Vice President of Engineering
V-retainer couplings are an economical and proven way to connect ducting, tubing, filters, and other components in high-temperature, high-pressure pneumatic and mechanical applications. In the aerospace industry, for instance, they connect bleed-air ducts, environmental control systems, and wing de-icing and exhaust systems, pumps, and filters. Compared to other fastening and joining techniques, V-retainer couplings bring substantial savings in assembly and maintenance to the bottom line, especially when connections are frequently put together and taken apart.
Compared to bolted flanges and welded joints, which are costly to manufacture and difficult to assemble, V-retainer couplings are relatively inexpensive and fasten with only one or two bolts. This is extremely important when access is limited or small envelopes are specified. During assembly, the coupling latch can be oriented at any convenient point around the flanges on the tubing or ducting and there are no bolt holes to carefully align. And unlike bolted flanges, coupling retainers apply and maintain uniform closing forces on the circumference of the flanges. Also, V-retainer couplings are more aesthetically pleasing and can be a designer's best friend when fighting a weight battle.
V-couplings come in two types: V-bands and V-retainers. V-bands have multiple segments and a full sheet-metal band either spot welded or riveted together. V-retainers, however, have a single-retainer segment with two sheet-metal loops spot welded to it. Today, the term "V-retainer couplings" refers to both types.
Although the design considerations in this article primarily concentrate on round applications (95% of usage), the same principals can successfully join a variety of square, oval, rectangular and other custom shapes made out of metal or composites.
On V-retainers, technicians apply torque to the coupling nut, generating an inward radial force on the retainer. The radial load created by band tension is transmitted as an axial load on the mating flanges due to the wedging action of the V-retainer. The retainer apex gap and foot clearance are typically .060 in., which must be maintained to prevent the V-retainer from bottoming out on the flanges before it generates the required loading.
The included angle can vary between 30 and 90°, but the industry standard is 40°. This angle provides optimum strength for drawing together a set of flanges and maintaining a tight seal. With a 40° angle, the retainer exerts an axial load on the flanges with an 8.6 to 1 mechanical advantage as the outside band is tightened. Although it is necessary to overcome friction when tightening V-retainers, once tightened, friction actually reduces the bolt load needed to prevent the flanges from separating.
Getting the right V-retainer
V-retainer couplings should be selected on the basis of the strength needed for the specific application. The coupling's strength is determined by retainer thickness, profile, and material. For proper performance, the nominal coupling apex should be 0.046 in. smaller than the flange apex (for a 40° included angle). The nominal coupling diameter should equal the flange diameter plus 0.12 in. (or 0.095 in. for couplings with retainer legs less than 0.19 in. high).
Designers should also know the loads and operating temperatures of the V-retainer's application. The following factors must be known to determine the coupling's total load.
where Ds = seal diameter; M = bending moment; A = axial tension; D = flange OD; FS = safety factor, p pressure (lb-in.); and CT = temperature conversion factor. NP = pressure load, NB = bending moment load, NA = axial tension, and N = total load. The total load then becomes:
This is based on a retainer made of 301 corrosion-resistant steel (CRES) and used at room temperature.
Once total load intensity is known and converted to an equivalent operator pressure, the proper retainer cross section can be selected from a manufacturer's burst pressure charts.
Flanges for V-retainer coupling
Flanges can be either sheet metal, machined, or a combination of both. Sheet-metal flanges are self-energizing and do not require seals or gaskets. They are generally used in low-pressure applications and come in aluminum, stainless, titanium and nickel 625 and 718. Sheet-metal flanges are based on narrow and wide-radius profiles. The difference is in the radius at the top of the flange apex. Bulge-formed sheet-metal flanges come in diameters ranging from 1 to 10 in. For larger diameters, flanges are roll formed and welded.
Machined flanges work well in high-pressure applications. Available in the same basic materials as sheet-metal flanges, they are more robust and use seals, O-rings for low temperatures, and Ni 718 E-seals for high temperature. Machined flange profiles are defined in the aerospace standards AS24563 and AS1895.
Other design considerations
The retainers total load and flanges are not the only criteria. Engineers should consider other design issues as well, including:
Number of retainer segments. V-retainer couplings are made with single and multiple retainer segments. Single-segment retainers are lightweight and generally the most economical. The stiffness of the continuous ring in single-segment retainers makes them suitable only in larger diameters (greater than 5 in.) and applications involving infrequent removal. And they are usually provided with strap loops rather than a full band.
Adding segments adds flexibility and simplifies installing and removing the coupling over flanges. The most popular design uses three segments. In addition to flexibility, this design provides more uniform clamp loading. For larger diameter couplings (8 in. and larger), a two-latch design works best. Two latches provide for uniform torquing and seating of the V-retainer onto the flanges. Two-latch designs have two halves and four retainer segments.
Coupling size. In general, sizes range from 1.5 to 12 in., but some have been made with 12-ft diameters. For couplings with diameters less than 3 in., a bent
T-bolt eases installation and prevents the bolt from hitting the top of the flanges.
Band-loop options. There are two types of band loops: the standard backbend and the tangential backbend. In aerospace and other critical applications, use tangential backbend loops. That's because standard backbend-band loops stretch when torqued and relax when temperatures rise causing some loss of bolt torque.
Construction method. V-retainer couplings are either spot welded together in accordance with AMS-W-6858 (a spot-welding specification), or have the loops and retainer segments riveted onto the band. Spot welding is more economical and does an excellent job. However, riveting is the preferred choice for critical applications.
Latch style. Before selecting a latch, designers should know how often the coupling will be removed and reassembled, as well as the pressure the application is under. There are several latch styles available.
Select a standard T-bolt and trunnion latch if the clamp isn't going to be removed frequently. They work on any size coupling and are some of the strongest standard latches available.
Use standard T-bolt quick-release latches if the coupling is going to be open and closed often. They can be quickly and easily opened by loosening the nut and raising the hinged latch. They work on couplings of any diameter and let operators replace the T-bolt.
Socket-head, cap-screw latches use barrel trunnion and barrel nut hardware and are suited to permanent and high-strength fastening jobs. They also have the most compact profile. The cap screw must be removed to open the coupling.
T-bolt over-center latches work well on couplings that are removed often. They have over-center handles and require no tools to open or close after making an initial adjustment. The latch's nut adjusts tension and its self-energizing clip helps prevent accidental opening. They are not recommended for pressurized applications.
Saddle latches with T-bolts are quick-release latches for light to medium-duty applications. They permit quick access for couplings that must be periodically removed.
There are many combinations of T-bolt and locknut materials. For aerospace and high-temperature applications, experts recommend A286 CRES, a high-temperature stainless steel for bolts and nuts with UNJF threads. A286 CRES is widely accepted as the material of choice throughout the industry. Nuts should be silver plated to add lubricity.
Coating the inside of the retainers with dry-film lubricants helps reduce friction between the coupling and flanges. Dry-film lubricants are also recommended for T-bolt threads to reduce galling and extend thread life. Dry lubricants also help in applying the true torque to the coupling, which contributes to proper joint performance, especially in high-vibration applications.
Material options. Several rules of thumb help engineers choose the right coupling material. For saltwater or corrosive environments, use 316 CRES, which has been passivated to remove any free iron. Do not use aluminum in applications over 400°F, titanium in applications over 600°F, stainless steel in applications over 800°F, A286 in applications over 1,000°F, or nickel 718 for applications over 1,200°F.
Installing V-retainer couplings. Proper installation is critical. Tubes and components to be joined must be aligned with each other before installing the coupling. Flanges must also be aligned correctly prior to coupling installation for good joint performance.