Engineered Casting Repair Service, Inc.

Grinding Mills

32 ft Diameter Sag Mill

This mill is located in Australia. A severe erosion problem was detected in this mill during a routine maintenance inspection. This is when ECRS was contacted and asked to take a look at it. After looking at the mill and doing preliminary calculations, it was decided that the mill could be effectively repaired.

survey of damages
The only damage to the mill was the erosion. The flange where the head and shell bolt together was the area of extreme erosion. The bolts through this flange were almost eroded completely in two at some locations. Product had slipped between the liner and the shell to cause this problem. Shown in the picture below is a bolt that is scalloped out from the long erosion process. This shows the extent of the erosion problem in this mill.

FEA analysis
Finite element modeling was used in the design of this repair. It was determined that reinforcement plates would mount to the outside of the head/shell joint to provide support. First, a model of the eroded mill was run without plates. Then the model was run with the plates installed. The model on the left is eroded without reinforcement plates. You can see the Von Mises stress exceeds 39,000psi. The model on the right is also eroded, but has the reinforcing plates installed. The stress here is under 17,000psi. This is less than 1/2 of the stress in the model without reinforcement. This proves our repair to be very effective.

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repair procedures performed
Once the design was proven to be both feasible and effective, ECRS employees traveled to Australia to perform the repair. Erosion was present over 3/4 of the circumference of the mill. Approximately 30 plates were mounted across the head/shell joint of the mill. These plates were bolted to both the head and the shell. Following is a model of one of these plates as well as pictures of these plates as they were installed.

The customer only wanted this repair to continue production for a couple of months while a new mill was being fabricated. It turned out that this mill held up in operation for almost 3 years after this repair. It was still operating fine when it was replaced by the new mill.

Redesign & Replacement of Trunnion

survey of damages
The trunnion and head on this mill was a one piece casting. It became badly eroded when product leaked behind the trunnion liner and raced around the ID until it eroded nearly through the 5" thickness of the trunnion wall. When the remaining thickness reached ¾", the trunnion broke completely off as shown in the image.

FEA analysis
Finite element modeling was used to simulate actual operation of the mill and to perform stress analysis. The repair consisted of converting the one piece trunnion/head design to a two piece configuration; therefore, FEA was first used to assure that the design for the attachment of the new trunnion to the old head was sufficient. Once the design of the attachment was finalized, the head and trunnion were analyzed as an assembly to assure that the design did not create any high stress locations, thus keeping the probability of failure very low.

repair procedures performed
A new trunnion was cast and then machined to rough dimensions. During this time, the head was also machined to receive the new trunnion. The trunnion was then attached with METALOCK® and METALACE components, as well as a series of high strength custom bolts.

Once the trunnion was securely attached to the head, the assembly was mounted onto a vertical boring mill for the machining of the OD and ID of the trunnion back to OEM specifications. The head/trunnion assembly was then fastened back to the mill and returned to service. This repair was performed over 5 years ago, and the mill is still in operation today.

Repair & Reinforcement of Trunnion

We have also repaired other trunnions with similar erosion problems, where the trunnion did not completely shear from the head but cracking had initiated. In these cases, we were able to offer a repair and reinforcement option along with the replacement option. When the repair option was chosen, we designed, fabricated, and installed a reinforcing sleeve on the ID of the trunnion. We have had very good success with these repairs.

Ring Gear for a 1250 HP Ball Mill

ECRS engineers were contacted when a crack was discovered in the rim of the gear between 2 teeth during a routine maintenance inspection. The crack completely severed the rim along the line of the root of the tooth.

FEA analysis
Finite element analysis was used in the repair design for this gear. One model was run with the crack present and no repair. These results are on the left. The model on the right also possesses the crack but has the repair & reinforcement plates installed on the inside. This FEA illustrates the effectiveness of our repair design.

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repair procedures performed
Based on the analysis performed and our previous experience in repairing cracks of this type we concluded that the crack was to be repaired utilizing metalock® and metalace repair components. Below is a picture of the crack before & after the repair. The holes you see are for the attachment of the reinforcement plates.

reinforcement installed
Once the metalock® & metalace components were installed, additional reinforcement was provided by the installation of two pieces of 1" thick plate that were bolted to the back side of the rim on either side of the crack and then welded together. This provides the gear with more cross-sectional area at the crack plane. These plates are shown mounted below. They have not yet been welded together.

repair complete
Once these plates were welded, the mill was ready to return to operation.