Approach & Benchmark

This design is based on a similar design made by Alstom and used for installing generator rotor poles at Wanapum Dam. There is a drawing for a device used by English Electric when Priest Rapids Dam was first constructed. Through discussion with a diverse group of people, a design similar to the one used at Wanapum was chosen because of the ease of use and the ability to build this design in a way that would allow it to be used when the rotor is installed in the unit as well as when it is outside the unit. The image to the right shows the device being used at Wanapum.

Performance Predictions

No part of the device will exceed a strain of 400 μs for A36 parts. This strain was determined based on a maximum stress of 12,000 psi and a modulus of elasticity of 29,000 ksi.

Design Parameters

For A36 Structural Steel parts:

Minimum Ultimate Strength of Structural Steel                  Sultimate = 60000 lbf/in2

Safety Factor Required                                                                  n = 5

Maximum Allowable Tensile Stress                                          σmax =  = 12000 lbf/in2

Maximum Allowable Shear Stress                                            τmax =  = 6924 lbf/in2

For Weld Joints:

Minimum Ultimate Strength of Weld Metal                         Sweld = 70000 lbf/in2

Safety Factor Required                                                                  n = 5

Maximum Allowable Tensile Stress                                          σmax weld = = 14000 lbf/in2

Maximum Allowable Shear Stress                                            τmax weld =  = 8078 lbf/in2

Scope of Testing and Evaluation

Testing of this device included two load tests; one test loaded the device when it was in a vertical position and the other when the device in a horizontal position.

Other Device Decisions

Most of the design decisions that were not made based on calculations were based on items similar to those found on Wanapum’s device. For instance, the size of the hole used in the crane lug is the same size as that used in Wanapum’s device to ensure that the proper equipment was available.

Technical Risk Analysis

The risks involved in this project are somewhat minimal. The most risk is within the manufacturing portion of this device. Getting a shop to build the device in the necessary time frame and getting them to actually deliver on time is the riskiest element of this build.

Failure Mode Analyses

The failure modes of each part and all connections were analyzed. Different parts have different critical load scenarios and therefore they were each analyzed based on this scenario.

Safety Factors

The rule of thumb at Grant County PUD is that any overhead lifting device requires a safety factor of 5. Although not all of the parts of this device are intended to be used overhead, a common safety factor of 5 was used throughout the analysis.

Operation Limits

This device is intended to be used with a crane rated at, at least 10,000 pounds. Using this device for a rotor pole on a smaller crane could result in failure.

Examples of Analysis

Below are examples of analysis completed on the side plate and the top plate. You can see that the predicted strain for the side plate was 27 μs axially and 103 μs in shear. You can also see it was predicted that the strain in the top plate at the normal working load would be 11 μs.

Note: See the vertical testing page for test values.