The back ferrule achieves the tube grip function in the two ferrule tube fitting. The back ferrule is case hardened to be substantially harder than the tube end. Tube fittings depend on a balance of factors to ensure proper installation and performance. In a two-ferrule tube fitting design, the back ferrule moves the front ferrule forward to spring load the fitting assembly, burnish and seal with the fitting body, and create the primary tubing seal. The front end of the back ferrule cams against a frusto-conical camming surface formed in the back end of the front ferrule. The ostensible angle of this camming surface is forty-five degrees, but due to the sliding movement of the front ferrule, the effective camming angle is actually a shallow angle of about fifteen to twenty degrees. Although the effective camming angle for the back ferrule is shallow, the back ferrule is not required to provide a primary seal (although it can form secondary or backup seals). The back ferrule also does not exhibit the undesired bowing action but rather grips
TECHNICALDOCUMENTONINSTRUMENTATIONTUBINGANDTHEIRCONNECTIONS 2008
Nirbhay Gupta 75
the tube end as a function of a radially inward hinging action. As used herein, the term "hinging" refers to a controlled deformation of the ferrule such that a central region or mid-portion of the ferrule body undergoes an inwardly radial compression, as distinctly contrasted to a bowing or radially outward displacement. Thus, the effective shallow camming angle not only does not compromise the fitting seal capability, it actually substantially enhances the overall performance of the tube fitting especially for stainless steel tubing.
By using separate ferrules for each to achieve primarily only one of the key tube fitting functions, the two ferrule tube fitting achieves tremendous tube grip and seal functions.
The back ferrule also swages the tube to provide the grip needed to keep the fitting and tubing firmly in place. To swage and grip the tube properly, the back ferrule’s leading edge must be sufficiently harder than the tube. Two methods of producing this differential hardness may be employed—
1. Complete surface hardening of the back ferrule:
The use of complete surface hardening on a conventional back ferrule can have several drawbacks.
First, it typically increases installation torque because a surface-hardened, conventional back ferrule is unable to flex or “hinge” downward to improve swaging action on the tube. Instead, it must be wedged into position using installer torque, and as a result, more torque typically is required.
Second, because it is not engineered to hinge and absorb installer torque on remakes, a conventional surface-hardened back ferrule can tend to overdrive the front ferrule when remade. This condition can potentially damage the tubing and fitting body and compromise the front ferrule action required for consistent gas-tight remakes.
2. Selectively hardened back ferrule: Use of a selectively hardened back ferrule, Swagelok reduced installation torque while providing the swaging and gripping action needed to perform in combination with a wide variation of commercial grade tubing. In manufacturing back ferrules selectively hardening the nose of the back ferrule is done, yet the center section and rear flange are left softer. During make-up, this softer center section acts as a hinge point when force is applied to the flange. This hinging mechanism helps limit the amount of torque required by the installer, yet delivers the right amount of swaging action through the nose of the back ferrule.
The improved engineered hinging action of the back ferrule (Figure 4, next page) provides several benefits:
TECHNICALDOCUMENTONINSTRUMENTATIONTUBINGANDTHEIRCONNECTIONS 2008
Nirbhay Gupta 76
It advances and seals the front ferrule predictably and accurately. It flexes to maintain installation torque at a predictable and manageable level, even on hard materials.
It smoothly and efficiently delivers more swaging energy earlier in the pull-up process. As a result, it reduces the potential for improper installation and leakage in cases where the fitting is less than properly tightened.
Its proprietary metallurgy and hinging action can absorb excess torque inputs to help prevent overdriving of the front ferrule, thus ensuring more predictable gas-tight sealing during remakes.
An important aspect of the choice of materials is that the ferrule preferably should be case or through hardened to a ratio of at least about 3.3 and preferably 4 or more times harder than the hardest tubing material that the fitting will be used with. Therefore, the ferrule need not be made of the same material as the tubing itself. For example, the ferrule may be selected from the stainless steel materials or other suitable materials that can be case hardened, such as magnesium, titanium and aluminum, to name some additional examples.
Figure 13-4: 316 SS Advanced S wage lok Tube Fitting Prior to Make -up
The elements of the fitting are depicted in cross-section prior to make-up: the fitting nut (top), the advanced geometry back ferrule (left), the front ferrule (center), and the fitting body (right). The tube wall section is shown below the ferrules and body.
TECHNICALDOCUMENTONINSTRUMENTATIONTUBINGANDTHEIRCONNECTIONS 2008
Nirbhay Gupta 77
Figure 13-5: 316 SS Advanced S wage lok Tube Fitting After Make -up
During make-up, the front ferrule (center) is driven into the body of the fitting (right) and the tube (bottom) to create primary seals (tube and body), while the back ferrule (left) hinges inward to create a strong grip on the tube. The back ferrule geometry allows for an improved engineering hinging action that translates axial (forward) motion into radial swaging action on the tube, yet operates with a low input force (torque) requirement. The improved radial colleting action of the back ferrule (the area to the left of the swage point) isolates and protects the swaged area of the tube, preventing the exposed vibration stress riser that is typical of bite- type fittings.
A distinct advantage of the contoured back ferrule is that pull up forces between the nut drive surface and the contoured face of the Back ferrule are more uniformly distributed across the surface of the back ferrule, thus reducing and substantially eliminating force concentrations. This further reduction of force concentrations on the drive nut reduces pull up torque and
TECHNICALDOCUMENTONINSTRUMENTATIONTUBINGANDTHEIRCONNECTIONS 2008
Nirbhay Gupta 78
13.8.4 Effect of Tube thickness on Swaging
The strength of the fitting is such that the tube contained will burst before the fitting shows any sign of a leak or movement. This is subject to certain constraints on the wall thickness of the tube. Tube thickness is decided by following factors
a. Pressure rating
b. Corrosion/Threading allowance c. Swaging considerations
For swaging over thickness may lead to unreliable joint and in very thin tube it may lead to distortion of tube leading to leakage. Thus considering all the above factors, optimal thickness should be selected when use of compression type of tube fittings is envisaged.
A heavy wall tube resists ferrule action more than a thin wall tube, allowing the ferrules to coin out minor surface imperfections. If the wall is too heavy the rings will not bite.
A thin wall tube offers less resistance to ferrule action during installation, reducing the chance of coining out surface defects, such as scratches. When the tube wall is too thin, the tube will collapse rather than allow the rings to bite fully. Within the applicable suggested allowable working pressure table, select a tube wall thickness whose working pressure is outside of the shaded areas. Reference to the manufacturers' product information should be made in all instances. The tube should generally have a hardness of no more than 80 on the Rockwell 'B' scale.
TECHNICALDOCUMENTONINSTRUMENTATIONTUBINGANDTHEIRCONNECTIONS 2008
Nirbhay Gupta 79