The threads on Fluid Systems’ HAM-LET range of hydraulic fittings are rolled. But what does this mean, and why is this a better method than cutting threads as some other manufacturers do? Rolled threads have improved physical characteristics, greater accuracy and a high degree of surface finish. They are produced with no wastage of material, which also results in cost savings. These advantages account for the increased adoption of rolled threads.
The cold forging that threads receive during the rolling process strengthens them in tension, shear and fatigue.
Tensile strength: the cold working of the surface increases the tensile strength of the metal, and static tensile tests have frequently recorded increases of the order of 30% in the breaking strength of the parts.
Shear strength: when a thread is rolled, the fibres of the material are not severed as they are in other methods of screw thread production, but are re-formed in continuous unbroken lines following the contours of the threads. Rolled threads resist stripping because shear failures must take place across rather than with the grain.
Resistance to fatigue: thread rolling increases the part’s resistance to fatigue failure in several different ways. Rolling between smooth dies leaves the thread with smooth burnished roots and flanks, free from tears, chatter or cutter marks that can serve as focal points of stress and therefore, starting points for fatigue failures. Rolling also leaves the surface layers of the thread, particularly those in the roots, stressed in the compression. These compressive stresses must be overcome before the tensile stresses can be built up, which alone, can cause fatigue failures. This increase in root hardness of up to 30% considerably adds to the parts’ resistance to fatigue.
It has been repeatedly demonstrated that any fitting that is properly tightened when it is installed, and remains tight throughout its life, is less likely to fail through fatigue than one that is assembled loosely, or that works loose during service. Threads produced by any of the cutting methods have a surface condition consisting of partly torn-away particles that gradually bear down in service, permitting the fitting to loosen. Rolled threads, on the other hand, are compacted and burnished during threading, making them less prone to loosening, which extends the fatigue life.
Rolled threads show no loss of fatigue strength when heated for several hours to temperatures up to 250°C, whereas fatigue strengths of threads produced by other means are lowered by as much as 25% when treated the same way. Improvements in fatigue strength resulting from all the above factors is reported to be of the order of 50-75%.
The production of accurate threads normally requires that close control be exercised over pitch diameter, thread angle, lead, taper and roundness. There are a number of reasons why it is inherently easier to achieve high accuracy in these parameters by rolling. Equally important, this accuracy is retained over long periods of time.
Thread angle and lead: the accuracy of the thread angle and the lead produced is almost entirely dependent upon the accuracy of the dies. In most cases, the thread angle and the lead on the die is exactly reproduced on the material rolled. The accuracy of the lead produced can also be influenced by the setup of the dies and the material being rolled. Some types of harder and stiffer materials have a tendency to ‘spring back’ after rolling, with the result that the lead may be contracted by a very small amount. In such cases, dies with expanded lead may be used, which will uniformly produce threads of the correct lead.
Roundness: this is dependent on the roundness and uniformity of hardness of the blank, and upon the rate of application and release of the die pressure. If the dies are designed and set up to apply and release pressure gradually and uniformly, close tolerances on roundness may be steadily maintained.
Uniformity: if sufficient care is taken, it is possible to produce extremely accurate threads by any of the common threading methods, but rolling is unique in its inherent ability to maintain the accuracy of the original setup during long runs of high speed production. The thread form of a set of thread rolling dies is faithfully reproduced on the parts and does not change appreciably during the entire life of the dies. Thread rolling dies do not wear out in the same manner as do other threading tools. Wear, instead of being concentrated on a sharp cutting edge, is distributed over a broad surface, and the rolling action is relatively free from friction. Therefore, the thread form of a rolling die is not changed by erosion, nor does it fail to reproduce itself because of dullness or adhesion. It cannot be altered by improper sharpening, as sharpening is never required.
The development of the thread rolling process and the thread rolling equipment is by no means complete. On the contrary, there is more interest and activity in the process now than at any time in its history. New types of machines and attachments are constantly being developed, and the process is finding new applications where its speed, accuracy, uniformity, and the strength that it adds to the parts rolled, can be used to reduce costs and improve the quality of an endless number of threaded parts.
The thread rolling process
Thread and form rolling is a simple cold forging process confined almost entirely to external threads. It is referred to as a cold forging process because most rolling is done on cold blanks. However, rolling of threads on heated blanks has been beneficial on some applications. Today, thread and form rolling is accepted by many industries as a preferred method of producing uniform smooth, precise threads of superior physical qualities.
Hardened steel dies are used to roll the threads. The threaded faces of the these dies are pressed against the periphery of plain cylindrical blanks and reform the surface into threads as the blank rolls on the die faces. The working faces of the dies have a thread form which is the reverse of the thread to be produced. In penetrating the surface of the blank, the dies displace the material to form the roots of the thread and force the displaced material radially outward to form the crests of the thread.
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