The threaded fastener has been a boon to engineering, allowing the application of axial loads to join a wide variety of components.  Inventors have struggled to come up with analogous devices to apply a uniform radial load to fasten and seal cylindrical components.  This has brought us press fits, thermal shrink fits, shaft collars, hose clamps, crimp rings, magnetic pulse forming, etc.  

UniLok rings offer new ways to join and seal cylindrical components.  These metal rings are supplied in the expanded condition and shrink 4.5% in diameter when heated.  Once shrunk, UniLok rings apply a uniform gripping pressure that is seamless, powerful and consistent. The gripping force can be set between 130 N (30 lb) and 180 kN (40,000 lb) by choice of the ring dimensions.  UniLok rings can clamp a holder to a delicate optical lens or swage a fitting onto a pipe to seal 400 bar (6000 psi).  No other fastener system provides seamless radial pressure over these force levels with comparable radial close-up. 

UniLok rings are made with nickel-titanium shape memory alloys, sometimes called memory metal or Nitinol. See the Technology page for a discussion of the shape memory effect.

We produce UniLok rings from several different alloys, each formulated and processed to achieve specific functionality.  However, we will restrict discussion here to our most commonly used material, Alloy H.  This NiTiNb alloy has a Heat-To-Recover characteristic that makes it suitable for most fastening and sealing applications.  If you find this alloy does not produce the behavior you need, consult the Alloy Selection page to see if any of our other materials are better suited.

Alloy H rings begin to shrink when heated above 45°C(113°F).  Full clamping force is achieved by heating to 165°C(330°F).  To insure full heating, the rings may be marked with temperature indicating paint that changes color at 165°C.  The rings maintain their grip over an operating range of -65°C (-85°F) to +300°C (+572°F).   If the rings are cooled with liquid nitrogen they will relax their grip on cooling below -120°C, allowing disassembly or repositioning.

Benefits
UniLok rings produce highly reliable joints because the shrinkage and clamping pressure generated are intrinsic properties of the ring material.   Benefits over conventional techniques such as crimping, welding, adhesives, elastic assembly and threaded fasteners include:

-seamless, stepless, gapless, 360° clamping pressure
-joins and seals dissimilar materials
-delivers radial clamping pressure up to 140 mPa (20,000 psi)
-low installation temperature
-simple verification of correct installation
-repeatability
-operator insensitive assembly
-immunity to vibration, shock, and thermal cycling
-chemical resistance
-rotary balance
-a unique product that enables efficient designs

 
Follow these simple steps to select the proper ring.

1)  Select the UniLok product drawing for the desired ring geometry.  Locate rings of the nominal diameter required.

2)  Based on the ring cross-section dimensions, and the nominal clamping forces tabulated, choose a ring that meets your space constraints and force requirement.  See PD-009 Clamping Force Calculation for the relationship between clamping force, pressure, and ring stress. 

3)  The outside diameter of your assembly where the ring will be installed is termed the substrate diameter.  Adjust the substrate diameter and its tolerance such that:

The maximum substrate diameter is smaller than the "Minimum As Supplied" ring ID to insure installation clearance.

The minimum substrate diameter, when compressed by the ring, is larger then the "Minimum Substrate Diameter" to insure clamping force generation.

4)   Get some UniLok rings and try them out!  The clamping force varies somewhat with the heating method, the substrate materials and geometry, and the operating temperatures.  Functional testing is required to confirm system performance.  Odds are, the ring you have selected will do the job.  

UniLok H-T-R rings are easy to install.
Position the ring on your assembly and heat until the thermochromic paint changes color.  Heat may be applied by oven, hot air gun, infrared, direct electric resistance heating, etc.

H-T-R Ring Removal
Rings may be removed by cutting with abrasive or carbide tools.  Alternately, HTR rings will relax their grip if cooled below -120°C (-185°F) using liquid nitrogen.  Rings will re-grip on warming to room temperature.  However, the gripping force is sensitive to changes in the substrate diameter.  Please consult Intrinsic if your application requires multiple mate / de-mate cycles.

See the UniLok HTR Installation Guide for more detailed information regarding installation and removal.

Qualification Testing
Once installed, Alloy H behaves like a conventional material, except at low temperatures where the clamping force is reduced.  Normally, minimum operating temperatures can be specified at -55°C to -65°C.  Qualification testing should always include functional testing at the minimum operating temperature of the assembly.

Sizes
Standard Alloy H UniLok rings are available in a variety of rectangular and circular cross-sections with inside diameters ranging from 1.4 mm (0.054") to 150 mm (6.0"). New rings of virtually any size can be produced for specific applications.