Planetary gear pinion shaft assembly system and method

Abstract

The application discloses a planetary gear pin shaft assembly system and an assembly method, and belongs to the technical field of mechanical assembly. The system comprises a normal-temperature pre-checking unit, a liquid nitrogen deep cooling device, a grabbing and transferring device, a servo press-fitting device and a control unit. The normal-temperature pre-checking unit is used for measuring the normal-temperature diameter and weight of the pin shaft; the liquid nitrogen deep cooling device cools the pin shaft to a preset low temperature by immersing the pin shaft in liquid nitrogen; the grabbing and transferring device mechanically scrapes the condensate on the surface of the pin shaft by using an annular scraper during the grabbing process; and the servo press-fitting device presses the pin shaft into an assembly hole. The control unit generates a personalized force-displacement envelope curve corridor as a reference model according to the pre-checking parameters of the single pin shaft, and compares the reference model during the press-fitting process, so as to determine the assembly quality on line. Through single-piece pre-checking, clean transferring, self-adaptive press-fitting and real-time monitoring, the system significantly improves the assembly qualified rate and consistency of the planetary gear pin shaft, and realizes whole-process data tracing.

Description

Technical Field

[0001] This invention relates to the field of mechanical assembly technology, specifically to a planetary gear set pin assembly system and assembly method. Background Technology

[0002] As a core component of reducers, coaxial electric drive systems, hybrid power systems, and heavy-duty transmissions, the assembly quality of planetary gear sets directly determines the transmission efficiency, NVH performance, and service life of the assembly. During the assembly of the planetary gear set assembly, pressing the planetary gear pins (or planetary gear shafts) into the mounting holes of the planet carrier is a crucial step in achieving a secure connection between the planetary gears and the planet carrier. An interference fit is typically used between the pins and the mounting holes to ensure that no relative rotation or axial movement occurs under high-speed, heavy-load conditions.

[0003] Currently, the assembly technology for planetary gear set pins has been developed. For example, there is an existing technology called "A Method for Assembling Planetary Gear Components," which involves: in the pre-assembly stage of the planetary gear assembly, thrust washers, needle roller bearings, and planetary gears are stacked first, and then a process pin is inserted. The upper limit structure of this process pin engages with the upper part of the planetary carrier to temporarily position and fix the above components. In the subsequent planetary gear shaft press-fitting process, the planetary gear shafts are press-fitted from top to bottom. During the press-fitting process, the process pin and the planetary gear shaft descend synchronously until the upper limit structure separates from the lower part of the planetary carrier. This scheme aims to improve the press-fitting accuracy and avoid damaging the needle roller bearings or thrust washers by guiding the process pin.

[0004] However, the aforementioned existing technologies and other assembly processes commonly used in the industry still have the following unresolved technical defects: First, there is a lack of compensation mechanism for the consistency of incoming pin materials. Existing technical solutions are all based on press-fitting under normal temperature conditions or control using uniform process parameters, without considering individual differences in materials and machining tolerances between different batches of pins. In particular, with the increasing requirements for planetary gear set power density, the interference fit design between the pin and the mounting hole has become increasingly precise. Even if there is a micron-level tolerance fluctuation in the diameter of the pin at room temperature, it may cause the interference fit after press-fitting to deviate from the design value, affecting the reliability of the connection.

[0005] Second, there is a lack of control methods for the surface condition of pins under cryogenic assembly processes. For planetary gearbox pin assemblies with large interference fits, the industry has gradually introduced liquid nitrogen cryogenic technology to reduce the pin diameter through the shrinkage effect, thereby lowering the resistance during the initial press-fitting process. However, after the cryogenically treated pin is removed from the liquid nitrogen, its extremely low-temperature surface will instantly condense a large amount of frost and condensate droplets when exposed to room temperature and humid air. If left untreated, these surface condensates will not only scratch the assembly hole walls during the press-fitting process but also alter the contact state of the actual mating surfaces, leading to uncontrollable effective interference fit and rendering theoretical calculations completely invalid. Existing technology does not provide any solution to this problem.

[0006] Third, the quality assessment methods for the pressing process are outdated and crude. Existing technologies rely solely on the mechanical limiting of process pins to passively prevent parts from being crushed, lacking real-time monitoring and intelligent judgment of pressing force and displacement. Traditional pressing processes often use whether the final pressing force reaches a certain fixed threshold as a pass / fail criterion, but this method cannot capture the abnormal fluctuations in interference caused by abnormal frost, material defects, or misalignment during the pressing process. This often leads to defective parts flowing into subsequent processes, causing assembly scrap or premature failure.

[0007] Fourth, assembly process data is isolated, lacking single-piece-level quality traceability capabilities. Existing technical solutions lack a complete data chain from incoming pin inspection, cryogenic treatment, press-fitting process to final quality assessment, making it impossible to trace the correlation between assembly quality and the physical characteristics of specific components (such as the actual diameter and weight of a single pin). Once a quality problem occurs, it is difficult to accurately pinpoint whether it is caused by incoming material defects, process deviations, or operational abnormalities. Summary of the Invention

[0008] The present invention aims to solve at least one technical problem existing in the above-mentioned background art, and provides a planetary gear pin assembly system and assembly method.

[0009] The technical solution of this application is: a planetary gear set pin assembly system, comprising: The liquid nitrogen cryogenic device is used to completely immerse the pin to be pressed in liquid nitrogen and cool it to a preset low temperature, and can remove a single cooled pin from the liquid nitrogen to a gripping position. A gripping and transferring device is used to grip the pin from the gripping position in an insulated environment, perform surface cleaning treatment during the gripping process, and then transfer the pin to the pressing station; A servo press-fitting device is used to press the cooled pin into the assembly hole when the gripping and transferring device transfers the pin to the assembly hole position of the planetary gear set. The control unit is configured as follows: Obtain the room temperature pre-test parameters for each pin, measured before it enters the liquid nitrogen cryogenic device; Based on the ambient temperature pre-inspection parameters and the preset material thermophysical database, the theoretical shrinkage or theoretical diameter of the pin at the preset low temperature is calculated, and a force-displacement reference model for controlling the press-fitting process is generated accordingly. The liquid nitrogen cryogenic device, gripping and transferring device, and servo pressing device are controlled to work in coordination. During the pressing process, the force and displacement signals of the servo pressing device are collected in real time to generate a measured force-displacement curve, which is then compared with the force-displacement reference model to determine the assembly quality.

