Existing Roll Bite Measurement Techniques

The nature of the rolling interface makes in-situ measurements difficult. Roll bite length was measured by Orowan (Orowan, 1943) using a 2-high mill to roll brass and steel strip. The mill was quickly stopped and the shape of the partially rolled strip studied. With this approach a noticeable bump was observed in the roll bite when rolling brass. This corresponded to a pressure peak part way through the roll bite as proposed by Orowan. Using this technique, it was relatively straightforward to determine the entry to the roll bite, due to the clear angle induced in the strip, however the exit point was much harder to ascertain, making accurate determination of the roll bite length impossible. This method is also unsuitable for measurements in an industrial setting.

Video of strip during rolling was taken and used to assess the length of the roll bite arc by Kobasa and Schultz (Kobasa & Schultz, 1968) as shown in Figure 2.22. This technique meant that the zones of elastic deformation and recovery, in addition to the zone of plastic deformation, could be observed.

Figure 2.22: High Speed Camera recording of Roll Bite by Kobasa and Schultz a) Video Equipment Used b) Side View of the Roll Bite (Kobasa & Schultz, 1968).

The pressure distribution acting on a roll surface has been the subject of many studies using a pressure pin technique, first developed by Siebel and Lueg (Seibel & Lueg, Untersuchungen über die Spannungsverteilung im Walzspalt [Investigations into the Distribution of Pressure at the Surface of the Material in Contact with the Rolls], 1933) and later refined Van Rooyen and Backofen (Van Rooyen & Backofen, 1957), as shown in Figure 2.23. This involves attaching a pin or cone onto a pressure transducer, and mounting this within the roll so that the pin is exposed to the rolling interface. This requires modification of the rolling surface and therefore results in marking of the rolled product. Using a similar radial pin technique, but including either a lateral force transducer or oblique pin, allows for the frictional stress profile on the roll bite to be recorded (Van Rooyen & Backofen, 1957) (Matsuoka & Motomura, 1968) (Liu, Tieu, Wang, &

Yuen, Friction Measurement in Cold Rolling, 2001). An example of such a setup is shown in Figure 2.24.

Figure 2.23: Pressure Pin Technique a) Developed by Seibel and Lueg (Seibel & Lueg, Untersuchungen über die Spannungsverteilung im Walzspalt [Investigations into the Distribution of Pressure at the Surface of the Material in Contact with the Rolls], 1933) b)

Refined by Van Rooyen and Backofen (Van Rooyen & Backofen, 1957).

a) b)

Figure 2.24: Radial and Oblique Pin Technique employed by Liu et. al. (Liu, Tieu, Wang, & Yuen, Friction Measurement in Cold Rolling, 2001).

Various instrumentation approaches have been attempted which have the aim of sensing stress in the roll, as shown in Figure 2.25. These typically use resistance strain gauges although piezoelectric elements have also been applied. These can be placed either in internal voids within the roll, or on the side of the roll outside of the roll bite. Both these approaches require the peak and surface stress to be extrapolated.

Figure 2.25: Strain Gauge and Piezo Techniques (Jeswiet, Arentoft, & Henningsen, 2006).

Strain gauges can also be used in conjunction with roll inserts designed to deform in a known manner. One such example is the ‘Portal Frame Sensor’ as shown below in Figure 2.26. The two legs of the frame are instrumented with strain gauges and deform under stress. The nature of this deformation can be detected by the strain gauges and the normal and friction forces inferred. As with the pin technique this still requires significant modification of the roll, and subsequently marks the strip. The rolling face of this sensor type is much larger than the roll bite contact length. Although this means it will not have the resolution achievable with the pin technique, it will also cause less disruption to the rolling interface.

Figure 2.26: A Portal Frame Sensor developed by Henningsen et. al. (Henningsen, Arentoft, &

Wanheim, 2006).

No studies have yet measured the roll bite lubricant film thickness in-situ. Several techniques have been employed in other applications which may be applicable to cold rolling. These include optical florescence (Ford & Foord, Laser-based Fluorescence Techniques for Measuring Thin Liquid Films, 1978) (Cameron & Gohar, Optical Measurement of Oil Film Thickness under Elasto-hydrodynamic Lubrication, 1963), capacitive (Grice N. , Sherrington, Smith, O'Donnell, &

Stringfellow, 1990) and resistive (Palacio & Bhushan, Surface potential and resistance measurements for detecting wear of chemically-bonded and unbounded molecularly-thick perfluoropolyether lubricant films using atomic force microscopy, 2007) approaches. Work jointly performed between the University of Sheffield and the University of Bristol used ultrasonic techniques to measure the lubricant films in a range of engineering components including hydrodynamic bearings (Dwyer-Joyce, Harper, Pritchard, & Drinkwater, 2006) and mechanical seals (Reddyhoff, Dwyer-Joyce, & Harper, A New Approach for the Measurement of Film Thickness in Liquid Face Seals, 2006).

2.10 Conclusions

It has been shown in this chapter that the theory behind cold rolling is well developed, with numerous authors having established models which can be used to estimate most key rolling parameters including friction, interface normal and shear stress, contact size, slip, strip and roll velocity and strip reduction. Experimental work has been completed that demonstrates the validity of these models under laboratory conditions. There is a benefit in measuring many of these parameters on real-world rolling mills to provide inputs to these models, allow tailoring of the models to specific rolling mills and materials, and therefore enable a more precise understanding and control of the rolling process.

Modern rolling mills are well instrumented and many of the parameters of interest are recorded as a matter of course. However, measurements taken on industrial mills are normally recorded away from the roll bite, such as strip entry and exit tensions, roll normal force and roll torque.

These measurements therefore only infer the roll bite condition. Few direct measurements of the roll bite are taken and no technology exists to measure some parameters, such as lubricant film thickness.

The lubrication conditions of the interface are important for efficient mill operation and product quality. These conditions are complex and highly dependent on a range of factors including rolling parameters, strip surface roughness, lubricant formulation, emulsion percentage and application method. All of these parameters, and their interactions, must be understood in order to effectively emulate a specific rolling case. This makes modelling of the interface a complex and involved process. Models do not exist that make case-by-case modelling practical for commercial applications. A more reliable approach would be to measure rather than model the lubricant conditions.

Of the measurement options for studying the roll bite that do exist, few are industrially applicable. This has limited these measurements of the roll bite to the research environment.

Rolling mill development and process monitoring would therefore benefit from in-situ measurements which could enable more precise monitoring and control of the rolling conditions.