6.3.1 Bucket Elevator Chain
Large conveyance: Vertical conveyance of grain and other bulk materials for the cement, chemical, and food industries
Bucket Elevator Chains convey bulk materials vertically. You might see this type of chain used to move cement, coal, or grain, for example. Buckets are installed at regular intervals, and the chain moves continuously, scooping and conveying the product. Because they are effective and economical, Bucket Elevator Chains are widely used (Figure 6.10).
When the chain engages the upper sprocket, the buckets are tipped, and conveyed objects are discharged. Discharging occurs either with centrifugal force or continuously, which uses the bottom side of the bucket as a guide for the next bucket.
The trend has been for bucket elevator equipment to become smaller, to economize on installation costs. To reduce the operating costs, the chain must travel faster (more than 80 m/min.). Therefore, the centrifugal discharge bucket elevator has become more common (Figure 6.11). Usually, the capacity of the conveyed material is within the range of 300 ton/h.
Figure 6.10 Bucket Elevator
Figure 6.11 Discharging of Buckets Occurs with Centrifugal Force or Continuously
In large-bucket elevators, two chains are installed, one on each side of the bucket. Small-bucket elevators use only one strand of chain. The two-strand arrangement is a preferable design, ensuring safer operation.
Construction and Features
Bucket Elevator Chain is based on standard large pitch conveyor chain with K-2 or G-4 attachments. Buckets are spaced evenly (usually every two links) over the length of chain.
Three important construction features include the following:
- Superior wear resistance of pins and bushings, which reduces chain elongation. This has become increasingly important as cement makers have increased the amount of slag in concrete.
- High fatigue resistance.
- Easy connecting and disconnecting. This is very important because of the limited space in the elevator housing. Chain must be easy to handle.
Usually, sprockets with 12 teeth are used in low-speed bucket elevators (chain speed less than 45 m/min.). High-speed bucket elevators normally require 24-tooth sprockets. It is important to choose a sprocket with suitable pitch-line clearance.
Excess conveyed material may accumulate in the bottom of the casing, which can cause accelerated wear. Worn chain and sprockets will not engage correctly; the sprockets may have to be replaced. Sometimes, welding material onto the tooth at the point of excessive wear will restore the sprocket, but it is not recommended. Additionally, this procedure is extremely difficult with the sprocket that is located at the top of the bucket elevator.
Even sprockets with hardened teeth are subject to excessive wear in bucket elevators, due to the abrasive nature of conveyed materials. For example, in cement conveyors, there is a point of sprocket hardness at which wear is virtually eliminated. However, it is impractical and expensive to make such hard teeth in standard sprockets. In the 1980s, detachable-tooth sprockets were developed, which permit the replacement of the tooth part only (Figure 6.12). The body of the sprocket remains on the shaft, which reduces repair time and costs. By using special materials in the tooth insert, high tooth hardness is achievable, and therefore, wear life is increased. Use of detachable-tooth sprockets is increasing, especially in the cement industry.
Figure 6.12 Detachable-Tooth Sprocket
Selection and Handling
- Choose Bucket Elevator Chain carefully. If the chain breaks, it is extremely difficult and time consuming to remove broken chain from the bottom of the casing. Re-installation is also very demanding. Rely on chain from manufacturers with proven records for quality and reliability.
- Use special tools to connect and disconnect chain links.
- Avoid grinding the pins, heating the plates, or increasing the size of side plate holes during chain assembly. These procedures, sometimes used at facilities, allow easier assembly of links; however, it compromises the strength of chain, which can lead to ultimate failure.
- Consider using detachable-tooth sprockets.
- For safety reasons, inspect chain and sprockets frequently, since chains and sprockets have a limited useful life.