[0010] According to the planetary gear pin assembly system provided by the present invention, a room temperature pre-inspection unit is further included, which is used to measure and record the room temperature diameter and weight of each pin before the pin enters the liquid nitrogen cryogenic device, and send the measurement data to the control unit as the room temperature pre-inspection parameters.

[0011] According to the present invention, a planetary gear pin assembly system is provided, wherein the liquid nitrogen cryogenic device comprises: Base; A liquid nitrogen bath is a hollow cylindrical structure with its lower end fixed to a base and its upper end open, used to house the pins to be cooled. The movable cover is vertically connected to the base at its lower end, and has a through hole at its upper end for the pin shaft to pass through. A cover plate is provided on the through hole to seal the through hole. When the movable cover is fully raised, it forms a sealed cavity with the base that covers the liquid nitrogen tank and the pin shaft placed inside. An injection tube, with its upper end extending through the base into the sealed cavity, is used to inject liquid nitrogen into the sealed cavity. The control unit determines the shortest forced immersion time based on the weight and material properties of the pin, and after the pin has been in liquid nitrogen for that time, it controls the movable cover to move down so that the pin opens the cover plate and waits to be grabbed.

[0012] According to the present invention, a planetary gear set pin assembly system is provided, wherein the gripping and transferring device comprises: A heat-insulating shell with an internal cavity and an opening at the lower end for a pin to enter; An annular scraper ring is coaxially fixed inside the opening. Its inner diameter is smaller than the diameter of the pin at the preset low temperature. It is used to mechanically scrape off the liquid nitrogen and frost adhering to the surface of the pin when the gripping and transferring device is put into the pin. A clamping mechanism is provided inside the heat insulation housing to clamp the root of the pin that has been scraped off. An air supply line, connected to the cavity, is used to continuously introduce slightly positive pressure dry cold nitrogen into the cavity during the grasping process to prevent external moisture from entering.

[0013] According to the present invention, a planetary gear pin assembly system is provided, wherein the annular scraper ring is made of a low-temperature resistant and elastic polymer material, and the clamping mechanism consists of a plurality of mechanical jaws evenly distributed along the circumferential direction.

[0014] According to the present invention, a planetary gear pin assembly system includes a servo press-fitting device comprising: a servo electric push rod; a force sensor and a displacement sensor connected in series between the end of the push rod and the pin contact component; and a tapered pre-guide sleeve that is vertically and vertically disposed above the workpiece assembly hole. In the initial stage of press-fitting, the control unit controls the tapered pre-guide sleeve to descend first and fit against the assembly hole opening, and then the push rod drives the pin to be guided into the hole through the guide sleeve.

[0015] According to the planetary gear pin assembly system provided by the present invention, the force-displacement reference model generated by the control unit is a nonlinear increasing envelope curve corridor that reflects the rapid increase of interference due to heat exchange during the pressing process of the cryogenic pin; the control unit is configured to compare the measured force-displacement curve with the envelope curve corridor in real time during the pressing process, and to determine an abnormality and execute an alarm or shutdown when the curve exceeds the upper boundary of the corridor.

[0016] According to the planetary gear pin assembly system provided by the present invention, the control unit is configured to generate a force-displacement envelope curve corridor reflecting the change of interference due to heat exchange during the pressing of the cryogenic pin as a reference model based on the ambient temperature pre-inspection parameters and material database, and to compare the measured curve with the corridor in real time during the pressing process.

[0017] This invention also relates to a method for assembling planetary gear pins, applied to the above-mentioned system, comprising the following steps: Under normal temperature conditions, measure and record the diameter and weight of each pin. The pin is sent into a liquid nitrogen cryogenic device and completely immersed in liquid nitrogen. The forced immersion time is determined according to the weight and material properties of the pin to ensure that its overall temperature reaches the preset low temperature. A gripper with mechanical scraping and micro-positive pressure cold nitrogen protection functions is used to grip the cooled pin. During the gripping action, the condensate on the surface of the pin is scraped off and clamped in a dry cold nitrogen atmosphere. The gripper holding the pin is quickly moved to the press-fitting station while in a heat-insulated state; The control unit calls the room temperature pre-inspection data of the pin, the material database and the measured dimensions of the workpiece assembly hole, calculates the theoretical diameter of the pin at the preset low temperature based on the thermal expansion theory, and generates a force-displacement envelope curve corridor with upper and lower limits and exponential growth shape as a reference model based on the thick-walled cylinder theory. Align the pin with the workpiece assembly hole, start the pressing process at a preset speed, and continuously press the pin into the target depth throughout the process; during this process, monitor the force and displacement in real time, generate a measured force-displacement curve, and compare it with the force-displacement reference model; Based on the comparison results, determine whether the assembly is qualified.

[0018] According to the present invention, a method for assembling planetary gear pins is provided, wherein the force-displacement reference model is an envelope corridor with an exponentially increasing shape; during the pressing process, if the measured force-displacement curve touches or exceeds the upper boundary of the envelope corridor, the pressing is immediately stopped and an alarm is issued; if the curve rises rapidly within the corridor and eventually enters the end pressure holding window corresponding to the target depth, it is determined to be qualified.

[0019] According to the planetary gear pin assembly method provided by the present invention, in the cryogenic treatment step, the forced immersion time is calculated based on the pin's material, weight, and liquid nitrogen temperature, by using a preset thermal equilibrium time model and multiplying it by a safety factor, thus forming a coarse closed-loop control in the time dimension.

