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Antenna Clamp Installation
How many mast clamps do I need? Many factors influence the number of mast clamps required for an antenna, including the antenna’s size, weight and surface area, and the prevailing weather conditions. Unbalanced antennas (where the surface area of the antenna on one side of the mast is greater than the surface area on the other side) pose yet another complication that produces unnecessary torque on the mast and rotor, and requires more mast clamps to keep the antenna in place. This torque can be easily offset by the use of small area balancing items, such as “sails,” which are small plates attached to the boom. A good book for people who build antennas is Physical Design of Yagi Antennas by D. Leeson, available at the ARRL bookstore among other places. This book covers the subject of torque balancing. DX Engineering has devised Yagi Mechanical Antenna Design Software, an easy-to-use program that calculates stresses on the elements, boom and mast. A version that will automate the procedure of torque balancing will be available soon. If you know the maximum torque your antenna is likely to encounter and the torque that each clamp can hold, then you can easily determine the number of clamps that you will need. EXAMPLE: A Yagi antenna designed with 2.5” clamps is expected to encounter torques up to 1550 ft-lb. From the table below, we see that each 2.5” clamp supports 245 ft-lb. Dividing the total force (1550 ft-lb) by 245 ft-lb/clamp results in 6.3 clamps, which of course, means you will need 7 clamps for that mast or boom.
*Important:
The use of molybdenum-based lubricant enables a uniform torque on a bolt or nut to always produce a uniform clamping load during installation and prevents galling. We recommend a molybdenum-based lubricant, such as Never-Seez, because it allows you to use the torque values in the table above with confidence, and it virtually eliminates seizing. When installing clamps, apply lubricant to the bolt threads and to BOTH sides of the washers and lock washers, and then tighten each nut to the recommended torque. Be careful to keep excess lubricant away from the mast, boom or antenna elements. When stainless steel hardware is installed without Never-Seez or another molybdenum-based lubricant, the torque values of the fastener do not uniformly correspond to the clamping load provided. Furthermore, for the same torque value on the fastener, the clamp load will vary greatly from one fastener to another. In addition, the potential for galling of the surfaces and ultimate seizing of the fastener is greatly increased without the lubricant. Our research indicates that stainless steel components work flawlessly when proper lubrication and torque are applied. Do I need a torque wrench? Yes. Perhaps in an effort to use fewer clamps many people have a tendency to over tighten them—which means going beyond the elastic limit of the material—which can cause bolts to weaken or break. The recommended torque values are based on independent laboratory studies and industry-accepted practices that allow for additional stresses and variations in instrument accuracy. What happens when I over-torque a fastener? When a fastener is over-torqued, the material of the fastener is stretched beyond its elastic limit. That means that when the fastener is removed, it will not return to its original size and/or shape. It has been changed forever—and not for the better. Furthermore, in many instances the load that it will hold is actually less than it would have held if it were installed properly; so, when you over-torque a fastener you may actually end up with even less clamping action then if you had installed it properly. |
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