Research on a kind of positioning anti-eccentric fixture for measuring thin wall gear with special big inner hole

This paper mainly studies and designs the clamping tool which is installed in the measuring of big gears with big inner holes and thin walls. This paper analyzes the disadvantages of conventional clamping of large gears with special inner holes, and presents an example of the research and development of the Assembly of the mounting and positioning device of the special large inner hole gear based on the gear measuring instrument. At present, in the field of gear measurement, the measurement of large gears is always a technical problem. The technical ability of gear testing in our country is restricted by various factors. Most gear high-precision instruments and their accessories still depend on imports, which cannot be a good solution to the measurement of special gear installation and positioning problems from the economy and timeliness. Especially, large gears with large bores and thin walls face the problem of determining the position of the measuring device and the measured gears, that is, the measuring datum is difficult to determine [1]. This paper proposes how to use the existing gear measuring instrument to develop and use small accessories, so as to help the production enterprises and measuring institutions solve the problem of reference positioning in the actual production measurement process and determine the accuracy of special gear samples.


Introduction
In China, the accelerated in-depth development of the high-end manufacturing industry, machinery industry, aerospace, railway transportation, automobile industry, agricultural electricity mining, and other industrial core transmission components, all involve the key mechanism of gear.At present, the production and manufacturing of gears has become the largest industry in mechanical basic components in China.In 2016, the output value of the domestic gear industry reached 230 billion yuan, ranking first in the world [1] .In the field of gear measurement, contact-type gear measuring instruments are mainly used for measuring precision gears at home and abroad [2][3] .The problem facing the gear measuring instrument is how to determine the position between the measuring device and the measured gear.Especially for some special thin-walled and large internal holes, the installation and positioning of oversized gears on gear measuring instruments is a complex project [4] .Most gear high-precision instruments and their supporting installation components rely on imports, so it is difficult to solve the problem of a wide variety of different shapes of special gear clamping from the economy and timeliness.The following paper describes the current general gear clamping method, analyzes the drawbacks of the current clamping method for special big gears and focuses on explaining the clamping fixture components for thin-walled large internal hole gears.

Conventional clamping for gear measurement
or thin gears with particularly large inner holes and large numbers of teeth, a non-standard gear with a tooth number of 580, a module of 1.02, a pressure angle of 30°, a tooth apex circle of 604 mm, a tooth root circle of 590 mm and a bore size of 580 mm is taken as an example.As shown in Figure 1, this gear requires a 580 mm diameter tapered mandrel to match.However, in actual production and use enterprises, non-standard gears involve various types, and each gear is matched with a mandrel.The coaxiality and straightness of the core shaft and the radial runout accuracy of the two ends are required to be within 2 ȝm, and the taper is 1:10,000 [6] .At present, it is difficult to process a 580 mm large-diameter high-precision mandrel and maintain a high-precision mandrel for a long time without corrosion and deformation.The preservation environment after processing needs to be dry and at 20ႏ constant temperature.Only a few constant temperature laboratories have harsh environmental conditions to maintain high-precision requirements.In order to maintain high precision and minimal deformation, large mandrels require the use of special metal materials.The commonly used materials for shaft parts include 35, 45, and 50 high-quality carbon steel, with 45 steel being the most widely used.Alloy steel can be used for heavy loads [6] , axial dimensions, weight limitations, or certain special requirements, and chromium plating treatment can be applied to the surface of the core shaft.The density of carbon steel and alloy steel is about 7.8 tons per cubic meter, the mandrel mass of the diameter d is 580 mm and the busbar height L is 150 mm.The calculation is as follows.

Mandrel with top clamping
ȡ=7.8 t/m 3 =7.8×10 - g/mm 3 m=ʌ×r2×L×ȡ=3.14×2402×150×7.8×10-3=211610g=211kg (1) At present, the main gear measuring instruments are from manufacturers such as Gleason in Germany, Mahr in Germany, and Klingelnberg Group in the United States.Most models of gear measuring instruments have a designed load-bearing capacity of less than 100 kg, which cannot withstand a core shaft with such a large mass of 211 kg.Large mass mandrels are prone to deformation and eccentricity of the instrument's center due to inertia during gear measurement rotation, resulting in damage to the instrument's center positioning system.At the same time, the axial eccentricity will cause the measurement error of gear radial runout to be amplified [7] , and it is not feasible to install this special gear core shaft with the center fitting. [8]igure 2 shows the large chucking gear.Because of the pressure brought by the weight of the large chuck to the turntable, especially in the measurement of gears with more than one tooth, the angle error of the turntable is caused.Taking 580 teeth as an example, the tooth profile and the tooth pitch are measured when the turntable rotates, and the turntable rotates 360°/580, that is 0.62069°.The angle deviation is easy to be caused by the increase of mass and inertia, and the uneven force on the contact tooth surface of the measuring needle is easy to wear and break [9] and finally leads to the uncertainty of the measurement results.