[0020] The planetary gear pin assembly method provided by the present invention further includes a full-process data traceability step: binding and uploading the pin's room temperature pre-inspection data, cryogenic immersion time, complete force-displacement curve of the press-fitting process, and final quality judgment results to the manufacturing execution system to form a single-piece level digital twin quality file.

[0021] The advantages of this invention are: 1. Personalized press-fitting control based on pre-inspection data of individual pins significantly improves assembly pass rate and product consistency. This invention abandons the practice of using uniform process parameters for the same batch of pins, and introduces a room-temperature pre-inspection unit to measure each pin individually. The control unit accurately calculates the theoretical shrinkage and theoretical diameter under cryogenic conditions for each pin based on its actual room-temperature diameter, weight, and material thermophysical database, and generates a customized force-displacement reference model accordingly. This one-to-one control strategy effectively compensates for the impact of pin material tolerances and batch variations on the interference fit, ensuring that each pin can be press-fitted under optimal interference conditions, significantly improving assembly quality and the service reliability of the planetary gear set.

[0022] 2. Surface cleaning treatment integrating mechanical scraping and micro-positive pressure cold nitrogen protection ensures the precision of interference fits in cryogenic assembly. This invention's gripping and transfer device creatively integrates gripping with surface cleaning processes. An annular scraper ring, with an inner diameter smaller than the diameter of the cryogenically cooled pin, mechanically scrapes away liquid nitrogen and frost adhering to the surface the instant the pin is gripped and inserted. Simultaneously, a slightly positive pressure of dry, cold nitrogen gas is introduced into the cavity, creating a locally clean and dry protective atmosphere to prevent external moisture intrusion. This design completes surface cleaning and gas-sealing protection in a very short time, avoiding rapid frosting and temperature rise of the cryogenically cooled pin in an exposed environment. This ensures that the actual interference fit during pressing closely matches the theoretical calculation value, a key prerequisite for the system to achieve precision assembly.

[0023] 3. Online force-displacement monitoring based on a nonlinear increasing envelope curve corridor enables real-time assessment of assembly quality and defect interception. This invention constructs a force-displacement envelope curve corridor as a reference model, reflecting the sharp increase in interference fit due to heat exchange during the press-in process of cryogenic pins, rather than a simple static threshold. During the press-in process, the force and displacement curves acquired in real time are compared point-by-point with this corridor. When the measured curve exceeds the upper boundary of the corridor, an alarm is immediately triggered or the machine is stopped. This allows for real-time detection of interference fit anomalies caused by poor surface scraping, abnormal temperature rise, material defects, etc. Simultaneously, a continuous, uninterrupted press-in action is employed throughout the process to avoid data distortion caused by local temperature field changes due to interruptions. This dynamic, full-process monitoring method achieves a leap from post-inspection to online judgment.

[0024] 4. Single-item-level full lifecycle data traceability and digital twin quality archives support zero-defect management in high-end manufacturing. This invention binds the ambient temperature pre-inspection data, cryogenic immersion time, complete press-fit force-displacement curve, and final quality judgment results of each pin shaft and uploads them to the manufacturing execution system, forming a single-piece-level digital twin quality archive. This data chain not only provides tamper-proof traceability evidence for the assembly quality of the planetary gear set assembly, but also optimizes the thermal balance model and force-displacement reference model through historical data mining, achieving a self-evolving closed loop in the process. This perfectly aligns with the stringent requirements of high-end transmission systems for full lifecycle traceability and continuous quality improvement.

[0025] 5. Multi-device timing coordination and modular design, balancing rigid process cycle time with flexible system expansion. The system of this invention tightly couples the forced immersion timing of the liquid nitrogen cryogenic device (based on a coarse closed-loop control with a thermal balance model and safety factor), the cover ejection mechanism of the movable hood, the clean gripping action of the gripping and transfer device, and the action of the servo pressing device into a coherent, fully automated process through a control unit, eliminating quality fluctuations caused by manual intervention. At the same time, each device is modular and relatively independent, facilitating adjustments based on production line rhythm or fault maintenance. It is also easy to embed this system into flexible production lines with different planetary gear models, exhibiting excellent industrial adaptability. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the liquid nitrogen cryogenic device of the present invention (the pin is in an immersion cooling state). Figure 2 This is a schematic diagram of the liquid nitrogen cryogenic device of the present invention (after the pin has been immersed and cooled, the movable cover moves down). Figure 3 This is a schematic diagram of the liquid nitrogen cryogenic device of the present invention (the pin is fully exposed and waiting to be grabbed). Figure 4 This is a schematic diagram of the liquid nitrogen cryogenic device of the present invention (the pin has been picked up and is waiting for the next pin to enter). Figure 5 This is a schematic diagram of the gripping and transferring device of the present invention; Figure 6 This is a flowchart of the planetary gear pin assembly method of the present invention; The components are: 1-base; 2-liquid nitrogen tank; 3-movable cover; 4-cover plate; 5-injection pipe; 6-shell; 7-scraper ring; 8-gripper; 9-telescopic arm; 10-slide rail; 11-gas supply line; 12-push rod. Detailed Implementation

[0027] The embodiments of this application are described in detail below, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0028] In the description of this application, it should be understood that the terms "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0029] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to specific embodiments. It should be understood that the description herein is for illustrative purposes only and is not intended to limit the scope of protection thereof.

[0031] Example 1: This example provides a planetary gear set pin assembly system and its corresponding basic assembly method.

