Why Shrink Fitting?

1. Shrink Fitting can often be performed in minutes.
2. Shrink Fitting can provide a precision fit.
3. Shrink Fitting will not damage the majority of ferrous and non-ferrous metals.
4. Heating components may take hours to achieve the necessary expansion.
5. Heating components can cause distortion and damage.
6. After shrinking, component reaches ambient temperature more rapidly than if heated.
7. Heating may introduce an imprecise fit between components.
8. Shrink Fitting maintains the interference fit for which the components were designed.
9. No discolouration of metal after Shrink-Fitting.
10. Shrink Fitting can eliminate the need for keyways or other fixing methods.
11. Shrink Fitting can also be employed to dismantle assemblies.

COST SAVING

• No capital equipment costs.
  
• No transport costs involved from manufacture site to assembly
  
• Damage free fitting
  
• Eliminate time lost with oil baths or heating furnaces.

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.

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.

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.

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 15ºC to -196ºC. 

             To calculate the maximum shrinkage use the above table and the following rule

S = Maximum Shrinkage

OD = Outside Diameter  (in inches)                                            S = OD x TPI   

TPI = Thous Per Inch

Example: For shrinking a 6inch cast iron pipe the maximum shrink age would look as follows

                        S = 6 x 1.8 = 10.8

so the maximum shrinkage of the pipe would be 10.8 thous.