ESP® Elastic Strain Preload Technology

ESP Elastic Strain Preload diagram

Lokring Elastic Strain Preload (ESP) Product Design Technology

Lokring fittings use a patented, elastic strain preload (ESP) technology to connect small diameter piping and tubing without threading or welding. Lokring fittings are designed to be used for field fabrication, for rapid repair, for installation in tight access areas, and for tie-ins of shop fabricated piping. The installation of Lokring fittings is simple, repeatable, and can be made in a fraction of the time required to weld the connection.

Fittings are installed with simple hydraulic tooling and form a leak-tight, metal-to-metal seal without O-rings or other elastomeric seals. The fittings are installed without hot work or open flame, making them ideal for installations in operating areas where welding is not feasible due to explosive atmospheres, etc. and where the use of a threaded connection is not acceptable. Because they require no hot work, the installation of Lokring fittings is the preferred method to connect pipe and tube than that of welding.

Lokring Installed Fitting and Sectioned Lokring Fitting/Pipe

Following insertion of the pipe end into the fitting, hydraulic tooling is used to advance the drive ring axially over the fitting body, radially compressing (swaging) the fitting body on to the outside diameter (OD) of the pipe. As the pipe is compressed first elastically and then plastically by the swaging action during installation, circumferential sealing lands, machined in the bore of the fitting body, grip and seal on the pipe OD, forming a gas-tight, metal-to-metal seal without O-rings or other elastomeric seals. The diagram shows the sealing lands on the coupling fitting body inside surface, indenting the pipe OD.

There is an inherent tendency for tensile, compressive and hoop forces/stresses to remain in state of equilibrium because of the elastic nature associated with the fittings. The pipe/tube wall resists this swaging action, generating high unit compressive loads at the contact points between narrow sealing lands inside the fitting body and the pipe/tube surface.

These contact stresses are sufficiently high to plastically yield the pipe/tube surface under the sealing lands, forming a perfect 360° circumferential, permanent, metal-to-metal seal between the pipe/tube and the fitting body. The driver, which experiences a small increase in diameter (elastic strain) during installation, exerts an elastic, radial preload on the metallic seals for the life of the connection.

Original Shape Before Install

As Driver is driven onto coupling Body the Driver is strained elastically inducing ESP to the assembly while the body is elastically/plastically deformed. The plastic strain permanently deforms the body while the elastic strain provides radial preload to the driver.

ESP Original Shape Install

The body is compressed onto the pipe forming a permanent bond while also forming a pressure seal at the 3 seal lands. The pipe is elastically/ plastically strained but to a lesser extent than the body.

The elastic pipe strain provides radial preload to the body permanently locking it to the coupling assembly.

To find out more about our ESP technology, contact your local distributor for technical assistance.

Learn More about ESP

Are you responsible for designing pressure piping systems according to ASME B31?  Get in touch with one of our global sales and service team for support in designing ASME B31 pressure piping using LOKRING® Elastic Strain Preload (ESP®) fittings.