[0032] like Figures 1-5 As shown, the system in this embodiment mainly consists of four functional modules: a liquid nitrogen cryogenic device, a gripping and transferring device, a servo pressing device, and a control unit. The liquid nitrogen cryogenic device is used to completely immerse the pin to be pressed in liquid nitrogen, cooling it to a preset low temperature (e.g., -196°C), and includes a mechanism to separate the cooled pin from the liquid nitrogen bath and move it to a predetermined gripping position. The gripping and transferring device is a robotic arm or gripper with a heat-insulated structure. Its function is to accurately grip the pin from the aforementioned gripping position under the maintained low-temperature heat-insulated environment, and to perform a surface cleaning process simultaneously with the gripping action. The pin is then quickly transferred to the planetary gear pressing station. The servo pressing device is located at the pressing station. When the gripping and transferring device moves the cooled pin to the target position directly above the assembly hole of the planetary gear, the device is controlled to perform a pressing action, pressing the pin into the assembly hole. The control unit, as the system's decision-making core, is signal-connected to each of the above devices and is responsible for coordinated scheduling and data processing.

[0033] The assembly method described in this embodiment is as follows: Figure 6 As shown, its core steps are: First, under normal temperature conditions, each pin that is about to enter the system is pre-inspected, and its key parameters such as diameter and weight are measured and recorded.

[0034] Afterwards, the pin is sent into a liquid nitrogen cryogenic device, where it is completely immersed in liquid nitrogen for cryogenic treatment, cooling it to a preset low temperature and stabilizing it.

[0035] After cryogenic treatment, a gripper equipped with mechanical scraping and micro-positive pressure cold nitrogen protection removes the condensate (frost and liquid nitrogen) that instantly condenses on the surface of the pin as it is removed and gripped. The pin is then clamped in a protective positive pressure atmosphere formed by dry cold nitrogen.

[0036] The gripper holding the low-temperature pin is quickly moved to the pressing station while in a heat-insulated state.

[0037] At the same time, the control unit retrieves the room temperature pre-inspection data of the pin, the built-in material thermophysical database, and the actual size of the planetary gear assembly hole measured in advance, accurately calculates the theoretical shrinkage and theoretical diameter of the specific pin at the preset low temperature, and generates a "force-displacement reference model" curve to control the pressing process.

[0038] At the start of the press fitting process, the pin is aligned with the workpiece assembly hole, and the process is initiated at a preset constant speed, executing a continuous pressing process without interruption until the pin reaches the target depth. During this process, the force sensor and displacement sensor on the servo press fitting device transmit signals back to the control unit in real time, generating a "measured force-displacement curve".

[0039] Finally, the control unit compares the measured curve with the pre-generated reference model in real time, and automatically determines whether the assembly is qualified based on the degree of matching between the two in the coordinate system.

[0040] The technical solution in this embodiment overturns the traditional one-size-fits-all press-fit process. Its core principle lies in creating a personalized process model, like a digital twin, for each pin. Due to unavoidable batches of raw materials and processing tolerances, each pin has slight differences in its room temperature diameter and weight, which directly affects its shrinkage under cryogenic conditions and the interference fit during press-fitting. This embodiment, through a control strategy of one measurement and one model per pin, transforms the press-fitting process from a passive, open-loop action into an active, closed-loop adaptive process. Its advantage lies in the system's ability to proactively compensate for inconsistencies in incoming materials, fundamentally ensuring an extremely high assembly pass rate and consistent final product performance, achieving a leap from qualified manufacturing to precision manufacturing.

[0041] Example 2: Based on Example 1, this example further clarifies the specific implementation method of room temperature pre-inspection.

[0042] This embodiment of the system explicitly adds a room temperature pre-inspection unit. This unit is an online workstation that integrates measurement, identification, and data communication functions. Its core components include a high-precision diameter measuring instrument (such as a laser micrometer), a high-precision dynamic weighing sensor, and an industrial control computer that is network-connected to the main control unit.

[0043] Before the pin is sent into the liquid nitrogen cryogenic device, it must pass through this ambient temperature pre-inspection unit. The pin is separated individually and placed at the measurement station, where its diameter and weight are simultaneously and accurately measured at ambient temperature. The measurement data is processed in real time by the industrial control computer and sent to the system's control unit as the ambient temperature pre-inspection parameter file for that pin.

[0044] The principle behind this embodiment is to move the starting point of quality traceability to the very beginning. The physical characteristics of each pin are digitized and recorded immutably before entering the core process. This not only provides the most basic input data for subsequent personalized press-fitting model calculations, but also provides the initial physical evidence for tracing the source of any quality problems that may arise later. Its advantage is that it avoids the errors and efficiency bottlenecks that may occur with manual measurement and recording.

[0045] Example 3: This example details the specific implementation of a cleverly structured liquid nitrogen cryogenic device, with particular optimization of the automatic removal and gripping interface of the pin shaft.

[0046] like Figures 1-4 As shown, the liquid nitrogen cryogenic device includes a base 1, several liquid nitrogen tanks 2, a movable cover 3, and an injection pipe 5. The base 1 is a fixed support component. The liquid nitrogen tanks 2 are hollow cylindrical structures with their lower ends fixed to the base 1 and their upper ends open. The diameter of the liquid nitrogen tank 2 is larger than the diameter of the pin, and the depth of the liquid nitrogen tank 2 is greater than 1 / 3 of the pin length and less than 1 / 2 of the pin length. This allows the pin to be easily placed into the liquid nitrogen tank 2 while its upper end is exposed, maintaining a near-vertical position, so that liquid nitrogen can be easily introduced into the liquid nitrogen tank 2 to cool the pin. The lower end of the movable cover 3 is connected to the base 1 in a liftable manner (e.g., driven by a cylinder or electric cylinder). The upper end of the movable cover 3 has a through hole that allows the liquid nitrogen tanks 2 to pass through. Above the through hole is a movable cover 4 that can be pushed open. When no external force is applied, the cover 4 closes the through hole by gravity or spring force. The cover plate 4 is positioned directly above the liquid nitrogen tank 2. When the movable cover 3 moves downwards, the cover plate 4 is lifted up by the liquid nitrogen tank 2, which passes through a through hole to facilitate the entry of the pin. When the movable cover 3 is fully raised, its lower end seals with the base 1, forming a sealed cavity that completely encloses the liquid nitrogen tank 2 and its internal pin. The upper end of an injection pipe 5 passes through the base 1 and extends into this sealed cavity to inject liquid nitrogen and maintain the liquid level.

