TPT July 2007

S traightening & F inishing T echnology

Understanding 6 and 10 roll rotary tube straightening machines

Rotary straightening machines configured with 3 pairs of vertical opposed rolls, all rolls driven, have been the tube industry standard for several decades. The 6-roll setting process remains complex and often the least understood process in a tube making plant. This has been further complicated by the introduction of machines with 10 rolls. Rotary tube straightening machines are termed ‘rotary’ due to the tube revolving as it passes through the machine. In the early days these machines were designed with a number of roll configurations. However, the 6-roll machine became the standard for modern tube straightening. This machine features 3 pairs of vertically opposed rolls, all rolls being driven. The tube is fed into the machine, after which the rolls grip the tube and rotate it while feeding through the rolls until the whole length has been straightened. While passing through the machine, the tube is subjected to two specific straightening forces: pressure straightening and bend (or offset) straightening. During pressure straightening, each pair of rolls can be adjusted so that the gap between them is slightly smaller that the outside diameter of the tube. fi As a tube is passed through the roll pairs, it is subjected to pressure which leads to a degree of straightening

As the tube passes through this restricted gap, it is subjected to pressure. If this pressure is sufficient, it will cause the tube walls to be strained past their elastic limit thereby causing some straightening of the tube. If this ability to squeeze the tube is used correctly, the tube will be ‘rounded up’ thus removing some or all of its ovality. Ovality correction is an important ability of multi-roll rotary straightening machinery. In the process of bend or offset straightening on the machine, one or more pairs of rolls can be adjusted to cause the tube to follow a curved path through the machine. Bending (or offsetting) the tube in this manner is the main straightening action performed. The amount of bend must be carefully controlled so that it is sufficient to take the tube past its elastic limit to achieve straightness, but without resulting in excessive work hardening of the tube material. As the revolving tube passes through the machine it is subject to pressure and bend straightening along its full length and in all axes. As mentioned, there have been a number of machine roll configurations over the history of rotary straightening. Although some staggered roll machines are still in › In offset straightening, one or more pairs of rolls can be adjusted to cause the tube to follow a curved path

› Roll configuration: the 6 roll machine

Ten roll machines are inevitably more complex to set and only a few were installed until computer-setting systems were developed in the 1980s. A 6-roll machine has a single bending moment, in comparison with its 10-roll counterpart that has two bending moments. All straightening machines use specially developed and carefully contoured hardened steel rolls. The profile is hyperbolical and is produced by a three dimensional process. The roll profile is specifically designed to enable a range of tube diameters to be accommodated within a single machine. The full range of tube diameters is accommodated by adjusting the angular position of the roll and the gaps between each pair of rolls. Rolls with worn profiles will not perform correctly and cause poor straightness and marks on the tube. However, rolls can be reprofiled several times during their life. It is important to understand that this generated profile is not a radius to prevent the tube from making contact with the roll along the profile. The machine can only perform well if the rolls are in good condition and have the correct profile. It is important to recognise roll problems, as the roll profiles must be in good condition for correct machine operation. If the profile is worn, the tube will be marked and good line contact will not be achievable. Once the profile has worn ‘off centre’, it will mark the tube and further rapid wear will occur.

operation, the tube industry has standardised in order to use vertical opposed pairs of rolls with all rolls being driven. Machines with 3 pairs of rolls (ie 6-roll machines) have been in production for several decades. In the late 1970s the first machines with 5 pairs of rolls were introduced, specifically where very high levels of straightness were required. Six roll machines require a high level of operator understanding to set the machine and achieve good straightening results.

fi Hyperbolic straightening rolls: the contact pattern on a simple radius is displayed

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J uly /A ugust 2007