95
94
EN
EN
maximum/necessary power for the pneumatic tool,
because by its limited inner diameter, the hose
will limit the necessary air supply to pneumatic
tools, e.g. air impact wrench, which have a large air
consumption.
y
Pressurised air has different dynamic properties
and behaviour than hydraulic fluid, e.g. transmi-
ssion of power and, therefore, in the case when
a system based on hydraulic fluid works for a cer-
tain process, the use of pressurised air of the same
pressure may not necessarily be sufficient, and it is
necessary to verify the given process by means of
a practical test.
y
The torque of the air impact wrench was
determined by a direct methodology without
the effect of friction forces acting on the area
of contact between the head of the bolt/nut
and the contact surface of the material into
which the bolt is screwed.
When determining the torque of the impact
wrench by means of a torque wrench, the
determined torque of the tightened bolt/
nut is usually smaller due to the effect of the
friction forces acting on the contact areas.
The friction force grows with an increasing
contact surface area and also significantly
depends on the sliding friction on the contact
surfaces, which depends on the type and
slipperiness of the material.
COMPARATIVE TABLE FOR MAXIMUM TORQUE VALUES
Size
bolt/nut
Standard bolts
High-strength
bolts
Strength classes according to DIN 267
3.6
4.6
5.6
4.8
6.6
5.8
6.8
6.9
8.8
10.9
12.9
M6
2.71
3.61
4.52
4.8
5.42
6.02
7.22
8.13
9.7
13.6
16.2
M8
6.57
8.7
11
11.6
13.1
14.6
17.5
19.7
23
33
39
M10
13
17.5
22
23
26
29
35
39
47
65
78
M12
22.6
30
37.6
40
45
50
60
67
80
113
135
M14
36
48
60
65
72
79
95
107
130
180
215
M16
55
73
92
98
110
122
147
165
196
275
330
M18
75
101
126
135
151
168
202
227
270
380
450
M20
107
143
178
190
214
238
286
320
385
540
635
M22
145
190
240
255
320
290
385
510
715
855
1010
M24
185
245
310
325
410
370
490
650
910
1100
1290
M27
275
365
455
480
605
445
725
960
1345
1615
1900
M30
370
495
615
650
820
740
990
1300
1830
2200
2600
Table 2
DETERMINING THE FLOW RATE
OF A COMPRESSOR
Measure the time that it takes to pressurise the
compressor‘s pressure vessel from the atmospheric
pressure to a pressure of 3, 4, 5, 6, 7, 8 bar with the
air outflow shut off.
y
The flow rate of the compressor for the given ope-
rating pressure can easily be calculated from one of
the formulae provided below.
For this calculation, it is necessary to know the
volume of the compressor‘s pressure vessel and
the time in seconds that it took to pressurise the
pressure vessel to the monitored pressure.
The formula for determining the flow rate of the
compressor for a given pressure is the following:
(Pressure in the pressure vessel
╳
volume of the pressure vessel
╳
60)
= Flow rate in L/min
Time for pressurisation to the given pressure in seconds
Example:
The time for pressurising the compressor‘s pressure
vessel with a volume of 24 litres to a pressure of
3 bar is 33 seconds.
The flow rate of the compressor for this pressure is
thus calculated from the aforementioned formula
in the following manner:
(3 bar
╳
24 litres
╳
60) / 33 seconds = 131 L/min.
The flow rate of the compressor at a pressure of
3 bar is 131 L/min.
If the time for pressurising the pressure vessel
of the same compressor to a pressure of 8 bar is
1 minute 55 seconds (115 seconds), the above for-
mula can be used to calculate that the flow rate at
a pressure of 8 bar is:
(8 bar
╳
24 litres
╳
60) / 115 seconds = 100 L/min.
From the above-described it is evident that the
flow rate (performance) of the compressor greatly
depends on the operating pressure, which applies
to all compressors without exception, as it is the
result of physical laws, where growing air pressure
in the pressure vessel leads to greater compression
of air pushed out of the pneumatic cylinder into
the pressure vessel and thereby also its volume.
With growing pressure in the pressure vessel, the-
refore, the volume of the air supplied by the pneu-
matic cylinder to the pressure vessel decreases due
to the effect of greater compression.
b) The air hose must have an inner diameter of at least
9 mm, e.g. Extol® Premium 8865142 (inner
∅
9 mm,
length 10 m, max. pressure 15 bar, PVC with inner
braiding and quick-connects, unwinding; fig. further
in the text), otherwise it will not provide sufficient air
supply to the impact wrench, thus preventing it from
achieving its maximum performance. Likewise, the
air hose should not be spiralled, but rather unwinda-
ble, as a spiral hose reduces air flow through its grea-
ter resistance. The hose should be as short as possible
with respect to feasible options. The longer the hose,
the greater the pressure loss at the hose outlet, which
may also reduce the performance of the impact
wrench. To demonstrate, we‘ll provide an example
available from literature, where if the inner diameter
of an air hose is 10 mm, then at a hose length of 5 m
the inflow pressure of 6.0 bar is reduced by 1.7 bar at
the outlet, and at a hose length of 15 m it is reduced
by 2.2 bar.
The pressure loss at the hose outlet resulting from
the length of the hose must be compensated for by
a greater pressure at the hose inlet and precisely set
using a pressure regulator at the inlet to the pneuma-
tic tool to prevent exceeding the maximum operating
pressure of the given tool.
In the event that a long hose is used, an impact wave
may result when the pneumatic tool is started.
•
ATTENTION
y
In order to achieve the require task (performance)
of the pneumatic tool, it is always necessary to
take into consideration the individual parts of the
entire pneumatic system, i.e. sufficient flow rate
of the compressor, its type and the necessary inner
diameter of the connected air hose - sufficiently
powerful tools. If, for example, the air hose has
an inner diameter of, for example, 6 mm and the
compressor is sufficiently powerful, it is possible
that such a system may not be able to provide the