[0047] In this embodiment, multiple liquid nitrogen tanks 2 are set on the base 1, and each liquid nitrogen tank 2 is individually controlled, allowing multiple pins to be immersed and cooled at one time, thus enabling batch assembly of pins. This significantly improves the efficiency of pin assembly.

[0048] During operation, the movable cover 3 is in its low position, and the cover plate 4 is lifted by the liquid nitrogen tank 2. The upper end of the liquid nitrogen tank 2 is open. The pin to be cooled is placed into the liquid nitrogen tank 2, and the movable cover 3 rises to form a sealed cavity. Liquid nitrogen is injected through the injection pipe 5 to completely immerse the pin. The control unit calculates the shortest forced immersion time to ensure that the core reaches the preset low temperature based on the weight and material properties (such as specific heat capacity and thermal conductivity) of the batch of pins using a preset thermal balance model. After the timing is completed, the system controls the movable cover 3 to move vertically downward. As the movable cover 3 descends, the liquid nitrogen level rises relatively and overflows from the through hole. The upper end of the pin will first contact and push open the cover plate 4, eventually extending out of the through hole and fixed in a relatively stable spatial position. This position is the gripping position. The cover plate 4 finally abuts against the side of the pin or the side of the liquid nitrogen tank 2.

[0049] The device in this embodiment utilizes the single action of lowering the movable cover 3 to simultaneously achieve four functions: breaking the sealed cavity, releasing liquid nitrogen, pushing the pin, and forming a gripping channel. In principle, the raising and lowering of the movable cover 3 controls the volume of the sealed space and the relative liquid nitrogen level, while the passive opening mechanism of the cover plate 4 eliminates the need for any additional active clamping or lifting mechanisms to remove the pin, resulting in an extremely compact and highly reliable structure. Its advantages include greatly simplifying the complexity of cryogenic mechanism design, avoiding the operation of drive components such as motors in a liquid nitrogen environment, and minimizing liquid nitrogen evaporation and cooling loss during non-gripping periods through the passive sealing of the cover plate 4, thus achieving energy saving and high efficiency.

[0050] Example 4: This example provides a detailed structure of a gripping and transferring device, which is a key element in ensuring precise interference fit assembly.

[0051] In this embodiment, the gripping and transferring device is designed as a gripper that can be mounted on the end of a robotic arm. For example... Figure 5 As shown, its core component is a heat-insulating shell 6, which forms a cavity inside. A circular opening at the lower end allows the pin to enter. Inside this opening, an annular scraper ring 7 is coaxially fixed. This scraper ring 7 is made of a low-temperature resistant and elastic polymer material (such as polytetrafluoroethylene or filled modified nylon). Fundamentally different from existing technologies, the inner diameter of the scraper ring 7 is precisely set to be slightly smaller than the theoretically calculated diameter of the pin at a preset low temperature. The cross-section of the scraper ring 7 is wedge-shaped or conical, with the smaller end facing its center and the larger end fixed inside the shell 6. This allows the pin to pass smoothly through the scraper ring 7 from both above and below.

[0052] The scraper ring 7 is located at the lower end of the internal cavity of the housing 6, and a clamping mechanism is provided near the upper end of the internal cavity of the housing 6. For example... Figure 5As shown, the clamping mechanism of this embodiment includes at least one set of grippers 8. Each set includes two grippers 8 symmetrically arranged around the axis of the housing 6. The gripper 8 has an arc-shaped end face facing the pin shaft, which can tightly fit the outer side of the pin shaft. The two grippers 8 in the same set are opposite each other to form a clamping mechanism for clamping the pin shaft. The end of the gripper 8 facing away from the pin shaft is provided with a radially extendable and retractable telescopic arm 9. The gripper 8 can clamp or release the pin shaft by extending and retracting radially through the telescopic arm 9. One radial end of the telescopic arm 9 is connected to the gripper 8, and the other radial end is provided with a slider. The slider cooperates with the slide rail 10 fixed on the inner wall of the housing 6. The slider is movably connected to the slide rail 10 along the axial direction of the housing, that is, the vertical height of the telescopic arm 9 can be adjusted by the slide rail 10. On the one hand, it can accommodate pin shafts of different specifications. On the other hand, after the gripper 8 clamps the pin shaft, it can drive the slider to move along the slide rail 10, moving the entire pin shaft axially, forcing the scraper ring 7 below to repeatedly scrape and clean the pin shaft.

[0053] For example, when the entire housing 6 is fitted onto the cooled pin from top to bottom, the scraper ring 7 first scrapes and cleans the pin surface from top to bottom. At this time, the gripper 8 may have moved to the position to be gripped, but the scraper ring 7 has not moved to the bottom of the pin. The pin can then be gripped by the gripper 8, and the pin can be driven to move axially, causing the scraper ring 7 to repeatedly scrape the pin surface. When transferring the pin to the mounting hole later, the gripper 8 can also be used to push the pin out from the opening at the bottom of the housing 6, making it easy to align with the mounting hole.

[0054] In addition, the device also includes a gas supply line 11, which is connected to the top or side of the cavity to continuously supply the cavity with dried and pre-cooled slightly positive pressure dry cold nitrogen gas with a pressure slightly higher than atmospheric pressure, from an external gas source.

[0055] After the pin emerges from the through hole and enters the gripping position, the robotic arm carries this device downwards and slides it onto the pin. At the moment of insertion, the inner edge of the annular scraper ring 7 at the opening forms a tight interference fit with the extremely cold surface of the pin. During the downward movement, the scraper ring 7, like a piston ring, mechanically and physically removes any liquid nitrogen beads and tiny frost crystals adhering to the pin's surface. The pin continues deeper into the cavity, passing through the scraper ring 7 to reach the gripping mechanism, where the grippers 8 immediately close, firmly clamping the cleaned pin root. Throughout the entire process from insertion to clamping, and then to subsequent transfer and pressing, the gas supply line continuously introduces slightly positive pressure dry, cold nitrogen into the cavity, forming an outward-flowing airflow barrier.

