Undergraduate Senior Project - GCPUD Lifting Device

Introduction

The rotor pole lifting device for Priest Rapids Dam was designed and analyzed by student intern Baylie Johnson of Grant County Public Utilities District. The device was manufactured by Busby International, Inc. out of Moses Lake, WA. Because this device is going to be used for lifting heavy object over people’s heads, it was necessary to do a load test to ensure safety. This load test was performed by student intern Baylie Johnson, hydro mechanic foreman Tom Marty and hydro mechanic Beau Campbell. The test consisted of assembling the device, attaching the device to the crane and the mounting surface, applying load to the device and measuring strain through strain gages in various places. The following report with explain the procedure in more detail, present the data taken and discuss the data.

Method/Approach

The resources needed for this test include the following:

Priest Rapids Dam Powerhouse Crane

2 Slings
2 Shackles
Access to a floor mount
Dynamometer
Camera
Strain gages
Strain indicator
Switch and balance unit

Extension cord

The device will be mounted to the floor and attached to the crane with a dynamometer between the device and the crane to properly load the device to specified loads. At each load, the strain will be recorded from each strain gage. Three strain gages were used for this test. Two were located on one of the side plates, one was oriented axially in the center of the plate and the other was underneath the bolt hole used to mount the side plate and bottom plates to measure shear. The third strain gage was located on the top of the top plate.

The side plates are what carry load between the top and bottom plates. They are a very sensitive part of the system and that it why they were chosen to be analyzed using strain gages. The orientation of the axial strain gage was chosen because the stress in the side plates is axial. The shear strain gage was used to measure the shear stress presented by the load on the bolt. The predictions for strain in these gages at a the normal working load (3500 pounds total, 1750 pounds per side plate) is 27 μs axially and 103 μs in shear. The limits of these predictions, based on the tolerance of the dynamometer are 26-28 μs axially and 100-106 μs in shear.

Procedure

The following procedure was used to conduct the vertical load test.

1. Lower the 30 ton crane hook from the powerhouse crane.

2. Fully assemble the device.

3. Wrap a sling around the hook.

4. Attach a shackle to the sling.

5. Attach a dynamometer to the shackle.

6. Attach another shackle between the bottom side of the dynamometer and the lifting lug on the device.

7. Attach the bottom plate to a suitable mounting point.

8. Record the dynamometer reading without any load applied (this is the weight of the device).

9. Connect the strain gages to the strain indicator and zero the amperage, input the correct gage factor and balance the gages to zero. This means that the load that is contributing to the strain reading is only the difference between the dynamometer reading and the weight of the device.

10. Load the device to a dynamometer reading of approximately 1500 lbs.

11. Record the actual load.

12. Record the strain of each gage.

13. Repeat steps 14 thru 16 for loads of approximately 3000, 5500 and 7000 lbs.

14. Release the load.

15. Lay the device down in a safe and accessible place.

16. Inspect the device for any cracking or deformation. Record anything seen.

Results

Axial Plate Strain

Discussion

Axial Plate Strain

The results from the strain recorded from the axially mounted rod strain gage were not favorable. The error was very high between the actual strain and the predicted strain, more than 200% in most cases, depending on the load. This amount of error is far too high to find this data acceptable. . The gages were not mounted in a particularly precise way. This means that the gage could have been crocked and that would have caused error. Also, the accuracy of the dynamometer is only ±100 pounds with a tolerance of ±50 pounds and was located fairly far overhead, so the ability to read the dynamometer with very much accuracy was difficult. Any difference in the load that was applied versus the load that was recorded would cause a small amount of error between the predicted strain and the actual strain.

The precision of these measurements was not required to be very high. The take away from this data is that the measured load was lower than the predicted, and in both cases, the load is so far underneath the maximum load with a safety factor of five.

Shear Plate Strain

The shear strain is slightly more critical than the axial strain. The load at this point is the highest of all points on the device. The error of these measurements is very high, 441% at the least. This amount of error makes this data unusable. The data did however, show actual strain much lower than predicted. This means that the stresses are so low that the device is still safe to use. Factors into the error are the same as those discussed with the axial plate strain.

Top Plate Strain

The top plate strain data also shows that the stresses are so low that it will not fail in an overload case. The error of this data is also high, up to 65%. This is likely for the same reasons stated in the discussion about the axial rod strain. The strain gage could be crocked or not applied completely correctly or the error could be caused by the tolerance of the dynamometer.

Conclusion

Although the error between the predicted strain and the measured strain gets to be quite high, up above 1400%, the resulting data of the vertical load test is sufficient enough to prove that this device can be safely used overhead with a safety factor in all tested parts of more than 5. The device can be loaded to twice its rated loading without failing. Failure in this case is defined as the yielding of the parts of this device. Modification of this device is not required.

Acknowledgments

Thank you to Tom Marty, Grant County PUD hydro mechanic foreman and Beau Campbell, GCPUD hydro mechanic.