# Study on the Position Deviation between the Iron Roughneck's Spin-Rollers and the Drilling Tool
## 1 Introduction  
With the development of the modern drilling process, the application of wellhead automation equipment is gradually increasing [1, 2]. The iron roughneck is a kind of wellhead equipment with a high degree of automation that is used to screw and unload the threaded connection of drilling tools with high efficiency [3, 4]. The spinner mechanism is the main mechanism of the iron roughneck and plays an important role in ensuring its working performance. The spinner mechanism has a variety of structural forms [5, 6], but the working principle of the spin action is basically similar among each form. After the drilling tool is held tightly, torque is applied by the spin-rollers and the drilling tool is driven to rotate by friction to realize the screwing action [7, 8]. With the application of an iron roughneck in actual construction, the following problems in the spinner mechanism gradually appear [9, 10]: first, the spin-roller may slip during operation, so it cannot reach the required torque, affecting the operation capacity of the spinning thread and resulting in unstable operation; second, the wear between the spin-rollers and the drilling tool is serious, and results in the reduction of the service life of the spin-roller; third, the axis of the upper and lower drilling tools deviates greatly during screwing, resulting in intense vibrations that affect the screwing process and increase the risk of thread wear. These problems will directlyaffect the working performance of the iron roughneck, reduce the reusability of spin-rollers and drilling tools, and even cause accidents such as drilling tool damage, which increases the cost of use and maintenance [11; 12; 13]. One of the main reasons for the above problems is that the position deviation occurs when the spin-rollers hold the drilling tool tightly, so that the upper drilling tool is not coaxial with the lower drilling tool. Therefore, the rollers have poor contact with the drilling tool during the screwing process, resulting in the uneven application of force and vibration [14; 15]. In consideration of the above problems, at present, the centering performance is mainly improved through adjusting the structural layout of the clamping arms, the use of a floating device, and the use of auxiliary detection. The TZG-130 iron roughneck produced by the Nov company adopts a clamping method consisting of the parallel clamping of two clamping arms. While this method ensures the structure has good centering performance, it consequently occupies a large space [16]. In order to realize the coaxiality of the upper drill pipe and the lower drill pipe, some iron roughness use a floating device to automatically adjust the position of the spinning wheels to achieve the adaptive adjustment function of the screwing process [17], but the centering effect of this method is affected by the performance of the floating device and the comprehensive force of the mechanism. The authors of [18] have proposed an automatic centering method for the iron roughneck based on a photoelectric sensor. The automatic centering method consists of measuring the signal of the photoelectric sensor, calculating the deviation, and then controlling the expansion or contraction of the oil cylinder to find the central position of the drill pipe and automatically complete the centering of the drill pipe. Other researchers have proposed a detection and control system based on vision to identify and locate drilling tools [19; 20]. The latter two methods increase the complexity of the system, and the detection accuracy is easily affected by the harsh wellhead environment [21]. In addition, this literature has not studied the deviation state of the drilling tool and the influence of different deviations.