[0056] The principle of this embodiment lies in integrating the separate process of surface cleaning into the continuous action cycle of gripping, creating a moving, positive-pressure local clean environment. The physical scraping of the annular scraper ring 7 is the primary means of removing existing contaminants, while the slightly positive-pressure dry cold nitrogen gas plays a dual crucial role: firstly, it prevents moisture from the external environment from invading the cavity and causing re-frost on the cleaned pin surface; secondly, its dry and low-temperature characteristics provide the pin with an inert protective atmosphere close to its own temperature, maximizing the suppression of heat exchange during the gripping and transfer process. Its advantage lies in the fact that this combined strategy of mechanical descaling and gas-sealed anti-fouling almost perfectly solves the industry problem of rapid frosting of low-temperature metal parts in air within a gripping cycle of a fraction of a second. This ensures that the interference fit surface of the pin is absolutely clean when it is pressed in, thus allowing the theoretically calculated interference to be truly converted into physical bonding force, an indispensable process guarantee for achieving precision assembly.

[0057] Example 5: This example optimizes the structure and operation details of the servo press-fitting device, and focuses on solving the problem of accurate guidance in the initial stage of deep cryogenic pin press-fitting.

[0058] like Figure 5 As shown, the servo press-fitting device includes a set of servo electric push rods 12 as the power source for pressing. A force sensor and a displacement sensor are connected in series between the end of the push rod 12 (the front end of the telescopic rod) and the pin contact component, respectively used to accurately sense the pressing resistance and the real-time travel position of the push rod 12. A key structural feature is the inclusion of an independently height-adjustable conical pre-guide sleeve (not shown in the figure). This guide sleeve is vertically and vertically positioned directly above the assembly hole of the workpiece (planetary gear set) via another small cylinder or linear module. Its internal conical hole has its large end facing upwards and its small end facing downwards, aligning with the opening of the assembly hole.

[0059] In the initial press-fit stage, the gripping and transferring device suspends the pin approximately above the assembly hole. At this point, the control unit first issues a pre-guiding command to the press-fit device. The tapered pre-guide sleeve descends independently first, causing its lower precision mating interface to mate and press against the opening of the workpiece's assembly hole, completing the precise alignment of the guide sleeve and the workpiece. Subsequently, the servo electric push rod 12 is activated, driving the clamped cryogenic pin downwards. The end of the pin first enters the large end opening of the tapered pre-guide sleeve, and under the mechanical constraint and guidance of its tapered surface, it is forcibly and smoothly guided into the small hole at the lower end of the guide sleeve, and finally precisely enters the assembly hole of the planetary gear set. Throughout the insertion and press-fit process, force and displacement sensors continuously provide feedback signals.

[0060] Because the surface temperature of the cryogenic pin is extremely low, repeated attempts to align it at the near-room-temperature assembly hole opening can easily cause localized sudden cooling at the opening, leading to thermal deformation or condensation, affecting the smoothness of press-fitting or even causing jamming. The tapered pre-guide sleeve first engages with the hole opening, essentially creating a temporary guide channel for the pin that is unaffected by low temperatures and has a large capture range. Then, the push rod 12 only needs to advance linearly. Its advantage lies in decoupling the capture action from the pin's approach to the workpiece, avoiding unnecessary low-temperature contact between the pin and the workpiece hole opening, protecting the workpiece quality, and significantly improving the success rate and reliability of the pin's entry, greatly reducing the risk of press-fitting failure due to alignment deviations.

[0061] Example 6: This example details the core control algorithm, quality judgment logic, process model, and data traceability system of the system, which constitute the core intellectual embodiment of this invention.

[0062] The method in this embodiment integrates real-time modeling, online judgment, and full-chain traceability. The specific steps are as follows: 1. Coarse closed-loop control of cryogenic immersion time: The control unit has a pre-set thermal equilibrium time model based on the lumped parameter method. The core of this algorithm is to calculate the time required for the temperature at the pin's center to drop to the target temperature (e.g., -180℃). The calculation formula can be simplified as follows: Where ρ is the density of the pin material. For specific heat capacity, V For volume (calculated from weight and density). For the submerged surface area, At room temperature The temperature of liquid nitrogen. The target core temperature is h, which is the liquid nitrogen boiling heat transfer coefficient (which needs to be retrieved from the database based on the material surface characteristics and the state of the liquid nitrogen). The minimum soaking time is calculated based on the weight and material of each batch of pins and is enforced, forming a coarse closed-loop control in the time dimension to ensure that no undercooling occurs.

[0063] 2. Generation of the force-displacement reference model (envelope curve corridor): Before pressing begins, the system determines the room temperature diameter of each pin. ), coefficient of thermal expansion of material ( α ), preset low temperature ( ) and room temperature ( ) Temperature difference, calculate its theoretical diameter after cooling ( = / (1+α( - Combined with the measured diameter of the assembly hole ( The maximum and minimum theoretical interference are calculated. Then, based on parameters such as the material's elastic modulus, friction coefficient, and fit length, the theoretical upper and lower boundaries of the press-fitting force are calculated using the thick-walled cylinder theory, and plotted as an envelope curve corridor with an exponentially increasing shape in the force-displacement coordinate system. The nonlinear increasing characteristics of this corridor accurately reflect the physical process of continuous heat exchange between the pin and the hole wall during the press-fitting process, temperature rise, and volume expansion, resulting in a sudden and rapid increase in the interference.

