The vast majority of fasteners available in the market utilize UNC or Unified Coarse Thread Series. Mass production and availability of UNC thread types have contributed to this series becoming the industry standard for industrial fasteners. While some distributors stock UNF or fine thread series, these are often very hard to locate and therefore are not used as often by engineers or designers. While there are some drawbacks to using fine thread, their utilization can open up a world of new possibilities on existing applications to achieve higher compressive load with the same targeted bolt stress.
Targeted bolt stresses are often calculated using the root area of the fastener (See ASME PCC-1). It is often pointed out in passing that because the root area of the fine thread fastener is larger than that of his course thread counterpart that greater loads can be achieved. Even though the numbers appear minimal, the resultant area typically ranges between 15% and 20% when calculated. To give an idea what this can do for an existing application, let’s look at an example.
Looking at a 1-3/8” nominal diameter bolt and targeting 60ksi bolt stress, you can see the calculated bolt stress based on the root diameters. The fine thread fastener with the same nominal diameter is able to achieve 20% more compressive load. In a 48 bolt flange, that is an increase of almost 600,000lbs of compressive force. This was achieved simply by converting to fine thread fasteners and since the nominal diameter is the same, they will fit in the existing hole of the flange with the same clearances.
There are some perceived drawbacks to using fine thread fasteners including commercial availability, susceptibility to thread damage in normal handling and a higher tendency to be cross threaded. When managed properly, there are also some significant benefits that can be derived from converting an existing application to fine thread fasteners. In addition to the increase in compressive load using similar bolt stress, fine threads will allow for greater accuracy in tightness adjustments and reduction in vibration loosening both attributed to the increased helix angle.
On our website, www.patriotbolt.com, in the technical section https://www.patriotbolt.com/technical-1 , we have included an Excel worksheet which you can download and see the calculations associated with the comparison. It is not protected and is a useful tool in evaluating any specific application you encounter. Simply enter the desired bolt stress (based on fastener material) and friction factor. If your particular bolt size is not listed, you can change nominal diameter and thread pitch and the rest of the math will be done for you.
Please get in touch with us if you have any questions and our engineers will be happy to assist