This commonly overlooked inspection item is critical for digger derricks
By Clint Davis
One of the most commonly overlooked inspection items for digger derricks is the rotation bearing. While some deflection in the system is expected, if the bearing is too loose or has excessive backlash, this may be an indication that the rotation bearing is worn and needs to be replaced. It could also be an indication that the boom has been shock loaded. Left unresolved, deflection exceeding the maximum that is allowed will create localized loads, causing permanent deformation and possibly early failure of the bearing.
It is common for digger derricks to perform most of their work in the same general rotation area, which leads to uneven wear patterns and excessive deflection, impacting accuracy and productivity of drilling activities. Rotation bearing deflection testing is also critical for aerial devices. Although the process is similar, instructions for boom positioning will vary depending on whether the unit is an Over-Center or Non-Over-Center Aerial Device. This article refers specifically to digger derricks.
It’s common for digger derricks to perform most of their work in the same general rotation area, which leads to uneven wear patterns and excessive deflection.
It’s important to measure the deflection of the boom according to manufacturer’s instructions. This test is recommended to be performed annually as the machine ages and wear increases or in accordance with manufacturer’s recommendations.
Step By Step
After setting the equipment up in a location free from overhead obstructions, position the boom in the location where it is most frequently used during operations. This is usually to the curbside of the truck or 45 degrees forward or backward of this position.
Fully extend the second boom section and lower the auger, placing it on an outrigger pad at the position where the auger contacts the ground (see Fig. 1). If the unit is not equipped with an auger, stack an adequate number of outrigger pads to allow the sheave head to make contact with the outrigger pads. Boom down slowly and stop just above the outrigger pads. This puts the boom in a position for consistent test methodologies each time rotation bearing deflection is measured.
Figure 1. Fully extend the second boom section and lower the auger, placing it on an outrigger pad at the position where the auger contacts the ground.
Next, attach the dial indicator with a magnetic base to the pedestal, positioning the tip perpendicular to the edge of the turntable bottom plate and at the specified indicator radius (see Fig. 2). Consult the unit-specific maintenance manual for the radius for your model. Make sure you know which way the dial indicator rotates when it moves in order to record the correct reading. Then, zero out the dial indicator, verifying that you have at least 0.25 inches of movement on the dial indicator in both directions.
Figure 2. Attach the dial indicator with a magnetic base to the pedestal, positioning the tip perpendicular to the edge of the turntable bottom plate and at the specified indicator radius.
Slowly and carefully lower the sheave head or auger onto the outrigger pad. Referencing the hydraulic pressure gauge, boom down until the gauge reads between 1,600 and 1,800 psi. (Note: These values are specific to Terex Utilities equipment and may vary depending on the make and model.)
The number on the dial indicator is your rotation bearing deflection. Record the number and check it against the maximum allowable bearing deflection in your manufacturer instructions. If the value exceeds the maximum allowable bearing deflection, the rotation bearing must be replaced.
One final tip: It’s a good idea to measure the deflection at the same boom location and dial indicator radius every time this test is performed in order to provide consistent measurements that can be compared over the life of the machine. Marking the dial indicator position with a center punch or paint spot will allow the test to be consistently repeated in the future. UP
Author’s Note: The information presented here is taken from Terex Utilities Tech Tip #4. Tech Tips provide additional insight into frequently asked questions that our service representatives receive and are a supplement to service manual instructions. To find the Tech Tips, visit terex.com/utilities, click on Support, then choose Tech Tips. Tech Tip #4 can be found under the Digger Derrick product category.
the Author: Clint Davis is a Terex National Service Technician with 32 years of experience at Terex. He has extensive knowledge on manufacturing and testing digger derricks and has been training for Terex for the past 7 years.