[0064] 3. Continuous input and real-time judgment: After starting at the preset speed, the control unit controls the servo push rod to perform a continuous pressing action without interruption. Legally, any active pressure-holding pauses during the pressing stroke are explicitly prohibited to prevent data distortion caused by abnormal local heat exchange due to pauses. During this process, the system acquires force and displacement signals at millisecond-level high frequency and plots the measured force-displacement curve in real time. A dynamic comparison algorithm compares this measured curve point-by-point with a pre-generated envelope curve corridor. The judgment logic is as follows: if any data point of the measured curve touches or exceeds the upper boundary of the corridor, it is immediately judged as abnormal, and the system immediately stops and issues an audible and visual alarm. This usually indicates an abnormally large interference fit, possibly caused by poor surface scraping, material defects, etc. If the measured curve rises rapidly and smoothly within the corridor boundary, and finally, after reaching the target depth, the force value enters a pressure-holding window corresponding to the target depth and stabilizes within that window, the system automatically determines that the assembly is qualified.

[0065] 4. End-to-end data traceability: Regardless of the assessment result, the system will generate a single-piece-level digital twin quality profile for this assembly task. Through the MES interface, the system irreversibly binds the pin's pre-inspection data at room temperature (diameter, weight), the calculated forced immersion time, the complete force-displacement curve of the entire press-fit process (including measured values ​​and reference model corridors), and the final quality assessment result, and uploads it to the enterprise Manufacturing Execution System (MES). This profile serves as a lifetime quality certificate for the planetary gear set.

[0066] This embodiment deeply integrates materials science (thermophysics), mechanics (thick-walled cylinder theory), control theory (real-time comparison), and information science (digital twins) across multiple dimensions. Its advantages are multifaceted: First, zero-defect quality: Through online determination of the nonlinear envelope corridor, real-time and precise interception of microscopic anomalies in the pressing process is achieved, preventing defects from flowing into the next process. Second, process self-optimization: The nesting of coarse and fine closed loops (force-displacement comparison) allows for the detection of short-term process anomalies, while long-term data accumulation can be used to optimize the soaking time model and adjust the envelope corridor, achieving a spiral increase in the process capability index. Third, absolute traceability: Single-piece-level digital twin archives provide a complete and reliable data goldmine for quality backtracking, failure analysis, and even design optimization throughout the product's entire lifecycle, representing a core element for achieving intelligent manufacturing in the high-end manufacturing field.

[0067] Specifically, such as Figure 6 Therefore, the planetary gear pin assembly method of the present invention is carried out according to the following steps: Step 1: Pre-inspection at room temperature.

[0068] The incoming pins are fed individually into the ambient temperature pre-inspection unit. The system reads and automatically measures the diameter and weight at ambient temperature, and sends the data to the control unit to form the initial digital profile of the pin.

[0069] Step 2: Adaptive cryogenic treatment.

[0070] The pin enters the liquid nitrogen tank 2 of the liquid nitrogen cryogenic device, and the movable cover 3 rises and is filled with liquid nitrogen. Based on the weight and material properties of the pin, the control unit calls the thermal balance model algorithm to calculate the minimum forced immersion time customized for the pin and starts timing to achieve coarse closed-loop control of the cooling time.

[0071] Step 3: Clean gripping and transfer.

[0072] After the soaking time reaches the standard, the movable cover 3 descends, and the pin opens the cover plate 4 to enter the gripping position. The gripping and transfer device immediately descends and fits into the pin. During the fitting process, the annular scraper ring 7 at its end mechanically scrapes away the surface condensate. At the same time, the air supply line introduces slightly positive pressure dry cold nitrogen into the housing 6 to form an air seal. After the clamping mechanism clamps the root of the pin, the robotic arm quickly transfers it to the upper part of the pressing station under heat insulation.

[0073] Step 4: Generate personalized pressing model.

[0074] During the transfer process, the control unit synchronously calls the room temperature pre-inspection data of the pin, the built-in material database and the measured dimensions of the workpiece assembly hole, calculates its cryogenic theoretical diameter, and generates an exponentially growing "force-displacement envelope curve corridor" with upper and lower boundaries based on the thick-walled cylinder theory, which serves as the unique reference benchmark for this press fitting.

[0075] Step 5: Pre-guidance and uninterrupted intelligent pressing.

[0076] At the press-fitting station, the tapered pre-guide sleeve descends first to fit against the workpiece's assembly hole opening. The servo-driven electric push rod then advances, guiding the pin through the guide sleeve into the hole, performing a continuous, uninterrupted press-fitting motion. Force and displacement sensors provide real-time feedback signals at high frequency.

[0077] Step Six: Online Quality Assessment.

[0078] The control unit dynamically receives force and displacement signals, generates a measured force-displacement curve, and compares it with the envelope curve corridor generated in step four in real time, point by point. Once the measured curve touches the upper boundary, the system immediately alarms and stops; if the curve runs within the corridor and eventually stabilizes within the end pressure holding window corresponding to the target depth, a "qualified" signal is issued.

[0079] Step 7: Archive the digital twin file.

[0080] Regardless of whether the product passes or fails, the system will package all data within this assembly cycle, including pin ID, room temperature pre-inspection data, soaking time, complete press-fit force-displacement measured curve and reference model, and final quality judgment result, into an unalterable single-piece-level digital twin quality file, and upload it to the MES system to complete the data loop and provide the product with lifelong quality traceability capabilities.

[0081] The system resets and enters the next loop.

[0082] The foregoing has shown and described the basic principles, main features, and advantages of this application. Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this application. Various changes and modifications can be made to this application without departing from the spirit and scope thereof, and all such changes and modifications fall within the scope of this application as claimed. The scope of protection of this application is defined by the appended claims and their equivalents.

