COST SAVING
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No capital equipment costs.
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No transport costs involved from manufacture site to assembly.
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Damage free fitting.
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Eliminate time lost with oil baths or heating furnaces.
LIQUID NITROGEN SHRINK FITTING
PROCESS EXPLANATION
For many applications where fitting is carried out by heating the outer
components or force fitting. Liquid Nitrogen Shrink fitting offers a better
alternative for fitting.
With Liquid Nitrogen shrink fitting the necessary clearance is obtained by
shrinking the inner component (instead of expanding the outer one). This is
achieved by immersing the inner component in a bath of Liquid Nitrogen at
a temperature of -196C.
After assembly the inner component warms up, expanding to form a tight
distortion free interference fit.
ADVANTAGES OVER HEAT EXPANSION
COST SAVINGS:
A 4" diameter mild steel shaft can be shrunk by 7.6thou in just 20minutes
(refer to metellurgical chart) smaller components such as valve guides, etc.,
take only a matter of seconds. No time is lost with oil baths or furnaces as
the component is simply immersed in a bath of Liquid Nitrogen.
The shrunken component is easier to work with than heat treated material
therefore, reduces handling time.
DAMAGE FREE COMPONENTS:
Liquid Nitrogen has no effect on ferrous or non ferrous materials therefore
problems of distortion and metallurgical change, such as loss of hardness
are eliminated (except for certain austenitic steels). Finished components
can be assembled using LN fitting without damage.
LOW CAPITAL COSTS:
Oil baths and heating furnaces are eliminated, resulting in savings on
capital costs. You also save on valuable workshop space. The equipment needed
for LN shrink fitting is both simple and inexpensive. One off jobs can be
carried out with complete success using a metal bucket, empty oil drum
or open tank as the LN bath. Small components can be handled using wires or
cords. Larger components may require ordinary slings.
ADVANTAGES OVER FORCE FITTING
COST SAVING:
LN fitting does not require force to be used therefore distortion is
eliminated resulting in damage free fitting.
Rejects due to misaligned pressings, etc., are eliminated with LN shink
fitting.
LOW CAPITAL COSTS:
Force fitting usually requires a heavy press with special dies. Compared
with these capital costs. LN shrink fitting costs are negligible.
TECHNICAL INFORMATION
SHRINKAGE OBTAINABLE WITH LN COOLING:
Inserts are cooled from ambient temperature (say 15ºC) down to LN temperature
(-196ºC): that is through 211ºC. The coefficients of expansion and contraction
for common metals over this range and the maximum shrinkages obtainable
with LN cooling are given in Table 1.
For Easy assembly clearance of not less than 0.025 mm for smaller inserts
from 25 mm diameter, to 0.05 mm for inserts of above 150 mm diameter, is
required between cold insert and hole. To find the maximum interference
available in shrink fitting, subtract this clearance from the maximum
shrinkage. The maximum shrinkage per unit diameter is shown in Table 1.
METALLURGICAL CONSIDERATIONS
Aluminium, copper and copper alloys (bronze, brass, Monel) are not
significantly affected by the low temperature of liquid nitrogen. But plain
carbon-steel and low-alloy steels are temporarily embrittled or lose impact
strength at LN temperature and should not then be subjected to severe blows
or shock. The steel returns to normal when it reaches ambient temperature
again. Our highly trained staff have the necessary expertise to carry out
Liquid Nitrogen shrink fitting in your workshop or construction sites if
required.
TABLE 1
Maximum shrinkage in metals cooled by LN. Cooling assumed to be 15°C to
-196°C.
|
|
Expansion Contraction Coefficient |
|
| Metal |
SI units
µm/
m/ºC |
British units
10¯6in/
in/ºF |
Maximum shrinkage thous per inch |
| Aluminium
(unalloyed) |
18.4 |
10.2 |
3.8 |
| Brass
(Cu/Zn 70/30 or 60/40) |
16.0 |
8.9 |
3.4 |
| Bronze
(Cu/Zn 88 10 2) |
16.0 |
8.9 |
3.5 |
| Cast Iron
(plain 3%C) |
8.5 |
4.7 |
1.8 |
| Copper
(unalloyed) |
14.1 |
7.8 |
3.0 |
| Magnesium
(unalloyed) |
21.4 |
11.9 |
4.5 |
| Monel
(cupronickel Cu Ni 70 30) |
13.0 |
7.2 |
2.7 |
| Nickel
(unalloyed) |
10.1 |
5.6 |
2.1 |
| Steel,
plain carbon (AS 1442, etc.) |
9.0 |
5.0 |
1.9 |
| Steel,
alloy (low alloy AS 618, AS 619,
etc.) |
9.0 |
5.0 |
1.9 |
| Steel,
alloy (high alloy austenitic AS 618,
AS 619, etc.) |
9.5 |
5.3 |
2.0 |
| Zinc (Zn
base die cast metal) |
26.4 |
14.6 |
4.1 |
To calculate the maximum shrinkage use the above table and the following rule:
S = OD x TPI
Where:
S = Maximum shrinkage
OD = Outside Diameter (in inches)
TPI = Thous Per Inch
Example: For shrinking a 6 inch cast iron pipe the maximum shrinkage would look as follows:
S = 6 x 1.8 = 10.8
so the maximum shrinkage of the pipe would be 10.8 thous.
For further information or obligation free quotation please contact us by
Clicking Here.
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