Turbo
Machinery International // July - August 2004 A bolting system's effectiveness depends on its ability to connect parts cost-efficiently and to generate proper clamping force in any environment. The bolted connection has to withstand the large and constant forces of vibration, heat, pressure and corrosion. The most common bolting problem is simply insufficient pre-load generated by the bolting method, which can cause the parts to come loose after only a short period of time. The tightening process merits careful consideration. Many applications require large amounts of pre-load (which is essentially the clamping force necessary to secure the connection) such as recent pressure vessels which handle large stresses. With the establishment of ASME Section 8, Division 3 pressure vessel code, extremely large pressures - 10,000 psi and over - are being allowed for various types of reactors. This means that both the sizes of these components and the studs that seal them have also dramatically increased. For example, an increasing number of pressure vessels call for 8-inch to 10-inch diameter studs. In a 10-inch stud, a stress level of 50,000 psi requires a preload of 3,700,000 lbs. torquing accurately a standard hex nut to this load is problematic. Clamping large bolts A bolt fastener's strength increases with the square of its diameter. In order to make full use of this strength, one must be able to torque the fastener until it is pre-stressed to a predetermined level. The fastener's strength increases in the second power of its diameter, but the torque required for pre-stressing inceases in the third power. Because of this reality, bolts larger than 1-inch in diameter cannot be torqued effectively with hand tools such as wrenches, torque wrenches and air impact wrenches. There are several methods available to apply clamping force on the joint and overcome this problem. Some are better than others, depending on the specific challenges presented by each application. Worker safety, speed of installation and removal, pre-load accuracy and durability are key factors to consider when choosing which method is best for your equipment. Hydraulic wrenches, a common method, have been used with some success. But they are expensive and require caution as they have been known to cause hand and arm injuries. Large wrenches are heavy and must be moved into place to function, which can be difficult in tight locations and possibly cause back injuries. Fasteners of 1-inch diameter and larger can be torqued to proper stress using ordinary hand tools with Multi-jackbolt Tensioners (MJT). They can be retrofitted into the same area as a standard OEM nut or bolt. The MJT has a series of jackbolts threaded through the body of the nut. To install, the washer is positioned over the existing stud, and the nut body is threaded onto the main thread of the stud, hand-tight, against the washer. The MJT's most important property is its high mechanical advantage where the clamping load is directly resultant from a relatively small torque input on the jackbolts. For example, the torque required to achieve 1,209,600 lb of preload on a 6 inch hex nut is 106,868 lb-ft. The torque required on an MJT is 189 lb-ft, which means that MJT has a mechanical advantage of 565:1. The worker applies the apropriate torque to each jackbolt using a standard hand-held torque wrench or air tool. Turning the jackbolts pushes the nut body away from the washer surface and creates clamping force on the joint by stretching the bolt or stud. MJTs stay in place and remain on equipment until removal for the next outage, assuming the joint was properly designed in the first place. The high preload (and subsequent clamping load) above the working load does the job of keeping the joint tight. Recently, tapered jackbolt threads have reduced the friction factor by distributing the load more evenly over the threads, which also relieves stress concentrations on the first few threads. Rounded jackbolt tips reduce friction and leave a smallmarking of the washer, also resulting in reduced torque requirements and easier removal. They are considered one of the safest bolting methods for tightening large diameter bolts and studs and have been recommended by insurance companies to their industrial accounts. Special MJTs have been designed for applications where traditional methods would not fit the prescribed space. |