Basic Principles Of Tube Expanding
Tube Expanding is based on engineering principles, involving elasticity and plasticity, to compress the OD of the tube against a fixed container, such as rolling tubes into tube sheets, drums, ferrules or flanges. To assure a proper tube joint, the tube wall must be reduced by a predetermined percentage. The following chart can be used for determining the correct tube wall reduction.
Determining Proper Wall Reduction
|Tube Test Number||1||2||3||4||5||6|
|A||Tube Sheet Hole Size||0.757|
|B||Tube Outside Diameter||0.750|
|C||Clearance (A – B)||0.007|
|D||Tube Inside Diameter||0.620|
|E||Tube Inside Diameter
When Metal-To-Metal Contact Is Reached (D + C)
|F||Tube Inside Diameter After Rolling||0.636|
|G||Actual Amount of Roll on Diameter (F – E)||0.009|
This chart shows a typical 3/4” – 16 gauge tube. Before rolling this tube you would find the proper rolling dimension as shown.
A. Determine the tube hole size.
B. Determine the tube outside diameter.
C. Subtract the tube outside diameter from the tube hole dimension.
D. Using a tube hole gauge, determine the inside diameter of the tube before rolling.
E. Add the dimension found in “D” to the clearance between the tube OD and the tube hole to get the tube’s inside diameter at metal to metal contact.
F. Roll the tube to what you feel is a good tube joint and check the I.D. of the tube with a tube gauge.
G. Subtract “E” from the rolled diameter to determine the actual amount of expansion (tube wall reduction) on the inside diameter of your tube. This can be converted to a % of wall reduction by dividing the actual wall thickness (“B minus D”) 0.130″ into the amount of roll 0.009.
H. This can be converted to a % of wall reduction by dividing the actual wall thickness (“B minus “D”) .130″ into the amount of roll .009 as shown in “G” above.
You can use this chart to your advantage by predetermining both the percent of wall reduction required and the actual inside diameter, which should be rolled. After the completion of “E” you realize any additional increase of the inside diameter of the tube will result in actual wall reduction. Since the amount of wall reduction greatly determines the quality of the tube joint, you should arrive at the percent required for your application prior to tube rolling.
By subtracting the tube inside diameter “D” from “B”, you determine actual wall thickness. This example would therefore be .130″. If you then take the 7% wall reduction times the wall thickness, you arrive at .0091″. Adding .0091″ (“G”) to .627″ (“F”) we get “F” the inside diameter of the tube after rolling (.636″).
This technique is an excellent way to set torque rolling devices. Once you have arrived at the rolled dimension for four or five tubes, you can roll them and very simply determine if more or less wall reduction is required. Knowing how to determine wall reduction is important; however, it is equally important to know the characteristics of the popular tubing materials. We should know the proper wall reduction, which would apply to each metal. A simple rule of thumb is the harder the material; the less wall reduction is required to obtain a tube joint.
Lubrication is critical in rolling a consistently tight tube joint. Tube expanding creates a great deal of heat and friction, so it is important to use the proper lubrication to obtain good tool life and consistent expansions.
The following are guidelines that have been used in the industry throughout the years. These are in no way recommendations for all heat transfer equipment. These suggestions are offered as a general practices. You should always consult the manufacturer of the heat transfer vessel for specific information before undertaking any maintenance procedures.