Rolls for tube straightening machines
A certain black art and mystery surrounds tube and pipe rotary straightening machines.
Like all machinery, the various components and systems join together to form the working straightener, however the heart of a tube straightener, is the set of work rolls, specially contoured to enable the machine to straighten a range of different tube diameters.
When a new straightener is put into service, for the first period of operation, the freshly trained personnel concerned with operating and maintenance are well versed in using and caring for the machine. But as time passes, recessions and good times come and go, but a well constructed straightening machine lives through it all giving 2, 3 or even 4 decades of useful life. Throughout it's life many personnel changes will occur and the techniques of operation will also change, usually for the worse, until a customer complains of poor tube straightness, barber pole marks along the tube and, or serious mechanical defects in the tube material.
With the machine in reasonable mechanical condition and roll drive systems operating correctly, most problems can be eliminated by ensuring that the rolls are in good condition, properly aligned in the machine and then correctly setup by a well trained operator.
Roll Profile
"We just send them out to our local shop and they regrind them with a radius". This comment is heard often. The fact that the profile should be a precise hyperbolic curve has got lost as time passed.
So what exactly is the correct profile for the rolls in a rotary tube straightening machine ?
The roll profile can be considered to be correct when:
The roll angular position can be set such that the range of tube diameters covered by the machine can all be nested into the roll to give 100% line contact along the profile of the roll; and,
When the "scroll pitch distance" of the largest tube is less than the length of the roll profile by a factor of at least 1.1
The first point will be understood, but an explanation may be needed for point 2.
The "scroll pitch distance" is the amount of forward linear movement that the tube makes, during which is completes a single revolution. The photographs show a means of simply doing a test to determine the scroll pitch distance for a specific tube diameter, which in the case pictured, the tube diameter exceeds the capacity of the machine.
The OEM of the straightening machine has designed the roll dimensions and distance between roll pairs to ensure that satisfactory straightening is possible throughout the diameter range of the machine.
The roll profile is a hyperbolic curve generated by computer. In order to generate the correct profile the exact OEM roll data must be used. It is vital that the correct mean roll angle and diameter are input to the profile generating mathematical equation. Without the OEM data it is not possible to create the correct profile.
If a local job shop re-profiles or makes new rolls using a simple radius for the roll profile the tube will only contact the roll in three places as depicted in the graphic.
So far we have only discussed the hyperbolic curve. But this exact curve must terminate at the roll shoulders on either side of the roll profile.
This transition from the hyperbolic curve to shoulder has undergone much development in recent years. Now using computer generated profiles a "soft" transition using perfectly blended dual tangential radii is possible. This "soft" transition reduces the marking of the tube when using maximum tube to roll line contact with the profile.
Roll Alignment In The Straightening Machine
Assuming the tube maker has just purchased a new set of rolls or received an existing set back from having them reprofiled. The profiles have been generated using the correct OEM data and the maintenance department have reassembling them into the machine.
The next step is vital and often overlooked � Align the rolls with a test bar.
To straighten correctly the rolls must be in near perfect alignment.
To align the rolls a solid steel ground "calibration bar" is used. The majority of machines have a means of adjusting the lateral position of the rolls.
If the rolls are not correctly aligned is will not be possible to gain correct line contact with the roll profiles. Poor alignment will also cause rapid roll wear.
Roll Drive Equipment
Straightening machines have been built with various methods of driving the rolls. Some machines have a separate drive motor for each roll, whereas the majority of modern machines usually use 2 drive motors, one motor for the top set of rolls and one for the bottom rolls.
Regardless of the way the rolls are driven it is important that all rolls revolve at the same speed within about 1 to 2 rpm at maximum machine speed. In addition it is usual for the manufacture to build in an electronic load sharing system to ensure that the power needed to straighten is shared equally between the driving motors.
Each time the rolls are changed a check of roll rpm and motor loading should be carried out.
If the rolls are not revolving at highly similar speeds, under load, tube straightness will be poor, roll wear will be rapid and under high straightening loads the motors may run out of power and stall.
Roll Wear
It is important to always keep rolls in a matched set.
The amount of roll wear should be checked on a monthly basis and logged. The easiest way is to measure and monitor the diameter of the roll in the center of the profile. When the difference in the diameter reaches the OEM advised maximum deviation, the rolls should be reprofiled to bring them back to standard.
The majority of rolls are manufactured from a high chrome, high carbon tool steel which is through hardened to values usually between 58 and 64 Rockwell C.
Rolls can often be reprofiled until the diameter at the center is down to about 80% of the original size.
The photograph of a small roll is typical of the abnormal wear than takes place when the rolls are not aligned correctly in the machine.
This specific roll has worn on the right side of the profile showing that the tube was running out of center.
Special Straightening Rolls
For unusual straightening operations where the surface of the tube must not be changed during the straightening operation special non marking rolls have been developed.
These rolls are coated with a variety of polymers and also fibrous materials.
Concluding Comment
Yes, there certainly is some black art surrounding tube and pipe rotary straightening machines, however, if the correct roll profiles are used and the rolls are properly aligned in the machine, a well trained operator will be able to achieve good straightness.