Figure 5. Typical steel band yoke. 



fabrication, and operation are 

 quite well understood and within 

 the scope of routine engineering 

 design. 



Although from the design 

 and fabrication viewpoint the use 

 of a yoke for retention of end 

 closures is a desirable design fea- 

 ture, from the operational 

 viewpoint it leaves a lot to be 

 desired. Regardless of whether 

 the vessel is placed horizontally 

 or vertically, cumbersome and 

 complicated mechanisms must 

 be employed to gain access to the 

 interior of the vessel for test speci- 

 men placement and removal. The 

 opening and closing of the vessel 



is a time-consuming operation, primarily because of the weights involved — 

 regardless of whether the yoke is stationary and the vessel movable, or vice 

 versa (Figure 6). 



The tie-rod system of restraining end closures is quite different from 

 the yoke type of restraint system. The tie-rod restraint system is first of all 

 not a continuous band that girds the vessel about the end closures (Figure 7). 

 It relies rather on a series of tie rods to act upon retaining flanges, that in turn 

 restrain the end closures. Because this restraint system is an assembly of sev- 

 eral structural components, it can be taken apart piecemeal for access to the 

 vessel's interior, rather than moving the whole restraint system assembly, or 

 pressure vessel, as is the case with the yoke restraint system. This possible 

 operational advantage, however, is coupled with serious structural disadvantages. 

 These include severe stress concentrations at load transfer points from one 

 restraint component to another, and the need for high-strength materials. The 

 low-grade structural steel generally employed in the yoke-restraint system is 

 inadequate to carry the axial loading distributed among a few tie rods whose 

 number is limited by the circumference of the vessel. Since in the tie-rod 

 restraint system the hydrostatic pressure on the end closures cannot be counter- 

 acted by the bearing stresses on the end closures provided by the yoke system 

 girdle, the design must be quite different from yoke restraint system design. 

 This difference not only extends to the shape of the end closure, which in this 

 case cannot be flat, but rather must be hemispherical, but also to the magni- 

 tude of, and complexity of stresses in it. With yoke restraint, the design and 



