Overhead Hoist Transport (OHT) Wheels Power for Wafer Transport

In the automated material handling system of semiconductor manufacturing, the Overhead Hoist Transport (OHT) crane has become a core equipment for wafer transportation in 300mm and next-generation FABs, thanks to its unique advantage of operating on overhead rails installed on the ceiling. The performance of its dedicated wheel assembly directly affects the OHT crane's transportation precision, operational stability, cleanliness control, and service life, serving as the core hub that connects the entire wafer transportation process.

Definition

An OHT crane is an automated system installed on the ceiling of semiconductor factories to transport Front-Opening Unified Pods (FOUPs) along overhead rails. Its core value lies in improving material transportation efficiency and reducing the risk of contamination caused by manual operations. The realization of these functions relies on the stable operation of the wheels: the wheels bear the entire weight of the crane body and the FOUP, and enable the high-speed movement of the crane through rolling cooperation with the rails. Meanwhile, their precise guiding performance ensures the crane does not deviate in complex rail layouts, laying a solid foundation for the safe transportation of wafers.

Core Position in the Composition System

The four core components of the OHT crane— rails, hoist, carrier, and control system— all need to achieve collaborative linkage through the wheels. The core connecting role of the wheels in the system is specifically reflected in the following aspects:

Precise Adaptation to Rails

As the running track of the crane, the straightness and flatness of the rails are converted into the operational precision of the crane through the fitting degree of the wheels. The wheels of OHT cranes adopt a customized contour design, and their contact surfaces with the rails are precision-ground to ensure tight fitting, preventing shaking or deviation during operation.

Power Transmission for Hoist Operation

The hoist is the lifting core of the crane. The moving power of the hoist along the rails is transmitted to the wheels through the drive motor, and then converted into driving kinetic energy by the friction between the wheels and the rails. The stability of the wheel's friction coefficient directly determines the efficiency of power transmission, avoiding power loss issues such as "slippage".

Indirect Guarantee for Carrier Safety

The carrier is used to hold and protect FOUPs, and its stability depends on the smooth operation of the crane. The buffering performance of the wheels can absorb the inertial impact during the crane's start-stop and steering, preventing vibration from being transmitted to the carrier and causing wafer damage.

Execution Terminal of the Control System

The control system monitors the crane's operating status in real time through sensors. When it is necessary to adjust speed, steer, or stop, the commands are finally implemented by controlling the driving and braking of the wheels. The response speed and braking precision of the wheels directly determine the regulation effect of the control system.

Core Carrier of Working Principle

The working process of OHT cranes— "precision control, efficient handling, and safety protection"— is essentially the implementation process of the wheels' technical characteristics. Through three key mechanisms, the performance of the wheels is converted into the operational advantages of the crane:

1. Precision Guiding Mechanism: The wheels of OHT cranes adopt a combined design of "main wheels and guide wheels". The main wheels are responsible for bearing and driving, while the guide wheels fit the side of the rails to achieve precise positioning. The rim of the guide wheels is precision-machined, and the gap with the rails is controlled within 0.1mm. Combined with real-time monitoring by the control system, the running deviation of the crane on complex rails is ensured to be no more than ±0.5mm, providing a guarantee for the precise docking of FOUPs;

2. High-Speed Stability Mechanism: The wheels are made of high-strength and lightweight materials, combined with high-precision bearings, which can achieve high-speed rolling of 3.5m/s. During start-stop, elastic structures absorb impact to avoid "bouncing". At the same time, the dynamic balance precision of the wheels is controlled above Grade G2.5, ensuring no abnormal vibration during high-speed operation and preventing wafers from shaking in FOUPs;

3. Cleanliness Protection Mechanism: The wheels adopt an oil-free lubrication design combined with a sealed structure to prevent organic contamination caused by volatilization of lubricating oil. The surface of the wheel body is coated with a special coating to reduce particles generated by rolling friction. At the same time, a built-in micro-dust suction structure absorbs trace impurities generated during operation, ensuring that the crane's operation has no impact on the Class 1 clean environment of the workshop.

With the intelligent upgrading of OHT cranes, the wheels are also integrated with wear detection sensors, which can upload operating status data to the control system in real time to realize "predictive maintenance". When the wheel wear reaches the threshold, the system automatically issues an early warning to avoid production interruption caused by sudden failures.

Conclusion

As semiconductor manufacturing advances to 2nm and below processes, wafers are becoming larger in size and higher in precision requirements, placing more stringent demands on the transportation performance of OHT cranes. The core of these demands lies in the upgrading of wheel technology. From load-bearing capacity to guiding precision, from cleanliness performance to service life guarantee, every technical indicator of the wheels is directly related to the yield rate and production efficiency of wafers.