Multi-jackbolt tensioners (MJT) are an alternative to traditional bolted joints. Rather than needing to tighten one large bolt, MJTs use several smaller jackbolts to significantly reduce the torque required to attain a certain preload. MJTs range in thread sizes from to and can achieve or more. MJTs only require hand-held tools, such as torque wrenches or air/electric impacts, for loading and unloading bolted joints.
Basics of multi-jackbolt tensioners
Multi-jackbolt tensioners (MJTs), registered under the trademark Superbolt or Supernut, are designed to decrease the torque required to tighten large bolted joints. One of the major problems associated with traditional bolt tightening methods is as the diameter of the bolt increases, the amount of torque required to tighten it increases in the third power of the diameter. Because of this, the largest size bolt a person can typically tighten by hand is . Multi-jackbolt tensioners reduce the amount of torque required to clamp the joint by using multiple jackbolts threaded through the nut or bolt head. The jackbolts, which are small enough to be tightened by simple hand tools, thrust against a hardened washer and generate clamping force on the joint. Loads up to and greater are attainable using only hand tools. MJTs and related products are produced by Superbolt, Inc. in Carnegie, PA.
Types of tensioners
Nut type
Nut type MJTs, otherwise known as Torquenuts, can be retrofitted onto existing bolts or studs. Because of this, they are the most common type of MJT. Bolt type Bolt type MJTs, otherwise known as Torquebolts, have a ring of jackbolts almost touching the bolt shank. They typically have a smaller outside diameter than Torquenuts. With Torquebolts, it is possible to have very close spacing even at high preloads. Thrust-collar type Thrust-collar type MJTs are used in applications where it is difficult to use threaded tensioners, such as on rolling mills where the joint would typically need to be tightened by crane wrenching. Jackbolts push a threadless nut against a retainer ring, which is fitted into a groove and transfers the jacking force to the main bolt or stud. Because there are no threads, they can also be used in press column applications, eliminating the problems and hassles associated with large nut threads.
Construction and operation
The main bolt or nut is assembled by hand, and there is no torque load on its thread as it is turned up about a quarter to eighth turn short of contact. Therefore it has no flats or splines, but is cylindrical and may have a knurled outer surface. There are six, eight or more hex headed jackbolts evenly spaced around the head, which are long enough to extend beyond the base surface to stretch the main bolt to the required pretension, and small enough diameter for manual tightening. When the fastener is assembled, a hardened washer is fitted under the head for the jackbolts to bear against without damaging the fitment surface. After loose assembly by hand, four of the jackbolts are torqued up to 50% pre-tension in a top-bottom-right-left sequence using a standard manual torque wrench, then retorqued in the same order to full nominal preload, After this they are all sequentially retightened to full torque until less than 20 degrees turn is needed to click the torque wrench, at which point the installation is complete. Removal is also in stages, loosening each jackbolt by one eighth turn on the first round, then one quarter turn per round repeatedly until loose and the main fastener can be turned by hand.
Mechanical advantage
MJTs have a high mechanical advantage for tightening large nuts. To calculate mechanical advantage over standard hex nuts, the following formula is used:
Where:
{ | border="0" |- |MA ||= overall mechanical advantage |- |T ||= main thread torque |- |Tj ||= jackbolt torque |- |n ||= number of jackbolts |- |D ||= main thread pitch diameter |- |d ||= jackbolt pitch diameter |- |A ||= torque advantage of jackbolts over tension bolts (with graphite lube A = 1.37) |}
Safety
MJTs are a relatively safe bolting method because only small hand or air tools are required for tightening. Other methods, including thermal tightening, crane wrenching, hydraulic wrenches and sledgehammers, can cause serious worker injuries due to heat, electricity, highly pressurized hydraulic fluid and brute force. Back and other injuries can also be avoided because no heavy tightening equipment needs to be lifted onto the bolt. Safety is also increased because MJTs prevent parts from becoming loose.