Design example of special gear fixture
The working turntable of the gear measuring instrument has three sets of positioning holes at an angle of 120°, and three assembly clamp components are developed.The whole gear clamping is realized by a turntable equipped with a laser measuring and positioning device.It can replace the traditional mandrel assembly method, which not only makes it less difficult to manufacture, but also has a small clamping contact surface, easy to process, easy to use, easy to disassemble and easy to store.It is not easy to cause deformation or damage due to motion interference with measuring instruments [10] and can be applied to different types of special gears.

Composition and working principle of a group of clamping components
A set of gear measuring clamping components includes a raised column (Figure 3), a t-shaped positioning rod (Figure 4), a pressure plate (Figure 5), a nut (Figure 6), and a positioning pallet (Figure 7).A chute with a scale is processed along the length direction of the positioning plate, the positioning plate is provided with a connecting part, and a supporting surface is formed on one side of the positioning plate.The raised column and the connecting part are fixed with nuts so that the positioning plate is supported on the external platform.The T-shaped positioning rod can slide along the groove direction of the chute.The clamping plate is connected to the positioning rod, and a clamping surface is formed on one side of the clamping plate close to the positioning bracket.The bearing surface and the clamping surface are parallel to each other.The positioning rod is connected to the chute vertically through the sliding part.The positioning rod is arranged with a scale along its length direction.The clamping plate is rotationally connected with the positioning rod, and can be rotated around the axis of the positioning rod, so as to realize the contact of the inner or outer end face of the gear and adapt to the clamping of different scenes.The connection part is the positioning hole, one end of the pad high column is embedded with the positioning hole, and the embedded end of the pad high column is processed with a screw hole.The screw hole is matched with a locking screw, and the nut of the locking screw is used for pressing the hole wall of the positioning hole.The bearing surface, the circumferential surface of the positioning rod and the clamping surface form a clamping space for clamping the inner or outer edge of the gear.The assembly diagram of a group of fixture components is shown in Figure 8.

Three assembly clamps on the turntable installation
As shown in Figure 9, the three assembly clamp components 100 are positioned in three directions of 120° on the gear measuring instrument turntable, with each fixed with the cushion height column respectively and the positioning plate fixed with locking screws.

Laser measuring device
The turntable 200 of the gear measuring instrument has a positioning center hole 201.The laser measuring device 300 is fixed in the center hole at the first installation station 201 (Figure 9).It has three sets of laser measuring heads 301 and a controling power supply.It is evenly distributed around the first installation station 201, and it is divided into 120° angles.Three laser beams match the radial line distribution of Turntable 200.In working condition, each set of the laser measuring head 301 can locate the position of the clamping assembly 100 in the corresponding direction.The emitted laser illuminates the clamping assembly 100 in the corresponding direction to form a positioning position.The position of the long pallet 100 is adjusted so that the laser beam is in the same position on the same part of the clamping assembly 100.The positioning plate distribution of the clamping assembly 100 is consistent with the radial line distribution of the turntable 200 to provide concentric clamping conditions for gear positioning.

New fixture installation measurement
The big gear is installed on the new fixture, and the beams of the three laser heads are directed to the right side of the long pallet.The left and right positions of the long pallet are adjusted so that the beams are aligned with the right side of the long pallet.The three data of placing the gear on the long tray scale are basically consistent.The pressure plate should be held down.At this time, the center of the gear basically coincides with the center of the turntable of the gear measuring instrument.Gear measurement is carried out, and the error diagram of tooth shape and tooth direction data is shown in Figure 10.

Advantages of the new fixture
After practical application, the new fixture has the following advantages: z When the big gear is installed, as long as the outer edge of the gear is placed at the same scale as the long support plate, it has been ensured that the initial installation of the gear and the gear measuring instrument turntable has been basically concentric [10] .When the measuring gear is adjusted, the coarse adjustment can be skipped, and the centering accuracy is high.z It can avoid the pressure brought by the large mandrel and the large chuck due to its own weight on the turntable, and it is not easy to cause deformation or damage due to movement interference with the measuring instrument [11] .

Conclusion
This paper analyzes the infeasibility of conventional clamping for special big gear.A case study of installing and positioning device components based on gear measuring instruments is presented.The clamping assembly in this paper solves the problem that the gear measuring instrument cannot clamp the big gear with a large inner hole [12] .The fixture can be adapted to different types of gear installation positioning by forming a clamping space with the inner or outer edge of the gear.It can also save the manufacturing and preservation costs of custom mandrels.

Figure 8 .
Figure 8.The assembly diagram of a group of fixture components.