Multi-Jackbolt Tensioners Designed for Superior Tightening

Diesel & Gas Turbine Worldwide / November 1997 / Pages 49 & 50 Article by: Joe Kane

Designers have been looking for ways to simplify bolted joints, a necessary part of machinery assembly, practically since the first machine was built. A simple solution to many bolting problems is now offered by Superbolt, Inc., of Carnegie, Pennsylvania, U.S.A.

Rolf Steinbock's striking solution to many bolting problems is the Superbolt. "All Superbolt products are based on our patented multi-jackbolt design," says Allan Steinbock, vice president of sales, " which provides superior tightening while requiring only a hand torque wrench - so hydraulic, thermal and sledgehammer methods are eliminated. Reduced downtime is accomplished because our products are designed not to vibrate loose. Installation time can be reduced and removal becomes a simple task, thus, greatly simplifying the maintenance associated with large fasteners."

The whole idea behind bolting is to join two or more machinery components - pressure vessel piping flanges, compressor casings or turbine casings - with an initial loading that will be greater than the operating load to which the joint will be subjected. The key is accuracy in preloading to avoid damaging stresses and joint leaks and to assure adequate preloading for the application. Other important keys are ease of making the joint and safety in assembly.

A more formal description of the Superbolt concept is "multi-jackbolt stud tensioner." Superbolt tensioners are designed as direct replacements for hex nuts. These devices can be threaded onto a new or existing bolt, stud, threaded rod or shaft. They can be used anywhere that a large hex nut is needed - and with greater security under fluctuating loads, temperature variations and extreme vibration.

The Superbolt stud tensioner consists of a tension ring which threads onto a stud or rod, a series of jackbolts arranged around and threaded through the tensioning ring and a hardened washer or bearing surface. The hardened washer is placed over the stud or rod first and then the tensioner is threaded onto it for positioning. With a simple hand torque wrench, the jackbolts are tightened uniformly until the proper preload is achieved. The tensioner flexes in order to accommodate fluctuating loads and temperatures that could tend to loosen the joint. The preload is obtained without thread welding or galling that is a common problem with large ordinary nuts being torqued on studs. The amount of torque required for adequate preload with jackbolt tensioners is a fraction of that needed for ordinary hex nuts. This accounts for the simple tooling required to install the Superbolt stud tensioner.

An elastic joint, which is characteristic of the Superbolt tensioner, can maintain sufficient preload when length changes occur in bolting systems because of thermal expansion or the settling of gaskets in gasketed joints. The superbolt tensioner will prevent joints from leaking or help seal leaking joints.

Since their introduction in 1984, over 30 variations of Superbolt tensioners have been developed to accommodate a multitude of applications. The various configurations are available in thread sizes from 3/4 in. to 32 in. and in load capacities ranging from five tons to 10,000 tons. This high total load tensioning can be achieved with a 3/4 inch wrench, permitting high-integrity joints even in locations with limited working clearances.

Although small in diameter, the jackbolts are many in number. The result is a high combined thrust force for relatively little individual torque, exerting a strong collective force. Also, the main thread does not slide under the load, eliminating the problem of galling. The main bolt, single-nut stress concentrations are reduced that would otherwise damage the first few threads inside the nut of a single hex-nut system.

Superbolt tensioners for applications under high-temperature conditions are available in AST A193-B7, B16, B8M, and ASTM A453 Grade 660 alloys as well as Inconel and other "super alloys." The company specializes in tailoring its high-temperature multi-jackbolt tensioners to the specific requirements of the application.

"We can point to many, many successful applications of the Superbolt tensioner," Allan Steinbock said. "These include joining high-pressure steam turbine casings, centrifugal compressor casings, joining pistons to piston rods or reciprocating compressors handling heated and high-pressure gases and a multitude of other without failures. If the bolting system using multi-jackbolt stud tensioners is matched properly to the application and the jackbolts are uniformly tightened to the proper preload, then the failure rate of the resulting joint will be zero. This is not a possible claim with the ordinary hex-nut and stud-type bolting system."