diameter measurements of the six tapping machines are given in figure 4.
The hammer outside the tolerance had visual damages close to the rim.
Number of hammers
14
Tolerance
12
10
8
6
4
2
0
29,80 29,85 29,90 29,95 30,00 30,05 30,10 30,15 30,20 30,25 30,30
Hammer head diameter (mm)
Figure 4. Results of hammer head diameter measurements.
Joint Baltic-Nordic Acoustics Meeting 2004, 8-10 June 2004, Mariehamn, Åland
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2.5. Falling direction
The falling direction of the hammers shall be perpendicular to the test surface to within ±0,5°. The method
used means that the blade of an angle meter is aligned to the vertical envelope surface of the hammer. To give
space for the angle meter the tapping machine is placed on distance cylinders.
All six tested tapping machines had falling directions within the required tolerance.
Picture 2. Measurement of falling direction.
3. CONCLUSIONS
The high ratio of tapping machines, five of six tested, found not to fulfil the requirements of ISO 140 any more
points out the need for regular verification. Checking the falling height with the supplied height adjustment
rod is not always sufficient due to increased friction of the hammer guidance. The verification interval needed
is dependent on the time and environment of use but should not be more than three years.
4. REFERENCES
[1] ISO 140 Acoustics – Measurements of sound insulation in buildings and of building elements, part 6-8,
Annex A.
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