56 THE DISTRIBUTOR’S LINK Industrial Fasteners Institute 636 Oak Tree Blvd. • Independence Ohio 44131 Phone: 216.241.1482 • Fax: 216.241.5901 www.indfast.org JAM NUTS PROVIDE A FALSE SENSE OF SECURITY by Joe Greenslade Director of Engineering Technology, IFI Every time I get a question about the use of jam nuts I get nervous because there is a good likelihood that the end user thinks their getting some resistance to loosening from vibration, which they are not. In short, the use of jam nuts does not provide any locking effect and can be dangerous if used alone in tension applications. A few weeks ago I was approached by two very experienced fastener people who wanted me to play referee in an argument they were having over the age-old question of, “When using a jam nut with a hex nut should the jam nut be on top or on the bottom” I stunned them both when I said, “In my opinion you are both wrong! As far as I am concerned jam nuts should never be used at all.” Some time ago I researched the question of whether the jam nut should go on top or the bottom. I found an old document on the subject that appeared to be very authoritative. It stated that the jam nut should go on first and be against the work piece. The document went on to state that the jam nut should be tightened to a torque value less than a hex nut to assure that the jam nut does not strip. Then the hex nut is run down until it touches the top of the jam nut. Then to prevent rotation, a thin open-end wrench is placed on the jam nut and not allowed to turn while the hex nut is tightened to its prescribed tightness. The theory is that the threads in the jam nut, being thinner than the hex nut, will distort causing the jam nut to be locked into place. I cannot agree with this. In the first place a jam nut can only withstand about 50 – 60% of the load of a standard hex nut. That is because there is a very close linear relationship of the thickness of a nut and its ability to carry the full load capacity of the properly mated bolt. Standard hex nuts are designed such that the proof load of the nut is equal to or greater than the minimum ultimate tensile strength of the mating bolt. In joint design the objective is to always break the bolts if they are over-tightened and to never strip the threads from the nut or other internally threaded component. Jam nut thickness ranges between approximately 50% and 60% of a standard hex nut. It is highly unlikely that a jam nut can be tightened constantly to the recommended torque tightening value prescribed for the mating bolt. In many cases the nut is likely to strip before reaching the desired bolt tension equal to 70 – 80% of the bolt’s yield strength. If the tightening torque is reduced to make sure the jam nut does not strip, then bolt tension is likely too low to create an ideal joint. One might ask, “If the jam nut is under-tightened to accommodate the jam nut, doesn’t the bolt tension increase if the hex nut on top is properly tighten to its higher value” The answer is “NO”. If any additional tension is created by holding the jam nut still and applying full torque with the top nut, all of the tension is created between the two nut faces and does NOT transfer through the jam nut to create more tension in the bolt. please turn to page 97
THE DISTRIBUTOR’S LINK 57 NATIONAL INDUSTRIAL FASTENER & MILL SUPPLY EXPO LAS VEGAS - OCTOBER 10, 2012 more photos on page 76
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