Claims

1. A planetary gear set pin assembly system, characterized in that, include: The liquid nitrogen cryogenic device is used to completely immerse the pin to be pressed in liquid nitrogen and cool it to a preset low temperature, and can remove a single cooled pin from the liquid nitrogen to a gripping position. A gripping and transferring device is used to grip the pin from the gripping position in an insulated environment, perform surface cleaning treatment during the gripping process, and then transfer the pin to the pressing station; A servo press-fitting device is used to press the cooled pin into the assembly hole when the gripping and transferring device transfers the pin to the assembly hole position of the planetary gear set. The control unit is configured as follows: Obtain the room temperature pre-test parameters for each pin, measured before it enters the liquid nitrogen cryogenic device; Based on the ambient temperature pre-inspection parameters and the preset material thermophysical database, the theoretical shrinkage or theoretical diameter of the pin at the preset low temperature is calculated, and a force-displacement reference model for controlling the press-fitting process is generated accordingly. The liquid nitrogen cryogenic device, gripping and transferring device, and servo pressing device are controlled to work in coordination. During the pressing process, the force and displacement signals of the servo pressing device are collected in real time to generate a measured force-displacement curve, which is then compared with the force-displacement reference model to determine the assembly quality.

2. The planetary gear set pin assembly system according to claim 1, characterized in that, It also includes a room temperature pre-inspection unit, which measures and records the room temperature diameter and weight of each pin before it enters the liquid nitrogen cryogenic device, and sends the measurement data to the control unit as the room temperature pre-inspection parameters.

3. The planetary gear set pin assembly system according to claim 1, characterized in that, The liquid nitrogen cryogenic device includes: Base; A liquid nitrogen bath is a hollow cylindrical structure with its lower end fixed to a base and its upper end open, used to house the pins to be cooled. The movable cover is vertically connected to the base at its lower end, and has a through hole at its upper end for the pin shaft to pass through. A cover plate is provided on the through hole to seal the through hole. When the movable cover is fully raised, it forms a sealed cavity with the base that covers the liquid nitrogen tank and the pin shaft placed inside. An injection tube, with its upper end extending through the base into the sealed cavity, is used to inject liquid nitrogen into the sealed cavity. The control unit determines the shortest forced immersion time based on the weight and material properties of the pin, and after the pin has been in liquid nitrogen for that time, it controls the movable cover to move down so that the pin opens the cover plate and waits to be grabbed.

4. The planetary gear set pin assembly system according to claim 1, characterized in that, The grasping and transferring device includes: A heat-insulating shell with an internal cavity and an opening at the lower end for a pin to enter; An annular scraper ring is coaxially fixed inside the opening. Its inner diameter is smaller than the diameter of the pin at the preset low temperature. It is used to mechanically scrape off the liquid nitrogen and frost adhering to the surface of the pin when the gripping and transferring device is put into the pin. A clamping mechanism is provided inside the heat insulation housing for clamping the root of the pin that has been scraped off. An air supply line, connected to the cavity, is used to continuously introduce slightly positive pressure dry cold nitrogen into the cavity during the grasping process to prevent external moisture from entering.

5. The planetary gear set pin assembly system according to claim 1, characterized in that, The servo press fitting device includes: a servo electric push rod; a force sensor and a displacement sensor installed in series between the end of the push rod and the pin contact component; and a tapered pre-guide sleeve that can be raised and lowered above the workpiece assembly hole; at the beginning of the press fitting stage, the control unit controls the tapered pre-guide sleeve to descend first and fit against the assembly hole opening, and then the push rod drives the pin to be guided into the hole through the guide sleeve.

6. The planetary gear set pin assembly system according to claim 1, characterized in that, The force-displacement reference model generated by the control unit is a nonlinear increasing envelope curve corridor that reflects the sharp increase in interference due to heat exchange during the pressing process of the cryogenic pin shaft. The control unit is configured to compare the measured force-displacement curve with the envelope curve corridor in real time during the pressing process. When the curve exceeds the upper boundary of the corridor, an anomaly is determined and an alarm or shutdown is executed.

7. The planetary gear pin assembly system according to claim 6, characterized in that, The control unit is configured to generate a force-displacement envelope curve corridor reflecting the change in interference due to heat exchange during the pressing of the cryogenic pin shaft, based on the ambient temperature pre-inspection parameters and the material database, as a reference model, and to compare the measured curve with the corridor in real time during the pressing process.

8. A method for assembling planetary gear pins, applied to the system as described in any one of claims 1 to 7, characterized in that, Includes the following steps: Under normal temperature conditions, measure and record the diameter and weight of each pin. The pin is sent into a liquid nitrogen cryogenic device and completely immersed in liquid nitrogen. The forced immersion time is determined according to the weight and material properties of the pin to ensure that its overall temperature reaches the preset low temperature. A gripper with mechanical scraping and micro-positive pressure cold nitrogen protection functions is used to grip the cooled pin. During the gripping action, the condensate on the surface of the pin is scraped off and clamped in a dry cold nitrogen atmosphere. The gripper holding the pin is quickly moved to the press-fitting station while in a heat-insulated state; The control unit calls the room temperature pre-inspection data of the pin, the material database and the measured dimensions of the workpiece assembly hole, calculates the theoretical diameter of the pin at the preset low temperature based on the thermal expansion theory, and generates a force-displacement envelope curve corridor with upper and lower limits and exponential growth shape as a reference model based on the thick-walled cylinder theory. Align the pin with the workpiece assembly hole, start the pressing process at a preset speed, and continuously press the pin into the target depth throughout the process; during this process, monitor the force and displacement in real time, generate a measured force-displacement curve, and compare it with the force-displacement reference model; Based on the comparison results, determine whether the assembly is qualified.

9. The planetary gear set pin assembly method according to claim 8, characterized in that, In the cryogenic treatment step, the forced immersion time is calculated based on the material, weight, and liquid nitrogen temperature of the pin, using a preset thermal equilibrium time model and multiplied by a safety factor, thus forming a coarse closed-loop control in the time dimension.

10. The planetary gear set pin assembly method according to claim 8, characterized in that, It also includes a full-process data traceability step: binding and uploading the pin's room temperature pre-inspection data, cryogenic immersion time, complete force-displacement curve of the press-fitting process, and the final quality judgment result to the manufacturing execution system to form a single-piece level digital twin quality file.

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